space science

2011-04-28T05:20:00.001-07:00

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2011-04-16T04:58:00.001-07:00

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ALIENS

2009-12-19T02:53:00.000-08:00

Close encounter

A close encounter in ufology is an event where a person witnesses an unidentified flying object. This terminology and the system of classification behind it was started by astronomer and UFO researcher J. Allen Hynek, and was first suggested in his 1972 book The UFO Experience: A Scientific Inquiry.^[1] He introduced the first three kinds of encounters; more sub-types of close encounters were later added by others, but these additional categories are not universally accepted by UFO researchers, mainly because they depart from the scientific rigor that Hynek aimed to bring to ufology.^[2]

Sightings more than 500 feet (160 m) from the witness are classified as "Daylight Discs," "Nocturnal Lights," or "Radar/Visual Reports."^[3] Sightings within about 500 feet are subclassified as various types of "close encounter." Hynek and others^[4] argued a claimed close encounter must occur within about 500 feet to greatly reduce or eliminate the possibility of misidentifying conventional aircraft or other known phenomena.

Close Encounter of the First kind

A sighting of one or more unidentified flying objects:

Flying saucers
Odd lights
Aerial objects that are not attributable to known human technology

Close Encounter of the Second kind

An observation of a UFO, and associated physical effects from the UFO, including:

Heat or radiation
Damage to terrain
Human paralysis (Catalepsy)
Frightened animals
Interference with engines or TV or radio reception.
Lost Time: a gap in one's memory associated with a UFO encounter.^[5]

Close Encounter of the Third kind

An observation of what Hynek termed "animate beings" observed in association with a UFO sighting.^[6] Hynek deliberately chose the somewhat vague term "animate beings" to describe beings associated with UFOs without making any unfounded assumptions regarding the beings' origins or nature. Hynek did not necessarily regard these beings as "extraterrestrials" or "aliens." Additionally, Hynek further expressed discomfort with such reports, but felt a scientific obligation to include them, at the very least because they represented a sizable minority of claimed UFO encounters.

Bloecher subtypes

The UFO researcher Ted Bloecher proposed seven subtypes for the close encounters of the third kind in the Hynek's scale.

A: An entity is observed only inside the UFO

B: An entity is observed inside and outside the UFO

C: An entity is observed near to a UFO, but not going in or out.

D: An entity is observed. No UFOs are seen by the observer, but UFO activity has been reported in the area at about the same time

E: An entity is observed. But no UFOs are seen and no UFO activity has been reported in the area at that time

F: No entity or UFOs are observed, but the subject experiences some kind of "intelligent communication"

Close Encounter of the Fourth kind

A human is abducted by a UFO or its occupants. This type was not included in Hynek's original close encounters scale.^[6]

Jacques Vallee,^[7] Hynek's erstwhile associate, argued that a CE4 should be described as "cases when witnesses experienced a transformation of their sense of reality," so as to also include non-abduction cases where absurd, hallucinatory or dreamlike events are associated with UFO encounters.

Also in some cases, The Fourth Kind is described as being in the presence or under possession. Described in various UFO abduction cases as the most fearful experience of any.. the presence, not true visibility, but the realm, or sense of their presence.

Close Encounter of the Fifth kind

Named by Steven M. Greer's CSETI group, these purported encounters are joint, bilateral contact events produced through the conscious, voluntary and proactive human-initiated or cooperative communication with Extraterrestrial intelligence. This is very similar to some "contactees" of the 1950s who claimed regular communication with benevolent aliens.

Close Encounter of the Sixth kind

On Michael Naisbitt's website, a sixth proposed CE scenario is described as UFO incidents that cause direct injury or death.^[8] This category was not included in Hynek's scale, and is furthermore redundant: a CE2 in Hynek's scale specifically included UFO encounters that leave direct physical evidence of any kind.

Close Encounter of the Seventh kind

The Black Vault Encyclopedia Project proposes a Close Encounter of the Seventh Kind as human-alien hybridisation,^[9]. This concept similar to ideas promoted by ancient astronauts theorists like Erich von Däniken, Zecharia Sitchin and Robert K.G. Temple, in that extraterrestrials interacted with, perhaps interbred with and influenced ancient human beings in the past^[10].

This concept of CE7 is at odds with Hynek's original concepts, however. Hynek's CE3 specifically avoided describing UFO occupants as "aliens" or "extraterrestrials," contending that there was not enough evidence to determine if beings associated with UFOs had an objective physical reality, let alone to confirm their origins or motives.

LATEST UFO SIGHTINGS

UFOs have been showing up, all over the world, from time to time. Some of the latest ufo sightings 2009 took place in Europe, and also in United States. Many people, believers and skeptics alike, now believe that this maybe a sign of imminent contact or revelation about life forms outside our planet.

Amongst the latest UFO sightings 2009, is the Mysterious craft which was spotted over a small town in the Czech Republic on 14th of February. This created quite a stir amongst the local population; many flocked to the local radio stations, newspapers and police departments with calls and enquiries regarding the strange objects which appeared in the sky. Though the flashes of strange light lasted for about a minute, it caught the attention of many locals. Many people who were witness to the incident said that there was no earth-like feeling about the light and the speed at which it vanished. Many of the residents, who were previously skeptical about UFOs, after this incident, believe that there is a possibility of extra terrestrial activity.

In Florida, USA, on 02-28-09 there was report of one of the latest UFO sighting 2009. The witnesses saw a huge cigar-shaped craft, followed by four triangular-shaped ones. The sightings lasted for 1-2 minutes. After this they also saw a series of balls of light in different formations. All the objects and lights seen by the witnesses allegedly were moving south to north, at the same speed, over the Atlantic Ocean for more than an hour. Many airplanes and helicopters which were in the area, seemed undisturbed by the flying objects. Conspicuously there was no report of this incident in the local papers.

Another one of the latest UFO sightings in 2009 was in Kentucky on 02-29-09. According to some of the witnesses, they saw a disc like flying object fling over the horizon. Surprisingly this object was flying at a low altitude, and looked like it was grazing over the rooftops, according to some observers. As it was raining that very day, not many people got a glimpse of it, but those who did, claim that the disc like object looked more like a giant Frisbee in the sky but made no noise and for such a large object, it was eerily silent.

Another report of the sighting was made in New Jersey, USA, on 02-27-09. It is one of the latest UFO sightings in 2009. The observers supposedly saw three star-shaped blinking lights, which appeared and disappeared constantly. Some even say that the lights on the street and the porches went off as soon as those blinking lights were seen. Following this, a brighter neon blue light appeared in the sky, and seemed to cast a dark shade all around the people. This looked like a regular thunderstorm to most people, but they were soon aware that there was no noise, no clouds and no sign of rain. It was also noticed by many who happened to drive by Ocean City, that there was a presence of five red star-like objects in the sky. Like before, the lights kept changing formation, going from a straight line to a triangle, from time to time. The lights seemingly appeared and disappeared, to the on lookers.

LIST OF UFO SIGHTINGS

This is a list of UFO sightings including cases of alleged alien sightings and abductions. Some cases are popularly known and have become folklore. Others are more obscure, and known mainly to ufologists and enthusiasts.

19th-20th centuries

Date	Name	City, State	Country	Description	Close enc.	Sources
1897-04-19	Aurora Texas UFO Incident	Aurora, Texas	United States	Alleged crash of a UFO and confirmed burial of the pilot, purported to be an alien, in the local cemetery.	3	^[1]^[2]
1908-06-30	Tunguska event	Podkamennaya Tunguska River	Russian Empire	Massive explosion, allegedly of a comet or meteoroid, also explained as an explosion of a UFO	2	^[3]
1917-08-13, 09-13, 10-13	The Miracle of the Sun	Fatima	Portugal	Thousands of people saw what they thought was the sun gyrate and descend. This was later reinterpreted by Jacques Vallee, Joaquim Fernandes and Fina d'Armada as a possible UFO sighting that was not originally recognized as such due to cultural differences.	1	^[4]^[5]^[6]
1926			Unknown, possibly Nepal (the Himalayas)	Nicholas Roerich's travel diary mentions that he and his companions encountered a silver metallic disc hovering above the Himalayas. They observed the disc through binoculars for some time until it disappeared beyond mountain peaks.	1	^[7]
1940s	Foo fighter			Small metallic spheres and colorful balls of light repeatedly spotted, and on occasion photographed, by military air crews around the world during World War II.	1	^[8]^[9]
1942	Hopeh Incident	Hopeh	China	A UFO was spotted and photographed.	1	^[10]
1942-02-24	Battle of Los Angeles	Los Angeles, California	United States	Unidentified aerial objects trigger the military to fire thousands of anti-aircraft rounds into the sky and raise the wartime alert status.	1	^[11]^[12]
1946	Ghost rockets		Scandinavia	Objects were sighted repeatedly over Scandinavia; Swedish Defense Staff expressed concern.	1	^[13]
1946-05-18	UFO-Memorial Ängelholm	Ängelholm Municipality	Sweden	Gösta Karlsson reports seeing a UFO landing and alien beings. A model of a flying saucer is now erected at the site.	3	^[14]
1947-06-21	Maury Island incident	Washington	United States	Harold A. Dahl reported an incident in which he claimed that his dog was killed and his son was injured by UFOs, and that a witness was threatened by Men in Black.	2	^[15]
1947-06-24	Kenneth Arnold	Washington	United States	The UFO sighting that sparked the name flying saucers.	2	^[16]
1947-07-08	Roswell UFO incident	Roswell, New Mexico	United States	United States Army Air Forces allegedly captures a flying saucer.	3	^[17]
1948	Green fireballs		United States	Objects reported over several United States military bases; an official investigation followed.	1	^[18]
1948	Kapustin Yar	Russian Soviet Federative Socialist Republic	Union of Soviet Socialist Republics	A "cigar" shaped UFO was supposedly shot down by a Russian MiG.	2	^{[citation needed]}
1948-01-07	Mantell UFO Incident	Kentucky	United States	US Air Force sent a fighter pilot to investigate a UFO sighting over Fort Knox, Kentucky; the pilot was killed while pursuing the UFO.	2	^[19]
1948-10-01	Gorman Dogfight	North Dakota	United States	A US Air Force pilot sighted and pursued a UFO for 27 minutes over Fargo, North Dakota.	1	^[20]^[21]
1950	Mariana UFO Incident	Great Falls, Montana	United States	The manager of Great Falls' pro baseball team took color film of two UFOs flying over Great Falls. The film was extensively analyzed by the US Air Force and independent investigators.	1	^[22]
1951-08-25	Lubbock Lights	Lubbock, Texas	United States	Lights were repeatedly spotted flying over the city. They were witnessed by science professors from Texas Tech University and photographed by a Texas Tech student.	1	^[23]
1952-07-13	1952 Washington D.C. UFO incident	Washington, D.C.	United States	A series of sightings in July 1952 accompanied radar/visual contacts at three separate airports in the Washington area. The sightings made front-page headlines around the nation, and ultimately lead to the formation of the Robertson Panel by the CIA.	1	^[24]
1952-07-24	Carson Sink UFO incident	Nevada	United States	Two pilots saw three unusual aircraft, flying in a V-formation and with a Delta-wing airfoil, over Carson Sink.	1	^[25]
1952-09-12	Flatwoods monster	Flatwoods, West Virginia	United States	6 local boys and a local woman report seeing a UFO landing and a bizarre-looking creature near the landing site.	3	^[26]^[27]^[28]
June, 1953	Otis AFB	Falmouth, Massachusetts	United States	A man claims that a U.S Air Force radar operator and pilot bailed out over Otis Air Force Base because of engine failure. They were chasing a UFO. Plane and operator never seen again. Pilot lives and the canopy is the only thing recovered.	1	^{[citation needed]}
1953-08-12	Ellsworth UFO Case	Bismarck, North Dakota	United States	A UFO appearing as a red glowing light is witnessed by forty-five people. The sighting takes place over a two night period.	1	^[29]
1953-11-23	Felix Moncla	Lake Superior	United States - Canada	U.S Air Force Pilot disappears while pursuing a UFO.	1	^[30]
1955-08-21	Kelly-Hopkinsville encounter	Kentucky	United States	A group of strange, goblin-like creatures are reported to have attacked a family, while the family shot at them.	3	^[31]
1957	Antonio Villas Boas	São Francisco de Sales	Brazil	Antonio Villas Boas claimed to have been abducted and examined by aliens. He also claimed to have had sex with an alien woman during his time aboard a UFO.	4	^[32]
1957-05-20	Milton Torres 1957 UFO Encounter	East Anglia	United Kingdom	US Air Force fighter pilot Milton Torres reports that he was ordered to interecept and fire on a UFO displaying "very unusual flight patterns" over East Anglia. Ground radar operators had tracked the object for some time before Torres' plane was scrambled to intercept.	1	^[33]
1957-11-02	Levelland UFO Case	Levelland, Texas	United States	Numerous motorists reported seeing a strange, glowing, egg-shaped object which caused their vehicle's engines to shut off. When the object flew away, their vehicles restarted and worked normally.	2	^[34]
1959	Dyatlov Pass incident	Russian Soviet Federative Socialist Republic	Union of Soviet Socialist Republics	Mysterious deaths of experienced skiers in the Urals is believed to have been caused by "unidentified orange spheres" and an "unknown compelling force".	2	^[35]
1959	Alor Incident	Alor Archipelago	Indonesia	This incident occurred on July 1959, in the Alor Islands area.	3	^{[citation needed]}
1961-02-17			Europe	NATO radar stations detected 50 objects coming from the Soviet Union, flying at high speed over Europe. Coming at the height of the Cold War, they feared the outbreak of World War III, but the objects vanished after 9 minutes. A Potsdam astronomer possibly observed the transit of an object over the sun's disc at the time.	1	^{[citation needed]}
1961-09-19	Betty and Barney Hill abduction	New Hampshire	United States	A widely publicized alien abduction experience.	4	^[36]
1961-11-10		Vukovar	Croatia	A UFO with flickering lights appeared. An increase in static electricity was observed.	2	^{[citation needed]}
1964-04-24	Lonnie Zamora	Socorro, New Mexico	United States	Zamora, a police officer, reports a close encounter.	3	^[37]
1965	Exeter incident	Exeter, New Hampshire	United States	A UFO was observed by a teenager and two police officers.	1	^[38]
1965	Project Serpo		United States	A reported secret exchange program between the U.S government and an extraterrestrial race from Planet Serpo.	5	^[39]
1965-12-01	Adhara Observatory UFO Lunar Transit	Greater Buenos Aires	Argentina	The Adhara Observatory took photographs of the moon after the public reported something unusual on the moon. Some of these pictures show disk shaped objects. Afterwards three luminous strange objects flew over La Plata.	1	^{[citation needed]}
1965-12-09	Kecksburg UFO incident	Kecksburg, Pennsylvania	United States	Mass sighting of a falling UFO, followed by a cordoning-off of the crash site.	1	^[40]
1966	The Mothman Prophecies	Point Pleasant, West Virginia	United States	A wave of sightings of a winged humanoid is reported to be connected to other mysterious events including sightings of UFOs.	1	^[41]
1966-01-11	Wanaque Reservoir UFO	Wanaque Reservoir, Wanaque, New Jersey	United States	UFO in the form of a bright light seen in the vicinity of Wanaque Reservoir, leading to traffic jams and overloaded police communications networks. UFO changed color on a continuing basis, although white was apparently one of the major colors; it shot a beam of light down towards the ice near the dam, stayed around there for about a half hour, and flew off to the southeast, where it was sighted by others. Another UFO sighting of a similar nature, flying in a back and forth motion, occurred the next night in Wanaque.	1	^[42]
1966-04-06	Westall UFO	Clayton South, Victoria	Australia	A sighting by hundreds of people. Witnesses of "The Clayton Incident" still gather for reunions.	1	^[43]
1966-04-17	Portage County UFO chase	Ohio	United States	Several police officers pursue a UFO for 30 minutes.	1	^[44]
1966-10-11	The Grinning Man	Elizabeth, New Jersey	United States	Strange tall man with no nose or ears is sighted in a neighborhood shortly after UFO sighting.	2
1967-03-05	Minot Air Force Base UFO Incidents	Minot/Minot A.F.B., North Dakota	United States	UFO hovers over area, strike teams are called in and confront the craft.	1	^[45]
1967-03-??	Malmstrom AFB Incident	Malmstrom A.F.B.	United States		1	^[45]
1967-05-20	Falcon Lake Incident	Falcon Lake, Manitoba	Canada	A UFO's exhaust allegedly burns a man.	2	^[46]
1967-08-29	Close encounter of Cussac	Cussac, Cantal	France	A young brother and sister claim to have witnessed a UFO and its occupants.	3	^[47]
1967-10-04	Shag Harbour incident	Shag Harbour, Nova Scotia	Canada	A UFO was seen crashing into Shag Harbor. A Canadian naval search followed, and is officially referred to as a UFO crash.	2	^[48]^[49]
1967-12-03	Schirmer Abduction	Ashland, Nebraska	United States	Sergeant Herbert Schirmer claimed he was abducted.	4	^[50]
1967	Minot Air Force Base UFO Incidents	Minot/Minot A.F.B., North Dakota	United States	Air Force claims unidentifiable craft "attacks" air base, specifically the missile silos. (the "attack" was two strange, beams of light, one of which seemed to be an offensive weapon. One of numerous cases involving the base, which is home to a nuclear weapons stockpile, and the missile silos are home to nuclear warhead missiles.)	1	^[45]
1968-6-6	Minot Air Force Base UFO Incidents	Minot/Minot A.F.B., North Dakota	United States	UFO hovers over a silo, then leaves. After the craft left, personnel discovered that the missile was armed and unlocked in launch mode, and the nuclear warhead was armed.	2	^[45]
1968-10-25	Minot Air Force Base UFO Incidents	Minot/Minot A.F.B., North Dakota	United States		1	^[45]^[51]^[52]
1969-01-01		Prince George, British Columbia	Canada	Three unrelated witnesses reported a strange, round object in the late afternoon sky. The sphere radiated a yellow-orange light and appeared to ascend from 2,000 feet (610 m) to 10,000 feet (3,000 m).	1	^[53]
1969	Jimmy Carter UFO incident	Leary, Georgia	United States	Jimmy Carter's sighting.	1	^[54]
1970s	Barry DeLong UFO incident	Somerset County, Maine	United States	Sheriff Barry A. DeLong witnessed a UFO: "They were hovering about 15 feet (4.6 m) from my cruiser, late at night. It had fixed lights that were spinning. It was huge, and oval-shaped. I knew it wasn't a jet fighter. It slowly started backing off toward Sugarloaf, and then at a terrific speed."	1	^[55]
1971-09-04	Lake Cote UFO	Arenal, Alajuela	Costa Rica	Considered the best UFO picture of all times. An UFO was photographed during an official mapping mission from an airborne camera, over Lake Cote, Costa Rica. No crew member saw the object at the time, but an image appeared on the developed film.		^[56]^[57]
1973-10-11	Pascagoula Abduction	Mississippi	United States	Alien abduction occurs while the victims were fishing on the Pascagoula River.	4	^[58]
1974-01-23	Berwyn Mountain UFO incident	Llandrillo, Merionethshire, North Wales	Wales	An alleged UFO crash. Some claim that the event was actually the result of an earthquake.	1	^[59]^[60]
1975-01-12	North Hudson Park UFO sightings	North Bergen, NJ	United States	Considered a close encounter of the second and third kind, the case introduced Budd Hopkins to UFO research, a later key figure in alien abduction research. Researcher Jerome Clark cites it as one of the best-documented, the core story being corroborated by numerous independent witnesses.	2	^[61]
1975-11-05	Travis Walton	Arizona	United States	Logger Travis Walton reports being abducted by aliens following his five-day disappearance. Walton's six workmates claimed to have witnessed the UFO at the start of his abduction. Walton described the event and its aftermath in The Walton Experience, which was dramatized in the film Fire in the Sky.	4	^[58]
1976-06-22	1976 Canary Isles sightings	(Several)	Canary Islands (Spain)	Several lights and a spherical, transparent blue craft, piloted by two beings was reported.	1	^{[citation needed]}
1976-08-20	Allagash Abductions	Maine	United States	Four campers claimed an abduction by alien beings in the Allagash wilderness.	4	^[62]
1976-09-19	1976 Tehran UFO incident	Tehran	Iran	A UFO disabled the electronic equipment of two F-4 interceptor aircraft, and ground control equipment as well. The event is well-documented in the U.S. DIA report and other documents. The Iranian generals involved in the incident said on public record that object was extraterrestrial.	2	^[63]
1977	Colares UFO flap	Colares	Brazil	A bewildering account of an island which was flooded by harmful UFOs.	2	^{[citation needed]}
1978-05-10	Emilcin Abduction	Emilcin	Poland	A man at Emilcin, Poland is said to be abducted by "grays." There is now a memorial at the site.	4	^[64]
1978-06-19	Frank Johnson	Oxfordshire, England	United Kingdom	While driving, a family is intercepted by a UFO. The family later wrote a book (The Janos People (book)) about the incident.	4	^{[citation needed]}
1978-10-21	Valentich Disappearance	Victoria	Australia	An Australian pilot reported a UFO sighting to air traffic control before he and his aircraft vanished without a trace.	1	^[65]^[66]
1978-12-21	Kaikoura lights	South Island	New Zealand	A series of sightings by a Safe Air freight plane; the airplane was escorted by strange lights that changed color and size.	1	^[67]
1979-08-27	Val Johnson Incident	Marshall County, Minnesota	United States	A deputy sheriff spotted a strange bright light which appeared to have collided with his patrol car and damaged it. The deputy also suffered temporary eye damage from the "light".	2	^[68]^[69]
1979-11-09	Robert Taylor incident	Livingston	Scotland	A forester, Bob Taylor, was pulled by two spiked globes towards a UFO, which stood on a clearing. He lost consciousness and afterwards had trouble walking and speaking and felt thirsty for days. At the clearing several traces were found.	4	^[70]
1979-11-11	Manises UFO Incident	Valencia	Spain	Three large UFOs forced a commercial flight to make an emergency landing at Manises airport.	1	^[71]
1980s	Hudson Valley UFO sightings	Hudson Valley	United States	A UFO flap during the eighties and early nineties, involving thousands of reports of similarly-shaped UFOs. They were first observed by a retired policeman in Kent, New York, late on New Year's Eve, 1981. The 1984 "Incident at Indian Point" was one of these sightings.	1	^[72]
1980-6-28	Disappearance of N3808H	Puerto Rico	Puerto Rico	Flight N3808H disappears without a trace at a flight from Santo Domingo, Dominicain Republic to San Jose, Puerto Rico	2	^[73]
1980-12-28	Rendlesham Forest Incident	Suffolk, England	United Kingdom	A sighting which was first thought to be a downed aircraft.	2	^[74]
1980-12-29	Cash-Landrum incident	Dayton, Texas	United States	A huge diamond-shaped UFO irradiates three witnesses, who were treated for radiation poisoning. The UFO was in the company of military helicopters and the victims have since sued the US Government.	2	^[75]
May 1981	Salyut 6 orbital station		Earthbound orbit	Cosmonauts Vladimir Kovalyonok and Viktor Savinykh supposedly established contact with a UFO while working at the Salyut 6.	1	^{[citation needed]}
1981-01-08	Trans-en-Provence Case	Trans-en-Provence, Var	France	A UFO-landing which left traces.	2	^{[citation needed]}
1985	Tbilisi-sighting	Georgian Soviet Socialist Republic	Union of Soviet Socialist Republics	In January, 1985, Soviet commercial airline crew and passengers observed a strange light during a flight from Tbilisi in Georgia to Tallinn in Estonia.	1	^{[citation needed]}
1985-12-26	Whitley Strieber	Toronto and New York state	Canada and the United States	A paranormal writer and alleged abductee, Strieber described his experiences in his national best-seller novel Communion. He later retracted his statements in a 1996 Larry King interview. He currently hosts a nighttime radio show Dreamland.	4	^{[citation needed]}
1986	1986 São Paulo UFO sighting		Brazil	Some 20 UFOs seen and detected by radar in various parts of Brazil. Colonel Silva pursues some in an aircraft, and later declares them to be “positive targets, a concrete thing”. They disappear as five other military aircraft are sent to intercept them. Afterwards Minister of Aeronautics, Brig. Lima, publicly declared: "...at least 21 objects were detected by Brazilian radars. [...] It’s fantastic, the radar signals were very clear."	1	^{[citation needed]}
1986-01-29	Height 611 UFO Incident	Dalnegorsk, Primorsky Krai	Union of Soviet Socialist Republics	An alleged UFO crash, followed by an alleged UFO landing in 1989.	1	^{[citation needed]}
1986-11-17	Japan Air Lines flight 1628 incident	Alaska	United States	A group of UFOs flew alongside Japan Air Lines Flight 1628 for 50 minutes above Northeastern Alaska. One of the objects trailing behind the Boeing 747 was detected by military radar.	1	^[76]
1987-11-11	Gulf Breeze UFO incident	Gulf Breeze, Florida	United States	A UFO wave started late 1987 over this town in western Florida. A fair number of these sightings, though by no means all, are attributed to Ed Walters, whose claims are now doubted.	1	^{[citation needed]}
1988-12-26		Villa Urquiza, Buenos Aires	Argentina	A silver UFO flew over this high density area in the city of Buenos Aires. It is maybe the most spectacular UFO incident of Argentina, with more than 7,500 witnesses. The local airport reported an object flying towards General Paz Ave. seen on the radars on that sunny afternoon.	1	^{[citation needed]}
1990-03-30	Belgian UFO wave	Ans, Wallonia	Belgium	Mass sighting of large, silent, low-flying black triangles, which were tracked by multiple NATO radar and jet interceptors, and investigated by Belgium's military. Photographic evidence exists.	1	^{[citation needed]}
1990-11-07	1990 Montreal UFO	Montreal, Quebec	Canada	For more than 3 hours, more than 40 people, including policemen, observe a UFO above a downtown hotel.	1	^[77]
1990 / 1992-07-08	Sassowo explosions	Sassowo	U.S.S.R. / Russian Federation	Two mysterious explosions, equivalent to 25 tons high explosive.	2	^{[citation needed]}
1991-04-21	Achille Zaghetti	London, England	United Kingdom	Alitalia pilot Achille Zaghetti reports a cigar-shaped UFO flying past his plane at high speed during his descent to Heathrow Airport on a flight from Milan to London. Immediately following the near-miss, Zaghetti contacted the area control center radar operator who confirmed an unidentified target was observed 10 nautical miles behind the plane. British defence officials ruled out a missile, but offered no explanation for the sighting.	1	^[78]^[79]
1991	STS-48 incident	Orbit, Space	Discovery, STS-48	A video apparently shot during STS-48 mission, appears to depict objects flying in an artificial fashion. NASA explained the objects as ice particles reacting to engine jets. Philip C. Plait briefly discussed it in Bad Astronomy, and agreed with NASA. A Florida Today article of 2000 quoted Dr. Jack Kasher as saying that found the footage to depict independently operated spacecraft, see "Journal for UFO Studies".	1	^[80]^[81]
1992	Steven M. Greer	Alton Barnes, Wiltshire, England	United Kingdom	While at a Woodborough Hill farm, a CSETI team experiences a close encounter with a "100-foot in diameter" UFO.	1	^{[citation needed]}
1993	Steven M. Greer		Mexico	Near Popocatépetl volcano, on the road towards Indian village Atlimayaya, (Atlixco and Metepec region, Mexico) a CSETI team experienced a close encounter with a triangular UFO and "extremely luminous and white" extraterrestrials within it.	5	^{[citation needed]}
1993	Kelly Cahill	Victoria	Australia	East of Melbourne's Dandenong foothills Kelly Cahill and five others were confronted by a flying saucer shortly after midnight, besides tall, slim, black aliens with glowing red eyes. A number of scientific organisations performed subsequent tests of the area and found significant anomalies in soil and vegetation samples.	5	^[82]
1994-06	Meng Zhaoguo Incident	Wuchang	China	Meng claimed to have had sexual intercourse with an alien, and to have been subsequently abducted.	4	^[83]
1994-12-14	Trumbull Ohio Disturbance	Vienna, Ohio	United States	Many citizens and at least 15 local police officers near Youngstown, Ohio, observed and pursued large, low-flying multicolored UFOs during the early hours of 12/14/1994. Citizens and police went on record giving testimonials on the 1999 NBC special "Confirmation". Publicly available 9-1-1 communications detail the event and officers' behavior. Local Air Force reps denounced local authorities saying the police were chasing Mercury for 7 hours at high speed. FAA also denied any anomalies. Local Youngstown media acknowledged the incident's importance, but refused coverage, reportedly out of fear. Gross abuse of media and military influence is claimed, which deliberately concealed the event. The principal investigator was Kenny Young of Cincinnati, OH.	1
1996-01-20	Varginha UFO incident	Varginha, Minas Gerais	Brazil	Multiple sightings and alleged capture of alien entity by the Brazilian military.	3	^[84]
1997	Area 51: The Alien Interview	Area 51, Nevada	United States	Video footage of an alleged interview with an alien.	5	^[85]
1997-03-13	Phoenix Lights	Nevada, Arizona, and New Mexico	United States	A strange set of lights are seen over Nevada, Arizona and possibly New Mexico.	1	^[86]
1997-08-06	Mexico City skyline UFO	Mexico City	Mexico	An amateur cameraman with a digital camera captured footage of a UFO passing behind and above several buildings. Air traffic was restricted that day except for two helicopters.	1	^[87]
2000	Eastern Afghanistan UFOs		Afghanistan	Residents of nw. Pakistan and e. Afghanistan claimed to have seen strange lights in the sky. The then-ruling Taliban regime suspected a US attacked on Al-Qaeda training camps, or Pakistani missile tests, which were denied by the respective countries. The suspicion was discounted as the lights were seen heading back towards the sky. The lights however returned after two weeks.	1	^{[citation needed]}
2000-01-05	Southern Illinois UFO	St. Clair County, Illinois	United States	Between the hours of 4am and 7am, six people, four of whom were police officers, observed a large, triangular shaped object in the night skies only a few hundred feet over St. Clair county. The object glided silently and slowly in a south-westerly direction over several towns before vanishing near the town of Dupo just before 7am. The object which was reported to have several bright lights, was also reported to be as tall as a two-story house and as long as a football field.	1	^[88]

[edit] 21st century

Date	Name	City, State	Country	Description	Close enc.	Sources
2004-03-05	2004 Mexican UFO Incident		Mexico	A drug-smuggling air patrol recorded on infrared camera what some claimed to be UFOs. The footage was released by Jaime Maussan. Others suggest the objects are oil platform burn-off flares.	1	^[89]
2004-10-27		Woodside, Queens	United States	Five flying saucers appeared and were observed by people throughout the Queens and Manhattan area. They disappeared one by one.	1	^{[citation needed]}
2004-10-31	The Tinley Park Lights	Tinley Park, Illinois	United States	A sequence of five mass UFO sightings, first on August 21, 2004, two months later on October 31, 2004, again on October 1 of 2005, and once again on October 31, 2006, in Tinley Park and Oak Park, Chicago.	1	^[90]^[91]^[92]
2005-04-27		Washington, D.C.	United States	The White House was evacuated when a UFO entered restricted air space, then disappeared. It was explained as: "probably a cloud or several birds."	1	^[93]
2006-05-25		Kent, England	United Kingdom	An oval orb was observed hovering above a location in Kent. It then moved off in a westerly direction.	1	^{[citation needed]}
2006-06-24			Vatican City	A Polish tourist photographed a UFO hovering near the Vatican Basilica's cuppola.	1	^{[citation needed]}
2006-08-17		Nanjing	China	A UFO was recorded hovering over an apartment building.	1	^{[citation needed]}
2006-10-23		North Carolina	United States	An object was sighted traveling northeast at night until, as it was claimed, it expelled a blue ring and split in two. The first piece continued to fly northeast, while the latter ricocheted in the opposite direction, and disappeared from view. The first part was seen to have light on its back, and shaped like a small blimp.	1	^{[citation needed]}
2006-11-07	Chicago O'Hare UFO sighting 2006	Chicago, Illinois	United States	United Airlines employees and pilots claimed sightings of a saucer-shaped, unlit craft hovering over a Chicago O'Hare Airport terminal, before shooting up vertically. The FAA initially denied having received reports, but information gained by means of the Freedom of Information Act revealed otherwise.	1	^[94]
2007-01-20			Poland	Seventeen or eighteen UFOs were spotted.	1	^{[citation needed]}
2007-02-02		Islington, London, England	United Kingdom	Ten to fifteen UFOs are spotted.	1	^[95]
2007-02-16		Kaliningrad	Russian Federation	A student records a UFO on his video camera.	1	^[96]
2007-03-03		New Delhi	India	A green fireball was seen over Delhi by ten pilots.	1	^[97]
2007-03-07		New Delhi	India	Two UFOs were detected near the prime minister's residence.	1	^[98]
2007-03-10		Cleveland, Ohio	United States	UFO seen over a peace rally.	1	^[99]
2007-04-23		Channel Islands		Two airline pilots on separate flights spot UFOs off the coast of Alderney.	1	^[100]
2007-05-02		Sutton, England	United Kingdom	John Gregory spotted a UFO after 10 pm. As he was observing the sky he noticed a very bright object, which at first he considered to be a star. Later he began to doubt this, and was very surprised by the very unusual and clear phenomenon, which was visible for miles around. The image, which seemed to be shaped like a face, nose and mouth, remained for about 20 minutes and then just vanished.	1	^[101]
2007-05-11		Massillon, Ohio	United States	At 10:40 pm, Rosemary Lyons and a co-worker were taking a smoke break outside the Loyal Order of Moose Lodge, located at 2935 Lincoln Way West, when they witnessed a UFO. They estimated its size as 60 feet (18 m) in diameter. Lyons: "The first thing I said was 'Oh my gosh. Look at that,' We saw it for 20 to 25 seconds. It was round and luminous underneath. It wasn't a plane, it wasn't a helicopter, it wasn’t a blimp and it wasn't a balloon. It made absolutely no sound. It was flying over just carefree – like it was no one's business."	1	^[102]
2007-05-12		Bangor, County Down, Northern Ireland	United Kingdom	Orange colored UFOs were spotted during the evening. Clifford Rossbottom said, "There were three orange globes, nearly in a straight line, ... an absolutely fascinating sight. I watched them for five minutes, and then very slowly, they just disappeared. The only thing I thought it could have been was three high-flying aircraft. If that is not the case, then I have no idea, and the only other thing I can think of is in fact that they were UFOs." Belfast International Airport's air traffic control received calls from people (including the Coastguard) who spotted the objects, but the airport reported no aircraft in the area during the time of the sightings. A company called UFO Balloons later claimed their product was responsible for the events. One month later two similar aircraft were sighted over Buela, Colorado.^[103]	1	^[104]
2007-05-27		Chilliwack, British Columbia	Canada	Dave Francis and Kelly McDonald witnessed a UFO over UCFV campus. Francis said, "I really think it was a UFO. I don’t really care if anybody else believes me ... it was the craziest thing I ever saw. Whatever it was, they were moving from place to place as a unit [and then] just faded away." McDonald said, "I know that I saw something that wasn’t from here. I’ve never seen anything move that way. It wasn’t birds. [As the UFO approached] it broke apart into 20 or more of these little spheres ... birds don’t dive-bomb in at each other. "	1	^[105]
2007-05-28		Bangalore	India	Afzal Khan told CNN IBN that he photographed a UFO, "Today (Monday) at around 9 pm (IST), I and my brothers saw a bright, slow-moving object which looked like a group of lights moving in a triangular formation in the sky towards west/north-west direction. This object was definitely not an aeroplane as it was moving very slowly. We observed the object from around 9 pm until 9.30 pm before it disappeared into the distance. During this time we managed to take some pictures with my camera. The astronomical/space/science departments of any activity in the sky during the above time can be confirmed. We live in Jayanagar area of Bangalore. Another thing that we noticed during this time was an aircraft that took-off from Bangalore Airport was flying very close to this object. It is possible that the pilot of that aircraft could have spotted this object as well."	1	^[106]
2007-07-23		Warwickshire, England	United Kingdom	Crowd of 100 persons saw a formation of strange lights in the sky for 30 minutes.	1	^[107]
2007-09-15	Double mothership sighting	Nassau	Bahamas	Event took place around 8 pm, two huge motherships in front of the moon, then suddenly vanish one after the next.	1	^{[citation needed]}
2007-09-25	Kodiak Island UFO, 2007	Kodiak, Alaska	United States	On September 25, 2007, around 6:45am, a bright red light was flying fast over Alaska's Kenai Peninsula. Within seconds, it travelled to Kodiak Island air space, over 300 miles away. Many claimed to have seen it descend behind a mountain. Local troopers and Coast Guard personnel were unable to find traces.	1	^[108]
2007-10-30		Calcutta	India	Many people claimed to see a UFO near Eastern Metropolitan Bypass, Calcutta. It was also recorded by an amateur videographer. It was like a ball of light which some time hovered, sometimes moved very fast and changed shape and size. The video was broadcasted on local news channels.	1	^[109]^[110]
2007-11-08		Larnaca	Cyprus	UFO reported to have come from the coast inland near a village called Catalkoy in small and rapid zigzags, then stopped and hovered for approximately 30 seconds before making a smooth ascent at 45 degrees east. Many witnesses have reported such events to a local newspaper.	1	^[111]
2008-01-01		San Diego, California	United States	Nine separate lights were spotted moving in an arc formation across the sky at around half past midnight on New Years Day. Around the globe, many people are said to have reported the same exact lights in the sky. A later Fox News San Diego report by anchor Jim Patten showed these lights to be Chinese New Year floating Lanterns. On February 16, 2008, two individuals reported seeing a large, pulsing orb of light hovering in place above Miramar Air Base.	1	^[112]
2008-01-08 to 2008-02-09	Stephenville, Texas UFO sightings	Stephenville, Texas, Dublin, Texas, Crawford, Texas	United States	UFOs were, and still are, sometimes reported in this area. One was an object that was 1 mile (1.6 km) by 1.5 miles (2.4 km) in size, spotted over Bush Ranch in Crawford, Texas. The Air Force has identified the objects as training fighter jets that went unreported due to a "communications problem".	1	^[113]
May–Sept, 2008	2008 Turkey UFO Sightings	Istanbul	Turkey	Over a four month span in 2008, a night guard at the Yeni Kent Compound videotaped one or more UFOs over Turkey at nighttime. Many witnesses confirmed the two and a half hours' worth of video, leading the Sirius UFO Space Science Research Center to dub it the "most important images of a UFO ever filmed".	1	^[114]
2008-05-14		Needles, California	United States	A flaming object crashed near the Colorado river. Several witnesses claim to have seen five helicopters picking up the strange object after 17 minutes and heading into direction of Las Vegas.	1	^[115]
2008-06-20	Wales UFO sightings	Different cities, Wales	United Kingdom	After media reports of a police helicopter that was almost hit by a UFO, before it tried to pursue it, hundreds of people reported to have witnessed a UFO on the same or preceding days, from different areas of Wales.	1	^[116]
2008-06-21	Moscow UFO sightings	Moscow	Russian Federation	Different people and media (including state-owned) reported sightings of 11 orange UFOs. A US blogger reported seeing the same phenomenon.^[117] Also see YouTube video:^[118] . Further confirmation came from Saint Petersburg and Novosibirsk. On June 25 a similar report, now with 13 objects, was claimed in the UK.	1	^[119]^[120]^[121]
2008-06-29		Weston-super-Mare, Somerset	United Kingdom	Royal Navy aircraft engineer Michael Madden watched a UFO hover above the M5 motorway near Weston-super-Mare for around three minutes before it disappeared at high speed. Madden described the UFO as looking "like alien aircraft in the films".	1	^[122]^[123]
2008-12-10		Zagreb	Croatia	Bright lights were seen over an insurance building in Zagreb. They were filmed and put on the news. The UFO was actually a misidentified media stunt.		^[124]
2009-01-05	Morristown UFO	Morristown, New Jersey	United States	A confirmed hoax in Morristown, New Jersey, perpetrated by Chris Russo & Joe Rudy who sent up different sets of helium balloons with attached flares.		^[125]^[126]^[127]
2009-03-18		London, England	United Kingdom	A group of UFOs appear on a photo, even though they were invisible to the naked eye.	1	^[128]
2009-05-31		Calcutta	India	UFOs detected by Kolkata ATC	1	^[129]
June, 2009		Wisconsin, Texas, Nevada, Mexico City, Tijuana	United States, Mexico	Scattered reports of UFO sightings all over the United States from June 3rd to 22nd, 2009.	1, 2	^[130]
2009-08-05		North England	United Kingdom	UFO captured on BBC webcam.	1	^[131]
2009-08-16		Russia	Ural	Alledged disappearance of an 11 year old girl at an UFO sighting	1	^[132]
2009-09-16		Cerro Azul	Panama	Confirmed hoax, four teenagers aged 15-16 claimed to have killed a small alien after spotting the creature at the entrance to a cave. The creature was then confirmed to be a type of sloth.		^[133]^[134]
2009-10-10		Pécs	Hungary	Either six or seven Spherical unidentified aircraft seen flying in sky.	1
2009-10-19		Florida	United States	200-foot-long triangle-shaped craft spotted flying at 100 feet altitude.	1	^[135

UFO sightings by Country

PROOF OF ALIEN LIFE

Are you looking for proof of alien life? Chances are if you found your way to this site that you are indeed searching for proof of alien life.

After all, the question if humans are alone in the universe or if there really is alien life out there is one of the most frequently pondered questions of all time.

The beauty of living in the times in which we do is that so much evidence pointing to proof of alien life has been collected from over the years and from around the world. Now, thanks to the beauty of the world wide web and the internet, all of this proof of alien life from around the globe can be collected and shared globally with a few clicks.

So, no longer do we have to sit around simply wondering if there is proof of alien life out there. We can see the evidence that shows proof of alien life for ourselves and make up our own minds.

It is much better to look at the proof of alien life and make up your own mind then to simply be told what you should believe. Remember, we were once told to believe the world was flat.

Is all the material that is put forth as proof of alien life real? Without a doubt the answer is no. It is, again, up to you to decide which you believe.

Proof Of Alien Life - UFO Sightings

For many people that believe there is proof of alien life the first proof they see in their own life comes in the form of of an UFO sighting. After all, if there is proof of alien life then the aliens have to arrive on Earth somehow!

Often when people want to excuse away accounts of UFO sightings they will say that most accounts happens so long ago that they should be discredited as they wouldn't stand up to modern scrutiny.

So, we have collected a few accounts of UFO sightings that have all taken place in the past few years.

UFO Sighting: South of France - Three possible UFO videos that document a UFO sighting that took place in Southern France in the spring of 2007.

UFO Sighting: Stephenville,Texas - Witness accounts on video of the Stephenville lights UFO sighting that was seen by over 20 witnesses in the opening months of 2008.

UFO Sighting: United Kingdom 2008 - Throughout the early summer of 2008 there are have a scattering of UFO sightings being reported throughout England and Wales. It alls started with a police helicopter having a near collision with a UFO over military airspace. These are video accounts and possible real UFO videos of those sightings. Proof of alien life in Europe?

UFO Sighting: O'Hare Airport, Chicago - In November of 2006 a UFO sighting was reported at Chicago's O'Hare International Airport by twelve airport employees. United Airlines tried to cover it up but these videos are proof the employees did see something.

Proof Of Alien Life - Alien Encounter Videos

One of the best ways to try to determine for yourself if there is proof of alien life is to actually listen to the people that claim to have met aliens. These are all alien encounter videos where people that feel they have had an alien encounter or been abducted by aliens talk about their experiences.

Alien Encounter Videos: Zimbabwe, Africa - In 1994, 62 school children from Zimbabwe had an alien encounter in their school yard. Despite not being influenced by pop culture depictions of aliens each child recounts the exact same story of the alien encounter.

Alien Encounter Videos: Alien Implant Story - A man from the western United States who reports repeat alien abductions that first started in 1957 when he was a young child. He recalls the discovery of an alien implant in his leg and more in this alien encounter interview video.

Alien Encounter Videos: Australian Highway UFO Encounter - Kelly Cahill believes that during a car journey with her family in 1993 she had a UFO sighting and was then the victim of an alien abduction. In this fascinating video interview, Kelly Cahill shares her experience of the alien abduction and what has happened to her life and her body after the alien encounter.

EAGLE NEBULA

2009-09-20T01:43:00.000-07:00

Eagle Nebula

This article is about the nebula in Serpens. For an object in the constellation of Canis Major, the Seagull Nebula also called Eagle nebula, see IC 2177. For the famous astrophotograph, see Pillars of Creation.

Eagle Nebula
A view of the "Spire" within M16, the Eagle Nebula. Courtesy of NASA/ESA
Observation data: J2000.0 epoch
Type	Emission
Right ascension	18^h 18^m 48^s^[1]
Declination	-13° 49′^[1]
Distance	7,000 ly
Apparent magnitude (V)	+6.0^[1]
Apparent dimensions (V)	7.0arcmins
Constellation	Serpens
Physical characteristics
Radius	70×55 ly (cluster 15 ly)
Absolute magnitude (V)	-8.21
Notable features	5.5 million years old
Other designations	Messier 16, NGC 6611,^[1], Sharpless 49, RCW 165, Gum 83
See also: Diffuse nebula, Lists of nebulae

The Eagle Nebula (catalogued as Messier 16 or M16, and as NGC 6611) is a young open cluster of stars in the constellation Serpens, discovered by Jean-Philippe de Cheseaux in 1745-46. Its name derives from its shape which is resemblant of an eagle. It is the subject of a famous photograph by the Hubble Space Telescope, which shows pillars of star-forming gas and dust within the nebula.

Characteristics

The Eagle Nebula is part of a diffuse emission nebula, or H II region, which is catalogued as IC 4703. This region of active current star formation is about 6,500 light-years distant. The tower of gas that can be seen coming off the nebula is approximately 57 trillion miles (97 trillion km) high.
The brightest star in the nebula has an apparent magnitude of +8.24, easily visible with good binocular 'Pillars of Creation' region

The "Pillars of Creation" within the Eagle Nebula. Courtesy of NASA/ESA

Images made in 1995 by Jeff Hester and Paul Scowen using the Hubble Space Telescope greatly improved scientific understanding of processes inside the nebula. One of these, a famous photograph known as the "Pillars of Creation", depicts a large region of star formation. Its small dark areas are believed to be protostars. The pillar structure of the region resembles that of a much larger star formation region, imaged with the Spitzer Space Telescope in 2005, in Cassiopeia, which is designated W5 and has been dubbed the "Mountains of Creation".^[2]
Combinations of X-ray images from the Chandra observatory with Hubble's "Pillars" image have shown that X-ray sources (from young stars) do not coincide with the pillars, but instead randomly dot the area.^[1] This suggests that star formation may have peaked approximately one million years ago in the Eagle Nebula and any protostars in the pillar's EGGs are not yet hot enough to emit X-rays.^{[citation needed]}
In early 2007, scientists using the Spitzer discovered evidence that potentially indicates the Pillars were destroyed by a nearby supernova explosion about 6,000 years ago, but the light showing the new shape of the nebula will not reach Earth for another millennium.^[3]

Detail of Hubble image. Courtesy of NASA/ESA

An infrared view of the pillars reveals different structures. Credits: ESO

Three-colour composite mosaic image of the Eagle Nebula. Credits: ESO

Head of Column in the Eagle Nebula. Credits: ESO

OWL NEBULA

2009-09-20T01:38:00.000-07:00

Owl Nebula

Owl Nebula
Owl Nebula by Fryns
Observation data (Epoch J2000.0)
Right ascension	11^h 14.8^m
Declination	+55° 01′
Distance	2,600 ly
Apparent magnitude (V)	+9.9
Apparent dimensions (V)	3.4 × 3.3 arcmin
Constellation	Ursa Major
Physical characteristics
Radius	1.5 ly
Absolute magnitude (V)	-
Notable features	Owl-like "eyes" visible through larger telescopes
Other designations	M97, NGC 3587
See also: Planetary nebula, Lists of nebulae

The Owl Nebula (also known as Messier Object 97 or NGC 3587) is a planetary nebula in the constellation Ursa Major. It was discovered by Pierre Méchain in 1781.
M97 is regarded as one of the more complex of the planetaries. The 16th magnitude central star has about 0.7 solar mass and the nebula itself about 0.15 solar mass. The nebula formed roughly 6,000 years ago.
The nebula gets it name due to the appearance of owl-like "eyes" when view through a large (>200 mm) telescope under dark sky conditions with the aid of a so-called "nebula filter." The "eyes" are also easily visible through photographs taken of the nebula.

Image of Messier 97 taken with red, green
and blue filters with the Faulkes Telescope North

DUMBELL NEBULA

2009-09-20T01:28:00.000-07:00

Dumbbell Nebula

Dumbbell Nebula
Dumbbell Nebula
Observation data (Epoch J2000)
Right ascension	19^h 59^m 36.340^s^[1]
Declination	+22° 43′ 16.09″^[1]
Distance	1,360⁺¹⁶⁰₋₂₁₂ ly (417⁺⁴⁹₋₆₅ pc)^[2]^[3]
Apparent magnitude (V)	7.5^[1]
Apparent dimensions (V)	8′.0^{[citation needed]} × 5′.6^[4]
Constellation	Vulpecula
Physical characteristics
Radius	1.44^+0.21_−0.16 ly^[a]
Absolute magnitude (V)	-0.6^+0.4_−0.3^[d]
Notable features	Central star radius is largest known for a white dwarf.
Other designations	NGC 6853,^[1] M 27,^[1] Diabolo Nebula,^[1] Dumb-Bell Nebula,^[1]
See also: Planetary nebula, Lists of nebulae

The Dumbbell Nebula (also known as Messier 27, M 27, or NGC 6853) is a planetary nebula (PN) in the constellation Vulpecula, at a distance of about 1,360 light years.
This object was the first planetary nebula to be discovered; by Charles Messier in 1764. At its brightness of visual magnitude 7.5 and its diameter of about 8 arcminutes, it is easily visible in binoculars, and a popular observing target in amateur telescopes.

Structure

ESO image showing extended structure and central star
Credit: ESO

This PN appears to be shaped like an prolate spheroid and is viewed from our perspective along the plane of its equator. In 1992, Moreno-Corral et al. computed that the rate of expansion in the plane of the sky of this PN was no more than 2″.3 per century. From this, an upper limit to the age of 14,600 yr may be determined. In 1970, Bohuski, Smith, and Weedman found an expansion velocity of 31 km/s. Given its semi-minor axis radius of 1.01 ly, this implies that the kinematic age of the nebula is some 9,800 years.^[4]^[5]

Knots

HST closeup of knots in M 27
Credit: C.R. O'Dell (Vanderbilt University)

Like many nearby planetary nebulae, the Dumbbell contains knots. Its central region is marked by a pattern of dark and bright cusped knots and their associated dark tails (see picture). The knots vary in appearance from symmetric objects with tails to rather irregular tail-less objects. Similarly to the Helix Nebula and the Eskimo Nebula, the heads of the knots have bright cusps which are local photoionization fronts.^[5]

Central star

The central star, a white dwarf, is estimated to have a radius which is 0.055 ± 0.02 R_☉ which gives it a size larger than any other known white dwarf.^[2] The central star mass was estimated in 1999 by Napiwotzki to be 0.56 ± 0.01 M_☉.^[2]

OMEGA NEBULA

2009-09-19T22:48:00.000-07:00

Omega Nebula

Omega Nebula
Omega Nebula from Hubble Space Telescope Credit: NASA/ESA/STScI
Observation data: J2000 epoch
Type	Emission
Right ascension	18^h 20^m 26^s^[1]
Declination	−16° 10′ 36″^[1]
Distance	5,000-6,000 ly
Apparent magnitude (V)	+6.0^[1]
Apparent dimensions (V)	11 arcmins
Constellation	Sagittarius
Physical characteristics
Radius	-
Absolute magnitude (V)	-
Notable features	-
Other designations	M17, NGC 6618, Swan Nebula^[1], Sharpless 45, RCW 160, Gum 81
See also: Diffuse nebula, Lists of nebulae

The Omega Nebula, also known as the Swan Nebula (catalogued as Messier 17 or M17 and as NGC 6618) is an H II region in the constellation Sagittarius. It was discovered by Philippe Loys de Chéseaux in 1745. Charles Messier catalogued it in 1764. It is located in the rich starfields of the Sagittarius area of the Milky Way.
The Omega Nebula is between 5,000 and 6,000 light-years from Earth and it spans some 15 light-years in diameter. The cloud of interstellar matter of which this nebula is a part is roughly 40 light-years in diameter. The total mass of the Omega Nebula is an estimated 800 solar masses.
A cluster of 35 stars lies embedded in the nebulosity and causes the gases of the nebula to shine due to radiation from these hot, young stars.

Three-colour composite image of the Omega Nebula (Messier 17, or NGC 6618). Credit ESO

A colour-coded composite image of a sky field in the south-western part of the galactic star-forming region Messier 17. Credit: ESO

TRIFID NEBULA

2009-09-19T22:35:00.000-07:00

Trifid Nebula

Trifid Nebula
Trifid Nebula by the Hubble Space Telescope (NASA/ESA)
Observation data: J2000 epoch
Type	Emission/Reflection
Right ascension	18^h 02^m 23^s^[1]
Declination	-23° 01′ 48″^[1]
Distance	2000 - 9000 ly
Apparent magnitude (V)	+6.3^[1]
Apparent dimensions (V)	~20 arcmins
Constellation	Sagittarius
Physical characteristics
Radius	-
Absolute magnitude (V)	-
Notable features	-
Other designations	M20, NGC 6514^[1], Sharpless 30, RCW 147, Gum 76
See also: Diffuse nebula, Lists of nebulae

The Trifid Nebula (catalogued as Messier 20 or M20 and as NGC 6514) is an H II region located in Sagittarius. Its name means 'divided into three lobes'. The object is an unusual combination of an open cluster of stars, an emission nebula (the lower, red portion), a reflection nebula (the upper, blue portion) and a dark nebula (the apparent 'gaps' within the emission nebula that cause the trifid appearance; these are also designated Barnard 85). Viewed through a small telescope, the Trifid Nebula is a bright and colorful object, and is thus a perennial favorite of amateur astronomers.

Characteristics

The Trifid Nebula was the subject of an investigation by astronomers using the Hubble Space Telescope in 1997, using filters that isolate emission from hydrogen atoms, ionized sulfur atoms, and doubly ionized oxygen atoms. The images were combined into a false-color composite picture to suggest how the nebula might look to the eye.
The close-up images show a dense cloud of dust and gas, which is a stellar nursery full of embryonic stars. This cloud is about 8 light years away from the nebula's central star. A stellar jet protrudes from the head of the cloud and is about 0.75 light-years long. The jet's source is a young stellar object deep within the cloud. Jets are the exhaust gasses of star formation. Radiation from the nebula's central star makes the jet glow.
The images also showed a finger-like stalk to the right of the jet. It points from the head of the dense cloud directly toward the star that powers the Trifid nebula. This stalk is a prominent example of an evaporating gaseous globules, or 'EGGs'. The stalk has survived because its tip is a knot of gas that is dense enough to resist being eaten away by the powerful radiation from the star.
In January, 2005, NASA's Spitzer Space Telescope discovered 30 embryonic stars and 120 newborn stars not seen in visible light images.

Gallery

M20 Amateur Image Courtesy Hunter Wilson

The Trifid nebula. The outlined area is enlarged right. Credit: NASA/ESA.

Hubble image of a stellar jet in the Trifid nebula. Credit: NASA/ESA.

Hubble image, a closer detail of last. Credit: NASA/ESA.

The Trifid Nebula in infrared as seen by the Spitzer Space Telescope

LAGOON NEBULA

2009-09-19T18:41:00.000-07:00

Lagoon Nebula

Lagoon Nebula
M8, the Lagoon Nebula Courtesy Hunter Wilson
Observation data: J2000 epoch
Type	Emission
Right ascension	18^h 03^m 37^s^[1]
Declination	−24° 23′ 12″^[1]
Distance	4,100 ly (1,250 pc)^[2]
Apparent magnitude (V)	6.0
Apparent dimensions (V)	90 × 40 arcmins
Constellation	Sagittarius
Physical characteristics
Radius	55 × 20 ly
Absolute magnitude (V)	???
Notable features	—
Other designations	Sharpless 25, RCW 146, Gum 72 M8 contains: NGC 6523, NGC 6530^[1], Hourglass nebula^[3]
See also: Diffuse nebula, Lists of nebulae

The Lagoon Nebula (catalogued as Messier 8 or M8, and as NGC 6523) is a giant interstellar cloud in the constellation Sagittarius. It is classified as an emission nebula and as an H II region.
The Lagoon Nebula was discovered by Guillaume Le Gentil in 1747 and is one of only two star-forming nebulae faintly visible to the naked eye from mid-northern latitudes. Seen with binoculars, it appears as a distinct oval cloudlike patch with a definite core. A fragile star cluster appears superimposed on it.

Characteristics

The Lagoon Nebula is estimated to be 4,100 light-years from the Earth. In the sky of Earth, it spans 90' by 40', translates to an actual dimension of 110 by 50 light years. Like many nebulas, it appears pink in time-exposure color photos but is gray to the eye peering through binoculars or a telescope, human vision having poor color sensitivity at low light levels.
The nebula contains a number of Bok globules - dark, collapsing clouds of protostellar material - the most prominent of which have been catalogued by E. E. Barnard as B88, B89 and B296. It also includes a funnel-like or tornado-like structure caused by a hot O-type star that pours out ultraviolet light, heating and ionizing gases on the surface of the nebula.

Central region of the Lagoon Nebula, showing the Hourglass Nebula to the right

The Lagoon Nebula also contains at its centre a structure known as the "Hourglass Nebula" (so named by John Herschel), which should not be confused with the better known Hourglass Nebula in the constellation of Musca. In 2006 the first four Herbig-Haro objects were detected within the Hourglass, also including HH 870. This provides the first direct evidence of active star formation by accretion within it.^[2]

COMETS

2009-09-18T08:33:00.000-07:00

COMETS

Background

Name and symbol

The word comet came to the English language through the Latin cometes from the Greek word komē, meaning "hair of the head"; Aristotle first used the derivation komētēs to depict comets as "stars with hair." The astronomical symbol for comets (☄) accordingly consists of a disc with a hairlike tail.

Orbits and origin

Comets have a variety of different orbital periods, ranging from a few years, to hundreds of thousands of years, while some are believed to pass only once through the inner Solar System before being thrown out into interstellar space. Short-period comets are thought to originate in the Kuiper Belt, or associated scattered disc,^[1] which lie beyond the orbit of Neptune. Long-period comets are believed to originate in the Oort cloud, consisting of debris left over from the condensation of the solar nebula, located well-beyond the Kuiper Belt. Comets are thrown from these outer reaches of the Solar System towards the Sun by gravitational perturbations from the outer planets (in the case of Kuiper Belt objects) or nearby stars (in the case of Oort Cloud objects), or as a result of collisions between objects within these regions.

Comets are distinguished from asteroids by the presence of a coma or tail, though very old comets that have lost all their volatile materials may come to resemble asteroids (see extinct comets).^[2] Asteroids are also believed to have a different origin from comets, having formed in the inner Solar System rather than the outer Solar System,^[3] but recent findings^[4] have somewhat blurred the distinction between asteroids and comets (see centaurs and asteroid terminology).

As of May 2009 there are a reported 3,648 known comets^[5] of which about 1500 are Kreutz Sungrazers and about 400 are short-period.^[6] This number is steadily increasing. However, this represents only a tiny fraction of the total potential comet population: the reservoir of comet-like bodies in the outer solar system may number one trillion.^[7] The number of comets visible to the naked-eye averages to roughly one per year, though many of these are faint and unspectacular.^[8] When a historically bright or notable naked-eye comet is witnessed by many people, it may be termed a Great Comet.

Physical characteristics

Nucleus

Nucleus of comet Tempel 1 imaged by the Deep Impact impactor. The nucleus measures about 6 kilometres across.

Comet Hyakutake

Comet Holmes (17P/Holmes) in 2007 showing blue ion tail on right

Main article: Comet nucleus

Comet nuclei are known to range from about 100 meters to more than 40 kilometers across. They are composed of rock, dust, water ice, and frozen gases such as carbon monoxide, carbon dioxide, methane and ammonia.^[9]

They are often popularly described as "dirty snowballs", though recent observations have revealed dry dusty or rocky surfaces, suggesting that the ices are hidden beneath the crust (see Debate over comet composition). Comets also contain a variety of organic compounds; in addition to the gases already mentioned, these may include methanol, hydrogen cyanide, formaldehyde, ethanol and ethane, and perhaps more complex molecules such as long-chain hydrocarbons and amino acids.^[10]^[11]^[12] Because of their low mass, comets cannot become spherical under their own gravity, and will thus have irregular shapes.

Surprisingly, cometary nuclei are among the darkest objects known to exist in the solar system. The Giotto probe found that Comet Halley's nucleus reflects approximately 4% of the light that falls on it,^[13] and Deep Space 1 discovered that Comet Borrelly's surface reflects 2.4–3.0% of the light that falls on it;^[13] by comparison, asphalt reflects 7% of the light that falls on it. It is thought that complex organic compounds are the dark surface material. Solar heating drives off volatile compounds leaving behind heavy long-chain organics^{[clarification needed]} that tend to be very dark, like tar or crude oil. The very darkness of cometary surfaces allows them to absorb the heat necessary to drive their outgassing.

Coma and tail

Main article: Coma (cometary)

Main article: Comet tail

In the outer solar system, comets remain frozen and are extremely difficult or impossible to detect from Earth due to their small size. Statistical detections of inactive comet nuclei in the Kuiper belt have been reported from the Hubble Space Telescope observations,^[14]^[15] but these detections have been questioned,^[16]^[17] and have not yet been independently confirmed. As a comet approaches the inner solar system, solar radiation causes the volatile materials within the comet to vaporize and stream out of the nucleus, carrying dust away with them. The streams of dust and gas thus released form a huge, extremely tenuous atmosphere around the comet called the coma, and the force exerted on the coma by the Sun's radiation pressure and solar wind cause an enormous tail to form, which points away from the sun.

Both the coma and tail are illuminated by the Sun and may become visible from Earth when a comet passes through the inner solar system, the dust reflecting sunlight directly and the gases glowing from ionisation. Most comets are too faint to be visible without the aid of a telescope, but a few each decade become bright enough to be visible to the naked eye. Occasionally a comet may experience a huge and sudden outburst of gas and dust, during which the size of the coma temporarily greatly increases in size. This happened in 2007 to Comet Holmes.^{[citation needed]}

The streams of dust and gas each form their own distinct tail, pointing in slightly different directions. The tail of dust is left behind in the comet's orbit in such a manner that it often forms a curved^{[citation needed]} tail called the antitail. At the same time, the ion tail, made of gases, always points directly away from the Sun, as this gas is more strongly affected by the solar wind than is dust, following magnetic field lines rather than an orbital trajectory. Parallax viewing from the Earth may sometimes mean the tails appear to point in opposite directions.^[18]

While the solid nucleus of comets is generally less than 50 km across, the coma may be larger than the Sun, and ion tails have been observed to extend 1 astronomical unit (150 million km) or more.^[9] The observation of antitails contributed significantly to the discovery of solar wind.^[19] The ion tail is formed as a result of the photoelectric effect^{[dubious – discuss]} of solar ultra-violet radiation acting on particles in the coma. Once the particles have been ionised, they attain a net positive electrical charge which in turn gives rise to an "induced magnetosphere" around the comet. The comet and its induced magnetic field form an obstacle to outward flowing solar wind particles. As the relative orbital speed of the comet and the solar wind is supersonic a bow shock is formed upstream of the comet, in the flow direction of the solar wind. In this bow shock, large concentrations of cometary ions (called "pick-up ions") congregate and act to "load" the solar magnetic field with plasma, such that the field lines "drape" around the comet forming the ion tail.^[20]

Comet Encke loses its tail

If the ion tail loading is sufficient, then the magnetic field lines are squeezed together to the point where, at some distance along the ion tail, magnetic reconnection occurs. This leads to a "tail disconnection event".^[20] This has been observed on a number of occasions, notable among which was on the 20th. April 2007 when the ion tail of comet Encke was completely severed as the comet passed through a coronal mass ejection. This event was observed by the STEREO spacecraft.^[21]

Comets were found to emit X-rays in 1996.^[22] This surprised researchers, because X-ray emission is usually associated with very high-temperature bodies. The X-rays are thought to be generated by the interaction between comets and the solar wind: when highly charged ions fly through a cometary atmosphere, they collide with cometary atoms and molecules, "ripping of" one or more electrons from the comet. This ripping off leads to the emission of X-rays and far ultraviolet photons.^[23]

Connection to meteor showers

As a result of outgassing, comets leave a trail of solid debris behind them. If the comet's path crosses Earth's path, then at that point there will likely be meteor showers as Earth passes through the trail of debris. The Perseid meteor shower occurs every year between August 9 and August 13, when Earth passes through the orbit of the Swift–Tuttle comet.^[24] Halley's comet is the source of the Orionid shower in October.^[24]

The fate of comets

Departure/ejection from Solar System

If a comet is traveling fast enough, it will enter and leave the solar system; such is the case for most non-periodic comets. In addition, comets can be ejected by interacting with another object in the solar system (see Perturbation), such as Jupiter.

Volatiles exhausted

Material coming off Component B of 73P/Schwassmann-Wachmann which broke up starting in 1995, as seen by the HST. This animation covers a span of three days.

Main article: Extinct comet

Jupiter family comets (JFC) and long period comets (LPC) (see "Orbital characteristics", below) appear to follow very different fading laws. The JFCs are active over a lifetime of about 10,000 years or ~1,000 revolutions while the LPCs disappear much faster. Only 10% of the LPCs survive more than 50 passages to small perihelion, while only 1% of them survives more than 2,000 passages.^[25] Eventually most of the volatile material contained in a comet nucleus evaporates away, and the comet becomes a small, dark, inert lump of rock or rubble that can resemble an asteroid.^[26]

Breakups/Disintegration

Comets are also known to break up into fragments, as happened with Comet 73P/Schwassmann-Wachmann 3 starting in 1995.^[27]

This breakup may be triggered by tidal gravitational forces from the Sun or a large planet, by an "explosion" of volatile material, or for other reasons not fully explained.

Collisions

Shoemaker-Levy 9 was broken up by tidal forces shortly before colliding with Jupiter

Some comets meet a more spectacular end—either falling into the Sun,^[28] or smashing into a planet or other body. Collisions between comets and planets or moons were common in the early Solar System: some of the many craters on the Earth's Moon, for example, may have been caused by comets. A recent collision of a comet with a planet occurred in 1994 when Comet Shoemaker-Levy 9 broke up into pieces and collided with Jupiter.

Many comets and asteroids collided into Earth in its early stages. Many scientists believe that comets bombarding the young Earth (about 4 billion years ago) brought the vast quantities of water that now fill the Earth's oceans, or at least a significant proportion of it. But other researchers have cast doubt on this theory.^[29] The detection of organic molecules in comets has led some to speculate that comets or meteorites may have brought the precursors of life—or even life itself—to Earth.^[11] There are still many near-Earth comets, although a collision with an asteroid is more likely than with a comet.

It is suspected that comet impacts have, over long timescales, also delivered significant quantities of water to the Earth's Moon, some of which may have survived as lunar ice.

Orbital characteristics

Orbits of Comet Kohoutek (red) and Earth (blue), illustrating the high eccentricity of the orbit and more rapid motion when closer to the Sun.

Histogram of the aphelia of the 2005 comets, showing the giant planet comet families. The abscissa is the natural logarithm of the aphelion expressed in AUs.

Most comets have elongated elliptical orbits (oval shaped) that take them close to the Sun for a part of their orbit, and then out into the further reaches of the Solar System for the remainder. Comets are often classified according to the length of their orbital period; the longer the period the more elongated the ellipse.

Short-period comets are generally defined as having orbital periods of less than 200 years. They usually orbit more-or-less in the ecliptic plane in the same direction as the planets. Their orbits typically take them out to the region of the outer planets (Jupiter and beyond) at aphelion; for example, Comet Halley's aphelion is a little way beyond the orbit of Neptune. At the shorter extreme, Comet Encke has an orbit which never places it farther from the Sun than Jupiter. Short-period comets are further divided into the Jupiter family (periods less than 20 years) and Halley family (periods between 20 and 200 years).
Long-period comets have highly eccentric (elongated) orbits and periods ranging from 200 years to thousands or even millions of years. (However, by definition they remain gravitationally bound to the Sun; those comets that are ejected from the solar system due to close passes by major planets are no longer properly considered as having "periods".) Their orbits take them far beyond the outer planets at aphelia, and the plane of their orbits need not lie near the ecliptic.
Single-apparition comets are similar to long-period comets, but have parabolic or hyperbolic trajectories which will cause them to permanently exit the solar system after passing the Sun once.^[30]
Some authorities use the term periodic comet to refer to any comet with a periodic orbit (that is, all short-period comets plus all long-period comets),^[31] while others use it to mean exclusively short-period comets.^[30] Similarly, although the literal meaning of non-periodic comet is the same as single-apparition comet, some use it to mean all comets that are not "periodic" in the second sense (that is, to also include all comets with a period greater than 200 years).
Recently discovered main-belt comets form a distinct class, orbiting in more circular orbits within the asteroid belt.^[32]^[33]

Based on their orbital characteristics, short-period comets are thought to originate from the centaurs and the Kuiper belt/scattered disk^[1]—a disk of objects in the transneptunian region—whereas the source of long-period comets is thought to be the far more distant spherical Oort cloud (after the Dutch astronomer Jan Hendrik Oort who hypothesised its existence).^[34] Vast swarms of comet-like bodies are believed to orbit the Sun in these distant regions in roughly circular orbits. Occasionally the gravitational influence of the outer planets (in the case of Kuiper Belt objects) or nearby stars (in the case of Oort cloud objects) may throw one of these bodies into an elliptical orbit that takes it inwards towards the Sun, to form a visible comet. Unlike the return of periodic comets whose orbits have been established by previous observations, the appearance of new comets by this mechanism is unpredictable.

Since their elliptical orbits frequently take them close to the giant planets, comets are subject to further gravitational perturbations. Short period comets display a tendency for their aphelia to coincide with a giant planet's orbital radius, with the Jupiter family of comets being the largest, as the histogram shows. It is clear that comets coming in from the Oort cloud often have their orbits strongly influenced by the gravity of giant planets as a result of a close encounter. Jupiter is the source of the greatest perturbations, being more than twice as massive as all the other planets combined, in addition to being the swiftest of the giant planets. These perturbations may sometimes deflect long-period comets into shorter orbital periods (Halley's Comet being a possible example).

Early observations have revealed a few genuinely hyperbolic (i.e. non-periodic) trajectories, but no more than could be accounted for by perturbations from Jupiter. If comets pervaded interstellar space, they would be moving with velocities of the same order as the relative velocities of stars near the Sun (a few tens of kilometres per second). If such objects entered the solar system, they would have positive total energies, and would be observed to have genuinely hyperbolic trajectories. A rough calculation shows that there might be four hyperbolic comets per century,^[35] within Jupiter's orbit, give or take one and perhaps two orders of magnitude.^{[citation needed]}

A number of periodic comets discovered in earlier decades or previous centuries are now "lost." Their orbits were never known well enough to predict future appearances. However, occasionally a "new" comet will be discovered and upon calculation of its orbit it turns out to be an old "lost" comet. An example is Comet 11P/Tempel-Swift-LINEAR, discovered in 1869 but unobservable after 1908 because of perturbations by Jupiter. It was not found again until accidentally rediscovered by LINEAR in 2001.^[3Comet nomenclature

The names given to comets have followed several different conventions over the past two centuries. Before any systematic naming convention was adopted, comets were named in a variety of ways. Prior to the early 20th century, most comets were simply referred to by the year in which they appeared, sometimes with additional adjectives for particularly bright comets; thus, the "Great Comet of 1680" (Kirch's Comet), the "Great September Comet of 1882," and the "Daylight Comet of 1910" ("Great January Comet of 1910"). After Edmund Halley demonstrated that the comets of 1531, 1607, and 1682 were the same body and successfully predicted its return in 1759, that comet became known as Comet Halley.^[37] Similarly, the second and third known periodic comets, Comet Encke^[38] and Comet Biela,^[39] were named after the astronomers who calculated their orbits rather than their original discoverers. Later, periodic comets were usually named after their discoverers, but comets that had appeared only once continued to be referred to by the year of their apparition.

In the early 20th century, the convention of naming comets after their discoverers became common, and this remains so today. A comet is named after up to three independent discoverers. In recent years, many comets have been discovered by instruments operated by large teams of astronomers, and in this case, comets may be named for the instrument. For example, Comet IRAS-Araki-Alcock was discovered independently by the IRAS satellite and amateur astronomers Genichi Araki and George Alcock. In the past, when multiple comets were discovered by the same individual, group of individuals, or team, the comets' names were distinguished by adding a numeral to the discoverers' names (but only for periodic comets); thus Comets Shoemaker-Levy 1–9. Today, the large numbers of comets discovered by some instruments has rendered this system impractical, and no attempt is made to ensure that each comet has a unique name. Instead, the comets' systematic designations are used to avoid confusion.

Until 1994, comets were first given a provisional designation consisting of the year of their discovery followed by a lowercase letter indicating its order of discovery in that year (for example, Comet 1969i (Bennett) was the 9th comet discovered in 1969). Once the comet had been observed through perihelion and its orbit had been established, the comet was given a permanent designation of the year of its perihelion, followed by a Roman numeral indicating its order of perihelion passage in that year, so that Comet 1969i became Comet 1970 II (it was the second comet to pass perihelion in 1970)^[40]

Increasing numbers of comet discoveries made this procedure awkward, and in 1994 the International Astronomical Union approved a new naming system. Comets are now designated by the year of their discovery followed by a letter indicating the half-month of the discovery and a number indicating the order of discovery (a system similar to that already used for asteroids), so that the fourth comet discovered in the second half of February 2006 would be designated 2006 D4. Prefixes are also added to indicate the nature of the comet:

P/ indicates a periodic comet (defined for these purposes as any comet with an orbital period of less than 200 years or confirmed observations at more than one perihelion passage);
C/ indicates a non-periodic comet (defined as any comet that is not periodic according to the preceding definition);
X/ indicates a comet for which no reliable orbit could be calculated (generally, historical comets);
D/ indicates a comet which has broken up or been lost, referred to as dark comet;^[41]
A/ indicates an object that was mistakenly identified as a comet, but is actually a minor planet.

After their second observed perihelion passage, periodic comets are also assigned a number indicating the order of their discovery.^[42] So Halley's Comet, the first comet to be identified as periodic, has the systematic designation 1P/1682 Q1. Comet Hale-Bopp's designation is C/1995 O1. Comets which first received a minor planet designation keep the latter, which leads to some odd names such as P/2004 EW₃₈ (Catalina-LINEAR).

There are only five objects that are cross-listed as both comets and asteroids: 2060 Chiron (95P/Chiron), 4015 Wilson-Harrington (107P/Wilson-Harrington), 7968 Elst-Pizarro (133P/Elst-Pizarro), 60558 Echeclus (174P/Echeclus), and 118401 LINEAR (176P/LINEAR).

History of comet studyEarly observations and thought

Comet Halley depicted on the Bayeux Tapestry which shows King Harold I being told of Halley's Comet before the Battle of Hastings in 1066.

Before the invention of the telescope, comets seemed to appear out of nowhere in the sky and gradually vanish out of sight. They were usually considered bad omens of deaths of kings or noble men, or coming catastrophes, or even interpreted as attacks by heavenly beings against terrestrial inhabitants.^[43] From ancient sources, such as Chinese oracle bones, it is known that their appearances have been noticed by humans for millennia. Some authorities interpret references to "falling stars" in Gilgamesh, the Book of Revelation and the Book of Enoch as references to comets, or possibly bolides.

In the first book of his Meteorology, Aristotle propounded the view of comets that would hold sway in Western thought for nearly two thousand years. He rejected the ideas of several earlier philosophers that comets were planets, or at least a phenomenon related to the planets, on the grounds that while the planets confined their motion to the circle of the Zodiac, comets could appear in any part of the sky.^[44] Instead, he described comets as a phenomenon of the upper atmosphere, where hot, dry exhalations gathered and occasionally burst into flame. Aristotle held this mechanism responsible for not only comets, but also meteors, the aurora borealis, and even the Milky Way.^[45]

A few later classical philosophers did dispute this view of comets. Seneca the Younger, in his Natural Questions, observed that comets moved regularly through the sky and were undisturbed by the wind, behavior more typical of celestial than atmospheric phenomena. While he conceded that the other planets do not appear outside the Zodiac, he saw no reason that a planet-like object could not move through any part of the sky, humanity's knowledge of celestial things being very limited.^[46] However, the Aristotelian viewpoint proved more influential, and it was not until the 16th century that it was demonstrated that comets must exist outside the Earth's atmosphere.

In 1577, a bright comet was visible for several months. The Danish astronomer Tycho Brahe used measurements of the comet's position taken by himself and other, geographically separated, observers to determine that the comet had no measurable parallax. Within the precision of the measurements, this implied the comet must be at least four times more distant from the earth than the moon.^[47]

One very famous old recording of a comet is the appearance of Halley's Comet on the Bayeux Tapestry, which records the Norman conquest of England in AD 1066.^[48]

Orbital studies

The orbit of the comet of 1680, fit to a parabola, as shown in Isaac Newton's Principia

Although comets had now been demonstrated to be in the heavens, the question of how they moved through the heavens would be debated for most of the next century. Even after Johannes Kepler had determined in 1609 that the planets moved about the sun in elliptical orbits, he was reluctant to believe that the laws that governed the motions of the planets should also influence the motion of other bodies—he believed that comets travel among the planets along straight lines. Galileo Galilei, although a staunch Copernicanist, rejected Tycho's parallax measurements and held to the Aristotelian notion of comets moving on straight lines through the upper atmosphere.^{[citation needed]}

The first suggestion that Kepler's laws of planetary motion should also apply to the comets was made by William Lower in 1610.^[47] In the following decades other astronomers, including Pierre Petit, Giovanni Borelli, Adrien Auzout, Robert Hooke, Johann Baptist Cysat, and Giovanni Domenico Cassini all argued for comets curving about the sun on elliptical or parabolic paths, while others, such as Christian Huygens and Johannes Hevelius, supported comets' linear motion.^{[citation needed]}

The matter was resolved by the bright comet that was discovered by Gottfried Kirch on November 14, 1680. Astronomers throughout Europe tracked its position for several months. In 1681, the Saxon pastor Georg Samuel Doerfel set forth his proofs that comets are heavenly bodies moving in parabolas of which the sun is the focus. Then Isaac Newton, in his Principia Mathematica of 1687, proved that an object moving under the influence of his inverse square law of universal gravitation must trace out an orbit shaped like one of the conic sections, and he demonstrated how to fit a comet's path through the sky to a parabolic orbit, using the comet of 1680 as an example.^[49]

In 1705, Edmond Halley applied Newton's method to twenty-three cometary apparitions that had occurred between 1337 and 1698. He noted that three of these, the comets of 1531, 1607, and 1682, had very similar orbital elements, and he was further able to account for the slight differences in their orbits in terms of gravitational perturbation by Jupiter and Saturn. Confident that these three apparitions had been three appearances of the same comet, he predicted that it would appear again in 1758–9.^[50] (Earlier, Robert Hooke had identified the comet of 1664 with that of 1618,^[51] while Jean-Dominique Cassini had suspected the identity of the comets of 1577, 1665, and 1680.^[52] Both were incorrect.) Halley's predicted return date was later refined by a team of three French mathematicians: Alexis Clairaut, Joseph Lalande, and Nicole-Reine Lepaute, who predicted the date of the comet's 1759 perihelion to within one month's accuracy.^[53] When the comet returned as predicted, it became known as Comet Halley or Halley's Comet (its official designation is 1P/Halley). Its next appearance will be in 2061.

Among the comets with short enough periods to have been observed several times in the historical record, Comet Halley is unique in consistently being bright enough to be visible to the naked eye. Since the confirmation of Comet Halley's periodicity, many other periodic comets have been discovered through the telescope. The second comet to be discovered to have a periodic orbit was Comet Encke (official designation 2P/Encke). Over the period 1819–1821 the German mathematician and physicist Johann Franz Encke computed orbits for a series of cometary apparitions observed in 1786, 1795, 1805, and 1818, concluded that they were same comet, and successfully predicted its return in 1822.^[38] By 1900, seventeen comets had been observed at more than one perihelion passage and recognized as periodic comets. As of April 2006, 175 comets have achieved this distinction, though several have since been destroyed or lost. In ephemerides, comets are often denoted by the symbol ☄.

Studies of physical characteristics

Comets have highly elliptical orbits (the motion is shown as uniform for convenience's sake). Note the two distinct tails: cyan for the gas tail, grey for the dust tail

Isaac Newton described comets as compact and durable solid bodies moving in oblique orbits, and their tails as thin streams of vapor emitted by their nuclei, ignited or heated by the sun. Newton suspected that comets were the origin of the life-supporting component of air. Newton also believed that the vapors given off by comets might replenish the planets' supplies of water (which was gradually being converted into soil by the growth and decay of plants), and the sun's supply of fuel.

“	From his huge vapouring train perhaps to shake Reviving moisture on the numerous orbs, Thro' which his long ellipsis winds; perhaps To lend new fuel to declining suns, To light up worlds, and feed th' ethereal fire."	”
	— James Thomson , "The Seasons" (1730; 1748)

As early as the 18th century, some scientists had made correct hypotheses as to comets' physical composition. In 1755, Immanuel Kant hypothesized that comets are composed of some volatile substance, whose vaporization gives rise to their brilliant displays near perihelion.^[54] In 1836, the German mathematician Friedrich Wilhelm Bessel, after observing streams of vapor in the 1835 apparition of Comet Halley, proposed that the jet forces of evaporating material could be great enough to significantly alter a comet's orbit and argued that the non-gravitational movements of Comet Encke resulted from this mechanism.^[55]

However, another comet-related discovery overshadowed these ideas for nearly a century. Over the period 1864–1866 the Italian astronomer Giovanni Schiaparelli computed the orbit of the Perseid meteors, and based on orbital similarities, correctly hypothesized that the Perseids were fragments of Comet Swift-Tuttle. The link between comets and meteor showers was dramatically underscored when in 1872, a major meteor shower occurred from the orbit of Comet Biela, which had been observed to split into two pieces during its 1846 apparition, and was never seen again after 1852.^[39] A "gravel bank" model of comet structure arose, according to which comets consist of loose piles of small rocky objects, coated with an icy layer.

By the middle of the twentieth century, this model suffered from a number of shortcomings: in particular, it failed to explain how a body that contained only a little ice could continue to put on a brilliant display of evaporating vapor after several perihelion passages. In 1950, Fred Lawrence Whipple proposed that rather than being rocky objects containing some ice, comets were icy objects containing some dust and rock.^[56] This "dirty snowball" model soon became accepted. It was confirmed when an armada of spacecraft (including the European Space Agency's Giotto probe and the Soviet Union's Vega 1 and Vega 2) flew through the coma of Halley's comet in 1986 to photograph the nucleus and observed the jets of evaporating material (though see also "Debate over comet composition", below). The American probe Deep Space 1 flew past the nucleus of Comet Borrelly on September 21, 2001 and confirmed that the characteristics of Comet Halley are common on other comets as well.

Although comets formed in the outer Solar System, radial mixing of material during the early formation of the Solar System is thought to have redistributed material throughout the proto-planetary disk,^[57] so comets also contain crystalline grains which were formed in the hot inner Solar System. This is seen in comet spectra as well as in sample return missions.

Comet Wild 2 exhibits jets on light side and dark side, stark relief, and is dry.

The Stardust spacecraft, launched in February 1999, collected particles from the coma of Comet Wild 2 in January 2004, and returned the samples to Earth in a capsule in January 2006. Claudia Alexander, a program scientist for Rosetta from NASA's Jet Propulsion Laboratory who has modeled comets for years, reported to space.com about her astonishment at the number of jets, their appearance on the dark side of the comet as well as on the light side, their ability to lift large chunks of rock from the surface of the comet and the fact that comet Wild 2 is not a loosely cemented rubble pile.^[58]
Forthcoming space missions will add greater detail to our understanding of what comets are made of. In July 2005, the Deep Impact probe blasted a crater on Comet Tempel 1 to study its interior. And in 2014, the European Rosetta probe will orbit Comet Churyumov-Gerasimenko and place a small lander on its surface.
Rosetta observed the Deep Impact event, and with its set of very sensitive instruments for cometary investigations, it used its capabilities to observe Tempel 1 before, during and after the impact. At a distance of about 80 million kilometres from the comet, Rosetta was the only spacecraft other than Deep Impact itself to view the comet.

Debate over comet composition

Comet Borrelly exhibits jets, yet is hot and dry.

Debate continues about how much ice is in a comet. In 2001, NASA's Deep Space 1 team, working at NASA's Jet Propulsion Lab, obtained high-resolution images of the surface of Comet Borrelly. They announced that comet Borrelly exhibits distinct jets, yet has a hot, dry surface. The assumption that comets contain water and other ices led Dr. Laurence Soderblom of the U.S. Geological Survey to say, "The spectrum suggests that the surface is hot and dry. It is surprising that we saw no traces of water ice." However, he goes on to suggest that the ice is probably hidden below the crust as "either the surface has been dried out by solar heating and maturation or perhaps the very dark soot-like material that covers Borrelly's surface masks any trace of surface ice".^[59]
The recent Deep Impact probe has also yielded results suggesting that the majority of a comet's water ice is below the surface, and that these reservoirs feed the jets of vaporised water that form the coma of Tempel 1.^[60]
However, more recent data from the Stardust mission show that materials retrieved from the tail of comet Wild 2 were crystalline and could only have been "born in fire."^[61]^[62] More recent still, the materials retrieved demonstrate that the "comet dust resembles asteroid materials."^[63] These new results have forced scientists to rethink the nature of comets and their distinction from asteroids.^[64]

Notable comets

Great comets

Main article: Great Comet

While hundreds of tiny comets pass through the inner solar system every year, very few are noticed by the general public. About every decade or so, a comet will become bright enough to be noticed by a casual observer—such comets are often designated Great Comets. In times past, bright comets often inspired panic and hysteria in the general population, being thought of as bad omens. More recently, during the passage of Halley's Comet in 1910, the Earth passed through the comet's tail, and erroneous newspaper reports inspired a fear that cyanogen in the tail might poison millions,^[65] while the appearance of Comet Hale-Bopp in 1997 triggered the mass suicide of the Heaven's Gate cult. To most people, however, a great comet is simply a beautiful spectacle.
Predicting whether a comet will become a great comet is notoriously difficult, as many factors may cause a comet's brightness to depart drastically from predictions. Broadly speaking, if a comet has a large and active nucleus, will pass close to the Sun, and is not obscured by the Sun as seen from the Earth when at its brightest, it will have a chance of becoming a great comet. However, Comet Kohoutek in 1973 fulfilled all the criteria and was expected to become spectacular, but failed to do so. Comet West, which appeared three years later, had much lower expectations (perhaps because scientists were much warier of glowing predictions after the Kohoutek fiasco), but became an extremely impressive comet.^[66]
The late 20th century saw a lengthy gap without the appearance of any great comets, followed by the arrival of two in quick succession—Comet Hyakutake in 1996, followed by Hale-Bopp, which reached maximum brightness in 1997 having been discovered two years earlier. The first great comet of the 21st century was Comet McNaught, which became visible to naked eye observers in January 2007. It was the brightest in over 40 years.

Sungrazing comets

Main article: Sungrazing comet

The Great Comet of 1882, is a member of the Kreutz group

A Sungrazing comet is a comet that passes extremely close to the Sun at perihelion, sometimes within a few thousand kilometres of the Sun's surface. While small sungrazers can be completely evaporated during such a close approach to the Sun, larger sungrazers can survive many perihelion passages. However, the strong tidal forces they experience often lead to their fragmentation.
About 90% of the sungrazers observed with SOHO are members of the Kreutz group, which all originate from one giant comet that broke up into many smaller comets during its first passage through the inner solar system.^[67] The other 10% contains some sporadic sungrazers, but four other related groups of comets have been identified among them: the Kracht, Kracht 2a, Marsden and Meyer groups. The Marsden and Kracht groups both appear to be related to Comet 96P/Machholz, which is also the parent of two meteor streams, the Quadrantids and the Arietids.^[68]

Unusual comets

The quasi-circular orbit of 29P/Schwassmann-Wachmann compared to Jupiter and Saturn.

Of the thousands of known comets, some are very unusual. Comet Encke orbits from outside the main asteroid belt to inside the orbit of Mercury while Comet 29P/Schwassmann-Wachmann currently travels in a nearly circular orbit entirely between Jupiter and Saturn.^[69] 2060 Chiron, whose unstable orbit is between Saturn and Uranus, was originally classified as an asteroid until a faint coma was noticed.^[70] Similarly, Comet Shoemaker-Levy 2 was originally designated asteroid 1990 UL₃.^[71] Roughly six percent of the near-earth asteroids are thought to be extinct nuclei of comets which no longer experience outgassing.^[25]
Some comets have been observed to break up during their perihelion passage, including great comets West and Ikeya-Seki. Comet Biela was one significant example, breaking into two during its 1846 perihelion passage. The two comets were seen separately in 1852, but never again afterward. Instead, spectacular meteor showers were seen in 1872 and 1885 when the comet should have been visible. A lesser meteor shower, the Andromedids, occurs annually in November, and is caused by the Earth crossing Biela's orbit.^[72]
Another significant cometary disruption was that of Comet Shoemaker-Levy 9, which was discovered in 1993. At the time of its discovery, the comet was in orbit around Jupiter, having been captured by the planet during a very close approach in 1992.^[73] This close approach had already broken the comet into hundreds of pieces, and over a period of 6 days in July 1994, these pieces slammed into Jupiter's atmosphere—the first time astronomers had observed a collision between two objects in the solar system.^[74] It has also been suggested that the object likely to have been responsible for the Tunguska event in 1908 was a fragment of Comet Encke.^[75]

Observation

Example of a comet's path plotted by planetarium software (Sky Map Pro)

A new comet may be discovered photographically using a wide-field telescope or visually with binoculars. However, even without access to optical equipment, it is still possible for the amateur astronomer to discover a Sun-grazing comet online by downloading images accumulated by some satellite observatories such as SOHO.^[76]
Comets visible to the naked eye are fairly infrequent, but comets that put on fine displays in amateur class telescopes (50 mm to 100 cm) occur fairly often—as often as several times a year, occasionally with more than one in the sky at the same time. Commonly available astronomical software will plot the orbits of these known comets. They are fast compared to other objects in the sky, but their movement is usually subtle in the eyepiece of a telescope. However, from night to night, they can move several degrees, which is why observers find it useful to have a sky chart such as the one in the adjoining illustration.
The type of display presented by the comet depends on its composition and how close it comes to the sun. Because the volatility of a comet's material decreases as it gets further from the sun, the comet becomes increasingly difficult to observe as a function of not only distance, but the progressive shrinking and eventual disappearance of its tail and the reflective elements it carries. Comets are most interesting when their nucleus is bright and they display a long tail, which to be seen sometimes requires a large field of view best provided by smaller telescopes. Therefore, large amateur instruments (apertures of 25 cm or larger) that have fainter light grasp do not necessarily confer an advantage in terms of viewing comets. The opportunity to view spectacular comets with relatively small aperture instruments in the 8 cm to 15 cm range is more frequent than might be guessed from the relatively rare attention they get in the mainstream press.

ASTEROID

2009-09-16T03:48:00.000-07:00

http://en.wikipedia.org/wiki/Asteroid

ASTEROID

Terminology

Traditionally, small bodies orbiting the Sun were classified as asteroids, comets or meteoroids, with anything smaller than ten metres across being called a meteoroid.^[1] The term "asteroid" is somewhat ill-defined. It never had a formal definition, with the broader term minor planet being preferred by the International Astronomical Union until 2006, when the term "small Solar System body" (SSSB) was introduced to cover both minor planets and comets. The 2006 definition of SSSB says that they "include most of the Solar System asteroids, most Trans-Neptunian Objects (TNOs), comets, and other small bodies".^[2] Other languages prefer "planetoid" (Greek for "planet-like"), and this term is occasionally used in English for the larger asteroids. The word "planetesimal" has a similar meaning, but refers specifically to the small building blocks of the planets that existed at the time the Solar System was forming. The term "planetule" was coined by the geologist William Daniel Conybeare to describe minor planets,^[3] but is not in common use.

When found, asteroids were seen as a class of objects distinct from comets, and there was no unified term for the two until "small Solar System body" was coined in 2006. The main difference between an asteroid and a comet is that a comet shows a coma due to sublimation of near surface ices by solar radiation. A few objects have ended up being dual-listed because they were first classified as minor planets but later showed evidence of cometary activity. Conversely, some (perhaps all) comets are eventually depleted of their surface volatile ices and become asteroids. A further distinction is that comets typically have more eccentric orbits than most asteroids; most "asteroids" with notably eccentric orbits are probably dormant or extinct comets.^{[citation needed]}

For almost two centuries, from the discovery of the first asteroid, 1 Ceres, in 1801 until the discovery of the first centaur, 2060 Chiron, in 1977, all known asteroids spent most of their time at or within the orbit of Jupiter, though a few such as 944 Hidalgo ventured far beyond Jupiter for part of their orbit. When astronomers started finding additional small bodies that permanently resided further out than Jupiter, now called centaurs, they numbered them among the traditional asteroids, though there was debate over whether they should be classified as asteroids or as a new type of object. Then, when the first trans-Neptunian object, 1992 QB1, was discovered in 1992, and especially when large numbers of similar objects started turning up, new terms were invented to sidestep the issue: Kuiper Belt object (KBO), trans-Neptunian object (TNO), scattered-disc object (SDO), and so on. These inhabit the cold outer reaches of the Solar System where ices remain solid and comet-like bodies are not expected to exhibit much cometary activity; if centaurs or TNOs were to venture close to the Sun, their volatile ices would sublimate, and traditional approaches would classify them as comets rather than asteroids.

The innermost of these are the Kuiper Belt Objects (KBOs), called "objects" partly to avoid the need to classify them as asteroids or comets.^[4] KBOs are believed to be predominantly comet-like in composition, though some may be more akin to asteroids.^[5] Furthermore, most do not have the highly eccentric orbits associated with comets, and the ones so far discovered are very much larger than traditional comet nuclei. (The much more distant Oort cloud is hypothesized to be the main reservoir of dormant comets.) Other recent observations, such as the analysis of the cometary dust collected by the Stardust probe, are increasingly blurring the distinction between comets and asteroids,^[6] suggesting "a continuum between asteroids and comets" rather than a sharp dividing line.^[7]

The minor planets beyond Jupiter's orbit are rarely directly referred to as "asteroids", but all are commonly lumped together under the term "asteroid" in popular presentations. For instance, a joint NASA-JPL public-outreach website states,

We include Trojans (bodies captured in Jupiter's 4th and 5th Lagrange points), Centaurs (bodies in orbit between Jupiter and Neptune), and trans-Neptunian objects (orbiting beyond Neptune) in our definition of "asteroid" as used on this site, even though they may more correctly be called "minor planets" instead of asteroids.^[8]

It is, however, becoming increasingly common for the term "asteroid" to be restricted to minor planets of the inner Solar System,^[9] and therefore this article will restrict itself for the most part to the classical asteroids: objects of the main asteroid belt, Jupiter trojans, and near-Earth objects.

When the IAU introduced the class small solar system bodies in 2006 to include most objects previously classified as minor planets and comets, they created the class of dwarf planets for the largest minor planets—those which have sufficient mass to have become ellipsoidal under their own gravity. According to the IAU, "the term 'minor planet' may still be used, but generally the term 'small solar system body' will be preferred."^[10] Currently only the largest object in the asteroid belt, Ceres, at about 950 km (590 mi) across, has been placed in the dwarf planet category, although there are several large asteroids (Vesta, Pallas, and Hygiea) that may be classified as dwarf planets when their shapes are better known.^[11]

Formation

It is believed that planetesimals in the main asteroid belt evolved much like the rest of the Solar Nebula until Jupiter neared its current mass, at which point excitation from orbital resonances with Jupiter ejected over 99% of planetesimals in the belt. Both simulations and a discontinuity in spin rate and spectral properties suggest that asteroids larger than approximately 120 km (75 mi) in diameter accreted during that early era, whereas smaller bodies are fragments from collisions between asteroids during or after the Jovian disruption.^[12] At least two asteroids, Ceres and Vesta, grew large enough to melt and differentiate, with heavy metallic elements sinking to the core, leaving rocky minerals in the crust.^[13]

In the Nice model, a large number of Kuiper Belt objects are captured in the outer Main Belt, at distances greater than 2.6 AU. Most were subsequently ejected by Jupiter, but those that remained may be the D-type asteroids, and possibly include Ceres.^[14]

Characteristics

Objects in the main asteroid belt vary greatly in size, from a diameter of 950 kilometres for the dwarf planet Ceres and over 500 kilometres for the asteroids 2 Pallas and 4 Vesta down to rocks just tens of metres across.^{[note 1]} A few of the largest are roughly spherical and are very much like miniature planets. The vast majority, however, are much smaller and are irregularly shaped.

The physical composition of asteroids is varied and in most cases poorly understood. Ceres appears to be composed of a rocky core covered by an icy mantle, whereas Vesta is thought to have a nickel-iron core, olivine mantle, and basaltic crust,^[15] and 10 Hygiea appears to have a primitive composition of undifferentiated carbonaceous chondrite. Many, perhaps most, of the smaller asteroids are piles of rubble held together loosely by gravity. Some have moons or are co-orbiting pairs of binary asteroids. All three conditions, as well as scattered asteroid families, may be the result of collisions which disrupted a parent asteroid.

Asteroids are believed to contain traces of amino-acids and other organic compounds, and some speculate that asteroid impacts may have seeded the early Earth with the chemicals necessary to initiate life, or may have even brought life itself to Earth (see also Panspermia).^[16]

Only one asteroid, 4 Vesta (which has a particularly reflective surface), is normally visible to the naked eye, and this only in very dark skies when it is favorably positioned. Very rarely, small asteroids passing close to Earth may be naked-eye visible for a short period of time.^[17]

The orbits of asteroids are often influenced by the gravity of other bodies in the solar system or the Yarkovsky effect.

The relative masses of the nine large main-belt asteroids for which precise (< 15% estimated error) data is available.^[18] Given the poor precision for asteroids estimated to be somewhat less massive than 16 Psyche, a few other may turn out to be more massive than Psyche, 3 Juno, or 15 Eunomia.	The same nine objects, compared to the remaining mass of the main belt.^[19]
		1 Ceres 4 Vesta 2 Pallas 10 Hygeia 704 Interamnia	511 Davida 15 Eunomia 3 Juno 16 Psyche other

Distribution within the Solar System

The Main asteroid belt (white) and the Trojan asteroids (green)

The vast majority of known asteroids orbit within the main asteroid belt between the orbits of Mars and Jupiter, generally in relatively low-eccentricity (i.e., not very elongated) orbits. This belt is currently estimated to contain between 1.1 and 1.9 million asteroids larger than 1 km (1 mi) in diameter,^[20] and millions of smaller ones.^[21] It is thought that these asteroids are remnants of the protoplanetary disk, and in this region the accretion of planetesimals into planets during the formative period of the solar system was prevented by large gravitational perturbations by Jupiter. Although fewer Trojan asteroids sharing Jupiter's orbit are currently known, it is thought that there are as many as there are asteroids in the main belt.
The dwarf planet Ceres is the largest object in the asteroid belt, with a diameter of over 975 km (606 mi). The next largest are the asteroids 2 Pallas and 4 Vesta, both with diameters of over 500 km (311 mi). Normally Vesta is the only main belt asteroid that can, on occasion, become visible to the naked eye. However, on some very rare occasions, a near-Earth asteroid may briefly become visible without technical aid; see 99942 Apophis.

Left to right: 4 Vesta, 1 Ceres, Earth's Moon

The mass of all the objects of the Main asteroid belt, lying between the orbits of Mars and Jupiter, is estimated to be about 3.0-3.6 × 10²¹ kg, or about 4 percent of the mass of the Moon. Of this, Ceres comprises 0.95 × 10²¹ kg, some 32 percent of the total.^[22]^[23] Adding in the next three most massive asteroids, 4 Vesta (9%), 2 Pallas (7%), and 10 Hygiea (3%), brings this figure up to 51%; while the three after that, 511 Davida (1.2%), 704 Interamnia (1.0%), and 52 Europa (0.9%), only add another 3% to the total mass. The number of asteroids then increases rapidly as their individual masses decrease.
Various classes of asteroid have been discovered outside the main asteroid belt. Near-Earth asteroids have orbits in the vicinity of Earth's orbit. Trojan asteroids are gravitationally locked into synchronisation with Jupiter, either leading or trailing the planet in its orbit. A couple trojans have been found orbiting with Mars.^{[note 2]} A group of asteroids called Vulcanoids are hypothesised by some to lie very close to the Sun, within the orbit of Mercury, but none has so far been found.

Classification

Asteroids are commonly classified according to two criteria: the characteristics of their orbits, and features of their reflectance spectrum.

Orbit groups and families

Main articles: Asteroid group and Asteroid family

Many asteroids have been placed in groups and families based on their orbital characteristics. Apart from the broadest divisions, it is customary to name a group of asteroids after the first member of that group to be discovered. Groups are relatively loose dynamical associations, whereas families are much tighter and result from the catastrophic break-up of a large parent asteroid sometime in the past.^[24] Families have only been recognized within the main asteroid belt. They were first recognised by Kiyotsugu Hirayama in 1918 and are often called Hirayama families in his honor.
About 30% to 35% of the bodies in the main belt belong to dynamical families each thought to have a common origin in a past collision between asteroids. A family has also been associated with the plutoid dwarf planet Haumea.

Quasi-satellites and horseshoe objects

Some asteroids have unusual horseshoe orbits that are co-orbital with the Earth or some other planet. Examples are 3753 Cruithne and 2002 AA₂₉. The first instance of this type of orbital arrangement was discovered between Saturn's moons Epimetheus and Janus.
Sometimes these horseshoe objects temporarily become quasi-satellites for a few decades or a few hundred years, before returning to their prior status. Both Earth and Venus are known to have quasi-satellites.
Such objects, if associated with Earth or Venus or even hypothetically Mercury, are a special class of Aten asteroids. However, such objects could be associated with outer planets as well.Spectral classification

This picture of 433 Eros shows the view looking from one end of the asteroid across the gouge on its underside and toward the opposite end. Features as small as 35 m across can be seen.

Main article: Asteroid spectral types

In 1975, an asteroid taxonomic system based on colour, albedo, and spectral shape was developed by Clark R. Chapman, David Morrison, and Ben Zellner.^[25] These properties are thought to correspond to the composition of the asteroid's surface material. The original classification system had three categories: C-types for dark carbonaceous objects (75% of known asteroids), S-types for stony (silicaceous) objects (17% of known asteroids) and U for those that did not fit into either C or S. This classification has since been expanded to include a number of other asteroid types. The number of types continues to grow as more asteroids are studied.
The two most widely used taxonomies currently used are the Tholen classification and SMASS classification. The former was proposed in 1984 by David J. Tholen, and was based on data collected from an eight-color asteroid survey performed in the 1980s. This resulted in 14 asteroid categories.^[26] In 2002, the Small Main-Belt Asteroid Spectroscopic Survey resulted in a modified version of the Tholen taxonomy with 24 different types. Both systems have three broad categories of C, S, and X asteroids, where X consists of mostly metallic asteroids, such as the M-type. There are also a number of smaller classes.^[27]
Note that the proportion of known asteroids falling into the various spectral types does not necessarily reflect the proportion of all asteroids that are of that type; some types are easier to detect than others, biasing the totals.

Problems with spectral classification

Originally, spectral designations were based on inferences of an asteroid's composition.^[28] However, the correspondence between spectral class and composition is not always very good, and there are a variety of classifications in use. This has led to significant confusion. While asteroids of different spectral classifications are likely to be composed of different materials, there are no assurances that asteroids within the same taxonomic class are composed of similar materials.
At present, the spectral classification based on several coarse resolution spectroscopic surveys in the 1990s is still the standard. Scientists have been unable to agree on a better taxonomic system,^{[citation needed]} largely due to the difficulty of obtaining detailed measurements consistently for a large sample of asteroids (e.g. finer resolution spectra, or non-spectral data such as densities would be very useful).

Discovery

243 Ida and its moon Dactyl, the first satellite of an asteroid to be discovered.

The first named minor planet, 1 Ceres, was discovered in 1801 by Giuseppe Piazzi, and was originally considered a new planet.^{[note 3]} This was followed by the discovery of other similar bodies, which with the equipment of the time appeared to be points of light, like stars, showing little or no planetary disc (though readily distinguishable from stars due to their apparent motions). This prompted the astronomer Sir William Herschel to propose the term "asteroid", from Greek αστεροειδής, asteroeidēs = star-like, star-shaped, from ancient Greek Aστήρ, astēr = star. In the early second half of the nineteenth century, the terms "asteroid" and "planet" (not always qualified as "minor") were still used interchangeably; for example, the Annual of Scientific Discovery for 1871, page 316, reads "Professor J. Watson has been awarded by the Paris Academy of Sciences, the astronomical prize, Lalande foundation, for the discovery of 8 new asteroids in one year. The planet Lydia (No. 110), discovered by M. Borelly at the Marseilles Observatory [...] M. Borelly had previously discovered 2 planets bearing the numbers 91 and 99 in the system of asteroids revolving between Mars and Jupiter" (emphasis added).

Historical methods

Asteroid discovery methods have dramatically improved over the past two centuries.
In the last years of the 18th century, Baron Franz Xaver von Zach organized a group of 24 astronomers to search the sky for the missing planet predicted at about 2.8 AU from the Sun by the Titius-Bode law, partly as a consequence of the discovery, by Sir William Herschel in 1781, of the planet Uranus at the distance predicted by the law. This task required that hand-drawn sky charts be prepared for all stars in the zodiacal band down to an agreed-upon limit of faintness. On subsequent nights, the sky would be charted again and any moving object would, hopefully, be spotted. The expected motion of the missing planet was about 30 seconds of arc per hour, readily discernible by observers.
The first asteroid, 1 Ceres, was not discovered by a member of the group, but rather by accident in 1801 by Giuseppe Piazzi, director of the observatory of Palermo in Sicily. He discovered a new star-like object in Taurus and followed the displacement of this object during several nights. His colleague, Carl Friedrich Gauss, used these observations to determine the exact distance from this unknown object to the Earth. Gauss' calculations placed the object between the planets Mars and Jupiter. Piazzi named it after Ceres, the Roman goddess of agriculture.
Three other asteroids (2 Pallas, 3 Juno, and 4 Vesta) were discovered over the next few years, with Vesta found in 1807. After eight more years of fruitless searches, most astronomers assumed that there were no more and abandoned any further searches.
However, Karl Ludwig Hencke persisted, and began searching for more asteroids in 1830. Fifteen years later, he found 5 Astraea, the first new asteroid in 38 years. He also found 6 Hebe less than two years later. After this, other astronomers joined in the search and at least one new asteroid was discovered every year after that (except the wartime year 1945). Notable asteroid hunters of this early era were J. R. Hind, Annibale de Gasparis, Robert Luther, H. M. S. Goldschmidt, Jean Chacornac, James Ferguson, Norman Robert Pogson, E. W. Tempel, J. C. Watson, C. H. F. Peters, A. Borrelly, J. Palisa, the Henry brothers and Auguste Charlois.
In 1891, however, Max Wolf pioneered the use of astrophotography to detect asteroids, which appeared as short streaks on long-exposure photographic plates. This dramatically increased the rate of detection compared with previous visual methods: Wolf alone discovered 248 asteroids, beginning with 323 Brucia, whereas only slightly more than 300 had been discovered up to that point. Still, a century later, only a few thousand asteroids were identified, numbered and named. It was known that there were many more, but most astronomers did not bother with them, calling them "vermin of the skies".

Manual methods of the 1900s and modern reporting

Until 1998, asteroids were discovered by a four-step process. First, a region of the sky was photographed by a wide-field telescope, or Astrograph. Pairs of photographs were taken, typically one hour apart. Multiple pairs could be taken over a series of days. Second, the two films of the same region were viewed under a stereoscope. Any body in orbit around the Sun would move slightly between the pair of films. Under the stereoscope, the image of the body would appear to float slightly above the background of stars. Third, once a moving body was identified, its location would be measured precisely using a digitizing microscope. The location would be measured relative to known star locations.^[29]
These first three steps do not constitute asteroid discovery: the observer has only found an apparition, which gets a provisional designation, made up of the year of discovery, a letter representing the half-month of discovery, and finally a letter and a number indicating the discovery's sequential number (example: 1998 FJ₇₄).
The final step of discovery is to send the locations and time of observations to the Minor Planet Center, where computer programs determine whether an apparition ties together previous apparitions into a single orbit. If so, the object receives a catalogue number and the observer of the first apparition with a calculated orbit is declared the discoverer, and granted the honor of naming the object subject to the approval of the International Astronomical Union.

Computerized methods

2004 FH is the center dot being followed by the sequence; the object that flashes by during the clip is an artificial satellite.

There is increasing interest in identifying asteroids whose orbits cross Earth's, and that could, given enough time, collide with Earth (see Earth-crosser asteroids). The three most important groups of near-Earth asteroids are the Apollos, Amors, and Atens. Various asteroid deflection strategies have been proposed, as early as the 1960s.
The near-Earth asteroid 433 Eros had been discovered as long ago as 1898, and the 1930s brought a flurry of similar objects. In order of discovery, these were: 1221 Amor, 1862 Apollo, 2101 Adonis, and finally 69230 Hermes, which approached within 0.005 AU of the Earth in 1937. Astronomers began to realize the possibilities of Earth impact.
Two events in later decades increased the level of alarm: the increasing acceptance of Walter Alvarez' hypothesis that an impact event resulted in the Cretaceous-Tertiary extinction, and the 1994 observation of Comet Shoemaker-Levy 9 crashing into Jupiter. The U.S. military also declassified the information that its military satellites, built to detect nuclear explosions, had detected hundreds of upper-atmosphere impacts by objects ranging from one to 10 metres across.
All of these considerations helped spur the launch of highly efficient automated systems that consist of Charge-Coupled Device (CCD) cameras and computers directly connected to telescopes. Since 1998, a large majority of the asteroids have been discovered by such automated systems. A list of teams using such automated systems includes:^[30]

The Lincoln Near-Earth Asteroid Research (LINEAR) team
The Near-Earth Asteroid Tracking (NEAT) team
Spacewatch
The Lowell Observatory Near-Earth-Object Search (LONEOS) team
The Catalina Sky Survey (CSS)
The Campo Imperatore Near-Earth Objects Survey (CINEOS) team
The Japanese Spaceguard Association
The Asiago-DLR Asteroid Survey (ADAS)

The LINEAR system alone has discovered 97,470 asteroids, as of September 18, 2008.^[31] Between all of the automated systems, 4711 near-Earth asteroids have been discovered^[32] including over 600 more than 1 km (1 mi) in diameter. The rate of discovery peaked in 2000, when 38,679 minor planets were numbered, and has been going down steadily since then (719 minor planets were numbered in 2007).^[33]

Naming

Main article: Minor planet#Naming

A newly discovered asteroid is given a provisional designation (such as 2002 AT₄) consisting of the year of discovery and an alphanumeric code indicating the half-month of discovery and the sequence within that half-month. Once an asteroid's orbit has been confirmed, it is given a number, and later may also be given a name (e.g. 433 Eros). The formal naming convention uses parentheses around the number (e.g. (433) Eros), but dropping the parentheses is quite common. Informally, it is common to drop the number altogether, or to drop it after the first mention when a name is repeated in running text.

Symbols

The first few asteroids discovered were assigned symbols like the ones traditionally used to designate Earth, the Moon, the Sun and planets. The symbols quickly became ungainly, hard to draw and recognise. By the end of 1851 there were 15 known asteroids, each (except one) with its own symbol(s).^[34]

Asteroid	Symbol
Ceres
2 Pallas
3 Juno
4 Vesta
5 Astraea
6 Hebe
7 Iris
8 Flora
9 Metis
10 Hygiea
11 Parthenope
12 Victoria
13 Egeria	Never assigned.
14 Irene	"A dove carrying an olive-branch, with a star on its head," never drawn.^[35]
15 Eunomia
28 Bellona
35 Leukothea
37 Fides

Johann Franz Encke made a major change in the Berliner Astronomisches Jahrbuch (BAJ, Berlin Astronomical Yearbook) for 1854. He introduced encircled numbers instead of symbols, although his numbering began with Astraea, the first four asteroids continuing to be denoted by their traditional symbols. This symbolic innovation was adopted very quickly by the astronomical community. The following year (1855), Astraea's number was bumped up to 5, but Ceres through Vesta would be listed by their numbers only in the 1867 edition. A few more asteroids (28 Bellona,^[36] 35 Leukothea,^[37] and 37 Fides^[38]) would be given symbols as well as using the numbering scheme. The circle would become a pair of parentheses, and the parentheses sometimes omitted altogether over the next few decades.^[35]

Exploration

Vesta, imaged by the Hubble Space Telescope

951 Gaspra, the first asteroid to be imaged in close up.

Until the age of space travel, objects in the asteroid belt were merely pinpricks of light in even the largest telescopes and their shapes and terrain remained a mystery. The best modern ground-based telescopes, as well as the Earth-orbiting Hubble Space Telescope, can resolve a small amount of detail on the surfaces of the very largest asteroids, but even these mostly remain little more than fuzzy blobs. Limited information about the shapes and compositions of asteroids can be inferred from their light curves (their variation in brightness as they rotate) and their spectral properties, and asteroid sizes can be estimated by timing the lengths of star occulations (when an asteroid passes directly in front of a star). Radar imaging can yield good information about asteroid shapes and orbital and rotational parameters, especially for near-Earth asteroids.
The first close-up photographs of asteroid-like objects were taken in 1971 when the Mariner 9 probe imaged Phobos and Deimos, the two small moons of Mars, which are probably captured asteroids. These images revealed the irregular, potato-like shapes of most asteroids, as did subsequent images from the Voyager probes of the small moons of the gas giants.
The first true asteroid to be photographed in close-up was 951 Gaspra in 1991, followed in 1993 by 243 Ida and its moon Dactyl, all of which were imaged by the Galileo probe en route to Jupiter.
The first dedicated asteroid probe was NEAR Shoemaker, which photographed 253 Mathilde in 1997, before entering into orbit around 433 Eros, finally landing on its surface in 2001.
Other asteroids briefly visited by spacecraft en route to other destinations include 9969 Braille (by Deep Space 1 in 1999), and 5535 Annefrank (by Stardust in 2002).
In September 2005, the Japanese Hayabusa probe started studying 25143 Itokawa in detail and may return samples of its surface to earth. The Hayabusa mission has been plagued with difficulties, including the failure of two of its three control wheels, rendering it difficult to maintain its orientation to the sun to collect solar energy. Following that, the next asteroid encounters will involve the European Rosetta probe (launched in 2004), which flew by 2867 Šteins in 2008 and will buzz 21 Lutetia in 2010.
In September 2007, NASA launched the Dawn Mission, which will orbit the dwarf planet Ceres and the asteroid 4 Vesta in 2011-2015, with its mission possibly then extended to 2 Pallas.
It has been suggested that asteroids might be used in the future as a source of materials which may be rare or exhausted on earth (asteroid mining), or materials for constructing space habitats (see Colonization of the asteroids). Materials that are heavy and expensive to launch from earth may someday be mined from asteroids and used for space manufacturing and construction.

CHANDRAYAN

2009-09-13T06:44:00.000-07:00

CHANDRAYAN

space science

ALIENS

Close encounter

Contents

Close Encounter of the First kind

Close Encounter of the Second kind

Close Encounter of the Third kind

Bloecher subtypes

Close Encounter of the Fourth kind

Close Encounter of the Fifth kind

Close Encounter of the Sixth kind

Close Encounter of the Seventh kind

Contents

19th-20th centuries

[edit] 21st century

UFO sightings by Country

Proof Of Alien Life - UFO Sightings

Proof Of Alien Life - Alien Encounter Videos

EAGLE NEBULA

Eagle Nebula

Contents

Characteristics

OWL NEBULA

Owl Nebula

DUMBELL NEBULA

Dumbbell Nebula

Contents

Structure

Knots

Central star

OMEGA NEBULA

Omega Nebula

TRIFID NEBULA

Trifid Nebula

Contents

Characteristics

Gallery

LAGOON NEBULA

Lagoon Nebula

Contents

Characteristics

COMETS

Background

Name and symbol

Orbits and origin

Physical characteristics

Nucleus

Coma and tail

Connection to meteor showers

The fate of comets

Departure/ejection from Solar System

Volatiles exhausted

Breakups/Disintegration

Collisions

Orbital characteristics

History of comet studyEarly observations and thought

Orbital studies

Studies of physical characteristics

Debate over comet composition

Notable comets

Great comets

Sungrazing comets

Unusual comets

Observation

ASTEROID

Terminology

Formation

Characteristics

Distribution within the Solar System

Classification

Orbit groups and families

Quasi-satellites and horseshoe objects

Problems with spectral classification

Discovery

Historical methods

Manual methods of the 1900s and modern reporting

Computerized methods

Naming

Symbols

Exploration