| Discovery | |
|---|---|
| Discovered by: | Charles Messier |
| Discovery date: | June 14, 1770 |
| Alternate designations: | 1770 I, P/Lexell, Lexell's Comet |
| Orbital characteristics A | |
| Perihelion: | 0.674449 |
| Eccentricity: | 0.786119 |
| Inclination: | 1.5517° |
| Last perihelion: | August 14, 1770 |
| Next perihelion: | Lost |
D/1770 L1, popularly known as Lexell's Comet after its orbit computer Anders Johan Lexell, was a comet discovered by astronomer Charles Messier in June 1770.[1] It is notable for having passed closer to the Earth than any other comet in recorded history, approaching to a distance of only 0.015 astronomical units (2,200,000 km; 1,400,000 mi).[2][3][4] The comet has not been seen since 1770 and is currently considered a lost comet.
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The comet was discovered on June 14, 1770, in the constellation Sagittarius by Messier, who had just completed an observation of Jupiter and was examining several nebulae.[2] At this time it was very faint, but his observations over the course of the next few days showed that it rapidly grew in size, its coma reaching 27 minutes across by 24 June: by this time it was of magnitude 2. The comet was also noted by several other astronomers.
The comet was observed in Japan. Surviving records identify it as an astronomical and historical phenomenon.[5]
On July 1, the comet passed just 0.015 astronomical units from Earth, approximately 6 times the radius of the Moon's orbit. Messier measured the coma as 2° 23' across, around four times the apparent angular size of the Moon. An English astronomer noted the comet crossing over 42° of sky in 24 hours; he described the nucleus as being as large as Jupiter, "surrounded with a coma of silver light, the brightest part of which was as large as the moon's orb".[2]
Messier was also the last astronomer to see the comet as it moved away from the Sun, on October 3.
A number of orbital calculations were made, some indicating a perihelion date (the date of the closest approach to the Sun) of August 9-10, and some a date of 13-14, depending on whether the orbital solutions were parabolic or elliptical. Anders Johan Lexell made four separate sets of calculations over a period of several years and deduced an orbital period of 5.58 years.[2] Lexell also noted that, despite this short-period orbit, by far the shortest known at the time, the comet was unlikely to have been seen previously because its orbit had been radically altered on a previous occasion by the gravitational forces of Jupiter.[6] It is, therefore, the earliest identified Jupiter family comet (as well as the first known Near-Earth Object).[7]
The comet was never seen again. Lexell, after conducting further work in cooperation with Pierre-Simon Laplace, argued that a subsequent interaction with Jupiter in 1779 had further perturbed its orbit, either placing it too far from the Earth to be seen or perhaps ejecting it from the Solar System altogether. The comet is now considered "lost in space".
Lexell's work on the orbit of the comet is considered to be the beginning of modern understanding of orbit determination.[8] In the 1840s Urbain Le Verrier carried out further work on the comet's orbit and demonstrated that despite potentially approaching Jupiter as close as three and a half radii from the planet's centre the comet could never have become a satellite of Jupiter [9]