Lexell's Comet

D/1770 L1 (Lexell)

Discovery
Discovered by:	Charles Messier
Discovery date:	June 14, 1770
Alternate designations:	1770 I, P/Lexell, Lexell's Comet
Orbital characteristics A
Perihelion distance:	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.

Discovery

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]

Close approach to Earth

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.

Orbit

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".^{[citation needed]}

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]

External links

• D/1770 L1 (Lexell) Orbit Diagram, NASA JPL

Notes

1. ^ Other comets named after their orbit computer, rather than discoverer, are 27P/Crommelin, 2P/Encke and 1P/Halley - Halley's Comet.
2. ^ ^{a b c d} Kronk, G. Cometography: D/1770 L1 (Lexell), accessed November 20, 2008
3. ^ Kronk, G. The Closest Approaches of Comets to Earth, accessed 20 November 20, 2008. It was thought that C/1491 B1 may have approached even closer on February 20, 1491, but its orbit was retracted in 2002 due to a misunderstanding of the records.
4. ^ Closest Approaches to the Earth by Comets
5. ^ Hall, John. (1955). Tanuma Okitsugu, 1719-1788, p. 120.
6. ^ Leverington, D. Babylon to Voyager and Beyond: A History of Planetary Astronomy, Cambridge University Press, 2003, p.193
7. ^ Valsecchi, G. 'A comet heading towards Earth: the first NEO', in Tumbling Stone, Issue 2, accessed November 21, 2008
8. ^ Valsecchi, G. '236 Years Ago...' in Near Earth Objects, Our Celestial Neighbors: Opportunity and Risk : Proceedings of the 236th Symposium of the International Astronomical Union, Cambridge University Press, 2006, xvii-xviii
9. ^ Valsecchi, G. 'Le Verrier's computations and the concept of Chaos', in Tumbling Stone, Issue 3, accessed February 11, 2011

References

• Hall, John Whitney. (1955). Tanuma Okitsugu, 1719-1788: Forerunner of Modern Japan. Cambridge: Harvard University Press. OCLC 445621