“Lexell's Comet”的意思、由来-开放百科全书

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 Earth than any other comet in recorded history, approaching to a distance of only {{convert|0.015|AU|lk=in}}.^[2]^[3]^[4] The comet has not been seen since 1770 and is considered a lost comet.

Lexell's Comet's 1770 passing still holds the record of closest observed approach of Earth by a comet.^[4] However, if approaches deduced from orbit calculations are included, it has been beaten by a small sungrazing comet, P/1999 J6 (SOHO), which passed even closer at about {{convert|0.012|AU|km mi|abbr=on|lk=off}} from Earth on June 12, 1999,^[5] albeit unobserved.^[6] Its estimated diameter was 48 meters in 1999 and 23 meters before its 2004 return.^[7]

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 arcminutes across by June 24: 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.^[8]

It was observed in the Hejaz in Safar 1184 AH (June 1770), where some believed it to be the comet predicted by the poet al-Fasi, portending future events.^[9]^[10]

Close approach to Earth

On July 1, 1770, the comet passed 0.015 astronomical units from Earth,^[4] or approximately 6 times the radius of the Moon's orbit. Charles Messier measured the coma as 2° 23' across, around four times the apparent angular size of the Moon. An English astronomer at the time 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 the last astronomer to observe the comet as it moved away from the Sun, on October 3, 1770.^[2]

Orbit

Scientists at the time largely believed that comets originated outside the solar system, and therefore initial attempts to model the comet's orbit assumed a parabolic trajectory, which indicated a perihelion date (the date of the closest approach to the Sun) of August 9–10.^[11] When it turned out that the parabolic solution was not a good fit to the comet's orbit, Anders Johan Lexell suggested that the comet followed an elliptical orbit. His calculations, made over a period of several years, gave a perihelion of August 13–14 and 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 in 1767 by the gravitational forces of Jupiter.^[12] It is, therefore, the earliest identified Jupiter family comet (as well as the first known near-Earth object).^[13]

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 Earth to be seen or perhaps ejecting it from the Solar System altogether.^[22] The comet likely no longer approaches any closer to the Sun than Jupiter's orbit.^[13]

Although Comet Lexell was never seen again, it remained interesting to astronomers. The Paris Academy of Sciences offered a prize for an investigation into the orbit of the comet. Johann Karl Burckhardt won in 1801, and confirmed the calculations of Lexell. He calculated that the 1779 close approach to Jupiter drastically altered its orbit and left it with a perihelion of 3.33 AU.^[14] 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.^[15] He showed that after the second encounter with Jupiter many different trajectories were possible, given the uncertainties of the observations, and the comet could even have been ejected from the Solar System. This foreshadowed the modern scientific idea of chaos.^[15]

Lexell's work on the orbit of the comet is considered to be the beginning of modern understanding of orbit determination.^[16]

2018 recalculation

In a 2018 paper, Quan-Zhi Ye et al. used recorded observations of the comet to recalculate the orbit, finding Le Verrier's 1844 calculations to be highly accurate. They simulated the orbit forwards to the year 2000, finding that 98% of possible orbits remained orbiting the Sun, 85% with a perihelion nearer than the asteroid belt, and 40% crossing Earth's orbit. The numbers remain consistent even when including non-gravitational parameters caused by pressures from a comet's jets.^[17]

Based on its apparent brightness in 1770, they estimate the comet to be between 4 and 50 kilometers in diameter, most likely less than 30. Additionally, based on a lack of meteor showers, they suggest that the comet may have ceased major activity before 1800 AD.^[17]

Identification

The aforementioned 2018 paper also attempted to identify if any discovered object may be a remnant of Lexell's comet. With an assumed size of >4 kilometers, it is highly unlikely that this comet would remain in the inner solar system and be undiscovered. Most new asteroids discovered even in the asteroid belt (as of 2018) are only 1–4 kilometers across. If Lexell's comet remains in the inner Solar System, it would most likely be an unidentified asteroid. The paper identified 4 potential asteroids which could be related: {{mpl|2010 JL|33}} (99.2% chance), {{mpl|1999 XK|136}} (74% chance), {{mpl|2011 LJ|1}} (0.2% chance), and {{mpl|2001 YV|3}} (~0% chance).^[17]

They find that {{mp|2010 JL|33}} is very likely to be a remnant of Lexell's comet, although due to a number of close approaches with Jupiter as well as uncertain non-gravitational parameters, a definite link cannot be made.^[17]

See also

Notes

1. ^Other comets named after their orbit computer, rather than discoverer, are 27P/Crommelin, 2P/Encke and 1P/Halley – Halley's Comet.
2. ^¹²³⁴⁵Kronk, 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. See [https://academic.oup.com/pasj/article/54/6/1091/2948908/Approximate-Orbits-of-Ancient-and-Medieval-Comets Approximate Orbits of Ancient and Medieval Comets: 3. Remarks and Discussion]
4. ^¹²{{cite web |title=Closest Approaches to the Earth by Comets |publisher=Minor Planet Center |url=http://www.minorplanetcenter.org/iau/lists/ClosestComets.html |accessdate=2018-01-10}}
5. ^{{cite web |type=2010-04-22 last obs (arc=10.9 yr; JFC) |title=JPL Close-Approach Data: P/1999 J6 (SOHO) |url=http://ssd.jpl.nasa.gov/sbdb.cgi?sstr=1999J6;cad=1#cad |accessdate=2012-06-28}}
6. ^{{cite journal |last1=Sekanina |first1=Zdenek |last2=Chodas |first2=Paul W. |title=Origin of the Marsden and Kracht Groups of Sunskirting Comets. I. Association with Comet 96P/Machholz and Its Interplanetary Complex |journal=The Astrophysical Journal Supplement Series |volume=151 |issue=2 |pages=551–586 |date=December 2005 |doi=10.1086/497374 |url=https://trs-new.jpl.nasa.gov/bitstream/handle/2014/39217/05-0507.pdf?sequence=1&isAllowed=y |accessdate=2018-01-11|bibcode=2005ApJS..161..551S }}
7. ^Studies of SOHO Comets dissertation by Matthew M. Knight, page 167)
8. ^Hall, John. (1955). Tanuma Okitsugu, 1719–1788, p. 120.
9. ^{{cite book|author-last=Daḥlan|author-first=Aḥmad Zaynī|title=Khulāṣat al-kalām fī bayān umarā’ al-Balad al-Ḥarām|script-title=خلاصة الكلام في بيان أمراء البلد الحرام|publisher=Dār Arḍ al-Ḥaramayn|year=2007|orig-year=1887/1888|language=Arabic|pages=274–276}}
10. ^{{cite journal|last1=Scheltema|first1=J. F.|title=Arabs and Turks|journal=Journal of the American Oriental Society|volume=37|page=156|url=https://books.google.com/books?id=8BQZAAAAYAAJ&pg=PA156|publisher=Yale University Press|location=New Haven, Connecticut|year=1917}}
11. ^{{cite-book|title=A History of Physical Theories of Comets, From Aristotle to Whipple|author=Tofigh Heidarzadeh|page=196-197|year=2008|publisher=Springer Science & Business Media|isbn=1402083238}}
12. ^Leverington, D. Babylon to Voyager and Beyond: A History of Planetary Astronomy, Cambridge University Press, 2003, p.193
13. ^¹Valsecchi, G. 'A comet heading towards Earth: the first NEO', in Tumbling Stone, Issue 2, accessed November 21, 2008
14. ^{{cite journal |last=Barnard |first=E. E. |title=Probable Return of Lexell's Comet |volume=2 |issue=6 |pages=21–24 |date=January 25, 1890 |url=https://archive.org/details/jstor-40666732 |accessdate=2018-01-11}}
15. ^¹²Valsecchi, G. 'Le Verrier's computations and the concept of Chaos', in Tumbling Stone, Issue 3, accessed February 11, 2011
16. ^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
17. ^¹²³⁴{{cite arxiv|last1=Ye|first1=Quan-Zhi|last2=Wiegert|first2=Paul A.|last3=Hui|first3=Man-To|title=Finding Long Lost Lexell's Comet: The Fate of the First Discovered Near-Earth Object|eprint=1802.08904 |date=24 February 2018|class=astro-ph.EP}}