Last updated 09 Oct 2016

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Comet 102P/Shoemaker

Discovered at magnitude 13 by Carolyn and Eugene Shoemaker in 1984, just two weeks after reaching perihelion and with the comet about 1.0 AU from Earth and 2.0 AU from the Sun, it was followed by visual observers through 1984 October and November and held its brightness well around mag 11-12 as it receded from both the Sun and the Earth.

It was recovered at its next return by Alan Gilmore and Pamela Kilmartin at Mount John Observatory (474) in New Zealand, recording it at mag +18 on 1991 June 8 when it was 1.8 AU form Earth and 2.5 AU from the Sun (IAUC 5286). It brightened up as predicted to mag 14 in the following months but the solar elongation was reducing all the time. A few astrometric positions were reported up to 1991 November, but with perihelion due in 1991 December the comet would be in conjunction with the Sun and very unfavourably placed. It was last picked up again a full year after perihelion, from the Steward Observatory, Kitt Peak.

Its next return in 1999 March was not particularly favourable, with southern hemisphere observatories favoured before perihelion in 1998 June/July and northern observatories in 1999 November/December with the comet predicted to be about 17th mag. at both oppositions. No confirmed sightings were made.

At its return in 2006 it was expected to be 14th-15th magnitude around its perihelion date of 2006 June 6th, although not very well placed, but initial searches for the comet from Great Shefford on 2006 July 21.1 revealed nothing at the predicted position. A search was made along the line of variation and on July 24.1 images of a faint, almost stellar object moving with the expected motion were located about 10' northeast of the prediction and re-observed on 2006 July 25.1 (see image above) and July 28.1 UT. At about magnitude +20 the comet was unexplainably 5-6 magnitudes fainter than at the previously observed apparitions.

Dr. Brian Marsden confirmed that the object was indeeed Comet 102P/Shoemaker and issued MPEC 2006-O54 later on 2006 July 28th, including 2 nights of previously unconfirmed positions of the comet from 1999 Dec 31 and 2000 Jan 1 from Szeged University, Piszkesteto Station (Konkoly), but noting that those positions were "somewhat questionable". Dr Marsden mentioned that a satisfactory orbit solution from observations from the four returns of 1984, 1991, 1999 and 2006 was not possible and the one published in the MPEC from the last three returns also included unusually large nongravitational parameters* (though fitting 1984, 1991 and 1999 together produced even larger nongravitational terms). Continued observations will be needed to show whether the bright appearances in 1984 and 1991 will be repeated in the future and also if nongravitational forces will continue to be unusually strong.


* A nongravitational orbital solution takes into account not only the Newtonian laws of motion but also the small but sometimes significant effects that gases and volatiles escaping from the nucleus of a comet can have on its motion. As the sun warms up the comet gases start to escape to form the coma and tail and act like small jet engines, thrusting the nucleus slightly off course. The direction these nongravitational forces act depends on the direction and speed the nucleus is spinning. The amount of outgassing can vary in short timescales and also from one apparition to another and can itself change the spin of the nucleus radically, even during just one trip to the sun, sometimes causing large differences in the nongravitational effects from one return to the next.


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