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 indeed 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 non-gravitational parameters*
(though fitting 1984, 1991 and 1999 together produced even larger
non-gravitational 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 non-gravitational forces will
continue to be unusually strong.