European Southern Observatory
A near-parabolic orbit with perihelion passage in April 1997
25 August 1995
- During the next few days, observers all over the world obtained additional positions
which allowed Brian Marsden to calculate a more accurate orbit. Thus, it also
became possible to trace the comet's motion backwards in time with some confidence.
As a result, Robert McNaught at Siding Spring Observatory (Australia) soon
found a possible image of Comet Hale-Bopp on a photographic plate obtained in
late April 1993 with the 1.2-metre Schmidt telescope at that site, i.e. more than two
years before the discovery. The estimated magnitude of this object was about 18. It
has not yet been possible to establish with absolute certainty that this image is indeed
of Comet Hale- Bopp, which was at that time nearly 2,000 million kilometres from
the Sun, but if the identification is correct, this would again indicate a most unusual
brightness at this enormous distance. [1]
- [1] Another relatively bright pre-discovery image of Comet Hale-Bopp (magnitude
11.7) has since been identified on a photograph taken on May 23, 1995, by Terry
Dickinson, a Canadian publicizer of astronomy observing in Arizona. Moreover,
Robert McNaught has found another Schmidt plate from Siding Spring, obtained on
September 1, 1991, which shows the sky region where the comet was located at that
time, but no image can be seen. The ESO collection has also been checked and no
such plates were found, although two ESO Schmidt plates obtained in May 1993
and August 1994 very narrowly 'miss' the comet.
Harvard's IAU
Comment on the 1993 Observation of C/1995 O1 (Hale-Bopp), by Brian G. Marsden
21 January, 1996
- The problem with fitting the comet's orbit seems to be that the excess weight of the
1995 data (more than 700 observations) can throw a large residual into the single
1993 position, presumably because of systematic errors in the GSC reference-star
system. By substantially reducing (e.g., by a factor of ten) the relative weight of
the 1995 data, it is possible to fit the 1993 position completely satisfactorily.
Harvard's IAU
PRESS INFORMATION SHEET: Comet C/1995 O1 (Hale-Bopp), by CfA
16 February, 1996
A single, apparent image of the comet was found by astronomer Robert H. McNaught of the
Anglo- Australian Observatory from a wide-field photographic plate taken in late April
1993 (when the comet was about 13 AU from both the sun and earth). This observation has
strengthened the orbital calculations by greatly extending the arc of observation. It allows
one to say that the comet has an orbital period of a few thousand years and extends out to
some ten times the distance of Neptune at its furthest point.