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.