The discovery of comet 1999 S1 (SOHO)


The University of Cambridge Cavendish Laboratory hosts a 'Physics at Work' Exhibition in mid September, which is designed to get prospective GCSE students enthusiastic about following a career in physics. I usually run an exhibit about work at the British Antarctic Survey (BAS), which shows the students the physics behind measuring ozone in the atmosphere. This year we were asked to put on two exhibits and for a variety of reasons I ended up teaching the students about 'geospace'. I had been looking at the real-time movies of the LASCO C3 camera for several months on an occasional basis and was impressed by how well they show the dynamic activity of the Sun. I thought it would be educational to show the students live images of the Sun, so I downloaded the images every morning and showed the current movie loop. There were no comets during the exhibition, but the planet Mercury was visible heading out from superior conjunction. I also showed an archival image, which did show a comet, to illustrate that comet tails always point away from the sun, thus demonstrating the existence of the solar wind.


Having packed everything up on Friday morning (September 17th), I cycled back to BAS and decided to have a look at the latest sequence from the wide field C3 camera. A quick scrutiny showed a star-like object heading towards the Sun and brightening, but without a tail. I guessed that it was a probable Kreutz group fragment, though expected someone else to have picked it up already. At 09:41 UT, I e-mailed Doug Biesecker of the SOHO-LASCO consortium to inform him of the object, with a copy to Dan Green at the CBAT. Doug responded at 12:32, and confirmed that it was a probable Kreutz group fragment and that I was the first to report it. He measured the images and quickly passed the details on to the CBAT. Brian Marsden was able to compute a preliminary orbit whilst the object was still visible in the coronagraphs. The positions and orbit appeared on MPEC 1999-S04, issued at 15:26 and a note recording the discovery appeared on IAUC 7256 at 17:08. The orbit shows that it is another member of group I of the Kreutz family of sungrazing comets. The fragment grew a short tail, visible on the C2 frames, but then faded as it dived towards the sun. The last image to show it was taken at 16:54, but that was the last image available on the web until the following week.


I was quite excited when I first spotted the object, which was sufficiently obvious that I was sure that someone else must have reported it. I wouldn't have been disappointed to find I wasn't the first, as the 'off my own bat' find was reward enough. I was even more elated when I heard that no-one else had reported it and have remained astonished ever since. I really had discovered a comet.