The discovery of the splitting of C/2005 A1 (LINEAR)


It seems that people are expressing a great deal of interest in this discovery and so the discoverers have asked me to recount the circumstances.


The splitting was discovered by Sensi Pastor and José Antonio Reyes, a husband and wife team, in Murcia at MPC station J76, just before dawn on 2005 July 2nd. Because of the similarity to images of C/2005 K2 they immediately suspected that the nucleus had split. They called me at about 6am local time (05UT) however I did not hear my mobile and missed the call. Having sent me an SMS message to alert me to what they had seen they waited for a response and finally decided to send the astrometry to the two components to the MPC with a brief note to Brian Marsden. Mid-morning on the 2nd we made contact and started to run through the standard checks (common motion in the sequence of images, separation, etc.). I contacted Ramón Naves at MPC 213 to see if his previous images of the comet from June 18th showed the fragments, finding that they were inconclusive and that while something might be there, it relied heavily on the eye of faith; he had intended to observe the comet that morning, but dawn and a desire to observe 29P had defeated him. Examination of the light curve from the very few observations of the comet in our archive showed a significant brightening which seemed to support a possible splitting even in the absence of confirmatory images.


When we were all satisfied that there was no mistake and there was good evidence of a real splitting I prepared a short note for Brian Marsden outlining the checks that I had made (this was basically the text on IAUC 8559) which was submitted at around 12:00UT that morning. This text was copied to Guy Hurst at The Astronomer and he released it as a circular in the hope that one of the 400+ subscribers worldwide would either have images or could observe and confirm the split. No feed-back was received though probably because this is an object that needs to be observed in the dawn sky.


Appeals to other Spanish observers did not turn up any additional observations. The fragment was re-observed by Sensi Pastor and José Antonio Reyes at dawn on July 3rd. However, in the absence of any other data, the splitting was not independently confirmed. Brian Marsden acknowledged the report on the night of July 2nd and requested a small clarification, which was submitted. As no independent confirmation was available (Brian, from past experience, normally expects to receive various near-simultaneous reports of a splitting like this, as had happened in the case of C/2005 K2 and, in the absence of multiple reports is sensibly cautious) and with so many other things happening, we completely understood that he was unwilling to put out the news on an IAUC. Meanwhile, Sensi and José Antonio checked their previous observations and found that the fragment was visible on June 18th, on the previous occasion that they had observed this comet; these observations were duly reported.


Finally, in the absence of any independent report, the MPC put out the observations plus the pre-discovery astrometry on MPEC N-21 on July 4th in the hope that someone would deliberately observe this comet, before releasing IAUC 8559 on the 5th.


What is unusual is that an object that is magnitude 10 and a very easy object to observe, can have a significant outburst and split without anyone noticing for several weeks.