SPA News August
Michael Oates is now THE leading comet discoverer with the all time record of over 60 SOHO comets. I've found a few more as well, though two had already been reported. Perhaps the most interesting of the recent discoveries was a non Kreutz group object that I discovered on C2 frames on July 31. A relatively bright object at 7th magnitude it was tracking across the top half of the image. The most surprising aspect was that none of the keen group of amateurs searching for Kreutz objects had spotted it before I did, showing that you see what you expect to see. This object was not moving on a Kreutz track where expected and was not seen despite being fairly obvious.
We had the brightest comet since Hale-Bopp this summer and it more or less lived up to expectations. Visible in the northern sky it became a technical naked eye object at 6th magnitude, though the smallest aperture that I've seen used was a pair of 8x30 binoculars. The surprise came as it approached perihelion.
The comet, LINEAR 1999 S4, was probably a new arrival from the Oort cloud and was making its first trip into the inner solar system. Such comets have been in cold storage for billions of years and this long spell at the fringe of the solar system seems to change them. They are often slower to brighten as they get close to the Sun than comets that have made several passes round our star. The infamous comet Kohoutek is perhaps the most notable example of this class of comets, however although it still has a bad reputation it was actually quite a nice comet to observe, easily visible to the naked eye and with a gas tail several degrees long.
As the comet crept out of conjunction in May observations began to come in, and these generally confirmed that 5th magnitude was the best we could expect. Due to the comet's position, UK observers couldn't pick it up until June and of course almost as soon as it appeared in our skies the weather, particularly in the east of England changed for the worse. Apart from a spell in mid July I was almost continually clouded out, particularly by cloud pouring in off the North Sea. Observers in the west were generally much better off and Denis Buczynski obtained a fine series of images that show the development of the comet.
Professional observers noted that the output of the comet was quite variable and it also showed large 'non-gravitational' effects. Many comets drift from the path predicted by purely Newtonian motion and the rocket like effect of the cometary emissions from the icy nucleus can explain this. The comet's absolute magnitude (the magnitude it would have when 1 AU from both the Earth and Sun) is quite faint, suggesting that it has a small nucleus, which would be particularly affected by the push of the material ejected from its surface. The variation in activity was quite substantial at times, with a notable outburst occurring between July 20.9 and 21.9. This seemed to be a final death throw as the comet then dramatically faded, the coma shrank and became diffuse and the tail collapsed. Observations from the Hubble Space Telescope and the ESO VLT eventually solved the mystery: the comet had fragmented into a number of cometisimals, each highly variable in activity and perhaps 100 metres in diameter. This graphically illustrates the current theory of comets, which postulates that each comet is an aggregate made up from smaller objects cemented together with ice and dust. These objects themselves are comprised of smaller objects and so on right down to meteoric sized particles.
By early August the comet was 9th magnitude and no longer visible from the UK, but it might be followed by more southerly located observers into September when it re-enters solar conjunction. By then another LINEAR comet, 1999 T1 should be brightening to within reach of binoculars, though it won't be until December that it moves far enough north for observation from the UK.