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Tue, 09 May 2006 - Comet 73/P Schwassmann-Wachmann 3

The third of Arnold Schwassmann and Arno Wachmann's comets has recently put on an astonishing display in our skies with over 63 fragments being identified and the comet visible to the naked eye at closest approach. As with the other two of the pair's comets, it was discovered on photographic plates taken at the Hamburg Observatory during routine minor planet surveys between 1927 and 1930. It was found in 1930 at a very close (0.062 AU) approach to Earth, which is 9th on the list of well determined cometary approaches to our planet. It was numbered 73 as it was the 73rd periodic comet to be recovered after having its orbit determined. This year's return is a little more distant with the comet passing at around 0.07 AU.

At the 1995 return the comet wasn't expected to become brighter than 12th magnitude, but much to most astronomers surprise the comet was found to be exceptionally bright with a tail in September and October 1995, reaching 5th magnitude and undergoing two outbursts. Computations by Zdenek Sekanina at JPL indicated that component C was the primary and intriguingly his scenario suggests that the first outburst preceded the splitting by at least six weeks whilst the second outburst followed the primary splitting. In 2025 the fragments will make a close approach to Jupiter, which will reduce their perihelion distance a little.

With the orbit approaching so closely to the Earth, an associated meteor shower might be expected, and the comet has been linked to the Tau Herculid shower, though the radiant now lies in the Bootes - Serpens region. Strong activity was reported in 1930 by a lone Japanese observer, but little has been seen since then. It is likely that any future activity would be in the form of a short lived outburst, confined to years when the comet is at perihelion. It is now very clear that there is a huge debris trail in the path of the comet, but it will be at least 50 years before we get another close approach to the Earth.

I have been literally deluged with observations and I must apologise for not having the time to respond to all observers. Instead I have been struggling to keep my web page up-to-date, so at least you will know how the comet has been behaving. Two of the fragments have been readily visible, the main component 'C' and fragment 'B'. The other fragments have generally been very short lived, most not exceeding 15th magnitude, and with erratic light curves. The main body has been well behaved, brightening in a fairly predictable fashion, becoming more condensed as perihelion approached and growing a short tail. A few observers with dark skies have seen it with the naked eye, but others using larger telescopes have found it as faint as 8th or 9th magnitude. This perfectly illustrates the old adage, known since before the founding of the SPA, that to make a reliable estimate of the magnitude of a comet you should use the smallest aperture that will show the comet clearly.

Fragment 'B' has also been bright enough to see in binoculars, but has been incredibly variable in appearance. On one night it has seemed well condensed and the next quite diffuse. It has clearly been in the state of more or less continuous fragmentation with fresh surfaces being exposed as boulders rupture from the conglomerate of dust and ice that is a comet nucleus. During the last few days it has undergone a substantial outburst. I observed it on the evening of May 9th from near the centre of Cambridge and it was 6.1 in 20x80B, with a 4' well-condensed coma, and very easy despite city light pollution, mist turning to low cloud and bright moonlight. Observers with better skies have reported much larger coma diameters up to 50' and a magnitude of 5. This evening (May10) it will be 6 degrees from Vega on a line running through epsilon.

Fragment 'C' is some 15 degrees away, crossing the neck of Cygnus and is about 6th magnitude. Both are rapidly moving out of the evening sky and will be lost to UK skies in a fortnight.


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