Over The Horizon

Little Lost Comet

In October, at least three New Zealand amateur astronomers thought that they had discovered a comet, only to find that it was one that has been lost and found repeatedly over the past 60 years. The difference this time is that the comet is 600 times brighter than it should be.

The comet is Schwassmann-Wachmann 3, first first discovered by Schwassmann and Wachmann of Bergedorf in May 1930. It came as close as 10 million kilometres and could be seen in binoculars. Calculations showed that the comet orbited the sun in the remarkably short period of 5.4 years. Thus it would return in 1935. It did, but was on the far side side of the Sun and not seen. Thereafter its position in the sky became more uncertain and it was lost.

It was rediscovered in 1979 and again its next return, in 1985, was invisible from Earth, but it was found at its 1990 return. In both these appearances the comet was faint and only visible in large telescopes.

It was expected to be similarly faint at this return. However after its closest approach to the Sun in late September, it began to brighten when it should have been fading. It has continued to brighten to the point where it is now visible in binoculars. It's not a spectacular sight by any means, but given that it should be hard to see in a big telescope, it's a quite remarkable flare.

What causes such brightenings? The essential part of a comet is its nucleus: a fragile mass of ice and dust ranging in size from a few hundred metres to many kilometres. When the comet is close to the Sun, the exposed ices evaporate, carrying away a cloud of dust and gas thousands of kilometres across. This reflects sunlight and, if enough dust and gas are produced, the comet grows a tail.

In some comets, the evaporating ice leaves behind a crust of dust. This acts as insulation and prevents the ice below from passing into a gas form, or sublimating. Less ice evaporates and the comet becomes faint.

If some of the dust crust is broken off, then fresh icy material is exposed to the sunshine and the comet brightens. This is probably what happened to Comet Schwassmann-Wachmann 3. Warming of the crust during its approach to the sun would cause expansion, producing stresses that resulted in part of the dust shell splitting off.

Alan Gilmore works at the Mt John Observatory in Tekapo
Pam Kilmartin is at Mt John Observatory.