In a textbook example of useful collaboration between ground- and space-based astronomical telescopes, the ESO Very Large Telescope has imaged the "shower" of "mini-comets" in Comet LINEAR, first seen by the Hubble Space Telescope (HST) a few days ago.

A computer-processed composite of three CCD exposures of the head of Comet LINEAR, obtained with VLT ANTU and the multi-mode FORS-1 instrument through a R(ed) optical filter on August 6, 2000. The telescope followed the comet's fast motion perfectly during the duration of the exposures (9, 10 and 8 min, respectively) and the images of the stars in the field appear as long trails. More than a dozen "mini-comets" are seen -- they are pieces from the comet's disintegrating nucleus. Photo: ESO

The disintegration and subsequent disappearance of the "dirty snowball" nucleus of Comet LINEAR (C/1999 S4), first observed at the Roque de los Muchachos Observatory (La Palma, Canary Islands) by Mark Kidger and his colleagues, was originally thought to be complete.

However, observations by the HST on August 5 showed that this was not really the case -- a swarm of extremely faint mini-comets was detected near the place where the nucleus would have been. Although this is reminiscent of Comet Shoemaker-Levy 9's break-up near Jupiter in 1992, the remaining pieces of Comet LINEAR's nucleus are much smaller and, presumably, much more short-lived. In fact, an event like this has never been observed in this detail before.

In order to take full advantage of this unique opportunity to study the disintegration of a cometary nucleus, American astronomer Hal Weaver (Johns Hopkins University) who was responsible for the HST observations, together with colleagues in several countries, urgently asked for "Target-of-Opportunity" observations of the comet with the ESO Very Large Telescope (VLT) at Paranal (Chile).

The proposal was successful and three short exposures were made by the ESO staff astronomers at Paranal in the early evening of August 6 with the FORS-1 multi-mode instrument, cf. ESO PR Photo 20/00. More than a dozen condensations are seen in Comet LINEAR, as "mini-comets" with tails. At the centre of each of them is a piece of the disintegrating cometary nucleus, perhaps some tens of metres across.

A comparison with the available HST images demonstrate the very dynamical state of this ensemble. The configuration has changed significantly in the course of only 24 hours and some condensations have apparently disappeared completely, while others seem to have appeared in the meantime.

The continued study of the behaviour (motion, brightness changes) of these objects will provide a most unique insight into the structure and composition (dust and ices) of a cometary nucleus. As the pieces of the original nucleus loose dust and gas, they will fade and disappear from view, possibly disintegrate completely. Additional observations with the HST and the VLT are therefore being planned while they are still bright enough to be observed.