This image, obtained on August 1, is a 100-second exposure with the Wide Field Camera of the 2.5 metre Isaac Newton Telescope. This section of the full image measures 4.5 arcminutes, equivalent to 110,000 km at the comet. This is by far the best and deepest image of the broken-up nucleus that exists and gives us a clear idea of how and why the comet disintegrated. No features are seen in the image, which implies that no significant individual fragments more than a few metres across still emit gas. This demonstrates the catastrophic disruption of the nucleus. Unlike comet Shoemaker-Levy 9, the only remnant that remains is an expanding dust cloud. Photo: Dr. Mark Kidger, Instituto de Astrofísica de Canarias

Continuous monitoring of Comet LINEAR by the Jacobus Kapteyn Telescope showed that it was disintegrating as it approached the Sun. The latest and best images from the larger aperture Isaac Newton Telescope now give us the clearest idea so far of how and why the comet disintegrated.

Having previously appeared completely normal, on the night of July 25th the comet was seen to undergo a rapid change. The initially compact comet nucleus evolved into a fuzzy, extended and much fainter object. This caused much speculation as to what the reason for the disruption of the comet might be. Further observations with the telescopes of the Isaac Newton Group at the Roque de los Muchachos Observatory, La Palma, Spain as well as telescopes elsewhere have confirmed the initial discovery and provided new insight into what the reason for the comet disruption could be: the evaporation of all the ice in the nucleus.

Cometary nuclei are a mixture of solid lumps of material of various sizes, held together by a cement of ices. When comets pass close to the Sun during their journey across the solar system the icy elements (mainly water ice and carbon monoxide ice) sublime, leaving loose material behind that forms the dust tail of the comet, while the sublimed ice forms its gas tails. As a result of this process, or due to the strong gravitational pull from a planet such as Jupiter, or from the Sun, a comet nucleus may sometimes split into two or more fragments. What was seen in the case of Comet LINEAR, however, was different.

From analysis of the images that lay out the recent sequence of events for Comet LINEAR, Dr. Mark Kidger from the Spanish Instituto de Astrofisica de Canarias concludes that this small comet probably ran out of ice altogether, leaving behind a loose conglomerate of particles that are now gradually dispersing into space. This model fits the observations well, as measurements have shown that the activity of the comet had been declining for several weeks as ice gradually sublimed away. During the comet's closest approach to the Sun, a burst of activity was recorded. Then, when all the ice was exhausted and nothing was holding together the solids, the nucleus began to fall apart.

Comet LINEAR's tail. Photo: Dr. Mark Kidger, Instituto de Astrofisica de Canarias

The latest images taken with the 2.5 metre Isaac Newton Telescope after break-up show no sign of the comet's original nucleus, nor of any active sub-nuclei larger than a few metres across. Any large remnants of the nucleus that remain cannot be subliming significantly or they would have been detected in these images. This corroborates the sequence of events proposed by Dr. Kidger. Other comets are known to have disappeared, but Comet LINEAR is the first one to have been caught in the act.

Comet LINEAR has maybe not been a spectacular night-time sight for most people, but for astronomers it presents an important and unique event of what can be described as the death of a comet.

The Isaac Newton Group of telescopes (ING) is an establishment of the Particle Physics and Astronomy Research Council (PPARC) of the United Kingdom and the Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO) of the Netherlands. The ING operates the 4.2 metre William Herschel Telescope, the 2.5 metre Isaac Newton Telescope, and the 1.0 metre Jacobus Kapteyn Telescope. The telescopes are located in the Spanish Observatorio del Roque de Los Muchachos on La Palma which is operated by the Instituto de Astrofisica de Canarias (IAC).