Comet's X-ray vision lights up sky at night

National Astronomical Meeting: Scientists surprised by brightness of `fluorescence' emitted by Hyakutake and changes on distant planet
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Science Editor

Comet Hyakutake - the most spectacular comet visible to the naked eye for 20 years - has pulled an major surprise on astronomers. It is not just bright with visible light, it is "fluorescent" with high-energy X- rays as well.

This is the first case of X-rays being emitted by a comet. They were picked up by the German orbiting X-ray satellite Rosat at the end of March.

According to Dr Alan Fitz simmons, an astrophysicist from Queen's University Belfast, who lectured on the comet in Liverpool yesterday: "Everybody's just sitting back and saying `Wow. X-rays from a comet!'."

Dr Fitzsimmons said that astronomers could not make sense of the observations. "We don't know where they come from," he said.

Dr Konrad Dennerl of the Max Planck Institute for Extra-terrestrial Physics at Garching in Germany, where the effect was observed, said "it was a thrilling moment".

The team observed the emissions over a period of 24 hours as the comet neared its closest approach to earth. It was less than 10 million miles away when Rosat started observing.

The X-rays seem to come from a crescent-shaped region on its sunward side. One theory is that X-rays from the sun were absorbed by the comet's "atmosphere" of water vapour surrounding the icy nucleus and then re-emitted by a process known as fluorescence.

A second theory is that the X-rays are produced from the violent collision between the comet and the supersonic "wind" of plasma and particles streaming away from the sun.

According to Dr Fitzsimmons, comets such as Hyakutake which venture past the Earth probably originate from Pluto. More than 30 large bodies, known as "Kuiper Belt Objects", have been found, he said. "Their surfaces are dark as coal or even darker. We think their surfaces are covered with a layer of dark carbon-based molecules . . . The colour varies. Some are almost grey and some a deep red."