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Wednesday, 5 April, 2000, 18:04 GMT 19:04 UK
Longest comet tail detected
David Hardy
Impression of Ulysses passing through the comet's tail
By BBC News Online science editor Dr David Whitehouse

UK scientists have identified the longest comet tail ever recorded.

The tail belonged to Comet Hyakutake and measured over 570 million kilometres (354 million miles) in length - four times the distance between the Earth and the Sun.

If the ghostly tail of gas could have been seen with the naked eye from Earth, it would have stretched almost halfway across the sky.

The gas came from ice in the nucleus of the comet. The nucleus, made of rock and frozen gas, was about eight kilometres (five miles) across. When it neared the Sun, it was warmed and shed gas and dust which then billowed away from our star.

The scientists, led by Dr Geraint Jones of Imperial College, London, made the discovery when analysing data from the magnetic field detector on the Ulysses spacecraft.

The probe, which is in a wide orbit around the Sun where it monitors the so-called solar wind, inadvertently passed through the comet's tail.

Odd readings

"As Ulysses crossed the comet's tail, the instruments recorded some very odd readings, and we couldn't work out why," said Dr Jones.

"Although the change in the magnetic field was typical of what we would expect in a comet tail, there was no known comet in the area, so we initially discounted the idea.

"When we looked again at the instrument readings, we were convinced it was a comet.

"So, we decided to look further into space and realised that Ulysses had crossed the tail of Comet Hyakutake, which was then far away in another part of the Solar System - making it the longest comet tail in history."

Professor Ian Halliday, of the UK's Particle Physics and Astronomy Research Council, said: "Only three comets have ever been encountered by spacecraft before and none so far from the Sun.

"This is a significant discovery by British scientists using technology that they developed."

The discovery has been announced in the journal Nature.

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09 Feb 99 | Sci/Tech
Comet-chaser mission postponed
17 Sep 99 | Sheffield 99
Small but deadly comets identified
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