Stargazers hope cloudbreak will give sight of Hyatukake approaches

Britain's clouded skies have so far denied thousands of amateur and professional astronomers any glimpse of the brightest comet to grace our skies for 20 years.

And the disappointment is likely to continue, according to weathermen. The BBC Radio forecaster Philip Eden reported that the only breaks in cloud covering Britain today are expected to be in south-east England and north-west Scotland. By tomorrow, only stargazers in the Highlands and Islands of Scotland will stand a chance of spotting the comet.

However by Monday - the day of comet Hyakutake's closest approach to Earth - there should be some hope for people in the north-west of England. Only by the middle of the week might there be a break in the cloud over eastern England.

If the clouds do lift next week, the comet should be among the brightest objects in the sky, easily bright enough to be seen with the naked eye. It will be visible virtually all night from Britain. Looking east, it will appear among the stars close to the constellation of the Plough, gradually changing its position in the sky in successive evenings before disappearing from view in the vicinity of the Pleiades at the end of April. Contrary to popular belief, comets do not flash across the sky, so Hyakutake will actually appear virtually stationary to the naked eye. The comet is basically a ball of dust and ice, just a few kilometres across, which was formed at around the same time as the planets. As it nears the sun, ice and dust stream off the nucleus, forming an atmosphere and a tail.

There is also a straighter, narrower "plasma" tail - consisting of ionised gases. This writhes as it is blown by the "solar wind" - a stream of charged atomic particles flowing outwards from the sun. Kinks often appear in this tail and sometimes it appears to drop off altogether.

Ionised water is an important constituent of the plasma tail and is formed by water molecules escaping the nucleus and then being bombarded by the sun's ultraviolet radiation.

Among the many observations which will be made of the comet around the world, at the Mullard Space Science Laboratory of University College London, the astronomer Geraint Jones will be looking just at the light from this ionised water molecule, by using a colour filter to block out all other wavelengths. The study will cast light on the chemistry of the comet and trace how the molecules are accelerated though space by the solar wind.

The comet was discovered in late January by a Japanese amateur astronomer, Yuji Hyakutake, using large binoculars. Its orbit is thought to take around 18,000 years to complete a circuit around the sun.