Sharp eyes on a tiny rock: Radar astronomers will track a rare visitor this week. John Davies explains their target

TELESCOPES around the world - and in orbit - are being focused on asteroid 4179 Toutatis, one of the smallest known objects in our solar system. Tomorrow Toutatis, which is about 4km (2.5 miles) in diameter, will be just three million kilometres (1.9 million miles) from Earth. This gives astronomers their best chance between now and the end of the century to observe an asteroid, as it is likely to be the only one to pass so close to Earth.

Most asteroids orbit the Sun in a broad belt between Mars and Jupiter. They were once thought to be the remains of an exploded planet, but astronomers now believe they are planet-building material - 'protoplanets' - that failed to join together during the formation of the solar system four billion years ago.

Most of the tens of thousands of asteroids remain in the main belt, but some, such as Toutatis, have escaped into elongated orbits that bring them in towards the Sun and across the Earth's orbit.

Small asteroids are of interest to astronomers who want to learn more about how the solar system formed, and Toutatis is one of the few that can be observed in detail from the Earth - small objects in the main belt are too distant to detect, even with very large telescopes.

Space engineers are interested in Earth- approaching asteroids because their nearness makes them relatively easy to reach. Toutatis, or something similar, could be the destination of the first astronauts to venture beyond the Moon.

The proponents of large space colonies also have their eyes on the asteroids as a resource that can be mined for large space manufacturing projects.

And since objects that loop across the Earth's orbit can, and sometimes do, crash into our planet, it seems prudent to learn as much about them as possible. Several British telescopes will be used to study this tiny object as it hurtles past.

Toutatis was discovered in 1934 by Eugene Delporte of the Royal Observatory in Belgium, but no attempt was made to keep track of it until it was rediscovered by chance in 1989. The object was linked with Delporte's sighting more than half a century earlier, confirming the details of the asteroid's orbit and making possible the prediction of this month's close approach. Once the orbit was confirmed, the asteroid was given the permanent designation 4179 Toutatis, the name of a god of ancient Gaul.

Because only a few observations of Toutatis were made in 1989, little is known about it. Measurements of its brightness suggest that it is between 2km and 6km across (1.2 to 3.7 miles), but this depends on how much sunlight its surface reflects. If Toutatis is stony and reflects sunlight well then it may be fairly small, but if its surface is dark it will be larger than expected, and may be one of the largest Earth-approaching asteroids yet discovered.

Observations made earlier this year from Chile show that the asteroid's brightness appears to wax and wane over a period of about a day. This is probably because it is irregularly shaped and its brightness depends on how big an area is facing the Earth. These observations also suggest that Toutatis rotates less than once a day, much more slowly than most small asteroids.

Determining the shape and rotation rate of Toutatis is one of the highest priority projects of the observation campaign, because these details are vital to interpreting many of the other observations that will be made.

Among those attempting this are astronomers in France, a US group using telescopes in California and Arizona, and a lecturer from the University of Kent, Simon Green, who will spend Christmas week observing from La Palma, in the Canaries.

Despite its nearness, Toutatis will appear only as a point of light in ground-based telescopes. But Nasa's Hubble Space Telescope will try to take pictures to reveal the shape of Toutatis directly. One image, taken during the period of closest approach, is expected to show a tiny crescent covering only a dozen or so of the 2.5 million elements in the Hubble's field of view.

In the unlikely event that Toutatis has a satellite of its own, the Hubble picture should also capture that. A second picture will be taken a few days later when Toutatis is fully illuminated like a tiny full moon.

Astronomers from the Royal Observatory, Edinburgh, will also attempt to determine the size of Toutatis. They will be using the UK Infrared Telescope in Hawaii to measure the amount of thermal, or heat radiation, emitted by the asteroid. Toutatis's temperature depends on how much of the Sun's radiation it reflects and how much it absorbs. Infrared signals from the asteroid will tell astronomers how reflective it is, which will enable them to estimate its size.

By observing when some of the asteroid's night side is visible, the infrared observations can also be used to discover if the surface is bare rock, which heats up and cools down quickly, or is covered by a thin layer of insulating dust, which would help to keep the night side fairly warm.

Other groups will try to determine the chemical composition of Toutatis. A group from Belfast and London will use Britain's William Herschel Telescope on La Palma, one of the world's largest optical telescopes, to examine the asteroid with a spectrometer. By splitting up the incoming light and measuring which wavelengths are absorbed when sunlight is reflected from the surface, it is possible to search for the signatures of specific minerals and so identify the rock types.

The International Ultraviolet Explorer satellite will make similar observations in wavelengths that cannot be studied from the ground, and may show the presence of iron-bearing minerals. Observations by both telescopes may reveal patches of different materials that swing into and out of view as the asteroid rotates.

Professional astronomers will follow Toutatis well into January to obtain as much information as possible, but for amateur astronomers with moderately large telescopes the best chance of seeing it is this week. Anyone who misses out should note that Toutatis will make an even closer approach in 2004, when it may be visible with binoculars.

The writer is an astronomer at the Royal Observatory, Edinburgh. He will use computer links with the UK Infrared Telescope in Hawaii to observe Toutatis without leaving his office.

(Photograph omitted)