Science: A world that came in from the cold: Heather Couper and Nigel Henbest explain why Nasa is booking for Pluto now

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OF ALL the planets, Pluto is the odd one out: it is the only world in our solar system that has not been visited by a space probe. All the others have been scrutinised, surveyed, prodded and sniffed - sometimes several times over - but not far-off Pluto. However, if Nasa, the American space agency, gets its way, two probes could be dispatched in 1999, reaching the planet in 2006. The mission could not come at a better time.

A lucky combination of circumstances is making Pluto an irresistible place to visit at the moment. But it is now or never: if Nasa does not get the funding this time, there will be little point in mounting another mission for another 250 years.

Pluto was discovered in 1930 after a long search for a planet beyond Neptune. At first, it was thought to be massive enough to exert a measurable gravitational pull on Uranus and Neptune. It soon became obvious that Pluto was far too small to have any effect, and a search for a weightier culprit lying beyond was underway. Some astronomers are still hot on the heels of the trans-Plutonian world 'Planet X'. However, evidence for it is rapidly disappearing in the wake of the Voyager journeys past Uranus and Neptune. These at last gave accurate masses for these giant worlds, allowing space scientists to make detailed orbit calculations - and these do not require the gravitational 'ballast' of a tenth planet.

So is Pluto the final frontier of the solar system? Not just at the moment. It has the most extreme orbit of all the planets, so oval that its farthest distance from the Sun is double that of its closest. It can even cross inside the orbit of Neptune, which has been the case for the past 14 years. So, until 1999, Neptune is actually the farthest planet in the solar system.

Pluto is circled by a moon, Charon, which is half the size of the tiny planet - 1,190km (740 miles) across, compared with 2,300km (1,400 miles). The two bodies together weigh just one-

fourhundredth the mass of the Earth. Charon was discovered just in time for astronomers to take advantage of a series of eclipses between Pluto and its moon that only occur every 120 years. By looking at the variations in light that took place as the two bodies repeatedly crossed each other, astronomers were able to make crude maps of their surfaces.

For a pair of worlds separated by only 20,000 kilometres (12,000 miles), Pluto and Charon are surprisingly different. Charon is less dense, with a dark surface, and is probably made almost entirely of ice. Pluto is up to three-quarters rock, and its surface shows interesting variations which may change with the planet's seasons. In particular, its south pole is now particularly bright. Astronomers suggest that we are seeing seasonal deposits of methane frost on the currently-shadowed pole.

Pluto's present proximity to the Sun (relatively speaking) is producing even more dramatic effects. Last year, astronomers in Hawaii discovered that Pluto has an atmosphere, made of nitrogen, methane and carbon monoxide. This is almost certainly a temporary one, and will refreeze when the planet moves out to the depths of space. Hence the urgency to get a space probe to Pluto now, while the planet is active. Nasa's chosen (but, as yet, unfunded) mission is the Pluto Fast Flyby - a matching pair of tiny probes, each with a communications antenna no bigger than a satellite dish, that would map both sides of the planet as they hurtle past at 16km a second (36,000 mph).

As well as getting the chance to see Pluto at its most active, astronomers are also keen to set this mystery world in context. Is it a bona fide planet, or just the largest of a swarm of minuscule 'ice dwarfs' (like the 'new planet' nicknamed Smiley) that live on the edges of the solar system? And what can Pluto, in its virtually unaltered state, tell us about the birth of the planets? Or about where icy comets come from? These days, far from being a cosmic afterthought, Pluto has at last come in from the cold.

The Moon and the planets

IF YOU happen to be visiting the Pacific Rim on 4 June, don't be too surprised if the Moon disappears - there is a total eclipse. The Moon moves into the Earth's shadow, and it may vanish entirely. Sometimes, however, enough sunlight is refracted round by the Earth's atmosphere and you can see the eclipsed moon as a dim dull-red. The eclipse begins at 11.50am and ends at 2.50pm; totality lasts from 12.13pm until 1.49pm (all times GMT).

Otherwise, evening skies in northern Europe are dominated by the same planets that have been there for the past few months - Mars and Jupiter. Jupiter is still the most brilliant object on view in the evenings, and is close to the bright star Spica in Virgo. Mars, fading fast as it and the Earth pull away from each other, passes very close to Leo's brightest star, Regulus, towards the end of the month. Look at the colour contrast between pure-white Regulus and the 'red planet'. Saturn is starting to put in an appearance again, but does not rise until around midnight.

Early risers will have no problems in spotting Venus putting on a spectacular show in the east before dawn. By the end of the month, the 'morning star' is coming up almost three hours ahead of the Sun. And Mercury, its fellow inner planet, is waiting a couple of hours after the Sun in the evenings in mid-month - but it will be hard to spot in the bright twilight.

The stars

CENTRE STAGE this month in the south are the constellations of Bootes and Corona - the first of the 'summertime' star patterns. Bootes, the herdsman, is an ancient constellation, seen as the man responsible for driving the bear (Ursa Major) across the sky. The name of its principal star, Arcturus, comes from the Greek 'bear guard'. The brightest star in the northern sky, Arcturus is a distended red giant, some 25 times wider than the Sun, and is now close to the end of its life.

Neighbouring Corona is equally ancient, and represents the jewelled crown given by Bacchus to Ariadne as a wedding present. Corona really does look like a tiny crown in the sky, with its circlet of white and blue-

white stars. The crown contains two bizarre stellar gems, neither of which are visible without a small telescope.

R Coronae normally lies just below naked eye visibility, but can drop in brightness to one- thousandth of its normal luminosity over a few weeks. Astronomers blame 'soot' grains in the star's outer atmosphere. Quite the opposite in its behaviour is T Coronae, the 'blaze star', which usually hovers 100 times or so below naked eye visibility. Yet every so often it flares up to become as bright as the Pole Star. The last outburst was in 1946, and the previous one was 80 years before.

Diary (all times BST)

4 2.03pm Full Moon; lunar eclipse (details above)

10 Venus at greatest western elongation

12 6.36am Moon at last quarter

17 Mercury at greatest eastern elongation

20 2.53am New Moon

21 10am Summer solstice

26 11.44pm Moon at first quarter.

(Maps omitted)