Giotto's close encounter of the second kind: Heather Couper and Nigel Henbest on comets, planets and shooting stars

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The Independent Online
THIS summer is a bonanza for comet-lovers. Comet Shoemaker- Levy can be seen with binoculars but is a particularly fine sight through a large telescope, and some debris from another comet can be expected to produce a spectacular display of shooting stars in a couple of weeks' time.

The big astronomical news of the summer was last month's encounter between the European probe Giotto and an obscure comet called Grigg-Skjellerup.

Giotto hit the headlines in 1986 when it flew through Halley's Comet and sent back the first pictures of the comet's 'dirty snowball' nucleus. The dust particles from Halley's nucleus sand- blasted Giotto, knocking it sideways so that controllers lost contact for a few minutes.

The sand-blasting blinded Giotto by damaging the baffle attached to its television camera. But seven of the 11 instruments survived, and the European Space Agency decided to send Giotto on to Grigg- Skjellerup.

While Halley is a young and active comet, Grigg-Skjellerup is trapped in an orbit so small that its frequent visits to the sun have melted much of its ice.

In 1986, Halley was a striking sight in the southern hemisphere. Even at its best this year, Grigg- Skjellerup was visible only through a moderately powerful telescope.

On 10 July, Giotto reached Grigg-Skjellerup. The first hint of contact with the comet came from a British instrument that measures ionised atoms. Six hours before the expected time of closest approach, the Johnstone Plasma Analyser picked up oxygen ions that had escaped from the comet.

As Giotto got closer, instruments measuring electrons and magnetic fields found that the probe was moving through giant 'waves' in the solar wind of particles streaming from the sun. The waves stretched 600 miles from crest to crest. Grigg-Skjellerup was raising huge ripples as it ploughed through the solar wind.

Then there was a major disturbance as Giotto crossed the comet's bow-shock. This region of highly compressed gas and magnetism corresponds roughly to the bow-wave in front of a ship. Here Giotto found a pile-up of magnetic fields and ions moving at almost the speed of light.

Although the blinded spacecraft could not see the comet's nucleus, the readings from the instruments suggested that Giotto passed only 120 miles from the comet's heart. This was less than half the closest distance it came to Halley - an impressive feat of celestial navigation, considering that astronomers had much less precise information about the position of Grigg-Skjellerup.

Just as it passed its closest point, Giotto was pelted with small specks of dust - but the bombardment was only one-thousandth as intense as the encounter with Halley. Giotto's dust detection system, built by astronomers at the University of Kent, picked up the impact of only three dust grains.

Giotto survived its second encounter without further damage. This means that the probe can be brought back past Earth in 1999, so that our planet's gravity can swing it round to a collision course with its next target.

But many scientists believe that a third encounter - perhaps in 2030 - would be more trouble than it is worth. By then the original researchers will have retired and the computer programmed for analysing Giotto data will be antiquated.

The European Space Agency has until September to decide whether to put Giotto into hibernation or to give it an honourable burial in space.

The planets

Saturn is August's planet of the month. You can see it low in the southern part of the sky all night long. On 7 August, Saturn is exactly opposite the Sun, as seen from Earth - a situation that astronomers call 'opposition'. On that date, Saturn is due south at 1am (midnight GMT).

At opposition, Saturn is also at its nearest point to Earth - a 'mere' 800m miles away. Although Saturn is the second largest planet, it is so far away that it is not a brilliant sight in our skies and does not rival the brightest stars. Even so, Saturn outshines the rather dull stars of the constellation Capricornus, where it currently lies.

On the night of 12 to 13 August, Saturn will be the bright 'star' near the full moon. Like all the planets, it shines with a steady light instead of twinkling like the stars. A small telescope will reveal Saturn's rings - so large that they would stretch two-thirds of the way from the Earth to the Moon. The rings are made of billions of particles of ice, ranging in size from snowballs to blocks the size of a lorry.

The only other planet you can spot easily this month is Mars, now rising at about midnight in the east. It lies in the constellation of Taurus, the bull. Mars now has the same brightness as Saturn, but is distinctly orange-red.

This month, Mars lies near the red giant star Aldebaran, which is similar to Mars in both colour and brightness. But they can be distinguished from each other because Mars does not twinkle, and its position changes slowly from night to night.

Comets and shooting stars

If you have a good pair of binoculars and the patience to track it down, Comet Shoemaker-Levy can now be seen. It was discovered by the American comet hunters Carolyn and Eugene Shoemaker and David Levy late last year, and is now at its maximum brightness. Look low in the north-west and sweep around with binoculars. The comet looks like a fuzzy blob - a telescope is needed to see its tail.

Debris from another comet will rain down as a shower of shooting stars during the first couple of weeks of August. This display, known as the Perseids, occurs every year, but this year's shower may be the best for several decades.

Last year, astronomers in Japan saw many more Perseid meteors than they expected - about six every minute. This suggests that Earth passed through a dense stream of dust particles from a comet called Swift-Tuttle, which was last seen in 1862. Brian Marsden, who calculates comet orbits for the International Astronomical Union, thinks that the comet may reappear towards the end of this year.

If Earth crosses the same stream of dust particles this year, the best display of shooting stars will be seen from Europe on 11 August. The meteors will appear in the the north-east part of the sky and will seem to head outwards from the constellation Perseus. Unfortunately, the moon is almost full that night and its light will allow only the brightest meteors to be seen.

The stars

Use Saturn as a guide to find Capricornus. The constellation of the sea-goat contains no bright stars and looks less like a goat than a rather bent triangle standing on its point. Saturn is now near the left-hand corner of this triangle. Look carefully at Algedi, the star marking the right-hand corner, and you may just be able to see that that it consists of two stars close together.

Many of the stars we see actually consist of two stars in orbit around each other, but Algedi is one of the rare cases in which the pair is unrelated and we just happen to see two stars lined up. The nearer star is 120 light-years from Earth, while the other is more than 10 times as far away.

Diary (all times BST)

2 Mercury at inferior conjunction

5 11.59am moon at first quarter

7 Saturn at opposition

11-12 maximum of Perseids meteor shower

13 11.28am full moon

21 11.02am moon at last quarter; Mercury at greatest western elongation

28 03.42am new moon