The Hell of the heavens: Beneath its beautiful veil of clouds, Venus is far from lovable; in fact, it is the most hostile of the planets, say Heather Couper and Nigel Henbest

After a year's absence, Venus is now making its comeback as the Evening Star. Since last March it has either been up before dawn, as the Morning Star, or hidden in the Sun's brilliant glow.

Venus is still almost on the opposite side of the Sun from the Earth, and you will need a powerful telescope to make out its shape. But the most powerful telescope on Earth won't show you what happens on the surface of Venus, for the planet is completely shrouded in veils of cloud.

Since the early Sixties, 21 spacecraft have reached Venus to pry out its secrets. Some have photographed its clouds, one mission dropped balloons into the turbulent atmosphere and several Soviet craft landed and photographed the surface.

To many astronomers, our new knowledge of Venus has come as a disappointment. It turns out to be the most hostile of all the planets - the closest place to Hell we know in the solar system. The beautiful clouds are made of concentrated sulphuric acid; the 'air' is composed of unbreathable carbon dioxide gas, at a pressure 90 times Earth's atmospheric pressure; and this 'greenhouse gas' has raised the temperature of its surface to 470C. Anyone who landed on Venus would be simultaneously corroded, suffocated, crushed and baked.

Although hopes of finding life on Venus have long since been dashed, researchers are intrigued by its geology. The planet is the Earth's twin in size, so the history of its rocks can cast new light on our geological past. The key to probing the surface has been radar - radio waves can easily penetrate the clouds and thick atmosphere. One of the first discoveries was that Venus rotates very slowly, and in the opposite direction to most planets.

Detailed views had to await the invention of spacecraft that could take radar sets into orbit around Venus. The breakthrough came with the US probe Magellan, which has been orbiting Venus since 1990. Its radar reveals details just 100 metres across.

Magellan has shown that most of Venus is covered with rolling lava plains, scarred by more than a thousand individual volcanoes, though radar cannot show whether any are now erupting. The highest are 5,000 metres - just over half the height of Mount Everest. Many of the volcanoes lie along a long bulge, Aphrodite Terra, that winds round Venus's equator.

Venus has other types of volcano not found on Earth. Coronas are large dish-shaped objects that look like failed souffles, and are probably volcanoes that have collapsed as lava has been withdrawn from under them. Tessarae look like parquet flooring, and show where the surface has been compressed and broken up by volcanic forces. Near the north pole is a highland region of mountain ridges and volcanoes. This region, Ishtar Terra, is about the size and height of the Tibetan plateau on Earth.

Craters made by infalling meteorites are very rare, partly because all but the largest meteorites are burnt up as they fall through the thick atmosphere. Geologists believe that craters are also uncommon because fresh lava flows and volcanoes have covered most of the old craters. By counting the craters - Magellan discovered 912 - geologists conclude that most of the present surface of Venus was laid down only 500 million years ago, less than one-tenth of the planet's age.

Magellan's detailed radar view shows that Venus is far from being Earth's geological twin. Earth has an obvious worldwide pattern of volcanoes and ridges, marking the edges of huge 'plates' on which the continents gradually move around the globe. The crust of Venus is certainly not split into distinct plates, and there is no movement of the surface around the planet.

With its radar mapping completed, Magellan's controllers have now moved the spacecraft into a tighter orbit. Here, its motion is affected by small disturbances in Venus's gravitational field. As a result, researchers can investigate whether there are concentrations of mass under mountains and other swellings. As the investigation of Venus moves under its surface, scientists hope not only to understand Venus's baffling geology, but also to gain new insight into geology on Earth.

The Planets

The planetary scene is beginning to hot up after a disappointing winter. Venus is an Evening Star for the next few months, and by the end of March you should be able to spot it hanging like a brilliant lamp in the western sky after sunset.

The second brightest planet, Jupiter, is now rising in the south-east around 10pm. In the constellation Libra, the scales, Jupiter is shining 20 times brighter than Virgo's main star, Spica, which lies to the planet's upper right. Mercury and Saturn are rising just before the Sun, and are hidden in the bright morning twilight.

The Stars

With the brilliant constellations of winter - Orion and his entourage - slipping towards the western horizon, try spotting two of the fainter constellations now taking up a large portion of the southern sky: Cancer and Hydra.

As signposts to Cancer, first pick out the twin bright stars Castor and Pollux in Gemini (the twins), high in the south- west. To their left is another star of roughly equal brightness, Regulus, in the constellation of Leo, the lion. In between the twins and Regulus, look for the pattern of faint stars marked on the chart as Cancer (the crab). You can just make out a small group of stars marking the crab's body, and others reaching out to make three legs. In the centre of the 'body' is a faint patch of light. Binoculars show it to be a cluster of hundreds of dim stars. The official name of the cluster is Praesepe ('the manger' in Latin), but its busy appearance has led astronomers to nickname it 'the beehive'.

In Greek mythology, Hera sent the crab to distract Hercules as he was laying into the many-headed Hydra. But Hercules merely crushed the crustacean underfoot. Hydra is immortalised in a constellation. Its only surviving head is a little group of stars under Cancer. The rest of the snake's body winds away as a line of faint stars below Leo and Virgo. Its only moderately bright star is Alphard, 'the solitary one'.

Diary (times BST after 27 March)

4 March 4.54pm Moon at last quarter

12 7.05am New Moon

19 Mercury at greatest western elongation

20 12.14pm Moon at first quarter; 8.28pm Spring equinox

27 2.00am British Summer Time begins; 12.10pm full Moon

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