Science: Stars and Planets: April

ON 24 APRIL, Mars reaches its closest point to the Earth this year. You can't miss seeing it in the sky - it shines brightly all night, an orange-red, untwinkling point of light. Mars is red because it literally is rusty. Particles of iron in its now-dry deserts were once washed by water, the elixir of life. How much water - and when it flowed - is not yet known. But we shall almost certainly find out within the next 10 years, and, in parallel, the answer to the Big Question: was there ever life on Mars?

Mars is now under invasion from our robotic planetary probes. The Mars Global Surveyor is currently collecting unprecedentedly detailed images of the planet from Mars's orbit, while Nasa's Mars Climate Orbiter is on course to reach the Red Planet on 23 September. Once in orbit, it will study Mars's water distribution, as well as keeping a weather eye on the Martian atmosphere.

A fellow Nasa probe, the Mars Polar Lander, is scheduled to rendezvous with the planet on 3 December. On its way down, it will drop off two tiny probes designed to penetrate two metres into the ground below Mars's southern icecap. The Polar Lander itself, equipped with a robotic arm for geological investigations, will also be able to dig down into the subsoil. Scientists hope that these "excavations" - the first ever on another world - will reveal 100,000 years of Martian history.

This is just the beginning. The Japanese Nozomi craft will reach Mars in 2004; and Nasa is planning combined Lander-Rover missions to be launched in 2001, 2003 and 2005. The first may deliver a robot aircraft to fly over the surface of Mars exactly 100 years after the Wright brothers took off at Kitty Hawk.

These missions are already provoking some hard thinking. The jewel in the crown, of course, would be to discover signs of life in the Martian rocks. But would we recognise it even if it were staring us in the face?

The tiny nodules in the famous "Martian meteorite" are still contentious, despite years of study. After the initial excitement surrounding these claimed "fossilised bacteria", most scientists ruled them out as being too small. Yet "nanobacteria" of a similar size have recently been discovered on Earth - so we may have to think again. A few scientists go so far as to believe that primitive organisms may still live on the Red Planet, from evidence collected by the Viking probes 20 years ago. Whatever the truth, there will be great care taken to sterilise the rock samples so that - if there is life on Mars - it won't contaminate life on Earth.

Inevitably, though, life on Earth will contaminate Mars. The final goal of all the robotic missions will be a crewed mission to Mars early next century. There will be so much more that humans can do than a probe, however intelligent. Some scientists believe that the mission could take place in 2014. But it's more likely to happen in July 2019 - the 50th anniversary of the first manned landing on the Moon. It will mark a new era in our relationship with the Red Planet and could be an important step on the road to its eventual colonisation.

What's up this month

The planet Venus is looking sensational this month, setting four hours after the Sun at about midnight in mid-April. In the spring twilight, it shines like a lantern. Catch it early enough with binoculars or a small telescope - when the contrast between planet and sky is least - and you'll see it showing a phase like a slightly bulging Moon.

The other object putting on a good show is this month's "star", the planet Mars, recognisable from its red colour. The other planets - Mercury, Jupiter and Saturn - are all too close to the Sun to be visible.

As for the stars, the spring constellations Leo and Virgo are holding centre stage. In our annual trek around the Sun, we have reached a point where our night-time "window" has moved past winter constellations such as Orion and Canis Major. These are now setting in the west before midnight. Follow the curve of the handle of the Plough (part of Ursa Major) downwards to hit the red giant Arcturus - fourth-brightest star in the sky, and a sure sign that spring is here.

Heather Couper and Nigel Henbest