They're going back to Mars - but they won't be looking for life

A fter a gap of 20 years, we're on our way back to Mars. On 6 November, the Mars Global Surveyor (MGS) will lift off from Cape Canaveral on a Delta launch vehicle; a month later, Mars Pathfinder will follow. Both probes have been planned for years, and are definitely not a knee- jerk reaction to the possible discovery of a microfossil in a Martian meteorite. In fact, neither probe is able to look for microfossils, although they are equipped to perform an otherwise impressive range of tasks.

The last successful mission to Mars was in 1976, when two US Viking craft landed on the planet's surface and searched for signs of life. The findings were negative, but the two orbiting craft captured images of an amazing world of geological extremes. Although Mars is only half Earth's size, it has canyons that would dwarf the Grand Canyon many times over, and a swarm of giant volcanoes - one three times the height of Mount Everest, and so vast that it would cover the whole of Spain.

Subsequent missions were dogged with failure. The Russian probe Phobos 2 - sent to rendezvous with Phobos, the larger of Mars's two moons - disappeared without trace in 1989, just after arriving at Mars. Four years later, the same fate befell the American Mars Observer. Fittingly, the MGS will carry spare instruments left over from Mars Observer when it begins its 10-month journey to Mars this week.

The MGS is the first mission in the decade-long Mars Surveyor Programme of robotic exploration of Mars. Every two years, when Earth and Mars draw close together, an orbiter-lander pair will be launched to explore every corner of the Red Planet. The MGS itself will arrive in September 1997, after which it will "aerobrake" in the thin Martian atmosphere, decelerating and descending without using large amounts of fuel. The final low orbit, only a few hundred kilometres up, will take it over the Martian poles every couple of hours. From this vantage point, the craft will be able to see every part of the planet as it spins underneath. For one Martian year (equivalent to two Earth years), the MGS will map the surface in unprecedented detail, monitor the weather and analyse the crust's chemical composition using infrared imaging. When the mapping work is done, the MGS will serve as a data relay station for both US and international landing craft.

The landing craft paired with the MGS - Mars Pathfinder - leaves Earth a month later, but will arrive two months earlier. The lander will separate from the main craft and parachute to an area close to the Viking 1 Lander site. Like its predecessor, Pathfinder will take panoramic images of the Martian landscape and deploy a small weather station. It will also carry a six-wheeled robotic rover called the Sojourner. This small, 10kg craft will be free to explore the Martian surface.

This is just the beginning. MGS and Mars Pathfinder are the first in an international flotilla of hi-tech, low-cost spacecraft that will be undertaking a determined assault on the Red Planet. By the end of the 10-year period, we will be in a position to judge whether we are ready to send humans there, too.

What's up

The most obvious planet on view this month is Saturn, just below the left-hand side of the Square of Pegasus. Jupiter, a notable feature of the summer skies this year, sets at about 7.30pm mid-month. In morning skies, look out for Mars itself. It is now rising at about midnight, and getting brighter as it and the Earth draw closer together. The dazzling planet Venus, however, is the main attraction of morning skies, rising three hours before the Sun. On 8 November, it will be three moonwidths to the north of the thin crescent Moon, while on the 16th, the planet passes close to the bright star Spica in Virgo.

Mid-month, look out for shooting stars from the Leonid meteor shower - although these will be best seen in the early hours of the morning. The shower lasts from 15-20 November, peaking on 17 November. The meteors appear to radiate from the "sickle" of Leo, the lion, and are getting more numerous as the comet which produces the stream, Comet Tempel-Tuttle, returns to the vicinity of the Sun. As many as 100 meteors an hour are predicted.

Star-wise, the heavens are still dominated by the barren Autumn constellations of Pegasus and Andromeda. But winter is approaching fast. Heralding the appearance of the bright winter star patterns is the tiny, glittering Pleiades star cluster, now high in the east. Taurus and Orion are not far behind.

Diary for November

(all times GMT)

3 (7.51am) Moon at last quarter

11 (4.17am) New Moon

17 Leonids meteor shower

18 (1.09am) Moon at first quarter

25 (4.10am) Full Moon