Satellite's mission to map the Red Planet: A space probe this week begins its journey to Mars, reports Peter Bond

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ONE OF humankind's rare visits to another world is about to begin. If all goes well, the American Mars Observer spacecraft will set off this Friday on an 11-month voyage to the Red Planet. Accidental contamination during efforts to protect the craft from Hurricane Andrew has already caused a nine-day delay, and if the launch is not achieved by 13 October, Nasa will have to wait two years until Mars returns close enough to Earth.

To the fertile imaginations of early 20th-century scientists and fiction writers alike, Mars beckoned like a beacon in the apparently barren emptiness of space. Viewed through a telescope, its polar ice-caps, Earth-like seasons, network of canals and carpet of vegetation inspired visions of alien civilisations and a refuge for human colonists.

Today, our picture has lost its romantic tinge and harsh reality has taken over. A series of American and Soviet spacecraft has revealed a dichotomy: a planet that is part-Moon, part-Earth, and which is now in the depths of a prolonged ice age. The apparent seasonal changes in 'vegetation' have turned out to be an optical illusion, probably caused by shifting sands in the arid Martian desert; the canals were just figments of the fertile human imagination.

Any visitor to Mars would need to be well prepared to cope with its hostile environment. Even in midsummer near the equator, the temperature would rarely rise above zero. The atmosphere is very thin - a barometer would register only six millibars, 160 times less than on Earth - and consists mainly of carbon dioxide. Yet, every Martian year, heat from the Sun stirs up the tenuous gases, generating dust storms that can engulf the entire planet. Mars even has an ozone hole, so that its surface is bathed in deadly ultraviolet radiation.

Although some water exists, it is insufficient to fill one of the imaginary canals, still less a network of them. The main constituent of the polar caps is now thought to be carbon dioxide ice.

To the best of our knowledge, no life exists on Mars, although some scientists believe that simple life forms may have had time to evolve during the first two billion years of the planet's evolution. They base such optimism on pictures of bizarre surface features sent back by orbiting spacecraft.

A canyon system about as wide as the United States is aligned, like a huge scar, parallel to the Martian equator. To the west lies a chain of shield volcanoes, which provide further evidence that Mars was once very active geologically. The largest of these, Olympus Mons, is the highest mountain in the solar system, almost three times the height of Everest.

At some time, perhaps when the active volcanoes were belching out greenhouse gases, the Martian atmosphere was dense enough to support liquid water. Today all we can see are intricate systems of channels that resemble river valleys on our planet. But the possibility remains that the temperate conditions prevailed long enough to enable some form of life to evolve.

What happened to the water? Strange splash marks around some craters suggest that much of it may be frozen into the rocks close to the surface; and the polar caps may contain water ice.

Mars Observer carries seven instruments that may help to provide some answers to the mysteries of this unique world. Its primary mission, which will begin about four months after arrival, will last one Martian year (687 Earth days).

Operating from a near-polar orbit about 390km (240 miles) above the surface, the satellite's camera system will map the entire planet. Daily low-resolution scans will give new insights into the global climate, while high-resolution images of selected areas will be used to study geology and small-scale changes, such as melting ice-caps and shifting dunes. The narrow- angle camera can show details as small as 56in (1.4m) across, the highest resolution obtained by a planetary orbiter.

The history of this ambitious mission has been plagued by problems and cost overruns. Mars Observer was due to be launched from the Space Shuttle in August 1990, but the Challenger disaster caused a two-year delay. The mission was conceived as the first of a cut-price series, using tried and trusted hardware from communications and weather satellites, but of that programme only Mars Observer remains.

Ironically, one reason for the premature termination of the series was escalating cost. In 1987 the cost of the Observer mission was estimated at dollars 212m; now it is four times that figure, with dollars 500m going on development of the spacecraft.

Originally, Nasa was looking forward to a decade of Mars exploration using ever more sophisticated technology. Mars Observer was to have paved the way with a comprehensive global survey, followed by a series of robotic landers and rovers. Before the turn of the century, an automatic lander would have returned to Earth with a sample of Martian rock.

In July 1989 President George Bush introduced his Space Exploration Initiative, under which the first humans were to land on Mars on 20 July 2019 - the 50th anniversary of Neil Armstrong's walk on the Moon. Times have changed. Far from exuding enthusiasm for another multi-billion-dollar star trek, Congress has undermined the project: after several years of minimal budgets, it looks as if the trend will continue next year.

Nasa's response has been to turn from such projects as Mars Observer to a 'small and cheap is beautiful' approach. One idea is to send a 100kg (220lb) Pathfinder probe to land on Mars in 1997. This may be followed, later in the decade, by 16 scattered surface stations that would send back data on a variety of subjects, including soil and weather.

Europe and Russia have shown considerable interest in this latest Mars survey. Nasa hopes that Mars Observer will still be operational in 1995 when a Russian- built craft called Mars 94 will drop two penetrators and two small stations on to the planet. A French relay system on board Mars Observer will then send their data back to Earth.

The same relay system may even be used in 1997 when Russia hopes to deliver a helium-filled balloon and a small roving vehicle to explore the Martian wastelands. Warmed by the rising Sun, the balloon would float over the rugged terrain by day, sending back close-up pictures and weather data; during the frigid night it would descend close to

the surface, recording data through a segmented tail packed with instruments.

(Photograph omitted)