All being well, the next mission to Mars will blast off this weekend. Among its prime objectives is to search for flowing water, the absolute requirement of life on Earth, so far as we know.
Success would rekindle hopes of finding life on Mars, even if only microbial. As this is 2001, there was only one name in the running for this new Nasa spaceprobe: Odyssey.
The orbiting satellite carries a range of instruments. One package will monitor the radiation environment, vital information for planning future manned missions. On Earth we are largely shielded from the dangerous flux of high-speed elementary particles made up of the solar wind and cosmic rays. Our magnetic field deflects many of these, producing the Van Allen belts thousands of miles above our heads. But Mars has a far thinner atmosphere, and a weaker magnetic field. Is it a safe planet even to visit, then, let alone colonise?
A gamma-ray spectrometer and a camera equipped with optical and infrared sensors will map its chemical composition. The camera will render fine resolution of fluvial patterns, indicating whether or not they are fresh. It will also allow the distribution of sub-surface ice, similar to permafrost, to be mapped. Such ice has been found using Earth-based radars, and we know that the Martian polar caps are partly made of frozen water, the rest being solid carbon dioxide, or "dry ice".
The possibility of liquid water on Mars was raised last summer when images from the Global Surveyor probe showed evidence of water seeping from the banks of valleys and canyons, especially in the lowest-lying regions of the planet. At such depressed altitudes the atmospheric pressure is greatest, and temperature highest, perhaps resulting in ice melting for a while when Mars is closest to the Sun.
Without huge fanfare, at the end of January Global Surveyor officially completed its mission, which began in September 1997, but data collection continues. In contrast, Nasa's Pathfinder lander which also arrived in 1997 carrying the little rover Sojourner was followed by millions of people over the internet. Not much bigger than a shoebox, checking progress of the tiny six-wheeled buggy set records for hit rates for the site, demonstrating the public popularity of Mars.
The last pair of Mars missions sent by Nasa were anything but successful. The Climate Orbiter probe was lost in September 1999 when it plunged into the Martian atmosphere, owing to a misunderstanding between project teams regarding the rocket burn required to insert it into orbit. One group had used imperial units, the other metric.
Three months later the Polar Lander probe was also a failure, contact being lost as it approached touchdown close to the south pole of Mars.
Critics argue that the history of Martian exploration has been littered with such failures. During the 1960s and 70s a dozen Soviet satellites were either lost completely, or returned very limited data. In 1988 radio contact with two further missions was lost during their cruise towards Mars. Another Russian satellite, including experiments contributed by various European nations, suffered a launch malfunction in 1996.
Japan launched its Nozomi probe to Mars in 1998, but a propulsion problem has delayed its arrival until late in 2003. The Americans may have suffered similar setbacks, but their successes have radically altered our view of the Red Planet.
The Mariner satellites 4, 6 and 7 all sent back pictures of Mars in 1960s flybys. In 1971 Mariner 9 was inserted into orbit around the planet, producing a bounty of information. That was followed in 1976 by the two Viking probes, each consisting of an orbiter module and a lander.
There followed a 17 year gap before Nasa again targeted Mars, with the Mars Orbiter probe. Contact was lost as it neared its destination in August 1993. The new mission due for launch soon - full name 2001 Mars Odyssey Orbiter - is carrying much the same suite of instruments as did Mars Orbiter, with a few improvements due to technological advances.
One lesson leant from the twin disasters in 1999 is that we can try to do too much, too quickly, with too little. There are launch windows to Mars every 26 months, set by the orbital clockwork, and Nasa had intended to send a pair of spacecraft at every opportunity for the next dozen years. The agency now has a long-term phased programme in which the missions alternate between landers and orbiters.
In June 2003 a pair of rover vehicles is to be sent, larger and better equipped that the lonely Sojourner, and also possessing far greater mobility. In 2005 it will be another mapper, the Mars Reconnaissance Orbiter, carrying a camera capable of detecting features as small as a basketball on the surface far below.
Then in 2007 the first of a new class of lander is planned, called Scout. Because rovers, whether using wheels or legs, can only cover limited terrain around their landing sites, eventually Nasa plans missions employing aerobots (balloon-borne instrument packages circumnavigating the planet with the ambient wind), or some form of aeroplane. Because of the tenuous Martian atmosphere these would need vast propellers, and thick wings to generate enough lift.
Nasa does not have a monopoly however. In 2003 the European Space Agency will launch its Mars Express mission, with the British Beagle 2 lander on board. To some extent this project depends on the imminent successful launch of Odyssey, because a functioning data relay satellite at Mars is needed to collect signals from the lander and then transmit the data back to Earth.
Although the nominal launch date for Odyssey is this Saturday, April 7, there is a window of some weeks. Storms over Cape Canaveral this weekend would not be disastrous, just a hiccup. With the cruise to Mars taking about 200 days, arrival is in late October. A retarding thruster burn during approach will slow Odyssey into a 25-hour orbit. Then aerobraking - flying the satellite through the Martian upper atmosphere to dump momentum - will drop it into a low orbit, taking two hours to loop the planet.
That should be completed by January. Data collection can then begin in earnest, and perhaps we will discover whether there is indeed liquid water on Mars. Remember, experience on Earth indicates that where there's water, there's life.
• Duncan Steel is a space researcher at the University of Salford. His latest book, Target Earth, was published recently by Time Life.
