Launched 10 months ago, the Mars Global Surveyor is now on the final stages of its journey, and is due to arrive in orbit around Mars on 11 September. It will not land, but instead will perform the most comprehensive photographic survey of another planet ever undertaken.
First, the spacecraft, which is about the size of a large garden shed and weighs about one tonne, will dive into the upper reaches of the Martian atmosphere to lose speed and height - a risky procedure known as aerobraking. Then it will position itself in a low orbit, taking high-resolution photographs which will show objects as small as 1.5 metres (5ft) across. These pictures will be sharp enough to help scientists conduct detailed geological studies without needing to set foot on the planet.
After Global Surveyor has done its work, a whole series of probes will be fired at Mars. Some will land, and bigger, better rovers are planned that will roam the surface for kilometres, not metres (as the Sojourner rover does), collecting rock samples and even sending them home to Earth as the search for extraterrestrial life intensifies. The US space agency, Nasa, is confident that Pathfinder has been such a hit with Vice-President Al Gore (who is responsible for overseeing the space programme) that the agency will receive funding for all its planned missions and will eventually get approval for a much more expensive, crewed trip to Mars, in the second decade of the next century.
Like its sister Pathfinder mission, which cost just $178m (pounds 103m), the Global Surveyor is a triumphant example of Nasa's new, "smaller, faster, cheaper" philosophy. The 1976 Viking Mars mission, which featured a pair of lander probes, cost more than $1bn. Pathfinder's success has shown that you don't need to spend a fortune to explore the solar system. Over the next decade, a whole fleet of small, cut-price probes to Mars are planned, one to be launched every 26 months. Another lander will set off from Earth next year, and is intended to touch down on the ice-cap of the Martian south pole, where it will drill through the layers of frozen water and carbon dioxide, looking for evidence of climatic change. The hope is that the robot missions will culminate in one in 2001 that will retrieve rock samples and return them to Earth.
Richard Taylor, of the British Interplanetary Society, has been following the Pathfinder mission from the start and has worked on the analysis of the data beamed back to Earth. "To be honest, there was an element of keeping the instrumentation on Pathfinder to an absolute minimum, just fairly cheap stuff," he comments. "No one will admit it now, but most people thought it had only a 50-50 chance of landing in one piece."
He says that Bob Zubrin, the designer of the Viking mission, thought that the landing method - crashing into the surface and bouncing off airbags - wouldn't work. "But it did. Pathfinder showed that it could be done that way, and that was what's important."
Some have wondered why Pathfinder is not looking for signs of past or present life, as Viking did; after all, interest in Mars exploded last August when Nasa announced its belief that the Martian meteorite ALH 84001 contained structures indicative of fossil bacteria.
But, Mr Taylor points out, "Pathfinder was not designed to look for life, or evidence of past life. The meteorite discovery coincided with the launch, so it was too late for that, but Nasa has milked the life-on-Mars angle for all it's worth, and when Al Gore saw the Pathfinder pictures he was so enthusiastic, he promised to give the go-ahead for more missions."
Pathfinder has, however, sent back far more to Earth than just pretty panoramic pictures of Mars. About 40 megabytes of data have been returned, in the form of pictures, weather data and chemical analyses of rocks and soil. According to Don Savage, a Nasa spokesman, the most important finding from Pathfinder has been that Mars is far more like our own planet than was previously thought.
"I would say the two headline findings from Pathfinder are that we now know that the geology of Mars is very Earth-like, and we also know that the surface undergoes huge and unexpected temperature variations in just a few minutes," he says.
The Earth was thought to be unique in the solar system in having a crust rich in the mineral quartz. Pathfinder, by analysing nearby rocks (nicknamed Yogi and Barnacle Bill), showed that Mars, too, is rich in silica. "This indicated that Mars had plate tectonics once, billions of years ago, just like Earth; this was totally unexpected," says Taylor.
Now Pathfinder is on half power, its batteries depleted to such an extent that the probe has to be completely shut down at night and operated only during the day, when the feeble Martian sunlight can provide its solar panels with just enough energy for experiments to be carried out. Nasa hopes, however, that there is still life in the Pathfinder mission. "The trouble is, the dust storm season is approaching, and dust in the atmosphere could seriously cut down the amount of sunlight available to Pathfinder," Mr Savage says. "Nevertheless, we hope to be able to keep it running for maybe a couple of months yet."
If the Global Surveyor mission is successful, in a few months' time we should have better maps of Mars than we had of Earth before the satellite era in the Sixties. Knowing the geology of Mars, say scientists, is the key to understanding whether or not life has ever existed there, and, indeed, whether it exists there to this day.
Mr Taylor is upbeat. "I think there is a good chance that life will be found there - living organisms, not just fossils - but it will be unicellular life, and it will be buried deep underground, where liquid water could exist. "The problem is that robots probably aren't going to be able to find it. You will need to send people there with a hammer to dig up the fossils."
The first mission to Mars with the capability to discover life - if it is, or was, there - is scheduled to land in 2005. Nasa says that even then, it is possible that the search will be fruitless. "To find life, it's very likely that we will have to wait for a sample to be returned from one of two rovers which will roam the surface. But even then, it's very difficult. Even a trained geologist may not be able to find what we are looking for, let alone a robot probe, however sophisticated," says Mr Savage.
The most bizarre outcome of the Mars programme could be the confirmation of an idea gaining credibility among some scientists that it was on Mars, not Earth, that life first took hold in the solar system. According to this theory, our planet was then "cross-fertilised" from its neighbour via bacteria-infested chunks of rock blasted off its surface in meteor impacts.
"You could say this isn't really an invasion of Mars," says Mr Taylor. "It could be that we are simply coming home".