The race to find life on Mars is set to begin on Monday with the launch of Europe's first voyage to another planet.
By Helen Briggs
BBC News Online science reporter
Three probes are leaving Earth this summer, starting with the European Space Agency's Mars Express mission.
Ready to go: Mars Express and its Beagle lander sit atop the launcher
It carries the Beagle 2 lander, which, if all goes well, will become the first British-built craft to touch down on another world.
The launch marks the start of a new golden age in Mars exploration.
The US space agency (Nasa) is sending two missions to the fourth planet. The first of its Mars Exploration Rovers should leave Earth in a week or so.
Another Mars traveller is destined to arrive early next year. Japan's Nozomi craft should reach the planet early in 2004 after a long journey beset by mishaps.
There has long been interest in exploring Mars because it is believed to be the planet most likely to harbour life.
Clues that Mars once had oceans, lakes and possibly microbes have sparked a "gold rush" to send unmanned space craft to visit the planet.
The United States and Russia have spent billions since the 1960s trying to land a dozen or so space craft on the Red Planet.
So-called because of speed and low cost of development
Will map Martian surface and analyse atmosphere
Radar instrument can detect water several km below surface
Only three have been successful so far: Nasa's two Viking probes, which landed in 1976, and its Mars Pathfinder, which explored the surface in 1997.
Mars Express is Europe's first solo mission to Mars and indeed any planet.
Final launch preparations are underway at the Russian Cosmodrome in Baikonur, Kazakhstan.
The first opportunity for the craft to be blasted into space comes on Monday at 1745 GMT (1845 BST).
The orbiter with the lander on board will go up on the Russian rocket that has become the workhorse of the space industry, a Soyuz/Fregat launcher.
The space craft will cover a distance of about 400 million kilometres (250 million miles) on the six-month journey to Mars.
Its main scientific goal is to detect vast reservoirs of water thought to be trapped under the Martian surface using a ground-penetrating radar. It will also take images of Mars and conduct a geological survey of the planet.
Between them, Mars Express and Beagle 2 could answer one of the biggest questions in science: Is there, or was there, life on Mars?
The chances depend much on what happened early in the planet's history when it was probably warm and wet like the Earth rather than the frozen, arid wilderness it is today.
BEAGLE 2 LANDER
Will search for signs of past and present life
Instrument "paw" can retrieve rocks and soil for examination
Carries the first microscope to travel to another planet
"The question is: where is that water? Has it disappeared or is it still there below the surface?" says Mars Express project scientist Agustin Chicarro. "We hope to find that out."
As Mars Express nears the planet, it will drop Beagle 2 into a basin that could once have contained water and life.
The small robotic probe, about the size of a garden barbecue, will dig into Martian rocks and soil to search for the chemical signature of life.
Hunting for life on Mars is a bold move for Beagle 2, a tiny space craft with a small budget.
Life detection experiments on Mars have only been conducted once before - during the Viking missions of 1976.
Mars missions have a high failure rate
While the Viking landers did not find any conclusive signs of life, technology has advanced since then.
"The experiments being used on Beagle are about as good as you can get," says Dr Charles Cockell, a Mars biologist at the British Antarctic Survey.
"It is difficult to know what Beagle might find but if it did find evidence of life, that would be incredible.
"The fun thing is, what questions will it send back and what will it answer?"