The mission is the first to try to determine if there is life on Mars since the United States sent the Viking I landing craft to Mars' surface in 1976.
"It's not looking for little green men, but it is looking for matter that might provide evidence of life. It is looking for clues," said David Southwood, the European Space Agency's director of Science.
The probe's launch is the first in a series of critical navigational maneuvers on which the success of the mission depends.
During the launch, the spacecraft gently pushes the probe away, setting it spinning to maintain stability as it heads toward Mars. Early on Dec. 25, the lander is expected to reach the surface.
At the same time the probe is to reach the surface, mission engineers plan to position the Mars Express craft to fire its main engine for about 30 minutes, sending it into Martian orbit, around 250 miles from the surface. Once there, the Express will use radar to penetrate the surface looking for layers of water or ice.
"This if the first time we will be looking under the surface of Mars using radar from Mars Express," Southwood said.
Should Friday's attempt to launch the lander fail, however, the mission could be doomed, project manager Rudolf Schmidt said.
"If we get the timing wrong, the spacecraft could burn up in the atmosphere or miss Mars altogether,'' Schmidt said. ``We just get one single chance."
Previous attempts haven't answered the question of whether there is life on Mars. Of 34 unmanned American, Soviet and Russian missions to Mars since 1960, two-thirds ended in failure. In 1976, twin U.S. Viking landers searched for life but sent back inconclusive results.
The Mars Explorer, which cost about $345 million, is an attempt to demonstrate that Europe can have an effective _ and relatively inexpensive _ space exploration program.
Launched atop a Russian Soyuz-Fregat rocket from the Baikonur cosmodrome in Kazakhstan on June 2, Mars Express has weathered solar eruptions that bombarded it with high-energy particles, temporarily disrupting its computers, as well as an unexpected drop in electrical power.
The 143-pound Beagle 2 -- named for the ship that carried naturalist Charles Darwin on his voyage of discovery in the 1830s -- will use a robotic arm to gather and sample rocks for evidence of organic matter and water, while Mars Express orbits overhead.
During its working life -- planned for one Martian year, or 687 Earth days -- engineers hope Mars Express will send back detailed overhead pictures of the planet's surface and use a powerful radar to scan for underground water.
Scientists think Mars, which still has frozen water in its ice caps, might have once had liquid water and appropriate conditions for life but lost it billions of years ago. It is thought water may also still exist as underground ice.
Earlier this month, Japan was forced to abandon its troubled mission to Mars, which was to determine whether the planet has a magnetic field, when officials failed in their attempts to position their Nozomi probe on course to orbit the planet.
U.S. officials are discussing a new course of space exploration, and debate has focused on whether the United States should set its sights on returning to the moon or landing on Mars.
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