The Beagle 2 lander was expected to touch down on Mars' surface early on Christmas Day, European time (Christmas Eve in the U.S.). Its companion, the Mars Express craft, was being prepared for a 34-minute burn of its engine to thrust it into orbit shortly afterward.
"From this point, the tension really starts to grow," flight director Michael McKay said in a written statement. "We don't have a lot more to do except watch and wait."
Mission control in the western German city of Darmstadt sent commands to heat the orbiter's fuel tanks before the engine is fired and also switched off its nonessential equipment.
The Beagle 2 lander is supposed to probe the planet's rocks and soil for evidence of organic matter.
Shaped like an oversize pocket watch, 143-pound craft is designed to fly through the Martian atmosphere, deploy parachutes and bounce to a landing on inflatable bags. Then it should open up, unfold its solar panels like the petals of a flower and begin sending a signal to let controllers know it has safely touched down.
The Mars Express orbiter is meant to send back overhead pictures of the planet's surface. It will also scan for underground water with a powerful radar system and relay information from the Beagle 2, which it released toward Mars Friday.
Mars Express was some 124,000 miles from Mars by mid-Wednesday and on course for the orbit maneuver, which was to be performed 250 miles above the surface, mission control said.
While Beagle 2 is scheduled to touch down at 9:45 p.m. EST on Wednesday, it will be several hours before controllers get a chance to confirm its landing.
Mars Express won't be in place to pick up the signal until Jan. 3, so the first chance of contact will be when NASA's Mars Odyssey passes overhead at around 12:15 a.m. EST on Thursday, Christmas Day.
If that fails, scientists at Britain's Jodrell Bank Observatory will have a chance to train its radio telescope on Mars several hours later to try to pick up the signal.
The Odyssey orbiter, which reached Mars in 2001, will have a daily chance to pick up the signal until Mars Express can make its own first contact.
Mission control spokesman Bernhard von Weyhe said controllers were "very confident" of pulling off the much-rehearsed Mars Express orbit sequence, but said the Beagle 2 landing carries greater risks.
Still, he said controllers wouldn't be too concerned if no signal from Beagle 2 is detected Thursday.
"It doesn't have to mean anything," von Weyhe said. "It can mean it needs more time to be unfolded, or it's at a funny angle."
Beagle 2, named for the ship that carried naturalist Charles Darwin on his voyage of discovery in the 1830s, will conduct experiments by scratching the surface with a robotic arm to test for signs of organic matter.
It is expected to transmit its first pictures from Mars between Dec. 29-31. The first radar pictures from Mars Express are expected in the spring.
Scientists believe that Mars, which still has frozen water in its ice caps, might have once had liquid water and suitable conditions for life, but lost them billions of years ago. It is believed that water may also still exist as underground ice.
The European mission is the first search for signs of life on Mars since two U.S. Viking landers probed the planet in 1976 but sent back inconclusive results. Of 34 unmanned American, Soviet and Russian missions to Mars since 1960, two-thirds ended in failure. Japan this month abandoned a mission to determine whether Mars has a magnetic field after its Nozomi probe failed to achieve planetary orbit.
By David McHugh