Mars Express fired its main engine for more than three minutes to switch from orbiting over the planet's equator to a route that will take it over the poles.

"Today was a very critical day," flight director Michael McKay said.

The craft's engine will be fired again Sunday to slow it down and put it into a lower orbit, positioning it to contact its missing companion, the Beagle, beginning on Jan. 7. The European mission is searching for evidence of life on Mars.

Mars Express went into orbit around Mars early on Christmas Day -- about the same time that the British-built Beagle was supposed to land north of the Martian equator. NASA's Mars Odyssey orbiter and British and U.S. radio telescopes have failed so far to pick up its transmissions.

Still, controllers remain hopeful that Mars Express will find the Beagle's signal when the craft passes over the landing site at an altitude of 196 miles on Jan. 7.

"The probability of communications (with Mars Express) is 100 times higher than having Mars Odyssey try," mission control spokesman Bernhard von Weyhe said.

European Space Agency officials note that the companion craft's radios have been tested together and shown to link up -- unlike those of Beagle and Mars Odyssey.

"I still have a little bit of hesitation to say we have no signal," McKay said at the control center in Darmstadt, south of Frankfurt. "I'm sure the Beagle is down there. I'm sure it's trying to communicate with something."

Controllers have said a problem with the software of Beagle's clock, confusing the timing of its planned transmissions, could be behind its silence. They also say the lander could have tumbled down a crater on the rocky Martian surface.

Mars Express is to use its powerful radar to search over the next two years for signs that there was once enough water on the planet to sustain life.

The orbiter is equipped with radar designed to search as far as 3 miles beneath the planet's surface and a high-resolution camera to take images of the surface. Early in January, controllers plan to start switching on the craft's scientific instruments.

The 143-pound Beagle is designed to sample soil and rocks with a mechanical arm. Even if the lander isn't located, the European Space Agency considers the mission a success because of the performance of the orbiter. "Here we have Europe, on its first mission to another planet, successfully in orbit around Mars," McKay said.

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