Scientists trying to find Europe's Beagle 2 Mars probe ruled out weather problems and a faulty onboard clock for its five-day silence, but considered a new possibility Monday — a crater that may be blocking its signal.

A new, detailed picture of the area of Mars where the Beagle 2 is believed to have landed revealed a crater a little more than a half-mile wide. It is possible — although unlikely — that the Beagle may be unable to communicate because it landed inside, chief Beagle scientist Colin Pillinger said at a news conference.

"This would be an incredibly unlucky situation," Pillinger said.

Several attempts to contact the Beagle 2 so far have failed five days after it was to descend to Mars. NASA's Mars Odyssey (search) orbiter has passed four times over the spot where scientists hope Beagle landed, without picking up a signal. Pillinger did not reveal the results of another overflight that happened at 2:40 a.m. EST on Monday, while he was speaking.

A British radio telescope at Jodrell Bank Observatory struck out again early Monday after sweeping the planet's surface for the craft's call sign, composed by the British band Blur (search).

The 143-pound probe, which has a robotic arm to take soil and rock samples, was supposed to unfold its solar panels and transmit a signal confirming its arrival within hours of landing on Christmas Day. An "analysis and recovery think tank" at Britain's space center considered several reasons for its silence.

They discounted two possible theories. One was that the weather may have played a role, and the other was that the lander's internal clock wasn't working, mission manager Mark Sims said.

The vessel is programmed to transmit its signal when its orbiter or telescopes on earth are in position to receive it. If the clock had been damaged, the Beagle could have been "talking" and staying quiet at the wrong times.

Sims said, however, a problem with the clock's software was still possible. The team planned to send the clock a reset command Wednesday.

The European Space Agency (search) will get a better idea about the Beagle 2 when the its mother ship, Mars Express, enters a lower orbit around Mars and tries to contact it on Jan. 4. Mars Express (search), which carried Beagle into space, is orbiting the planet as high as 117,00 miles above its equator.

"They are keeping up their spirits," Peter Barratt, spokesman for the British government's physics and astronomy research agency, said Sunday. "The big crunch will be when Mars Express comes into the frame. That was always the prime chance for communication. But if we get negative responses after a few tries, we will start to become concerned."

On Tuesday, European Space Agency scientists at mission control in Darmstadt, Germany, will fire Mars Express' engine to shift its orbit. They hope to move it over one of the Martian poles — where it will survey the entire planet with its high-resolution camera and a radar that can look for underground water.

Scientists hope to gradually reduce the polar orbit and say that by Jan. 4, it should pass as low as 125-155 miles above the surface, enabling it to take close-up pictures and listen for Beagle.

If the Beagle fails to transmit its call sign, Wells said a radio telescope at Stanford University in California could help determine if it survived the descent to Mars. It would scan the planet's surface for low levels of radiation emitted by the probe.

Efforts to get a lander on Mars have proven difficult. Of 34 unmanned American, Soviet and Russian missions to Mars since 1960, two-thirds have ended in failure. Beagle 2 would be only the fourth successful Mars landing if all goes well.