It is too early to know the fate of Beagle 2, but space scientists believe there could be a host of reasons why they have not heard from the British Mars lander that was due to touch down on the red planet in the early hours of Christmas morning.
Beagle 2 may be working fine, but landed way off course, or touched down on such uneven ground that its antenna is pointing in the wrong direction, either of which could make it impossible for Nasa's Mars Odyssey, which is in orbit around Mars, to pick up Beagle's call sign.
The communications link between Mars Odyssey and Beagle 2 was never tested, so there is also a chance Beagle's calls are simply being ignored by the orbiting spacecraft.
But it is likely Beagle 2 is silent for another reason. Scientists operating the giant Jodrell Bank radio telescope in Cheshire have scoured the entire planet for signs of Beagle's transmissions and have so far found nothing.
Andrew Coates, a space scientist at University College London whose team built the stereo cameras for Beagle admits the outlook is not good: "That Jodrell Bank did not see any sign of Beagle is worrying."
Another possibility is that Beagle's clock might have been reset during its descent to the Martian surface, and so it is trying to call home when nobody is listening. Just in case, the search for Beagle has been broadened to look outside the times the probe was programmed to call home.
The probe may easily have been destroyed during its descent, the most perilous part of its journey. Less than 10 minutes before landing, Beagle 2 would have slammed into the Martian atmosphere at 13,000mph, raising its heat shield to 1,600C. If the heat shield survived and successfully protected Beagle, the probe would have used computer-controlled split second timing to fire charges, releasing parachutes to slow it to 200mph. Releasing the parachutes a second too early could damage them, but a second too late and the probe could begin to tumble and burn up in the atmosphere.
If Beagle survived this far, it would have inflated a set of huge airbags, encasing it in a soft cocoon before striking the surface at around 36mph. Finally the bags would be jettisoned and the probe, the size of a large dustbin lid, would have dropped to the rocky surface with the jolt of a briefcase being shoved off a desk.
The descent happens out of radio contact, so scientists have no way of knowing whether something has gone wrong.
The smallest of glitches in any one of the landing stages could have spelled disaster. Richard Zurek at Nasa's Jet propulsion Laboratory in Pasadena, California, led two Nasa missions that were both lost on Mars in 1999, the Mars Climate Orbiter and the Mars Polar Lander: "You try to do everything you can to make sure it'll work, but one mistake somewhere along the line can be enough to bring it down."
According to Dr Zurek, the time spent trying to get in touch with a missing space probe is excruciating. "At least if it blows up on the launchpad, you can just go 'ok, it's gone' because it's very obvious, but when you know it's probably there and it could be working, but you can't talk to it, that's really hard," he said.
Even if Beagle 2 is lost for good, the search for life on Mars will go on. Two US rovers called Spirit and Opportunity are due to touch down on the red planet in the next four weeks.
And the European Space Agency is working on an audacious robot mission to land on Mars in 2009, grab some soil and bring it back to Earth. There are US plans to explore Mars with robot aircraft. And far beyond that - perhaps by 2030 - there could be a human mission to Earth's nearest neighbour: both US and European space agencies have begun systematically thinking about the challenge.
This is in part because space research has a way of paying off as big business. The British government invests £192m a year on space science, but the UK space industry last year turned over £2.9bn. Beagle 2 is the brainchild of Professor Colin Pillinger but the £50m project is backed by the space giant EADS-Astrium and the hardware has involved 39 other British businesses, plus four European and eight US companies.
But money is only part of the story. For more than 100 years humanity has been teased by the biggest question of all: is Earth the only planet with life?
In 1976 two Nasa Viking lander missions examined the dusty horizons of the red planet and found nothing to indicate life: no water, no organic compounds linked with life, no green plants, no little green men. The mission seemed to end more than a century of speculation about the fourth rock from the sun - but only for a while.
Scientists later discovered that microbial life on Earth survived even in the most hostile places: on high altitude cloud surfaces; in vents of boiling water at the bottom of the deepest oceans; in lakes of caustic soda and baths of sulphuric acid; a mile below the ocean floor; underneath Arctic ice floes down in volcanic craters. Wherever there was liquid water, life could hang on. So all a planet needed for life was light, rock and liquid water.
Then in 1996, 20 years after the Viking missions, a team backed by Nasa announced that they had detected fossilised bacteria inside a meteorite known to have come from Mars. But the meteorite evidence failed to clinch the argument. In the US, Europe and Japan, researchers began working on a flotilla of experiments designed to explore the history of Mars.
Beagle 2 is the first overtly designed to look for life. It may yet survive to answer the question: Britain's scientists have their fingers firmly crossed.
Hunting for the lander
· Nasa's Mars Odyssey probe is in orbit around Mars and is listening out for Beagle 2's call sign.
If it hears anything, it will send it via Nasa's Jet Propulsion Lab in California, from where the news will be relayed to the Beagle 2 Operations Control Centre in Leicester.
· The giant Jodrell Bank radiotelescope in Cheshire is designed to pick up extremely weak radio signals from space and should be able to detect Beagle 2 on Mars if it is transmitting properly.
· Mars Express, Beagle 2's mothership is set to enter the correct orbit around Mars to listen out for the probe on January 4.
If it picks up Beagle 2's broadcasts it will relay them to the Operations Control Centre in Leicester via the European Space Agency's deep space ground station in Western Australia.