Nasa probe set for Mars landing as Beagle's handlers carry on listening

After sending a spacecraft on a journey of 300 million miles to Mars, scientists at the US National Aeronautics and Space Administration (Nasa) could wake up today with an awful feeling of déjà vu.

If all goes according to plan, the first of a pair of robot landers should have arrived safely on the surface of the Red Planet in the early hours of this morning.

Nasa will be hoping its lander is more successful than Britain's Martian explorer, Beagle 2, which went missing on Christmas Day.

The best new hope of contacting Beagle 2 will also come today when the orbit of its mother ship, Mars Express, finally comes close to the probe's presumed landing site. Beagle 2 should by now be transmitting a continuous SOS during daylight hours, rather than the intermittent pulses controlled by its computer clock. If its transmitter is working, then Mars Express should be able to hear it.

Scientists believe there will be even better opportunities for contact in about two days' time when the orbiter is due to fly directly over the landing site. Their biggest fear is that it has crashed or landed in a crater or crevice, thus making it unable to open its solar panels and transmit any signals.

Although Beagle 2 and Nasa's Mer mission share much in common in terms of measures designed to ensure a soft landing, such as heat shields, parachutes and a set of inflatable gas bags, the similarities mask a very different approach to the technical and engineering challenges of putting a space probe safely on the Martian surface.

Nasa spent $820m (£512m) on the Mer mission, whereas the European Space Agency's total budget for the Mars Express orbiter was just over £100m, and a relatively paltry £45m for the Beagle 2 lander.

The British designers had to cope with a pitifully small payload of 65kg for the Beagle 2 hardware, while their US counterparts had the luxury of playing with a payload of 365kg for the Mer lander.

In addition to parachutes and gas bags, the Mer had rocket boosters to slow down its descent and cameras to gauge the spacecraft's horizontal velocity before landing.

It also employed a complex programme of radio call signs to its controllers on Earth during landing, while Beagle 2 had to land in radio silence.

Even if it did land successfully, Beagle 2 could only operate beyond the landing site with a Heath Robinsonesque "mole'', a hollow tube containing a spring-loaded weight. Mer has a six-wheel-drive vehicle called Spirit, which is about the size of a golf buggy.

Beagle 2 is nevertheless capable of carrying out an analysis of Martian rocks that equals or even surpasses its American cousin. Professor Colin Pillinger of the Open University said that Beagle 2 was the only probe designed specifically to search for the direct signs of life, rather than indirect evidence, such as the presence of water.

Whereas Spirit will look for the geological signs that water once flowed on Mars, Beagle 2 has the theoretical power to detect directly the chemical signature of microscopic fossils that may have been preserved for billions of years.

But the relative merits of the scientific instruments on board the two Martian landers mean nothing unless the probes can communicate what they find. Beagle 2's day looks as if it may soon be declared over before it ever began; now it is the chance of Spirit to steal the Martian limelight.