The European Space Agency (Esa) is making a last-ditch attempt to locate the missing Beagle 2 probe.
Beagle 2 was designed to hunt for signs of life
Mission scientists are pinning their hopes on Beagle's mothership, Mars Express, which flew over the presumed landing site at 1213 GMT on Wednesday.
Numerous attempts to communicate with the lander through the US satellite Mars Odyssey and radio telescopes on Earth have all ended in failure.
If Mars Express cannot find Beagle, the mission will be classed as lost.
The results of the attempt to contact Beagle should be known sometime after 1500 GMT.
After 9 January, Mars Express will be able to take pictures of the site with two hi-resolution cameras. These may be able to spot signs of Beagle's airbags.
Prof Colin Pillinger, lead scientist on the Beagle 2 project said that the transmitter on Mars Express may have to be shut off if contact has not been made after 22 January.
"We hope that the size of the search part today will be enough to prompt the dog to come back to its kennel," said Prof Pillinger.
He conceded that the team may have to give up "some time in February."
There will be further opportunities to contact Beagle with the radio telescope at Jodrell Bank. A spectrometer on Mars Express could also pick up signs of ammonia in the Martian atmosphere from Beagle's airbags.
"By then we will be into the very last minutes of extra time," said Prof Pillinger.
Mars Global Surveyor has also taken pictures of the landing site and this may help Beagle mission scientists discover what happened to it.
The comparison between Beagle's arrival at Mars and that of the US space agency's (Nasa) probe Spirit could not be more stark.
While Spirit sent back spectacular images of the barren, dust-strewn surface of Mars within hours, nothing has been heard from Beagle since 25 December.
Scientists are now focusing on three main reasons for Beagle's continued silence: a software glitch, a problem with the probe's receiver or transmitter, or the growing possibility that it was destroyed on landing.
But the British experts behind the Beagle project, and their counterparts at Esa have refused to give up hope.
"If there's any chance for Beagle, we can pick it up. If not tomorrow, within the next few weeks," said Alan Moorhouse, spacecraft operations manager at Esoc on Tuesday. "We don't give up yet."
Hopes now rest on the communications link between Mars Express and Beagle 2, the only method of contact that has been fully tested.
Mars Express carried the 60-kilogram lander most of the way to the fourth planet, and the two spacecraft are well used to "talking" to each other.
There is a possibility that Mars Odyssey and radio telescopes have been unable to contact the £45m probe because of an incompatibility problem.
Mars Express - Europe's first solo mission to Mars - has been unable to search for its lost "baby" until now because it has not been in the correct orbit.
On Wednesday, it will swoop over the vast plain, Isidis Planitia, where Beagle should have landed, at a height of 375 km (233 miles) above the surface - the closest it can get.
Mars Express will send out a radio signal to the probe in the hope that Beagle will hear it and cry out.
Continued silence would be a huge blow for the mission. But there are other chances on 8, 9 and 10 January for flyovers of about five to eight minutes each time.
On 9 January, the orbiter will fly down the centre of the landing zone and use its high-resolution stereo camera to hunt for signs of Beagle's parachutes and airbags.
Other passes by Mars Express on 12 and 14 January are potentially much longer, providing even better possibilities to talk to Beagle 2.