There is still no sign of life from the British-built Mars probe, Beagle 2.
If Beagle got down why has it not called? (Image rights reserved by Beagle 2)
All attempts to contact the lander with the Mars Odyssey craft in orbit around the Red Planet and with large radio telescopes on Earth have drawn a blank.
Scientists have now set up a "tiger team" of top experts to work through all possible reasons for the silence.
The small group, based at the National Space Centre in Leicester, is drawing up a list of "blind" commands to send to Beagle that might prompt it to respond.
Both the radio telescopes at Jodrell Bank in the UK and at Stanford University in the US listened out for Beagle late on Sunday - but with no positive result.
Mars Odyssey has passed four times over
the spot where scientists hope Beagle landed, without
picking up a signal. Another overflight was due to take place at
around 0740 GMT on Monday.
But the Beagle team think their best hope of raising the robot will come next weekend when Mars Express - the "mothership" which carried the "pocket watch" lander to the Red Planet - gets into position to contact its "baby".
At the moment Mars Express is heading away from the planet, preparing for a major engine burn on Tuesday that will sweep it back into a polar orbit of Earth's near neighbour.
In the meantime, the Beagle tiger team is working overtime to try to find a way to break the lander's silence.
The team is closeted away from other mission scientists at their Leicester base. They have a full-sized engineering model of Beagle and reams of technical documents and drawings to hand.
"We've pulled out a number of specialists into a think-tank," said senior Beagle team scientist Professor Alan Wells. "These guys are looking at what information we're getting so far from the contact attempts, analysing that data and looking at recovery options we can pursue."
There are a great many technical issues for the team to work through (Image rights reserved by Beagle 2)
He added: "What we're doing is looking at any possibility where Beagle 2 is still functioning and we're not able to communicate with it for some reason."
These scenarios include a failure of the pocket watch design to open fully; a failure to deploy properly the solar "petals" that charge the Beagle battery system; obstruction of the antenna; computer glitches that affect the Beagle clock and other electronic systems.
The tiger team has already had one blind command sent to Beagle via Odyssey, telling the lander to reset its timer.
WHY MIGHT BEAGLE NOT CALL?
A computer glitch has affected transmission timings
The probe has a misaligned or obstructed antenna
There was some catastrophic failure during landing
Beagle made it down but is in a hole or tilting badly
Scientists had suspected that a clock error might have meant Beagle 2 was simply transmitting at the wrong time and this could explain the inability of the US orbiter and the telescopes to pick up its signals.
"We're going to send a blind command to drive the lid mechanism again - to see if that produces a response," said Professor Wells. "These commands are blind in the sense we have no idea if they are getting through - but we're going to try."
Beagle lead scientist Professor Colin Pillinger added: "We have a model of the spacecraft so the sensible thing is to try out a command on it first - not just send the commands anyway because you want to understand the possible repercussions."
One obvious explanation for silence is that Beagle was destroyed as it attempted to land on Christmas Day, but no one connected with European Space Agency (Esa) mission is prepared yet to contemplate this awful possibility.
Professor David Southwood, head of Esa's science directorate, said: "I certainly haven't written Beagle off - none of the team has. We believe Beagle is on the surface and the mother, Mars Express, is very anxious to get in touch with her baby again.
Mars Express should be in a good position to try to contact Beagle on 4 January, just as the first US Mars Exploration Rover, called Spirit, arrives on the surface.
Mars Express carried Beagle to the Red Planet
It is at this stage that Mars Odyssey will have to divert its attention away from the Beagle hunt to relay communications with Spirit.
More radio telescopes around the world, though, will have been recruited by then to join the search for Beagle. It is hoped the Molonglo radio telescope near Canberra in Australia can soon start scanning the Red Planet.
Scientists caution such searches can be slow to produce results. Computers are used to identify potential coherent signals in the cascade of noise detected by the telescopes.
Some of the radio telescope facilities are able to do this in reasonably quick order; others may take a few days to process fully their gathered data.