The British team behind the Beagle 2 mission to Mars has failed to contact the lander after several attempts.
Beagle's only hope may be Mars Express, its mothership
There has been no signal since the probe's planned touchdown on the Red Planet early on Christmas Day.
Team leader Professor Colin Pillinger says they are now pinning their hopes on the mothership, Mars Express.
He told reporters: "Mars Express is our primary route of communication. It's the one we spent most of our time over the last four years testing.
"Really and truly now we're waiting until 4 January for a really big attempt with Mars Express."
That date is when the spacecraft - which carried Beagle to Mars - gets itself into the correctly defined orbit to start communicating with and studying the planet's surface.
It is also the time when Mars Odyssey, which is already in orbit and has been hunting for Beagle over the past three days, will have to divert attention to two US rovers due to land on the planet - the first arriving next weekend.
Professor Alan Wells, a senior consultant at the British National Space Centre and a member of the Beagle team, acknowledged a "setback" in the silence that greeted Mars Odyssey on its latest overflight.
He told a news conference in London that a command signal had been sent via Odyssey to reset Beagle 2's timer.
Scientists had suspected that a clock error might have meant Beagle 2 was transmitting at the wrong time.
In addition, US space agency scientists are being asked to check there is nothing wrong with Odyssey.
The spacecraft's communications system was temporarily shut down by radiation from a massive solar flare in October. And one of its instruments was totally lost.
Mars Express carried Beagle to the Red Planet
The other principal avenue scientists have been using to detect Beagle is the 76-metre Lovell Telescope at the Jodrell Bank Observatory in Cheshire, UK.
Unlike Odyssey which is searching for Beagle's tonal communication written by the rock group Blur, Jodrell is simply trying to track down any radio emission that might come from the lander's electronic equipment.
However, the Cheshire dish - which listened out for signals on Friday and Saturday night - has also drawn a blank.
Further opportunities to scan for a signal from Beagle 2 will be undertaken over the coming days.
These will include efforts by the Westerbork radio telescope array in the Netherlands and by the super-sensitive 45-metre dish at Stanford University in California, US.
"The Dish", as Stanford's radio telescope is commonly known, is said, theoretically, to be capable of detecting radio emissions from Beagle 2's central processor microchip.
But, in truth, many now suspect the European Space Agency's (Esa) Mars Express spacecraft may be the last and only hope of finding Beagle.
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
"We need to get Beagle 2 into a period when it can broadcast for a much longer period," said Professor Colin Pillinger.
"This will happen around the 4 January after the spacecraft has experienced a sufficient number of communication failures to switch to automatic transmission mode."
This means Beagle will then be transmitting permanently during daylight hours. And, by then, Mars Express will be in prime position to listen for its "baby".
Currently, the mothership is far from Mars and preparing to fire its engines for a major trajectory change that will move it into a polar orbit around the planet.
"We haven't yet played all our cards," said Professor David Southwood, the head of Esa's science directorate. "With Mars Express we will be using a system that we have fully tested and understand.
He added: "At the moment, I am frustrated rather than concerned."
Meanwhile, the Beagle team continues to investigate other potential reasons for the communications failure.
These include a possible landing off course, a tilting of the spacecraft and a problem in fully opening the solar "petals" which could result in a blockage of the weak signal from Beagle's antenna.
Jodrell and other radio telescopes on Earth will continue searching
Professor Southwood urged everyone to be positive: "Mars Express is working extremely well - we're as happy as Larry about it. But don't get me wrong, Beagle is very important to us. We want the full story and we're hanging in there."
Mars Express is the major part of the European mission - Beagle was a late add-on.
It will perform detailed studies of the planet's surface, subsurface structures and atmosphere.
Commissioning of some of the on-board scientific instruments will begin towards mid-January and the first scientific data are expected later in the month.
It has a powerful camera system which could at some stage search the planet for signs of Beagle's parachutes and airbags.