The British-built Mars probe Beagle 2 has failed to call Earth, dashing hopes that its mothership Mars Express would establish contact with the robot.
All along Beagle's best hope has been Mars Express
Wednesday's bid to reach Beagle with its mothership was considered the best hope of locating the missing lander.
But the European orbiter picked up no signal as it passed over the presumed landing site at 1213 GMT, mission controllers in Germany have announced.
If other attempts to communicate fail, the mission will be classed as lost.
At a news conference held at 1500 GMT in Darmstadt, Germany, Professor David Southwood, head of science at the European Space Agency (Esa), gave journalists the sad announcement.
"We did not get any content of a signal or indeed a signal from the surface of Mars," he said.
"This is not the end of the story; we have many more shots to play. But I have to say, this is a setback. I have to say it makes me very sad."
The blow means that hopes for finding the probe intact are fading fast.
Professor Colin Pillinger, lead scientist on the Beagle team, was disappointed but was determined not to give up yet.
"We will play to the final whistle. It only takes a fraction of a second to score a goal. Let's not give up yet. But it really is a moment when we have to start looking at the future as well.
Dr Mark Sims, mission manager on Beagle 2, said: "There was no RF [radio frequency] signal seen by Mars Express."
The bad news was delivered through Europe's mission controllers
Further flyovers of the presumed landing site by Mars Express will occur on 8, 9, 10, 12 and 14 January.
But Professor Pillinger said Mars Express' transmitter may have to be shut off if contact has not been made after 22 January.
Other attempts may be made with the Jodrell Bank telescope after Beagle switches into another communications backup mode on 2 February.
"My personal view is if we have not received a signal within 5-10 days after that event, we have to assume Beagle is lost," Dr Sims explained.
Professor Southwood told BBC News Online: "The prospects took a dive today. Until today, I had a candle alight for Beagle, but today I feel it's guttering in the wind."
Although this is not the last chance to contact Beagle, Wednesday's conference was marked by much talk of future missions. Previously, scientists had insisted on focussing on options for the recovery mission.
After digesting the news, Professor Pillinger extolled the merits of continuing with British space research.
"I think this is the time when such managers start thinking along the lines of: 'what am I going to say to the chairman and the board in the morning?'
"What you expect to say, really and truly is: 'I have a fantastic team here. If you give me a little bit more support and instead of playing just the occasional cup run like this, we could be playing in the Premier Division.
"Let's go back to Mars with Beagle 2. Let's have a second voyage," he later told BBC News 24. "But the next time, we must have redundancy by having more than one lander."
"The big prize isn't won, yet. There's still everything to play for. Seeking past and present life on Mars is still available for us."
Professor Pillinger added that, although Esa's prospective Aurora project for 2009 was a good opportunity to return to Mars, Britain should try to send another Beagle mission for 2007.
On 9 January, the orbiter will use its high-resolution stereo camera to hunt for signs of Beagle's parachutes and airbags.
A spectrometer on Mars Express may also be able to search for signs of ammonia in the atmosphere from Beagle's airbags.
The US space agency's Mars Global Surveyor orbiter has already imaged the landing site and the data sent to Professor Pillinger to analyse.