A European robot probe has made the first, historic landing on a comet, but its status is uncertain after harpoons failed to anchor it to the surface.
Officials said the craft may have lifted off the comet after touchdown before returning to the surface.
Lander project manager Stephan Ulamec said: "Maybe we didn't just land once, we landed twice."
The European Space Agency's director general described the landing as "a big step for human civilisation".
Further analysis is needed to fully understand the status of the probe, known as Philae.
However, Dr Ulamec told the BBC that at last radio contact with the probe, he believed it to be in a stable configuration.
"This is the indication right now," he explained. "We really have to wait until tomorrow morning and then we will know a lot more."
The "first" landing on Comet 67P/Churyumov-Gerasimenko was confirmed at about 16:05 GMT.
There were cheers and hugs at the European Space Agency (Esa) mission control in Darmstadt, Germany, after the signal came through.
Director general Jean-Jacques Dordain described it as "a great great day, not only for Esa, but... I think for the world".
The moment the Philae comet landing was confirmed
Early data started to come back from instruments, and one team could see that the lander had sunk about 4cm (1.5 inches) into the surface, suggesting a relatively soft top layer.
But shortly after, engineers could see that the harpoons, designed to fasten the spacecraft to the 4km-wide (2.5 miles) ball of ice and dust, had not fired as planned.
Mission facts:
Philae lander
- Travelled 6.4 billion km (four billion miles) to reach the comet
- Journey took 10 years
- Planning for the journey began 25 years ago
Comet 67P
- More than four billion years old
- Mass of 10 billion tonnes
- Hurtling through space at 18km/s (40,000mph)
- Shaped like a rubber duck
In a later media briefing, Dr Ulamec said: "What we know is we touched down, we landed at the comet at the time when you all saw us cheering and when it was announced.
"We had a very clear signal there; we received data from the landing - housekeeping and science data. That's the good news."
But then Dr Ulamec delivered the "bad news". He said telemetry from the craft suggested it might have drifted off the surface after landing and started to turn. This subsequently came to an end, which the German Space Agency official interpreted as a possible "second landing" on Comet 67P.
In fact, even later data would indicate that the Philae robot may have bounced twice, taking a full two hours to come to a rest.
This bouncing was always a possibility, but had been made more likely by the failure of the harpoons to deploy, and the failure of a thruster intended to push the robot into the surface.
'Close enough to lick'Pictures from the surface have been retrieved at Earth and are being processed in preparation for release.
Scientists were initially elated following the confirmation of a landing.
Canadian astronaut Chris Hadfield, famous for performing David Bowie's Space Oddity on the space station, said of the comet: "Now we're close enough to lick it, and see what it's really made of."
Prof Monica Grady of the UK's Open University, who has worked on the project from its earliest days, was jumping for joy at Darmstadt when the news came through.
She told BBC News: "I can't believe it, it's fantastic, we've landed - we've waited so long for this."
But the news about the unanchored state of Philae has cast a shadow over the celebrations.
The mission team must decide if the harpoons can now be commanded to fire without unsteadying the robot still further.
Esa's Rosetta satellite, which had carried Philae on a 10-year, 6.4-billion-km journey to the comet, and which relayed its communications to Earth throughout the descent, has now moved out of radio "visibility".
If this connection is re-established on cue on Thursday, the team will feel much more confident.
One of the cameras on Philae sent this image of the comet during the descentWhat is clear is that Philae touched down very close to the targeted zone on the head of the rubber-duck-shaped comet.
Paolo Ferri, head of operations at Esa, said: "The camera teams used [descent] images to show that we landed very, very close to the planned centre of our big error ellipse. And I think some people already have the co-ordinates of the first landing."
If Philae remains stable and can be properly secured, it will engage in several months of science experiments on 67P.
It will take pictures of the cometscape and analyse the surface chemical composition.
Scientists are hoping 67P's surface materials will hold fresh insights into the origins of our Solar System more than 4.5 billion years ago.
One theory holds that comets were responsible for delivering water to the planets. Another idea is that they could have "seeded" the Earth with the chemistry needed to help kick-start life. Philae will test some of this thinking.
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