A European robot probe has made the first, historic landing on a comet, but its status remains 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.
Mission scientist Stephan Ulamec said: "Maybe we didn't just land once, we landed twice."
He said it would take further analysis to understand what has happened to it.
A landing on Comet 67P/Churyumov-Gerasimenko was confirmed at about 1605 GMT.
There were cheers and hugs at the control room in Darmstadt, Germany, after the signal was confirmed.
It was designed to shine a light on some of the mysteries of these icy relics from the formation of the Solar System.
The landing caps a 6.4 billion-kilometre journey that was begun a decade ago.
The lander sank about 4cm into the surface, suggesting a relatively soft surface.
But shortly after, scientists confirmed that the harpoons, designed to fasten the spacecraft to the ball of ice, did not fire as intended.
In a media briefing, Dr Ulamec, the mission's landing chief, 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 may have drifted off the surface after landing and started to turn. This subsequently came to an end, which Dr Ulamec interpreted as a possible "second landing" on Comet 67P.
This "bounce" was always a possibility, but had been made more likely by the failure of the harpoons to deploy.
The first pictures from the surface have already reached Earth and are being processed in preparation for release.
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 Open University, who has worked on the project from its earliest days, was at mission control in Darmstadt and was jumping for joy 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 harpoons has cast a pall over the celebrations. Scientists will now take a decision on whether to re-fire them.
Scientists believe Philae was in a stable configuration when they last had contact with the probe. But they have now lost radio "visibility" and will only re-establish contact on Thursday.
Earlier, a thruster system designed to push the robot down into the surface of the comet failed.
Part of the difficulty is the very low gravity on the 4km-wide ice mountain.
