|This photo from Philae shows the surface during the lander’s approach|
Officials said the craft may have briefly lifted off from the comet before touching back down again.
Lander project manager Stephan Ulamec said: “Maybe we didn’t just land once, we landed twice.”
Further analysis is needed to fully understand the situation.
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 1605 GMT.
Early data started to come back from instruments, and one team could see that the lander, known as Philae, had sunk about 4cm into the surface, suggesting a relatively soft top layer.
In a 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 may 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.
This “bounce” was always a possibility, but had been made more likely by the failure of the harpoons to deploy, and the failure of a thruster supposed to push the robot into the surface.
Pictures from the surface have been retrieved at Earth and are being processed in preparation for release.
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.
Philae could have alighted upon terrain whose constitution is anything between rock hard and puff-powder soft.
Controllers in Darmstadt have already received pictures from the surface of the comet, but are getting intermittent drop-out in the lander’s signal.
Philae was deployed to take pictures of the comet’s landscape and to analyse its chemical composition.
They are hoping its surface materials will hold fresh insights into the origins of our Solar System more than 4.5 billion years ago.