Rosetta: Waiting game after comet lander glitch
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”.
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. (Courtesy to BBC)