Europe's robot lab Philae has fallen "silent" on the surface of a comet zipping towards the Sun, said ground controllers on Monday who are afraid that it may have shifted out of radio contact.
"The lander could have moved," the German Aerospace Center (DLR) said in a statement, adding: "even a slight change in its position could mean that its antennas are now obstructed".
"It is also possible that one of the landers' two radio receiver units is damaged and that one of the transmitter units is not fully functional."
Philae, which touched down on comet 67P/Churyumov-Gerasimenko on November 12 last year, went into hibernation three days later, and woke up again on June 13 for intermittent communications with Earth via its orbiting mothership Rosetta.
The washing machine-sized lander has since called home eight times, the last on July 9, when it uploaded critical data obtained from Philae's prodding and probing of its alien world.
Since then, the robot probe has gone "back to 'silent mode'," said the statement.
The DLR Lander Control Center "team has been working hard to get back in contact with the lander and operate it to conduct scientific measurements," it added.
But from some of the data received, "we have observed signs that Philae could have moved and that its antennas are thus perhaps more concealed or their orientation may have changed," said project leader Stephan Ulamec.
"At the moment we have some concerns about this," DLR spokesperson Manuela Braun told AFP. "We are trying to understand."