The Philae comet lander has fallen silent, according to scientists working on the European Rosetta mission.
The fridge-sized spacecraft, which landed on Comet 67P in November, last made contact on 9 July.
But efforts to contact it again since then have failed, scientists have said.
The first craft to perform a soft landing on a comet, Philae initially bounced, landing in a position too dark for sunlight to reach its solar panels.
It woke up in June as the comet moved closer to the sun. But the latest data suggests something, perhaps gas emission from the comet's surface, may have moved it again.
"The profile of how strongly the sun is falling on which panels has changed from June to July, and this does not seem to be explained by the course of the seasons on the comet alone," said Stephan Ulamec, Philae project manager at the German Aerospace Center (DLR).
Philae's antenna may have been obstructed, and one of its transmitters appears to have stopped working, Rosetta team members said.