Europe's robot lab Philae has fallen "silent" on the surface of a comet zipping towards the sun, said ground controllers Monday who fear it may have shifted out of radio contact.

The European Space Agency says it hasn't received data from the lander since July 9.

Philae's project manager, Stephan Ulamec, said Monday the pattern of sunlight on the lander's solar panels appears to have changed, possibly due to a slight shift in position triggered by gas coming out of the comet.

One of Philae's two transmission units also appears to be faulty.

Scientists plan to send further commands to the lander and hope it responds again, as has happened before.

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 spokeswoman Manuela Braun told AFP. "We are trying to understand."

(FRANCE 24 with AFP)