Philae lander has gone silent — and we might never hear from it again

Gas or faulty radio equipment might have led to the loss of signal

The Philae comet lander has gone quiet — leading to worries that it might have got stuck and we will never hear from it again.

Scientists fear that the dishwasher-sized robot — which made an historic descent to comet 67P/Churyumov-Gerasimenko in November — has been nudged out of sight by gas, or that its radio equipment has broken. Earth has not been from it since July 9.

Scientists believe the probe may have been nudged by out-gassing from the comet, which is becoming more active as it travels closer to the sun and heats up.

Alternatively something might be wrong with Philae's radio equipment.

On July 9, the lander sent a signal to mission scientists at the German Aerospace Centre (DLR) but then went into "silent mode".

A command to switch on one of the probe's scientific instruments, the Rosetta Lander Magnetometer and Plasma Monitor (Romap) was ignored.

DLR's Philae project leader Dr Stephen Ulamec said: "In the telemetry received, we have observed signs that Philae could have moved and that its antennas are thus perhaps more concealed or their orientation might have changed."

The probe detached from its Rosetta mother ship and bounced onto the surface of the comet on November 12, ending up in the shadow of a crater wall or rock face that prevented sufficient sunlight reaching its solar panels.

After hibernating for months it "woke up" in June as the comet moved closer to the sun and the solar panels began generating more power.

The latest information about the amount of sunlight reaching the panels suggests that the probe has shifted position, said Dr Ulamec.

"This profile - where panels are receiving a great deal of sunlight - has clearly changed between June and July," he said. "This cannot be explained only by the course of the seasons on the comet."

Philae has two transmitters and is programmed to switch back and forth between them periodically.

Mission controllers have sent a "blind command" to the probe telling it to use only the functional transmitter if one of them is faulty.

The lander has an "emergency programme" that will allow it to operate five instruments without any communication from Earth.

Meanwhile the increasing amount of gas and dust being ejected from the comet has forced controllers to move the Rosetta orbiter to a safer distance 170 to 190 kilometres (105.6 - 118 miles) away.

Dr Ulamec remains hopeful.

He added: "The lander is obviously still functional, because it sends us data, albeit at irregular intervals and at surprising times. There have been several times when we feared that the lander would not switch back on, but it has repeatedly taught us otherwise."