ESA’s Philae comet lander has once again gone silent. According to the space agency, the first spacecraft to make a soft landing on a comet lost radio contact with the Rosetta orbiter mothership on July 9. Despite this setback, engineers are still transmitting commands to the unmanned probe in the hope of reestablishing communications and getting it to continue observations.

Though the cause of the radio silence remains a mystery, ESA scientists and engineers have a number of ideas about what may have happened. Based on telemetry returned by the lander, the amount of sunlight falling on Philae’s solar panels has altered in a way that cannot be explained by changing cometary seasons. They theorize, as one answer, that gas emissions from beneath surface could have shifted the washing machine-sized probe, so it's antenna is no longer aiming at Rosetta’s flight path.

Another factor that ESA cites as a potential source of problems is that, though Philae has two transmitters and two receivers, one of each is out of order, so mission control is sending blind orders to the probe to switch to the good transmitter rather than swapping between the two as it was originally programmed.

"Philae is obviously still functional, because it sends us data, even if it does so at irregular intervals and at surprising times," says project manager, Stephan Ulamec. "Several times we were afraid that the lander would remain off – but it has repeatedly taught us otherwise."

The Philae lander was the first manmade object to make a soft landing on a comet. On November 12, 2014, it made contact with the surface of Comet 67P/Churyumov-Gerasimenko, but due to a malfunction in its landing systems it was unable to anchor itself. As a result, it rebounded four times before finally coming to rest at an unconfirmed location that has been called Abydos.

Unfortunately, the spacecraft landed on its side next to a cliff wall, where not enough sunlight could reach its solar panels and provide power. After 54 hours, the batteries went dead and Philae went into hibernation on November 15, 2014 at 01:15 CET. After a failed attempt to reestablish contact in March, Rosetta kept a watching brief until contact was reestablished on June 14.

ESA says that Rosetta had been flying along comet 67P' s terminator plane at an altitude of 153 to 180 km (95 to 112 mi) in hopes of contacting the lander, but the increasing presence of dust as the comet approaches the Sun has forced it to withdraw to 170 to 190 km (106 to 118 mi). Though it will continue to attempt to re-contact Philae, the orbiter will spend more time on its scientific mission.

Philae’s present condition is unknown, but mission control hopes that the lander is receiving orders to carry out a suite of low-power instrument measurements and will send back the results if contact is reestablished.

"Although the mission will now focus its scientific priority on the orbiter, Rosetta will continue attempting – up to and past perihelion – to obtain Philae science packets once a stable link has been acquired,” says Patrick Martin, Rosetta mission manager.

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