Photo of Philae's landing site with a sketch of the lander superimposed to show its orientation. Credit: ESA/Rosetta/Philae/CIVA

PARIS — Europe’s Philae comet lander, which for unexplained reasons had been silent since June 24, reawakened July 9 for a nearly uninterrupted period of about 20 minutes, sending signals through the Rosetta orbiter, the French and German space agencies said July 10.

The communications raised hopes that stable links between Philae and Rosetta, and then to ground controllers, would be established in time to allow the lander to provide data as Comet 67P makes its closest approach to the sun on Aug. 12-13.

Ground teams had sent commands to Philae that it switch on its CONSERT instrument, the Comet Nucleus Sounding Experiment by Radio Wave Transmission, which is designed to take soundings of the comet interior to derive its composition.

The French space agency, CNES, and the German Aerospace Center, DLR, said they could not explain why Philae chose July 9 to reactivate. The Rosetta orbit, which has been modified to maximize the chances of communications, had not made any changes that could explain Philae’s behavior.

The lander’s internal temperature of zero degrees Celsius should permit it to charge its batteries, especially as Comet 687P gets closer to the Sun.

Peter B. de Selding was the Paris bureau chief for SpaceNews.