Lack of contact with comet lander Philae raises concerns

(CNN) Philae, the probe that landed on a comet, has been uncommunicative for more than a week, and scientists are trying to figure out what's wrong.

"No contact has been made with Philae since Thursday 9 July," the European Space Agency, which is overseeing the mission, said in a blog entry. "The data acquired at that time are being investigated by the lander team to try to better understand Philae's situation."

Rosetta, Philae's orbiter, has been flying around Comet 67P in order to find the best location to communicate with the probe, the ESA said.

However, lander system engineer Laurence O'Rourke points out that such a gap in communications isn't unusual. Before the contact of July 9, the previous signal had been made on June 24 -- 15 days earlier.

"It's not overworrying in the perspective we haven't heard from the lander in a few days," he said from the ESA's Space Astronomy Centre in Madrid, Spain.

O'Rourke also observes that some latitudes that Rosetta flies are less receptive to signals than others.

"(In one range), we've never received a signal, and this would suggest that this is some sort of surface feature in the way, which is blocking any signal from the lander reaching the orbiter," he said.

Rosetta and its mission

The ESA has a couple of theories on Philae's silence.

One is that the lander's position shifted, putting Philae in a more challenging position to be contacted by Rosetta.

Another is that one of Philae's two transmission units isn't working. The lander also has two receiving units, and one of those was damaged earlier.

However, Philae has coped with communications issues before.

In November, the washing machine-size probe lost contact with Rosetta after 60 hours on the surface of the comet. At the time, plans called for Philae to operate until March, at which point the fierce heat of the sun would probably burn it out.

Instead, the lander ended up in a sheltered spot, one that prevented communications for months but also protected it from the worst of the sun's might. In June, the lander suddenly resumed contact.

At the time, O'Rourke said that the renewed communications "allows us to do more science." Scientists had hopes that Philae would be operational until August or September so it could do experiments while the comet makes its closest approach to the sun, called perihelion.

O'Rourke is still hopeful. The communications issues have prompted the Rosetta team to try new ideas, he said, including taking advantage of commands already aboard the lander to do science. As soon as a signal appears, the lander immediately switches on instruments.

"This is not something that was in our plan originally, but being aware of the lander communication issues means that we are able to take steps (like this one) to overcome them and get science irrespective," he said.

He adds that the fact that the lander is still going is a positive sign.

"If the lander had landed where it was supposed to land in November, it would have lasted until March, because it had no way to cool itself down," he said. "The fact that it is where it is -- in the shade -- in some ways keeps it cool. It's created longevity, which is absolutely amazing."