The rotation of a comet being studied by a European Space Agency (ESA) probe is gradually slowing down as it moves close to the sun, scientists say.

Whether this will help allow the Philae lander, the probe put on the comet's surface last November, to emerge from "hibernation" is still unknown, they say.

The agency's Rosetta spacecraft has determined the rotation of Comet 67P/Churyumov-Gerasimenko, which takes about 12.4 hours for one turn around, is getting longer by about a second a day.

The cause is the increased activity of the comet as it is gradually warmed by its approach to the sun, throwing out gas and dust, mission experts report.

"The gas jets coming out of the comet - they are acting like thrusters and are slowing down the comet," said ESA mission flight director Andrea Accomazzo.

"To give you an idea, these gases come out of the comet for a few kilometers and are moving at 800 meters per second," he explained.

Mission scientists are still attempting to make contact with Philae, the small robotic lander Rosetta placed on the surface of the 2.4-mile-wide comet.

In its landing attempt in November, the lander bounced away from its intended touchdown spot and ended up in a shaded area of the surface that kept sunlight from reaching its solar panels. The event caused its batteries to run down to the point where Philae went into sleep mode.

Although the lander is receiving twice as much sunlight now as it did in November, repeated attempts by mission controllers to contact Philae have generated no response.

A team at the German Aerospace Center tried last week, to no avail, but the scientists there say they're not discouraged.

"It was a very early attempt," explained project manager Stephan Ulamec. "We will repeat this process until we receive a response from Philae. We have to be patient."

The question is whether, even with the increased sunlight, Philae can charge its batteries sufficiently for a reboot, as Rosetta tries to contact it by radio.

"The problem is that even if Philae hears Rosetta, it has to have enough charge to turn on its radio transmitter," said Accomazzo, adding he's a bit pessimistic.

"I put it at 50-50, but I will be the happiest person in the world if it happens," he said.

Philae needs at least five watts to automatically turn back on and would need a total of 19 watts to send data to Earth through the Rosetta spacecraft.

Officials say they hope that by this summer, when the comet will be much closer to the sun, the lander may finally show signs of life.

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