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The most active pit, known as Seth_01, which scientists believe is one of several sinkholes on the comet 67P/Churyumov-Gerasimenko.

AP

Europe's comet lander, Philae, may be a sophisticated work of space engineering, but for mission controllers here at the German Aerospace Centre, Philae is behaving more like a difficult teenager. It only speaks when it feels like it and it never really connects.

Three weeks after the lander surprisingly sent word to Earth that it's still alive and well on the surface of 67P/Comet Churyumov-Gerasimenko, engineers have yet to establish a stable link with lander that would allow them to restart science operations on the distant comet.

"This is the frustrating part," says Koen Geurts, technical manager for the lander team. "The battery is charged, the lander is warm, everything is ready to go. It's just that we need a bit more communication."

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There is much at stake. Philae wowed the world last November when it became the first spacecraft to land on a comet. But it was unable to accomplish much of its mission before going silent. The lander's reawakening has now raised the possibility of a second chance for the mission, which, in theory, could continue for months and deliver a massive scientific return.

Philae has now made contact at least seven times since June 13, most recently on June 24. But the signal has been fleeting or – when it lasts longer – continually broken up, like a radio station that's just out of range. So far, Dr. Geurts and his colleagues have not been able to respond to the lander and upload commands that would allow them to restart its science mission.

Scientists at the sprawling, leafy aerospace campus south of Cologne are standing by with a laundry list of tasks they want the lander to perform if it starts to co-operate, including capturing a new set of image from the comet's surface. Dr. Geurts said those tasks have been broken down into intervals as short as 10 minutes to take advantage of narrow communications windows.

On Thursday, he and his colleagues will again try to make contact. In addition to listening for a signal, they will send a blind instruction to restart one of the lander's science instruments, a radio sounder, that has its own dedicated antenna. If the instrument shows signs of turning on, "then we would know that the lander can pick up a signal," says Valentina Lommatsch, an operations engineer with the mission.

"We're really hoping, hoping that we do get another link now," she said.

After seven months of hibernation, Philae has come back to life after at a crucial time, just as the comet is nearing its closest point to the sun on Aug. 13. The comet has become more active as the sunlight warms its frozen surface, releasing gasses and dust. It's a perfect moment to study one of the solar system's ancient building blocks, with the promise of revealing what comets are made of, how they work and what they can tell us about our own cosmic origins.

Public interest in the mission remains high. Philae's Twitter account has 442,000 followers. Earlier this week, Chandra Wickramasinghe, a U.K. astrobiologist who is not a member of the lander's science team, triggered headlines by claiming the comet shows evidence of hosting microbial life.

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Other researchers were quick to dismiss the idea, though there is ongoing scientific interest in what more Philae could reveal about the carbon-chain molecules it detected shortly after it landed. Such molecules are the raw materials for life and comets may have played a key role in transporting them to Earth billions of years ago.

Philae's journey began in 2004 when it was launched toward the comet aboard the European Space Agency's Rosetta mission. After Rosetta caught up to and begin orbiting the comet last summer, scientists used its powerful camera to scout out a suitable landing site for Philae. They ultimately selected a patch of flat, sunny terrain at one end of the four-kilometre-long comet.

Considering that no one has tried to land on a comet before, the descent went remarkably well – until Philae failed to anchor itself to the comet's surface and instead bounced into a dark area, where it ended up on its side, wedged against a cliff or large boulder and without enough sunlight to keep its batteries charged.

But Philae's unlucky bounce also set up the possibility of a dramatic resurrection.

The comet, now 240 million kilometres from Earth, is far closer to the sun that it was last November. The lander is getting about four hours of sunlight every 12 1/2 hours as the comet spins on its axis – more than enough for it to operate at full power, a situation that has scientists clamouring for data.

"The lander's revival was considered a long-shot, and if it can conduct science during the comet's closest approach to the sun, it will really have gone beyond the call of duty," said Paul Wiegert, an astronomer at Western University in London, Ont.

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The obstacle is the radio link. Philae is designed to communicate directly with Rosetta, which relays its signal back to Earth. But because Philae is on its side, its transmitter is likely obstructed or pointing into the ground. That makes it hard to know exactly where Rosetta should be to intercept the signal. To complicate matters further, Rosetta is giving the comet a wide berth at this point to avoid the jets of dust spewing from its surface. Most of the time it may not be in range of Philae's signal at all.

When the signal was picked up last month after Philae's long silence, team members scrambled to get what information they could.

"The first days were really intense," said Cinzia Fantinati of the operations team. "We had to interpret the data as best we could and put together a picture of the situation."

Since then, the lander operations team has been working with colleagues controlling Rosetta to see how the orbiter's trajectory might be tweaked to improve and lengthen contact with the lander. But Philae has so far defied expectations and not responded at times when it was expected to.

Another problem may be that the lander simply can't hear. After analyzing the data received from last month's sporadic contacts, engineers have determined that at least one of the lander's two receivers is broken. If the second receiver is similarly compromised, then the mission is effectively over.

But there are reasons for optimism. Philae has already beaten long odds by switching back on and, though taciturn, it seems to be in good shape. Each brings new opportunities to hear a signal. Team members have put summer vacations on hold and are keeping an eye on their phones during the weekends.

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After all, Dr. Geurts said, talking to Philae may have its up and downs, "but it's not boring."

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