"Our Lander's Asleep": Philae Delivers Science as Batteries Fade

A misfire of harpoons, an ancient technology, may have spelled the end of one of the most advanced: Philae, the first spacecraft to send images from the surface of a comet. Unanchored to its comet, the spacecraft drifted into darkness, sending back scientific data even as its solar-powered batteries faded.

The lander successfully delivered data on the composition of the comet, before losing contact early Friday with the European Space Agency at the Rosetta Mission Operations Centre in Darmstadt, Germany.

"With its batteries depleted and not enough sunlight available to recharge, Philae has fallen into 'idle mode' for a potentially long silence," an ESA statement announced.

"It was a very successful mission; it has returned a lot of great data already," said space scientist Alan Stern of the Southwest Research Institute in Boulder, Colorado, a member of the Rosetta science team.

When Rosetta was originally designed more than a decade ago, the comet lander Philae was conceived of as a 24-hour mission, he says. "People are focusing on the wrong thing to ignore the success of carrying a lander to a comet and getting it on the surface."

The lander detached from the Rosetta spacecraft orbiting 13.7 miles (22 kilometers) above comet 67P Churyumov–Gerasimenko on Wednesday, landing on the 3.1-mile-long (5-kilometer-long) comet that same day.

During the landing at its intended site, called Agilkia, the lander's stabilizing rocket and two anchoring harpoons failed to fire.

On Earth, Philae weighs 187 pounds (85 kilograms), but gravity is so weak on Comet 67P—hundreds of thousands of times weaker than Earth—that it weighs about an ounce there. The harpoons were intended to keep the probe from drifting off the surface of the comet.

Instead, the unsecured lander bounced off the comet's surface, touching down again nearly two hours later, and then bouncing once more before finally settling, apparently in a shadowed crater beside a cliff. (See "Space Probe Philae Bounces on a Comet.")

Japan's Hayabusa spacecraft, which landed on an asteroid in 2005, encountered similar travails. Its MINERVA mini-lander was designed to hop on the space rock but instead drifted into space.

In the shadows, the solar panels of Philae failed to recharge, leaving it helpless after its 2.5-day batteries died. "From now on, no contact would be possible unless sufficient sunlight falls on the solar panels to generate enough power to wake it up," said mission controllers.

There is some hope of rousing Philae from its sleep. Before the lander shut down, mission controllers rotated Philae's main body about 125 degrees (some 35 percent of the way around its axis), which should have shifted its main solar panel. "This may have exposed more panel area to sunlight," said the ESA, perhaps allowing the lander's batteries to recharge.

Even if Philae is never revived, the mission will continue to deliver more new science. Scientists are analyzing data from Philae's final communication, and will receive much more information from the Rosetta orbiter, expected to keep orbiting the comet until December 2015.

The orbiter, currently about 14 miles (22.5 kilometers) away from 67P, will slowly spiral in until it's only a mile or two from the comet's surface, at which point its cameras will be able to pick out details just a few inches across.

Light reflected from the surface and into spectrographs will tell scientists the contents of 67P's outer crust.

Rosetta will also add to Philae's data on the composition of gases released from the interior. Already, the orbiter has detected hydrogen sulfide, ammonia, formaldehyde, and hydrogen cyanide along with water and carbon dioxide.

As the comet makes its closest approach to the sun next summer, the orbiter will study the comet as it heats up.

Michael D. Lemonick contributed to this article.

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