Rosetta Mission’s Probe Lands on Comet

 European Space Agency’s Philae Is First Craft to Land on Comet in Historic Moment for Space Exploration

The Rosetta robotic space probe has landed on a comet. WSJ’s Gautam Naik explains how a feat like this is achieved. Photo: ESA

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Gautam Naik

Updated Nov. 12, 2014 1:43 p.m. ET

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DARMSTADT, Germany—A robotic probe has become the first craft to land on a comet, an unprecedented moment for space exploration and one that offers the promise of fresh insights into what comets are made of and how they behave.

ENLARGE The navigation camera on Rosetta shows the boulder-strewn neck region of comet 67P/Churyumov-Gerasimenko on Oct. 28. Associated Press

Applause in the European Space Agency’s headquarters in Darmstadt, Germany, after Philae’s successful landing. Reuters

Rocket scientists at the European Space Agency’s mission control here erupted in cheers as they received the first signal that the Rosetta mission’s probe, called Philae, had touched down more than 300 million miles away on the forbidding landscape of a small comet known as 67P/Churyumov-Gerasimenko. There were even more cheers, hugs and handshakes when it became clear that it had done so safely.
The landing follows a decadelong trek through the solar system to get the up-close-and-personal visit with a comet for a lengthy period of time as it hurtles closer to the sun.
“We made history today,” said Matt Taylor, project scientist for the Rosetta mission, who sported a pair of shorts revealing a tattoo on his thigh depicting a successful Philae landing. “I can’t see anyone doing this again anytime soon.”
Philae is expected to quickly snap a few photographs and beam them back to Earth—in the first images ever taken from the surface of a comet. (It takes nearly half an hour for a signal from the comet to reach earth.) Made of ancient ice, dust and other materials, comets are objects of scientific curiosity because they have survived virtually intact from the earliest days of the solar system, more than 4.6 billion years ago.
The signals indicate that Philae landed on its three legs and was sitting safely in the zone where it was aimed, a relatively flat elliptical landing area about 550 yards in diameter, away from deep crevices, large boulders and sharp peaks.

Rosetta’s lander, Philae, took this parting shot of its mothership shortly after separation. ESA/Rosetta/Philae/CIVA

But the process wasn't without its hitches. A thruster aboard Philae that would have helped anchor the lander to the comet’s surface couldn’t be switched on Tuesday night and harpoons meant to do the job didn’t fire. Ice screws would likely have pushed out of the legs and anchored Philae to the surface.
But if they failed the lander would be an in unstable position, imperiling one of the lander’s more important experiments, which involved drilling beneath the surface and studying the material there, said Stephan UIamec, the lander manager.
The latest data beamed back from the lander is coming back in an intermittent basis, suggesting that it may be resting on a slope. If the slope was greater than 30 degrees, the lander would likely toppled over—but it appears to be on firm footing, ESA scientists said.
Philae separated from the Rosetta spacecraft and drifted slowly toward the comet’s surface over a seven-hour period. Scientists were holding their breath in the a few minutes before the first signal from the lander arrived saying it had reached the comet surface.
“I’m nervous but excited,” said Andrew Coates, a planetary scientist at University College London, whose research prediction on the comet’s nucleus could be confirmed by data collected by a properly functioning lander. Noting that the lander was still drifting down to the comet’s surface purely on the basis of gravity, he added: “At this point, Isaac Newton is in the driving seat.”

Because comets carry water and organic molecules, scientists also hope that the Rosetta mission will provide insights into whether comets could have brought water to Earth and possibly kick-started life here. The most important series of experiments will be done over the first 2 1/2 days, the lifetime of Philae’s primary battery. During that time, a variety of scientific instruments on board will provide high-resolution images and data about the shape, density, temperature and chemical composition of the comet. Corrections & Amplifications Confirmation of the Rosetta probe’s successful touchdown on the comet was expected in a one-hour window around 1602 GMT (11:02 a.m. ET). An earlier version of this article and a headline incorrectly said confirmation was expected around 1702 GMT (12:02 ET). (Nov. 12, 2014)