A camera on Rosetta captures the Philae lander as it falls to the surface of comet 67P/Churyumov-Gerasimenko.

A camera on Rosetta captures the Philae lander as it falls to the surface of comet 67P/Churyumov-Gerasimenko.

ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

Rosetta mission's lander settles on a comet and makes history

DARMSTADT, GERMANY—The Rosetta orbiter on Wednesday dropped a spidery, three-legged robot the size of a small refrigerator and watched as it tentatively set down on a comet—the first time that the surface of these primordial balls of dust and ice has ever been explored.

In the hours after the 11:03 a.m. U.S. Eastern Standard Time touchdown, European scientists and engineers struggled to make sense of how stably the lander was resting on comet 67P/Churyumov-Gerasimenko. Radio connections between the lander and the orbiter were tenuous, solar power generation was fluctuating, and the lander was not anchored by two harpoons that were supposed to have fired on touchdown. It was even possible that the lander, called Philae, had slipped or bounced—but not hard enough to fling it back into the void. Stephan Ulamec, the Philae project manager for the European Space Agency (ESA), said that the lander’s scientific instruments were taking data and that he was confident Philae would live for another day. “Maybe today, we didn’t just land once, but twice,” he said at a press briefing at ESA’s control center here.

In addition to being a first for humanity, the €1.4 billion Rosetta mission marked a strikingly ambitious effort for ESA’s planetary exploration program. It was the first successful landing for the agency since January 2005, when ESA dropped the Huygens probe onto the surface of Saturn’s moon Titan. To a packed audience here, ESA Director General Jean-Jacques Dordain celebrated the historic aspect of the comet landing with obvious pride. “We’re the first to have done that, and it will stay that way forever,” he said.

If healthy, the lander will now pursue a preprogrammed science routine on battery power lasting about 2.5 days. With sufficient solar power, Philae could continue to work for months. And already, 11 science instruments on the Rosetta orbiter have been taking measurements of 67P’s coal-black, organic-rich crust. But Philae, equipped with a drill that can sample more than 20 centimeters deep, has the potential to reveal the nature of material lying underneath the crust—where ice, dust, and organic molecules have been less altered by sunlight and outgassing. The sun’s heat causes buried ices of water, carbon dioxide, and other molecules to burst forth in diffuse jets of gas and dust.

Moments after the landing, Philae lead scientist Jean-Pierre Bibring, of the Institute for Space Astrophysics in Orsay, France, was passing out toasts of Nicolas Feuillatte, a Champagne he had on hand for the occasion. He said he hadn’t been so nervous about the outcome. “As when you go to a plane, you are not the driver,” he says. “We couldn’t do more than what we did.”

Getting to the moment was by no means easy. The Rosetta spacecraft launched in 2004 and whirled around the solar system on one of the longest planetary journeys ever, traveling 6.5 billion kilometers. In August, Rosetta arrived at 67P, and scientists began a frantic effort to map the feeble gravity of the comet’s mountain-sized nucleus even as they searched for boulder-free, level landing terrains. The time pressures were severe because Rosetta had to drop Philae before 67P got too close to the sun’s heat. An instrument on Rosetta is already measuring 5 liters per second of water vapor production, but that rate is expected to rise to 500 liters per second by the time 67P makes its closest approach to the sun in August 2015.

The target area of comet 67P/Churyumov-Gerasimenko looms 3 kilometers below the Philae lander as it descends to the surface.

The target area of comet 67P/Churyumov-Gerasimenko looms 3 kilometers below the Philae lander as it descends to the surface.

ESA/Rosetta/Philae/ROLIS/DLR

The mission team settled on a relatively risk-free region on the “head” of the duck-shaped comet, where only 20% of the terrain contained potentially hazardous slopes greater than 30°. Plans were hatched for the 7-hour landing descent and were loaded up to the spacecraft.

The landing almost didn’t take place. In the night before separation, software trouble nearly forced Ulamec to postpone it for 2 weeks. Ulamec also learned that gas thrusters—meant to pin the lander to the surface while its harpoons fired—hadn’t pressurized correctly. But mission managers decided to go ahead anyway, and just past 4 a.m. EST on Wednesday, Philae slipped out of Rosetta’s grasp at walking speeds.

Step by step, Rosetta’s instruments followed the lander down and found it to be right on track. A picture taken by a camera on the underbelly of the lander at a 3-kilometer altitude showed it to be nearly in the middle of a 1-kilometer-wide landing ellipse. “The real error was much smaller than the error ellipse,” says ESA Head of Mission Operations Paolo Ferri. Sensors on the legs of the lander indicated that the surface was soft and had helped dampen Philae’s fall.

But team members were still confused by conflicting data coming back from the lander, and ESA officials planned to reconvene on Thursday in Darmstadt—hopefully with more answers. “It’s complicated to land on a comet,” Ulamec says. “It’s also very complicated to understand what has happened.”

To read more Rosetta coverage, visit our Rosetta collection page.

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