New results from Philae lander offer first close-up of a comet

Ice and dust inside, 67P’s surface marked by mix of sand, hard rock, craters and cliffs


TAKE TWO   Philae’s intended landing spot (left, imaged from 40 meters above the comet’s surface) was soft and sandy. A two-hour-long bounce dropped it in a craggy pit (right, with lander leg at bottom) where the ground is as hard as pumice.

DLR/ROLIS/Philae/Rosetta/ESA (left), CIVA/Philae/Rosetta/ESA (right)

During its brief time awake on comet 67P/Churyumov-Gerasimenko, the Philae lander documented a diverse world. New analyses of lander data reveal the comet as uniform on the inside, but full of variety on the outside. Pebbles, boulders, cliffs and pits blanket the forbidding landscape. Complex organic molecules float above a surface that is as soft as sand in some places and as hard as rock in others.

Not too shabby for a lander that bounced, tumbled, bounced again, fell in a hole and landed on its side. For nearly 60 hours, Philae learned all it could about its new home before running out of power and slipping into a seven-month slumber from which it only recently awoke. The specifics of Philae’s rough landing along with a first look at data from last year appear in seven papers online July 30 in Science.

“Every time we take a good look at a comet, it looks totally different than what we’ve looked at before,” says Anita Cochran, a planetary scientist at the University of Texas at Austin. “This is no exception.”

The European Space Agency’s Rosetta spacecraft arrived at comet 67P last August and released Philae on November 12. The plan was for Philae to touch down gently on a smooth plain and secure itself with harpoons. The harpoons failed to fire, sending Philae on a two-hour trek across the comet. Along the way, it nicked the rim of a crater and bounced once more before settling into a Philae-sized pit.

“The lander didn’t do what it was supposed to do, but it did do something spectacular,” says Cochran, who is not involved with the Rosetta mission. “It took a lot of guts to try this. They did a fabulous job.”

The multiple landings turned out to be a mixed blessing: Mission scientists got to prod the surface at two additonal sites. The first is soft like fine sand, says Jens Biele, a physicist at the German Aerospace Center in Cologne, Germany. The other two are as hard as pumice.

“That was a stunning difference,” Cochran says.

Seven cameras on Philae got a good look at its final stop. A cracked cliff lies one meter away, casting a shadow that prevented the lander’s solar panels from recharging its batteries and cutting short its observations. One of its three legs is pointing up; the other two are resting on the surface. Three of the cameras see mostly shadows. Boulders and pebbles are strewn about a sharp, speckled terrain.

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BOUNCE-BY-BOUNCE After its first touchdown, Philae nicked a crater rim, bounced one more time and finally settled somewhere in a rocky landscape two hours after first contacting the comet, as shown in this image adapted from a figure in J. Biele et al/Science 2015. IDA/DASP/UPM/INTA/SSO/IAA/LAM/UPD/MPS for OSIRIS Team MPS/Rosetta/ESA; adapted by M. Telfer

TRAJECTORY OF A CRASH LANDING Two teams of scientists independently calculated Philae’s trajectory (blue and green lines). The lines diverge slightly because the groups used different datasets for their calculations. The landing is shown from two different perspectives.SONC/ESOC/ESA

“We had no idea what comets looked like close up,” says Scott Sheppard, a planetary scientist at the Carnegie Institution for Science in Washington, D.C. “These are the first images that show things like that.”

As Philae dropped toward 67P, it saw a landscape shaped by erosion on a world devoid of wind and running water. Impacts from tiny meteorites might sandblast the surface, the researchers speculate, and erupting gases could create gentle winds. The lander also got a whiff of a cornucopia of organic molecules, some of which had never before been detected on a comet.

Philae blasted radio waves through the comet that were picked up on the other side by the Rosetta orbiter. The strength and delay of the signal gave researchers a peek through a narrow slice of the comet, revealing what it’s made of and how it’s put together.

“It’s the first time that’s ever been done,” says Sheppard. The interior appears to be assembled from primordial ice and dust, which researchers have long assumed. “But it’s a good thing to have it finally confirmed.”

The insides appear uniform, suggesting that the comet formed from similar building blocks rather than a hodgepodge of material. The comet is a snapshot of a particular time and place roughly 4.6 billion years ago as the solar system came together.

“I am very happy that the experiment worked,” says Wlodek Kofman, a planetary scientist at the Institut de Planétologie et d’Astrophysique de Grenoble in France. “This was never done before. It shows what’s possible for the next missions to explore asteroids and comets.”

The probe woke up on June 13 once its solar panels started receiving enough sunlight to recharge its batteries, and has been in intermittent contact with Rosetta ever since. If mission scientists can get the radar experiment working again, Kofman says, then they could start to piece together a 3-D image of the inside of the comet. “I hope that we have this chance,” he says, “but I’m not so sure.”

Recent data from the lander indicate it may have shifted as the comet cracks and crumbles on its race toward its closest approach to the sun on August 13. Rosetta is out of contact for a few weeks while it scopes out the comet’s southern hemisphere, but will move back north in late August. With any luck, Philae will be waiting and ready to work. “The lander has surprised us before,” Biele says.

Editor’s Note: This story was updated August 4, 2015, to clarify that Philae touched down in three spots, not two, and to correct a photo credit.

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