the chaotic landing of the spacecraft reveals the softness of the comet

The chaotic landing of a robotic spaceship called Philae produced fortuitous insights into the softness of comets.

In 2014, the European Space Agency’s pioneering lander landed on Comet 67P / Churyumov-Gerasimenko, after a ten-year voyage aboard its mother ship, Rosetta. But rather than staring at the surface, Philae bounced twice and landed on her side under a shadowy overhang, disrupting her mission.

After meticulous research, an ESA team has now discovered the previously unknown site of Philae’s second touchdown – and with it a footprint the aircraft left in the ice of the billion-year-old comet (see “Boulder bounce”). .

The fingerprint allowed the researchers to measure the strength of the ice beneath the comet’s surface, and they found that it is exceptionally soft. “It’s softer than the lightest snow, the frothy cappuccino, or even the bubbles in your bubble bath,” says Laurence O’Rourke, an ESA scientist at the European Space Astronomy Center in Madrid, who conducted research to locate the rebel lander, which was found in 2016.

This is “wonderful detective work,” says Jessica Sunshine, who studies comets at the University of Maryland at College Park, and was not involved in the work. The study is important, he says, because previous data from Philae had suggested that the 67P’s surface could be very hard, which could hamper future attempts to recover comet ice samples. The latter result provides evidence that ice is weak and compressible. “I’m very excited to have proof that we can go back out there and get a 4.5 billion year old ice sample,” says Sunshine. The results were published¹ in Nature on October 28.

ESA scientists knew that Philae bounced off on landing and cut the edge of a cliff. Then it rolled to a mysterious place before stopping. The landing would not have been violent: in the comet’s low gravity, the 100-kilogram probe would have weighed one gram and would have taken 10 seconds to drift one meter, O’Rourke says.

To search for the second landing site, O’Rourke’s team analyzed images around the lander taken by Rosetta. After spotting telltale signs of artificially cut bright ice about 30 meters from the probe’s resting place, the researchers used images taken from all angles to build a 3D model of the area, which they dubbed the crest of the top of the skull due to of the boulders’ shape. The team compared the landscape before and after Philae’s passage and analyzed the probe’s internal data to reconstruct the aircraft’s likely trajectory across the ridge.

They think Philae hit the surface in four places in two minutes: she slid down a slope, rolled a wheel through a crack and hit a boulder, then bounced off her head before setting off for her resting place. The third impact was the most revealing. The top of the aircraft left a 25cm deep footprint in the boulder ice. By comparing this depth to the time it took to create the imprint – which figured out how long Philae’s magnetic sensor had been moved – the researchers calculated the material’s compressive strength to be just 12 pascals. It’s softer than freshly fallen snow, they say.

Although Philae’s final tilted position meant the drill on its underside never reached the comet’s surface, the instrument housing, a tower on the aircraft’s surface, ended up penetrating the ice far enough to make similar measurements. “We were able to probe the interior of the comet and figure out what it is made of, all because of Philae’s movements, rather than a dedicated instrument on board,” O’Rourke says. “This is the icing on the cake.”