'Like froth on a cappuccino': spacecraft's chaotic landing reveals comet's softness - Nature.com

Rosetta’s lander Philae on the surface of a comet (artist’s impression).Credit: ESA/ATG medialab.

In 2014, the European Space Agency’s pioneering lander touched down on comet 67P/Churyumov-Gerasimenko, after a ten-year journey aboard its mothership, Rosetta.

But rather than fix itself to the surface, Philae bounced twice and ended up on its side under a shady overhang, cutting its mission short.

After a meticulous search, an ESA team has now discovered the previously unknown site of Philae’s second touchdown — and with it an imprint that the craft left in comet ice that is billions of years old (see ‘Boulder bounce’).

“It’s softer than the lightest snow, the froth on your cappuccino or even the bubbles in your bubble bath,” says Laurence O’Rourke, an ESA scientist at the European Space Astronomy Centre in Madrid, who led a search to locate the wayward lander, which was found in 2016.

The study is important, she says, because some previous data from Philae had suggested that 67P’s surface could be very hard — which might hinder future attempts to retrieve samples of comet ice.

The landing would not have been violent: in the comet’s low gravity, the 100-kilogram probe would have weighed one gram and taken 10 seconds to drift one metre, says O’Rourke.

To hunt for the second touchdown site, O’Rourke’s team analysed images around the lander taken by Rosetta.

They think that Philae touched the surface at four points over two minutes: it slid down a slope, cartwheeled through a crevice and hit a boulder, then bounced on its head before departing for its resting place.

“We’ve been able to probe the interior of the comet and understand what it’s made of, all because of Philae’s movements, rather than a dedicated instrument on board,” says O’Rourke