The Rosetta mission’s try and drop the Philae lander on a comet in 2014 didn’t go in line with plan. The harpoon mechanism meant to stay Philae to terra-not-quite-firma didn’t work, and poor Philae ended up bouncing round and touchdown below a darkish cliff overhang, unable to deploy its photo voltaic panels and full its duties. But let it not be stated that Philae failed to depart its mark. Because it did. Quite actually.
To extract worth from Philae’s unintentional journey, researchers have labored exhausting to determine the spots the place the craft impacted the floor of the comet. This required painstaking evaluation of Philae’s movement sensors to reconstruct its trajectory, alongside with a terrifically complicated sport of “one of these things is not like the others” performed with before-and-after photographs of the comet’s jumbled floor.
The website of the preliminary bounce was straightforward sufficient to find, however the path from there to its resting place was one other story. A new examine led by the European Space Agency’s Laurence O’Rourke reveals one other spot the place Philae dented comet 67P. And the dimension of that dent really tells us one thing outstanding about what comets are like.
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Researchers ultimately discovered a spot they dubbed “skull-top ridge” the place a pair of boulders separated by a crevice appeared to have met Philae. After the touchdown, a vibrant spot appeared in that crevice, as if floor mud had been eliminated to show water ice in the boulder. And certainly, spectral information from imagery confirms that the vibrant spot is largely water ice. While water ice makes up a substantial portion of comets—which are sometimes considerably rudely referred to as “dirty snowballs”—a comet’s floor is composed of a layer of mud left behind as daylight drives off the outermost ice, so really seeing ice there is telling.
Philae’s preliminary landing location was in a flat spot seemingly lined by a thick layer of that mud. Its encounter with this boulder represents an interplay with one thing more much like the comet’s inside.
The group estimates the depth of the dent it left behind at about 25 centimeters. Using the recorded velocity of the 100-kilogram craft, this allowed them to calculate the boulder’s sturdiness—or lack thereof, as it seems. They discovered that the boulder was really about as comfortable as fluffy snow on Earth.
This illustrates one thing that Rosetta efficiently measured: the comet is extraordinarily porous. The excessive water ice and CO2 ice content material may make you assume the comet is a exhausting, frozen block, however round 75 p.c of its quantity is void house in between grains of ice and mud. Without robust gravity to tug issues collectively, comets simply aren’t that dense.
This isn’t the first estimate of the comet’s materials energy to return out of the Philae touchdown try. The estimate primarily based on Philae’s preliminary landing crater was considerably sturdier, more much like the regolith of our Moon, however that was seemingly the thick mud layer. A drilling experiment carried out by Philae returned a practically rock-like stretch estimate, far surpassing any others. But it’s not clear what materials that might have been drilling, or if it was even correct.
But as University of Arizona researcher Erik Asphaug writes in a abstract accompanying the new examine in the journal Nature, this all highlights the issue of missions concentrating on comets. A pattern assortment mission, for instance, must consider carefully about what to pattern and how that materials may behave.
“This is not the first misadventure to occur on the surface of a small planetary body, nor will it be the last,” Asphaug writes. “Unfortunately, the findings also suggest that the best places to sample comets will not be the flat plains, but along the newly exposed ridges, cliffs and boulder piles, which are more difficult to land on.”
And touchdown on a flat plain proved troublesome sufficient.