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The doomed Philae lander accidentally performed science by sinking the comet



When Philae scored his second touchdown, the close-up shot highlighted the bright ice exposed by the boulder (green box above).
enlarge / When Philae scored his second touchdown, the close-up shot highlighted the bright ice exposed by the boulder (green box above).

The attempt by the Rosetta mission to land the Philae lander on the comet in 201

4 did not go as planned. The harpoon mechanism was intended to fix Philae on rather unstable ground, but this method did not work properly. Poor Philae eventually bounced up and landed under a dark cliff overhang, unable to deploy his solar panels and complete his mission. . But let’s not say that Philae failed to leave his mark. Because it did. without any exaggeration.

In order to get value from Philae’s accidental adventure, researchers have been working to determine where the spacecraft hit the surface of the comet. This requires painstaking analysis of Philae’s motion sensor to reconstruct its trajectory, and a complex game of “one of which is different from the others”, playing with the front and back images of the comet mixed with the surface.

The location of the initial rebound is easy to find, but the route from there to its resting place is another matter. A new study led by Laurence O’Rourke of the European Space Agency has revealed another location for Philae Dent Comet 67P. The size of the dent actually tells us about the extraordinary appearance of the comet.

Sunken

The researchers finally found a place they called “the crest of the skull”, where a pair of boulders separated by a gap seemed to meet Philae. After landing, a bright spot appeared in the gap, as if the surface dust had been removed to expose the water ice in the boulder. In fact, the spectral data from the image confirms that the bright spots are mainly water ice. Although water ice occupies most of the comet (often rudely called “dirty snowball”), the surface of the comet is composed of a layer of dust left by sunlight driving away the outermost layer of ice. What I saw above is Bing talk.

A simple animation demonstrates how the team thought Philae interacted with the pair of boulders.

Philae’s initial touchdown was on a flat spot, possibly covered by thick dust. Its encounter with the boulder represents a more similar interaction with the interior of the comet.

The research team estimated that the depth of the dent it left was about 25 cm. It turns out that using the recorded speed of the 100 kg aircraft, they can calculate the solidity (or lack of solidity) of the boulder. They discovered that the boulders are actually as soft as the fluffy snow on the earth.

Mixed measurement

This illustrates what Rosetta successfully measured: comets are very porous. High water ice and CO2 The ice content may make you think that a comet is a hard block of ice, but about 75% of its volume is the space between ice particles and dust. Without strong gravity to bring things together, comets would not be so dense.

This is not the first estimate of the comet’s material strength derived from Philae’s landing attempt. Estimates based on Philae’s initial touchdown crater are stronger and more similar to our lunar heavy lava, but this may be a thicker layer of dust. A drilling experiment conducted by Philae estimated that the elongation of the rock is almost close to that of the rock, far exceeding that of any other rock. However, it is not clear what material may be drilled, or even if it is accurate.

But, as Erik Asphaug, a researcher at the University of Arizona, wrote in a new research abstract in the journal Nature, all of this highlights the difficulty of targeting comets. For example, a sample collection task must carefully consider the sample to be collected and how the material behaves.

“This is not the first misfortune on the surface of an asteroid, nor the last,” Asphaug wrote. “Unfortunately, the research results also show that the best place to sample comets will not be flat plains, but along newly exposed ridges, cliffs and boulders, which are more difficult to fall on.”

Landing on a flat plain is very difficult.

Naturally, 2020. DOI: 10.1038 / s41586-020-2834-3, 10.1038 / d41586-020-02941-x (about DOI).


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