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Comet 67P is brushing dirt off its shoulder as it moves closer to the Sun

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'But all the comets they love it just to see one of us make it.'

It’s almost showtime: Comet 67P/Churyumov–Gerasimenko, the target of ESA’s Rosetta mission, is finally shedding its outer coating of dust. This event has never been directly observed before. Particles collected by Rosetta will offer new information on the composition of comets, according to a study in the journal Nature. This shedding also sets the stage for the reawakening of the Philae lander as the comet moves closer to the Sun and is exposed to more sunlight.

Comets get dusty in the outer solar system — but not because dust lands on them

Comets are made up of a mixture of rock, dust, frozen gas, and water ice. Even when a comet is traveling through the colder outer regions of the solar system, these frozen elements sublimate — changing directly from a solid to a gas. This process isn't forceful enough to dislodge any solids on the surface though, and so the comet’s exterior becomes gradually drier and dustier as the ice disappears.

When the comet draws closer to the heat of the sun, the dust that had congealed on the surface is blown off by more forceful sublimation. Soon, the dust mantle will be gone entirely to reveal 67P’s icy surface, reports Rita Schulz, lead author of the paper and former Rosetta project scientist. The process is predicted to take about four weeks based on previous observations of the comet.

Rosetta with 67P in the background: that's close for a comet.

"Because we have never before had a spacecraft at a comet so far away from the Sun, this is the first time we can determine the properties of the mantle," says Schulz. She explains that while previous cometary flybys like the 2004 Stardust mission have taken samples from comets after the shedding occurs, this time scientists will have front row seats. "It’s imminent, it’s on the verge of doing it right now," says Schulz.

67P's crust dust contains the parents of interplanetary dust particles

Samples collected so far have been high in sodium and appear porous or "fluffy" under Rosetta’s microscope cameras. Many shattered into pieces when they hit the 1cm by 1cm target plate on the spacecraft, suggesting that they were fragile and had not been glued together by water ice. "From this we can conclude that the dry dust particles that we see in the dried out crust of the comet are actually the parents of interplanetary dust particles," says Schulz. "These particles are everywhere — the solar system is filled with them. They’re what make shooting stars, for example."

The work by Schulz and her colleagues corroborates a string of reports published last week. One of these confirmed the lack of ice on the comet’s surface (caused by the same low-level sublimation that created its dusty exterior), while another observed the ratio of gas to dust being expelled. However, even better for the scientists involved, is that as 67P moves closer to the Sun it's getting more and more sunlight — good news for the Philae lander, which is currently shut down due to insufficient power from its solar panels. "This means the solar cells might unfreeze and recharge and the lander might say, ‘Hello, I’m back!’" says Schulz. "We believe that this may happen within the next few months."