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SYFY WIRE Bad Astronomy

A Distant, Moody Comet

By Phil Plait

I just thought y’all deserved to see this stunning and moody portrait of the comet 67P/Churyumov-Gerasimenko, taken by the Rosetta spacecraft on March 27, when it was about 329 kilometers away from the comet’s rocky and icy solid nucleus.

Although it looks like the Sun is directly behind the comet, it’s not quite; the angle from the spacecraft to the comet to the Sun is about 160°, with the Sun off the frame above and a bit to the right. You can see the lit “crescent” comet, its two lobes mostly backlit. Vents in the surface are warmed by the Sun, and ice there sublimates, turning directly into a gas, expanding, and blowing away from the comet. That’s what forms those lovely fanlike jets of material.

Some of the jets look curved, and I suspect that’s due to the rotation of the comet; the material flung away moves in a straight line, but as the comet spins, each particle is flung away in a slightly different direction, creating curved or spiral patterns. This is called the “sprinkler head effect” and is seen in many objects, including comets, stars, and even rocket flights.

To give you a sense of scale, the big lobe of the comet is about 4.1 kilometers across, and the smaller one about 2.6 kilometers. The jets fan out, and eventually become the comet’s coma—the fuzzy cloud around the solid nucleus—and the tail.

This image was taken specifically to study the material around the comet; it’s a relatively long four-second exposure, and the camera was set to be more sensitive to light to capture faint details. And yes, I’m pretty sure those are actual stars in the background. This really is quite a lovely picture.

After several months of close passes, Rosetta was recently sent on a long-distance excursion that went to 1,000 kilometers from the nucleus to study the space environment surrounding the comet now that it’s passed its closest point to the Sun and is moving outward again in its orbit. The spacecraft is on its way back to the comet now, and will pass just 30 kilometers over the surface in a few more days—this is referred to as a “zero phase flyby,” which means the Sun will be shining directly down on the comet from Rosetta’s view (I wonder if the spacecraft might even see its own shadow, which it’s done before, though it may be too distant for the shadow to be clear).

It’s nice to see Rosetta still performing well. But not for much longer: The end of the mission is planned for September 2016, when the spacecraft will be commanded to land (hopefully gently) on the comet’s surface.

But that’s months away, and there’s much more to be learned from this spectacular and frankly weird object as it heads away from the Sun and its warmth, back into the colder depths of the solar system past Mars and Jupiter.

I wonder … the comet orbits the Sun every six years or so. Will we ever venture back? Will future robots, or even humans, come to find Rosetta resting on the comet’s surface, dusty and frosty from the constant exhalations of the active object? It’s a nice thought. I hope so.

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