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SYFY WIRE Bad Astronomy

Rosetta Arrives at a Comet!

By Phil Plait

After a journey of more than 10 years and a total of many billions of kilometers, the Rosetta spacecraft has finally arrived at the comet 67/P Churyumov-Gerasimenko.


The European Space Agency has only released a few close-ups of the comet, but they're simply stunning. They've also released images taken on Aug. 3, when Rosetta was just a few hundred kilometers from the comet.

That shot shows the comet in all its glory. As we saw in pictures taken over the past week, the solid nucleus is a weird, lumpy, two-lobed structure. But these new shots have many more details!

You can see the overall landscape is oddly sculpted, sharp-edged and spiky. Unlike more monolithic and structurally strong asteroids, there are no obvious craters, either. There are circular features, which may be impact sites, though it’s hard to say just yet. Comets are like dirty snowballs—rock, gravel, and dust mixed together with frozen water, carbon dioxide, and other substances. This gives the nucleus a crunchy, softer texture than solid rock. Impacts would leave craters wholly different in something like that.

You can also see boulders scattered across the surface. That’s even more clear in this spectacular end-on view that was taken today, Aug. 6:

That reminds me strongly of Apollo pictures of the Moon! Judging by eye, those rocks are about the size of houses, to give you a sense of scale. The smooth surface is unusual, too, but as I said, comets have lots of frozen ices in them. As a comet nears the Sun, this material warms up and can sublimate (turn directly from a solid into a gas), spewing out into space. The comet is 540 million (340 million miles) kilometers away from the Sun—nearly as far out as Jupiter—and it’s cold there, but still warm enough to sublimate some of the ices.

You can see that very clearly in this image, taken on Aug. 2:

The nucleus is heavily overexposed, and you can see fans of material blowing off the surface. It’s possible this material sputtering out of the vents in the surface may be responsible for the smooth regions; we know the surfaces of comets change on very short timescales. Smoothing out of rough terrain would be expected as the ices replate onto the surface.

Another image, taken on Aug. 6, shows (if you imagine the overall shape of the comet as a rubber ducky) the smaller "head" of the comet on the left, the "body" on the right, and the narrow neck in between:

The neck region is brighter overall than the rest of the surface, for reasons I'm hoping astronomers will be able to figure out soon. It's not clear at the moment. Also, note the jagged shadow at the bottom center of the image; that's the head's shadow cast on the body.

You can also see all sorts of other features in the images, too: sinuous cracks, sharp cliffs, huge hills towering a hundred meters high over the surface … it’s as alien a place as I can imagine.

And we’ll have plenty of time to take a look; Rosetta will be in orbit around 67/P for over a year. Though to be fair, it’s not really an orbit right now, it’s more of a forced companionship. The spacecraft is actually making a smoothed triangle around the comet, passing it and then doing a short, weak thruster burn to change its path to pass it again on another side, over and over. We don’t know much about the gravity of the comet—the mass and shape of the nucleus need to be better understood first—and these passes will help scientists understand it. They can then sidle the probe closer over time as confidence builds.

In a few weeks, Rosetta will be in a more circular orbit just 30 or so kilometers (20 miles) from the surface. In November it will then drop even closer, and engineers will command the lander named Philae to deploy and move to the nucleus. For the first time in history, we’ll have set down gently on the surface of a comet! Philae will then examine the comet in detail, learning about its surface composition and structure. It will even take samples of the surface to examine.

The pictures here are amazing, but remember, they were taken when Rosetta was still on its way in. Much higher-resolution images will be coming very soon.

But for now, congratulations to the many, many people who worked on this amazing mission. Imagine spending 10 or more years of your life working on a project, dealing with the problems, the delays, the scares, the long periods of hibernation as Rosetta sailed between the planets, and now, finally, seeing the result of your labor.

Rosetta and Philae may be robots, machines, but the minds and hearts of a lot of real, emotional human beings are behind them. And now we get to share with them the excitement and joy of exploring a new world.

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