A new angle on Churyumov-Gerasimenko brings circular features into focus
It's just two days now until Rosetta arrives in its initial 100-kilometer "orbit" of comet Churyumov-Gerasimenko, and the latest view from Rosetta's NavCam is fascinating. It doesn't look like a rubber duck anymore; several people have told me that they see the Sphinx in it now.
Look at all those circular features! Let's magnify the image by a factor of 3 to appreciate them better:
ESA / Rosetta / NavCam / Emily Lakdawalla
NavCam view of comet Churyumov-Gerasimenko on August 3, 2014 (original)
Rosetta took this photo of the comet from a distance of only 300 kilometers away. The image has a resolution of 25.6 meters per pixel. It has been cleaned of artifacts. The long axis of the comet measures 5 kilometers.
The circular features remind me strongly of the ones seen on comet Tempel 1 by Deep Impact and Stardust:
NASA / JPL / Cornell / animation by Emily Lakdawalla
Stardust's closeup views of Tempel 1
Here, the 10 images taken by Stardust nearest its closest approach to Tempel 1 have been resized and rotated to match each other, taking out some of the distracting effects of spacecraft motion, making it easier to follow morphological features as Stardust passes by the comet.
Are they impact craters? They might be. But they might also be some uniquely cometary feature. We don't really know. Rosetta is the first mission to orbit a comet rather than just fly by one. So it really represents our first opportunity to study such features from every angle to try to understand how they form and evolve. Comets can change fast, as we saw with Tempel 1, so Rosetta may even get to see surface features change over time.
NASA / JPL / Cornell
Surface changes on Tempel 1
There were changes in the surface of comet Tempel 1 between the time it was first observed by Deep Impact on July 4, 2005 (top right), and by Stardust on February 15, 2011 (bottom right). Between the two visits, the comet made one trip around the sun. The image at top left is a wider shot from Deep Impact. The smooth terrain is at a higher elevation than the more textured surface around it. Scientists think that cliffs, illustrated with yellow lines to the right, are being eroded back to the left in this view. The cliffs appear to have eroded as much as 20 to 30 meters in some places, since Deep Impact took the initial image. The box shows depressions that have merged together over time, also from erosion. This erosion is caused by volatile substances evaporating away from the comet.
We can see the surface features in the latest NavCam image more clearly partially because Rosetta is closer to the comet than before, but also because it has shifted its position with respect to the comet and the Sun so that shadows now outline its features. I liked this higher-angle view so much that I decided it was time to update my compet comparison montage with it:
Image credits: Halley: Russian Academy of Sciences / Ted Stryk. Borrelly: NASA / JPL / Ted Stryk. Tempel 1 and Hartley 2: NASA / JPL / UMD. Churyumov-Gerasimenko: ESA / Rosetta / NavCam / Emily Lakdawalla. Wild 2: NASA / JPL. Montage by Emily Lakdawalla.
Scale comparison of comets visited by spacecraft as of 2014
As of 2014, six comets have been visited by spacecraft; the most recent addition is Rosetta's 67P/Churyumov-Gerasimenko.
One thing I noticed while updating this view is that, if I've done my math right, the comet's long axis is a bit longer than previously estimated, about 5 kilometers. That's not unusual. A new estimate for the dimensions of the comet will be one of the first scientific results from this mission!
There is going to be a media event for the arrival at the comet on August 6, so stay tuned for news from that.