You do know about ESA’s Rosetta probe don’t you? This european mission to the comet 67P/Churyumov-Gerasimenko (say that three times fast!) launched in 2004 and has one of the most convoluted mission timelines I’ve seen. Here’s a graphical version, followed by a list of key events. You can also click the image to read the text better.

• First Earth flyby (March 4, 2005)
• Mars flyby (February 25, 2007)
• Second Earth flyby (November 13, 2007 )
• Flyby of asteroid 2867 Šteins (September 5, 2008)
• Third Earth flyby (November 13, 2009)
• Flyby of asteroid 21 Lutetia (July 10, 2010)
• Deep-space hibernation (May 2011 – January 2014)
• Comet approach (January-May 2014)
• Comet mapping / Characterisation (August 2014)
• Landing on the comet (November 2014)
• Escorting the comet around the Sun (November 2014 – December 2015)

As you can see, yesterday, Rosetta had a flyby of the asteroid Lutetia, and there are some stunning images coming down. Let’s take a look at the closest approach image:

Of course, the first thing to notice is that Lutetia has a very irregular shape, as expected for an asteroid. It is about 100km in diameter, and is peppered with craters ranging from big basins that change the whole shape of the asteroid, down to tiny pits. One thing that stands out to me is that the surface appears to have lots of linear features. Similar lines of pits are visible on Phobos, one of the moons of Mars. The grooves on Phobos are more pronounced than the ones on Lutetia, but the similarity is still there. One hypothesis for the Phobos grooves is that they are formed when impacts on Mars, launch streamers of debris into orbit, which then impact Phobos and form a chain of craters. This process wouldn’t work for Lutetia because it isn’t orbiting a convenient source of ejecta.

The grooves could also be cracks, possibly caused by the big impacts that formed the giant craters on Lutetia. Another possibility is that the lines have to do with the way asteroids are held together. I heard a talk at LPSC this year about the role of van Der Walls forces in asteroids. (For those who have forgotten their high school chemistry, van Der Waals forces are what makes small particles cling together even if they have no net charge). The idea is that on asteroids, the particles are much bigger than we usually consider with van Der Waals forces, but gravity is so low that the forces are still important. In other words, the best analogy for rubble pile asteroids might actually be something like dry flour rather than a pile of rocks and boulders! And if you’ve ever packed flour in a measuring cup, you’ve probably noticed that it can “crack” even though it’s barely held together. The same thing might be true of objects like Lutetia and Phobos!

I’m not an asteroid expert at all, so I can only imagine what the true asteroid afficionados are getting out of these pictures. I’ll keep an eye out for results from this flyby at conferences!

As for Rosetta, it’s back out into lonely space for a few years until finally catching up with comet 67P and conducting its primary mission: orbiting and landing on a comet!