21 Lutetia, Rosetta's July target

While I was waiting for President Obama's speech yesterday, I read over a paper by I. N. Belskaya et al titled "Puzzling asteroid 21 Lutetia: our knowledge prior to the Rosetta fly-by." Rosetta is ESA's comet rendezvous mission, which will enter orbit around comet 67P/Churyumov-Gerasimenko in May 2014 after a decade-long cruise. On the way to the comet, it's had three flybys of Earth and two planned encounters with asteroids, one with Steins on September 5, 2008 and the other planned for 21 Lutetia on July 10, 2010 -- that is, in less than three months. 

Lutetia is a main belt asteroid with an elliptical orbit that takes it from 2.0 to 2.8 astronomical units from the Sun. As you can tell from its low number (and thus early discovery date, in this case 1852), Lutetia is a relatively big asteroid. Belskaya and coworkers list its size as 132 by 101 by 76 kilometers, making it similar in dimensions to Saturn's moon Epimetheus. That makes it bigger, by far, than any asteroid yet visited; in fact, it's bigger than all the previously visited asteroids combined. If you drew the shape of Lutetia atop the montage I posted before, it could easily cover up the pictures of all the previously visited bodies. 

The two equatorial dimensions (the larger two) are fairly well constrained -- there's an uncertainty of only 1 kilometer in those dimensions -- but the polar dimension has large uncertainty of about 31 kilometers. That's because Lutetia's spin pole is nearly horizontal, and most Earth observations have looked more or less directly down onto the pole. 

Apart from that, it has been difficult to learn much about Lutetia because its spectrum is nearly featureless. What does that mean? When astronomers use prisms to spread the light from Lutetia and see how bright it is at different wavelengths, they find very little variation in its brightness from wavelength to wavelength. When surfaces are freshly broken, a graph of the brightness versus wavelength will have dips in it where specific minerals absorb light. Lutetia shows no such dips, so it's hard to determine what it may be made of. One conclusion they could draw is that the asteroid is very likely covered with a layer of fine-grained dust (that is, dust particles smaller than 20 microns), which would help to obscure any spectral features. 

While there isn't much spectral variation, there is a lot of variation in the polarization of the light reflected by the asteroid as it rotates, and that suggests that Lutetia probably has at least one huge impact crater, and likely more, giving it a distinctly non-round shape. 

Although it's hard to conclude much from the spectral information, the spectral data and polarimetric data, taken together, make Lutetia unusual among asteroids, so we're likely to see something distinctly different from asteroids that have been visited before. 

And that's pretty much it. How much more will we know about Lutetia by the end of July? Stay tuned!