Planetary Society Blog | 16 Apr 2010
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 Šteins 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.
The Puzzle of 21 Lutetia
Technology Review | the physics arXiv blog | 11 Mar 2010
21 Lutetia has puzzled astronomers since its discovery. Now they have made a daring set of predictions about what the Rosetta spacecraft will find when it flies past this mysterious asteroid in July.
On 10 July, the European Space Agency's Rosetta spacecraft will fly within a few thousand kilometres of 21 Lutetia, a main belt asteroid that orbits the Sun between Mars and Jupiter.
Lutetia is an unusual object. It is classified as an M-type asteroid, which are thought to be made mainly of nickel and iron. However, Lutetia's spectrum does not seem to show any evidence of metals. In fact, exactly what Lutetia is made of puzzles astronomers. That's partly why it was chosen for the fly by.
So come July, astronomers should know the answer to this conundrum. But in the run up, they're indulging in a little fun. The game they've invented is to see how good a prediction they can make about what Rosetta will find.
Today, Irina Belskaya at the Observatoire de Paris and a few friends take a stab. They make several detailed predictions about Lutetia based partly on observations dating back to the 1960s but mostly on data taken since 2004, when interest picked up after the asteroid was chosen as a flyby target.
So what do they think Rosetta will find?
Belskaya and company say that Lutetia will be 132x101x76 km in size (that's technically known as potato-shaped). They say its texture and mineral content will vary across its surface. At least part of Lutetia's surface will be covered by a layer of loose dust having a mean grain size less than 20 micrometres across. And Lutetia's surface will be made of stuff that has more in common with the carbonaceous chondrite meteorites found on Earth than the iron-nickel ones.
But they're most interesting prediction is that Lutetia will be "non-convex" in shape. That means a large crater will be visible on its surface. In fact its shape will be dominated by this crater.
Puzzling asteroid 21 Lutetia: our knowledge prior to the Rosetta fly-by
- arXiv.org > astro-ph > arXiv:1003.1845 > 09 Mar 2010
).
