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.

Great fun to see a daring set of forecasts like this. And only four months until we find out how well they’ve done.

Ref: arxiv.org/abs/1003.1845: Puzzling Asteroid 21 Lutetia: Our Knowledge Prior To The Rosetta Fly-By