Los Angeles ? Hurtling more than 33,500 miles an hour toward a comet coming from the far reaches of our solar system, a European spacecraft today will take a relative moment out of its long journey to rendezvous with a mysterious asteroid, 21 Lutetia.

Flying to within about 2,000 miles of Lutetia’s surface, the Rosetta orbiter will analyze the asteroid’s surface and composition to discover whether it is metallic or filled with organic, carbon-based molecules.

The fly-by isn’t the spacecraft’s ultimate purpose, but that makes it all the more exciting to some astronomers. It’s “basically a goodie — an add-on we can do, which is very interesting,” said Rosetta mission manager Gerhard Schwehm. Not only might scientists finally be able to solve the “what’s-it-made-of” question, they’ll get up-close-and-personal images of the asteroid.

Lutetia — one of the largest asteroids in the belt that sits roughly between Mars and Jupiter — was discovered more than 200 years ago, but much about the asteroid remains unknown. Scientists aren’t quite sure what its length is; estimates have ranged from 59 to 83 miles. And its potential composition — metallic, organic or perhaps a combination — has proved puzzling. Previous scans using such tools as radar and spectroscopy have showed signatures of both carbonaceous and metallic asteroids, Schwehm said.

In the two hours that the comet-chaser will be watching the asteroid, Rosetta will try to determine Lutetia’s age, shape and whether the asteroid has a magnetic field and an exosphere. An exosphere is a very thin layer of gases between a celestial body’s surface and the rest of space; if it exists around Lutetia, analyzing it could help determine whether the asteroid is carbonaceous or metallic.

If Lutetia turns out to be metallic, that would make this fly-by the first to document a metallic asteroid. This would be of great help to astronomers, because metallic asteroids are thought to be shards from the cores of far larger bodies and thus could reveal much about long-gone giants.