Lutetia is a large, primitive asteroid left on the shelf for billions of years, presumably because no planet consumed it as the Solar System formed.  Most measurements affirm this, making the asteroid out to be a 'C-type', which contains primitive compounds of carbon, but some others suggest that Lutetia is an 'M-type', which could mean there are metals in its surface. 

If the second is true, it could be a real find in space science because although metallic asteroids do exist, they are thought to be fragments of the metallic core of larger asteroids that have since been shattered into pieces. If Lutetia is made of metal or even contains large amounts of metal, the traditional asteroid classification scheme may need rethinking. 

One issue; no one knows what Lutetia looks like. Orbiting in the main belt of asteroids between Mars and Jupiter, it appears as a single point of light to ground telescopes.  Initially it was thought that Lutetia has a diameter of around 95 km, mildly elliptical, but a more recent estimate suggests 134 km, with a pronounced elongation. 

Enter Rosetta, ESA’s comet-chaser, which is heading for asteroid Lutetia as you read this. Like any good sport, a lack of knowledge about what Lutetia looks like hasn't prevented Rosetta from a blind date on July 10th, a Saturday night (naturally), flying to within 3200 km, though Rosetta started taking navigational sightings of Lutetia at the end of May so that ground controllers can determine any course corrections required to achieve their intended flyby distance. 

Animation of asteroid (2867) Steins, which was encountered by ESA’s Rosetta on 5 September 2008 at a distance of 800 km. The spacecraft is now on its way to Comet 67P/Churyumov-Gerasimenko via asteroid Lutetia.   Steins, the firs target of Rosetta, is a relatively rare type of asteroid (E-type) about 5 km across, but its properties were not yet known in detail before Rosetta’s encounter. Credits: ESA, animation by C.Carreau

The close pass will allow around 2 hours of good imaging. The spacecraft will instantly begin beaming the data back to Earth and the first pictures will be released later that evening.

Asteroid science stands to gain once this observational conundrum about Lutetia is resolved because Rosetta's data will provide a valuable collection of 'ground truths' that can be used to resolve conflicting ground-based observations not just for Lutetia but for other asteroids as well.

For 36 hours around the moment of closest approach, Rosetta will be in almost continuous contact with the ground. The only breaks will come as Earth rotates and engineers have to switch from one tracking station to another.

Good contact is essential because the uncertainties in the asteroid's position and shape may demand last minute fine-tuning to keep it centered in Rosetta's instruments during the flyby. "The skeleton of the operation is in place, and we have the ability to update our plans at any time," says Andrea Accomazzo, ESA Rosetta Spacecraft Operations Manager.

Want to know more about the blind date as it happens?  Rosetta is willing to kiss and tell on her blog.