Discovered in Paris by Hermann Goldschmidt in November 1852, asteroid (21) Lutetia has been a cosmic riddle for astronomers. In an attempt to pin down its properties once and for all, ESA's Rosetta spacecraft flew past Lutetia at a distance of 3162 km, at a relative speed of 15 km/s on 10 July 2010 at  18:10 CEST.

Asteroid (21) Lutetia - pre Rosetta flyby

The asteroid flyby of 10 July 2010 was expected to address a number of open questions about Lutetia; in particular, the observations and measurements obtained by instruments on board Rosetta were to be used to:

• Attempt to settle the ongoing debate as to the asteroid's true composition. In particular to ascertain if it is a C-type or M-type asteroid.
• Determine the mass and density of the asteroid with unprecedented precision.
• Search for an exosphere around the asteroid and determine its composition.
• Provide ground-truth for the better calibration of existing observations obtained by ground-based telescopes.
• Test out the scientific instruments on board Rosetta as it continues to travel to its final destination: comet 67P/Churyumov-Gerasimenko.
• Carry out a close-up study of a primitive building block of the Solar System, with the intention of using it to decode how our solar neighbourhood formed.

Situated in the main asteroid belt and with estimated dimensions of 132 × 101 × 76 km (see Belaskaya et al., [2010]), asteroid (21) Lutetia has been subjected to intense ground-based scrutiny since it was announced as a target for Rosetta in 2004. Initial observations recorded a high albedo, suggesting a high metallic content, and led to the body being classified as an M-type asteroid (see Bowell et al., [1978]). Should (21) Lutetia indeed turn out to be M-type, the Rosetta flyby would be the first close encounter of a spacecraft with this class of asteroid.

However, Lutetia's true nature has always been far from clear-cut. One difficulty in unambiguously classifying Lutetia is the lack of clear features in the spectrum of this asteroid. Recent visual spectroscopic studies, reported in Belaskaya et al., and Perna et al., have noted different spectral slopes at different rotation phases. This has been interpreted as arising from inhomogeneities in the asteroid's make up, perhaps caused by local differences in mineralogical or chemical content of the surface.

Some researchers have suggested the closest analogue to Lutetia's surface is a type of carbonaceous chondrite meteorite (see Barucci et al.). When Lutetia was at opposition in 2008/2009 the opportunity was taken to test this theory further. A team of researchers used the VLT and Keck telescopes to estimate Lutetia's bulk density, finding it to be in the range 3.98 to 5.00 g cm^-3, depending on the model that is adopted. Although no precise value could be determined this range of density would support a carbonaceous composition (see Drummond et al., [2010]).

The ground-based observations in preparation for the flyby have also allowed astronomers to construct Lutetia's light curve. Most asteroids tend to be irregularly shaped and therefore different amounts of sunlight are reflected towards the Earth as they rotate. Hence the ratio between the three major axes defining the asteroid as well as its rotational properties can be determined from measuring how this reflected light changes with time. Assuming a certain reflectivity (albedo) the dimensions of the asteroid can also be estimated. Knowing, from this preparatory work, that Lutetia rotates with a period close to 8.17 hours was of great help in planning the scientific measurements for the flyby.

The encounter of Rosetta with asteroid (21) Lutetia is key to understanding the true nature of this puzzling member of the main asteroid belt. Only with the close inspection that is possible with a flyby can the riddles of Lutetia be solved, as this provides the opportunity to measure and analyse many of the asteroid's properties including its shape, density, composition and surface topography. The instruments on board Rosetta have been designed specifically for such tasks and will be able to provide the answers that are sought.

The flyby at Lutetia was the second time Rosetta had studied an asteroid up-close. In 2008 the spacecraft flew past asteroid (2867) Steins at a distance of just 802.6 km, only 2.6 km further out than baselined. However, these two asteroids are just stepping stones on the journey to Rosetta's ultimate goal, the rendezvous with comet 67P/Churyumov-Gerasimenko, scheduled for 2014. The Rosetta team hopes that with this rendezvous they can decipher the enigmas of the formation of our Solar System, just as its namesake helped unscramble ancient Egyptian hieroglyphics.