2010

Rosetta Flies by 21 Lutetia

Spectroscopy and color measurements from telescopic observations during the second half of the twentieth century revealed that main belt and near-Earth asteroids can be grouped into a small alphabet soup of compositional classes. For example, asteroids that show colors and spectra indicating the presence of typical planet-forming volcanic minerals, like those found in stony meteorites, are known as S-type asteroids; darker objects with grayer colors and spectra that are more like those of carbonaceous (carbon-bearing) meteorites are known as C-type asteroids; objects with spectra similar to metallic meteorites are known as M-types, and so on. Dozens of different asteroid types have been proposed, depending on the classification scheme and research group.

Prior to 2010, spacecraft had encountered both S-type (for example, Eros, Gaspra, Ida, and Itokawa) and C-type (253 Mathilde) asteroids, but no others. Thus it was especially exciting when the European Space Agency’s Rosetta spacecraft made a close flyby past the M-type asteroid 21 Lutetia on July 10, 2010. Rosetta is a comet rendezvous mission that was launched in 2004 and will rendezvous and deploy a lander onto the surface of periodic comet Churyumov-Gerasimenko in 2014. In addition, like many other space mission teams, the Rosetta team has been able to do some excellent “bonus” science by flying past other objects on the way.

The Rosetta images of Lutetia revealed it to be the largest asteroid then encountered by spacecraft (at 82 × 63 × 48 miles [132 × 101 × 76 kilometers] in size). It is also one of the densest (at 3.4 grams per cubic centimeter), suggesting a possible rocky, metallic composition consistent with its M-type classification. In terms of its visual appearance and geology, however, Lutetia shares many similarities with the other asteroids that have been photographed up close: a lumpy, irregular surface heavily covered by both relatively fresh and relatively degraded impact craters in a variety of sizes. Lutetia also shows evidence of a surface layer of fine-grained, mobile, impact-generated debris—what planetary scientists call a regolith. Why Lutetia’s spectrum appears similar to metallic meteorites, and how such small objects with such low gravity (less than 0.3 percent of Earth’s) can retain fine-grained regolith materials, are active areas of research and debate motivated by Rosetta’s flyby measurements.

Photo of the asteroid Lutetia, taken by the European Space Agency’s Rosetta spacecraft during its July 2010 flyby. Lutetia is a heavily cratered Main Belt asteroid and was the largest asteroid yet encountered, prior to the Dawn mission’s exploration of Vesta and Ceres.