July 12, 2010
Asteroid Lutetia has been revealed as a battered world of many craters. The European Space Agency's (ESA) Rosetta mission has returned the first close-up images of the asteroid showing it is probably a primitive survivor from the violent birth of the solar system.
The July 10 flyby was a spectacular success with Rosetta performing faultlessly. Closest approach took place at a distance of 1,965 miles (3,162 kilometers).
The images show that Lutetia is heavily cratered, having suffered many impacts during its 4.5 billion years of existence. As Rosetta drew close, a giant bowl-shaped depression stretching across much of the asteroid rotated into view. The images confirm that Lutetia is an elongated body, with its longest side around 81 miles (130 km).
The pictures come from Rosetta's Optical, Spectroscopic, and Infrared Remote Imaging System (OSIRIS) instrument, which combines a wide-angle and a narrow-angle camera. At closest approach, details down to a scale of 200 feet (60 meters) can be seen over the entire surface of Lutetia.
"I think this is a very old object," said Holger Sierks from Max Planck Institute for Solar System Research in Lindau, Germany. "We have seen a remnant of the solar system's creation."
Racing past an asteroid
Rosetta raced past the asteroid at 9 miles per second (15 km/s), completing the flyby in just a minute. But the cameras and other instruments had been working for hours and, in some cases, days beforehand. Shortly after closest approach, Rosetta began transmitting data to Earth for processing.
Lutetia has been a mystery for many years. Ground telescopes have shown that it presents confusing characteristics. In some respects, it resembles a C-type asteroid, a primitive body left over from the formation of the solar system. In others, it looks like an M-type asteroid. These have been associated with iron meteorites, usually reddish and thought to be fragments of the cores of much larger objects.
The new images and data from Rosetta's other instruments will help to decide. Compositional information is needed for that.
Sensors investigate Lutetia
Rosetta operated a full suite of sensors at the encounter, including remote sensing and in-situ measurements. Some of the payload of its Philae lander was also switched on. Together they looked for evidence of a highly tenuous atmosphere, magnetic effects, and studied the surface composition as well as the asteroid's density.
They also attempted to catch any dust grains that may have been floating in space near the asteroid for onboard analysis. The results from these instruments will come later.
The flyby marks the attainment of one of Rosetta's main scientific objectives. The spacecraft will now continue to a 2014 rendezvous with its primary target, Comet Churyumov-Gerasimenko. It will then accompany the comet for months, from near the orbit of Jupiter down to its closest approach to the Sun. In November 2014, Rosetta will release Philae to land on the comet nucleus.