Asteroid Lutetia: Close-up images

Shape model of asteroid (21) Lutetia developed from adaptive optics images and lightcurves. The "depressions" or flat spots, labeled 1, 2, 3 are predictions of such flat spots prior to the flyby.

On July 10, 2010, the European Space Agency (ESA) Rosetta spacecraft beamed back to Earth a dramatic set of close-up images as it flew past the 100-kilometer-sized asteroid (21) Lutetia, on its way to a comet rendezvous in 2014.

But even before Rosetta made its close encounter with Lutetia, astronomers, using three of the world's largest telescopes, were busy making their own assessment of the asteroid's shape and size, as well as searching for satellites.

Their pre-flyby images are being compared this week with those from Rosetta at a meeting of the Division for Planetary Science of the American Astronomical Society in Pasadena, California, revealing that the ground-based images are amazingly accurate.

These telescopes all use adaptive optics (AO), which removes the blurring caused by the Earth's atmosphere. Using something like a fun-house mirror in reverse, AO allows clear pictures to be made, from Earth's surface, of distant astronomical objects that were impossible to see previously.

"Adaptive optics has set in motion an astronomical revolution, bringing new worlds into better view, ranging from asteroids that were previously unresolved pinpoints of light, to the discovery of new planets in other solar systems," said Dr. William Merline of Southwest Research Institute (SwRI) in Boulder, Colorado, lead scientist of the international team that made the observations, funded by NASA and the National Science Foundation.

Two of the telescopes are atop Mauna Kea in Hawaii: the W.M. Keck telescope with its 10-meter mirror and the Gemini telescope, equipped with an 8-meter mirror. The 8-meter Very Large Telescope (VLT) of the European Southern Observatory in Chile was also used.

"We carefully evaluated the size and shape of Lutetia, and pinned down the orientation of its spin pole using telescopes on Earth, prior to the flyby," reported Dr. Jack Drummond, an astronomer at Starfire Optical Range in Albuquerque, New Mexico, where AO was first developed in the early 1990s and can be considered the cradle of adaptive optics. Drummond is an expert in turning AO images into models of asteroids, detailing their shapes and sizes.