Random Walk
Dawn Over Vesta
At around 10 p.m. Pasadena time on July 15, JPL’s Dawn spacecraft slipped into orbit around the giant asteroid Vesta. The spacecraft that was the first to use ion drive—a system that accelerates ions to generate thrust instead of relying on chemical fuels—to boldly go beyond Earth orbit is now the first spacecraft ever to orbit a main-belt asteroid; in a year, Dawn is slated to beome the first spacecraft to leave orbit around a main-belt asteroid and go orbit a second one.
Vesta is 530 kilometers in diameter, and the second most massive object in the asteroid belt. Discovered in 1807, it is thought to be a protoplanet that never quite formed, offering us a look at how a rocky hunk like Earth might have appeared during its infancy.
Dawn will continue to tighten its orbit around Vesta for about three weeks, during which time the navigation team will measure Vesta’s gravitational pull in order to make a highly accurate calculation of the asteroid’s mass. This will reveal whether Vesta, like Earth, has a nickel-iron core and an olivine mantle—a so-called differentiated interior, which would be a consequence of that interior having once been molten. Meanwhile, the science team will continue its search for possible moons orbiting the asteroid while calibrating the spacecraft’s camera and spectrometers. Detailed mapping of Vesta’s surface commences in August.
In July 2012, Dawn will restart its ion engine (another first!) and head off to the dwarf planet Ceres, the biggest object in the asteroid belt, arriving in February 2015. —DS
This shot of Vesta was taken on July 17 from a distance of about 15,000 kilometers. The resolution is about 1.4 kilometers per pixel.
