2011

Dawn at Vesta

The demotion of Pluto from the domain of planethood in 2006 was accompanied by the promotion of one Main Belt Asteroid, 1 Ceres, and probably a second, 4 Vesta, to dwarf planet status. According to the current International Astronomical Union definition, a dwarf planet is a small body that has sufficient mass and self-gravity to have formed itself into a near-spherical shape. In the process, dwarf planets, like full-fledged planets, have probably differentiated into core, mantle, and crust, and thus may have had active interior and surface geologic processes during their histories.

Even the best Hubble Space Telescope (HST) views do not reveal much detail about Ceres and Vesta, and so more thorough exploration of these worlds requires close-up space missions. In 2007, NASA launched a mission that would do both. The novel mission, called Dawn (in honor of its primary targets, which formed at the dawn of the solar system), uses xenon ion thrusters to gently alter its trajectory to match that of Vesta (2011 encounter) and then Ceres (2015 encounter). Dawn became the first mission to orbit one planetary object, then leave, and go into orbit around a second.

Dawn went into orbit around Vesta (330 miles [530 kilometers] in diameter) in July 2011. High-resolution images reveal a heavily cratered surface, and confirmed the presence of two enormous, overlapping south polar impact basins. A series of enormous, deep grooves circle Vesta’s equator, and appear to have been caused by the south polar impacts.

Dawn’s spectroscopic measurements revealed the presence of ancient hydrated (OH-bearing) minerals on the asteroid’s surface, and confirmed that Vesta is the source of a set of meteorites that came from a large, differentiated, volcanically active parent body. Mass and volume estimates reveal a density of about 3.4 grams per cubic centimeter—comparable to the densities of the Moon and Mars. Vesta thus appears to be a rare, preserved example of a protoplanet—an ancient, transitional solar-system body that is part asteroid and part planet, a frozen-in-time survivor harboring relics of the initial formation of the terrestrial planets.

The Dawn mission (the logo of which is shown at left) obtained this photo of Vesta after going into orbit around the large main belt asteroid in July 2011. The view is centered on the large and deep south polar impact basin called Rheasilvia and its enormous central peak.