4 Vesta was the first large asteroid to be visited by the Dawn mission, a mission that was the first to use ion propulsion, and the first to orbit two separate objects. Vesta is the only asteroid identified as a source of meteorites on Earth. In contrast to 1 Ceres, Dawn's other target, Vesta seems to have remained pristine since its formation, and is now considered a protoplanet. Although Vesta is massive enough for gravity to deform it into a round shape, it is not in hydrostatic equilibrium so does not qualify as a dwarf planet like Ceres.
Locations on Vesta are complicated by the use of two coordinate systems. The International Astronomical Union uses a coordinate system with its prime meridian defined by a dark patch informally known as Olbers Regio (by convention, all objects discovered in historic times have a feature named after the discoverer of the object). The IAU prime meridian has the advantage that it could be located even on the relatively coarse imagery from Earth-based telescopes. The Dawn mission chose a prime meridian based on a tiny craterlet called Claudia, about 150 degrees away. Also, measurements by Dawn showed that the earth-based pole position estimated for Vesta was about 10 degrees off, and that Olbers Regio is not obvious on close-up imagery. The current IAU recommendation assigns Claudia a longitude of 146 degrees and is the system used on these maps. Unfortunately, many earlier maps of Vesta use the older system.
As annoying as the IAU can be at times, like their screwy decision to define poles in terms of the ecliptic plane rather than rotation sense, they were solidly in the right on this issue. First and foremost, their system has priority. Second, there was absolutely no reason the Dawn team couldn't have defined a coordinate system to match the IAU definition. Instead of putting the prime meridian close to Claudia, they could have, oh, let's say, assigned it a longitude of 146 degrees. The clash was solely due to sloppiness and poor planning on the part of the Dawn mission.
Vesta would fit comfortably into some of the larger impact basins on the Moon, so the scale on these maps is a mere 600 meters per pixel.
Vesta's geologic history seems to have been one of accretion, differentiation, basaltic volcanism and later cratering. Vesta is dominated by two huge impact basins, both at the south pole. The older basin is Veneneia and the younger Rheasilvia (all place names on Vesta are based on Roman names, particularly figures associated with the Vestal Virgins). Rheasilvia has a gigantic central peak that was imaged from Earth. The total topographic relief on Vesta is about 40 km, about 15% of its radius. Vesta is massive enough to have accreted into a sphere but is rigid enough to support very large departures from sphericity. Therefore it does not meet the definition of a dwarf planet. With Vesta flattened on one side by large impacts, tidal friction from the Sun and Jupiter probably caused it to assume an orientation with its largest diameter on the equator and the craters at one pole.
Vesta's proposed geologic time scale is based on major impacts. The oldest epoch is the pre-Veneneian, followed by the Veneneian, Rheasilvian and Marcian. The dating of the spochs is extremely rough and estimates based on presumed asteroid collision rates and lunar cratering rates yield very different results. The Marcian epoch probably began several hundred million years ago and the Rheasilvian and Veneneian epochs probably lasted about a billion years each.
Created 11 April 2014, Last Update
28 January 2015
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