Dawn and 4 Vesta

In the newest e&s issue (Engineering & Science, published by Caltech) I discovered that on July 15, around 10 pm, JPL's Dawn spacecraft got into orbit around the brightest asteroid 4 Vesta.

Vesta has a mean diameter of about 530 km, and only smaller in the asteroid belt than the dwarf planet Ceres. It's about 9% (estimated, as these estimates were recently downgraded a lot) of the mass of asteroid belt.

The northern hemisphere from 5,200km

Dawn has a two-part mission. It will orbit Vesta for a year, and then is scheduled to reach Ceres in 2015. This has never been done before as there has never before been the right kind of propulsion system to let this happen. All former multi-target missions using conventional drives, such as the Voyager program, were restricted to flybys of the bodies that they wanted to study. Conventional drives rely on chemical fuels.

The ion drive used for this mission is innovative. It accelerates xenon ions to generate thrusts from three thrusters, and everything is powered by a 10 kW (at 1AU) triple-junction pv array.

It has already tried to tightly constrain and calculate the asteroids mass as well by its gravitational pull, which will reveal whether Vesta, like Earth, has a nickel-iron core and an olivine mantle. Earth and, perhaps, Vesta, has this differentiated interior because of a formerly molten interior.

Craters in various states of degradation.
Taken in August.

Detailed mapping of the entire surface of the asteroid will continue to study the apparently dry/rocky protoplanet, to help our understanding of rocky planet formation. Vesta is achondritic (basaltic instead of filled with molten droplets found in space and accreted to asteroids), so it seems that it has experienced significant heating and differentiation, with a Mars-like density and lunar-like basaltic flows. However, it probably differentiated quickly (from analyzing radionuclide dating of pieces thought to come from Vesta). All these theories will be tested.

Again, the plasma drive enables it to be the first spacecraft to orbit two extraterrestrial bodies (and the sun). Dawn used 275 kg of zenon to get to Vesta. With the propellant it carries, it can perform a velocity change of over 10 km/s!