Controllers at the Jet Propulsion Laboratory (JPL) are using 12-in. ion thrusters to ease NASA’s Dawn spacecraft into orbit around Vesta, a protoplanet in the asteroid belt between Mars and Jupiter that may have one of the oldest planetary surfaces in the Solar System.

The probe has switched from radio navigation to the use of guide stars for the final leg of its 4.8-billion-km (3-billion-mi.) journey. Now only about three times the distance from Vesta as the Moon is from Earth, the spacecraft will use its solar-electric thrusters to chase its target as it orbits the Sun. On July 16, when the probe draws within a range of 9,900 mi., Vesta’s gravity will capture Dawn and it will “spiral” into orbit around it.

An early order of business in the approach phase is surveying the space around Vesta for any moons that may be orbiting there, using the spacecraft’s German-built framing cameras for a closer view than the Hubble Space Telescope can provide. Later those cameras will map the surface — starting with a large crater at the south pole that may show how the differentiated object evolved.

Unlike most asteroids, Vesta apparently has the core-mantle-crust structure of larger planets like Earth. With an average diameter of 530 km, Vesta is too small to be considered a dwarf planet like Ceres — Dawn’s second target — but is large enough to have survived as an artifact of the early Solar System dating back 4.5 billion years.

Dawn was launched on a Delta II rocket on Sept. 27, 2007, and took a gravity assist from Mars during a flyby on Feb. 17, 2009, to send it on toward Vesta. After orbiting there for a year, the spacecraft’s three xenon ion thrusters will take turns pushing it on to Ceres.

Unlike rocky Vesta, Ceres has an icy surface and is spherical. At 950 km in diameter, it is the largest object in the asteroid belt, but it is not considered large enough to be designated a planet because it does not clear debris from its orbit.

Dawn was built by Orbital Sciences Corp. Other partners in its development include the Los Alamos National Laboratory (gamma ray and neutron detector), German Aerospace Center DLR (framing camera fabrication, integration and test), Max Planck Institute for Solar System Studies (framing camera development) and the Italian Space Agency/Italian National Astrophysical Institute.