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Huge parallel troughs, probably related to the stress of the huge impact that created the south polar crater Rheasilvia, stripe the 326-mile-diameter asteroid’s equator. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

On July 25 at 11:45 a.m. (CDT) the Dawn spacecraft, which has been orbiting the asteroid Vesta since July 16, 2011, powered up its xenon ion engine and began raising its altitude in preparation for departure. Dawn’s exit is a drawn-out affair – not until August  26 will the probe slip free of Vesta’s gravity and once again orbit the sun. Its next and final target is the asteroid Ceres, which along with Pluto and a couple other asteroids, is now classified as a dwarf planet.

Artist’s view of Dawn in orbit around Vesta. Credit: NASA/JPL-CalTech

The mission’s not over yet. Thrusting will be stopped four times during the month-long ascent to allow Dawn to photograph regions of the asteroid’s northern hemisphere that have been in seasonal shadow for most of the mission.

I can’t wait for Wednesday August 22. On that day mission controllers will halt the craft to look back and photograph Vesta as a thin crescent “moon”, a perspective never before seen. The purpose of the photos will be to measure the reflective properties of Vestan dust, but we’ll all appreciate the aesthetic bonus.

The south polar mountain inside the gigantic impact crater Rheasilvia rises 13 miles above the surrounding terrain. That’s almost three times higher than Mt. Everest. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

The scientific bonus from the mission has been immense – over 4,700 photos, 9 million spectra of the surface to help scientists unravel the composition of the asteroid’s minerals, measurements of Vesta’s gravity field to determine how matter is distributed in the planetoid’s interior and identifying many individual atomic elements in the uppermost yard of soil with a gamma ray and neutron detector.

Circling the asteroid at an altitude of only 420 miles (about twice the height the space station flies over Earth) every 4 hours, 21 minutes, Dawn snapped photos of Vesta’s sunlight hemisphere and then beamed the images back to Earth during its night side flight. Over a year’s time, the probe completely mapped the surface six times. Four of the maps were made with the camera pointed at an angle instead of straight down; scientists combined the two views to create a boatload of 3-D images.

The crust of Vesta is pocked with all variety of craters that have excavated both light and dark materials from beneath the surface. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

The biggest thrill has been seeing what was a pinpoint of light in most telescopes evolve into a misshapen sphere with craters, tall mountains, and a strange series of parallel troughs ringing the equator. Scientists believe the grooves were created by an impact that left a 314-mile diameter crater in the asteroid’s southern hemisphere called Rheasilvia.

This Howardite meteorite called NWA 3149 was found in the Sahara Desert. It almost certainly came from Vesta. Notice all the fragments. These were broken during earlier impacts on the asteroid and welded together under heat and pressure. Photo: Bob King

Material blown into space from the blast has even managed to make its way to Earth as HED meteorites. Known as Howardites, eucrites and diogenites, they’re made of the same materials Dawn found on the surface and within the crust of Vesta.

On September 1 Dawn will cast one last glance at Vesta from a vantage point 24,000 miles away before setting its sights on Ceres. We learned patience on the journey to Vesta, and we’ll need it again as we wait for the hardy craft to arrive at Ceres in February 2015.

The title of this blog refers not only to Dawn’s departure from Vesta but also to a live Twitter and Facebook event on September 8 to celebrate the mission. Click HERE for more information on how you can participate.