NASA's Dawn spacecraft obtained this image of giant asteroid Vesta on July 9, 2011, from a distance of about 26,000 miles (41,000 kilometers). Each pixel in the image corresponds to roughly 2.4 miles (3.8 kilometers). Photo courtesy of NASA.

On Friday, July 15, NASA's ion-propelled Dawn probe will become the first spacecraft to enter orbit around a main-belt asteroid.

Dawn will orbit Vesta for one Earth-year, studying the giant space rock at close range to help scientists understand the earliest chapter of our solar system's history.

As Dawn approaches Vesta, surface details are coming into focus, as seen in a recent image taken from a distance of about 26,000 miles (41,000 kilometers).

Engineers expect the spacecraft to be captured into orbit at approximately 10 p.m. Pacific Daylight Time Friday, July 15.

They expect to hear from the spacecraft and confirm that it performed as planned during a scheduled communications pass that starts at approximately 11:30 p.m. Pacific Daylight Time on Saturday, July 16.

When Vesta captures Dawn into its orbit, engineers estimate there will be approximately 9,900 miles (16,000 kilometers) between them. At that point, the spacecraft and asteroid will be approximately 117 million miles (188 million kilometers) from Earth.

“It has taken nearly four years to get to this point,” said Robert Mase, Dawn project manager at NASA's Jet Propulsion Laboratory in Pasadena, Calif. “Our latest tests and check-outs show that Dawn is right on target and performing normally.”

Engineers have been subtly shaping Dawn's trajectory for years to match Vesta's orbit around the sun.

Unlike other missions, where dramatic propulsive burns put spacecraft into orbit around a planet, Dawn will ease up next to Vesta. Then the asteroid's gravity will capture the spacecraft into orbit.

However, until Dawn nears Vesta and makes accurate measurements, the asteroid's mass and gravity will only be estimates.

The Dawn team will refine the exact moment of orbit capture over the next few days.

Launched in September 2007, Dawn will depart for its second destination, the dwarf planet Ceres, in July 2012. The spacecraft will be the first to orbit two bodies in our solar system.

Dr. Tony Phillips works for the National Aeronautics and Space Administration.

Follow Lake County News on Twitter at http://twitter.com/LakeCoNews, on Tumblr at www.lakeconews.tumblr.com, on Facebook at http://www.facebook.com/pages/Lake-County-News/143156775604?ref=mf and on YouTube at http://www.youtube.com/user/LakeCoNews.