Today, after a four-year journey, NASA's Dawn spacecraft will go into orbit around the asteroid Vesta, where it will stay and make observations for a year.
It should go into orbit at around 10 pm PDT tonight, and NASA expects to hear from the spacecraft and confirm that it performed as planned at approximately 11:30 pm tomorrow. Dawn will be about 9,900 miles from Vesta - and 117 million miles from Earth.
"It has taken nearly four years to get to this point," says Robert Mase, Dawn project manager at NASA's Jet Propulsion Laboratory. "Our latest tests and check-outs show that Dawn is right on target and performing normally."
Unlike most missions, where dramatic propulsive burns put spacecraft into orbit around a planet, Dawn will ease up to Vesta gradually, allowing the asteroid's gravity to capture it into orbit.
However, until Dawn gets closer to Vesta and can make accurate measurements, the team is having to work from estimates. They'll refine the exact moment of orbit capture over the next few days, says NASA.
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.
Ceres and Vesta are the most massive of the protoplanets - baby planets whose growth was interrupted by the formation of Jupiter. Ceres is very primitive and wet while Vesta is evolved and dry.
The mission is intended to help establish the role of size and water in planetary evolution, and the team has suspicions of what it will find.
Ceres may have active hydrological processes leading to seasonal polar caps of water frost, says NASA, and a thin, permanent atmosphere. Vesta may have rocks more strongly magnetized than on Mars, giving insight into how and when dynamos arise.