A NASA spacecraft has slipped into orbit around the giant asteroid Vesta to begin a year-long study of the second-largest object in the asteroid belt.
The robotic probe Dawn relayed a signal to the US space agency's Jet Propulsion Laboratory in Pasadena, California, to confirm it had entered Vesta's orbit, about 188 million kilometres from Earth.
NASA says Dawn is the first probe to orbit an object in the main asteroid belt between Mars and Jupiter, and is on track to be the first spacecraft to orbit two solar system destinations beyond Earth.
Dawn was dispatched to Vesta in 2007, the first stop in its $US466 million quest to learn more about how the solar system formed 4.5 billion years ago.
It will study the 530km-wide asteroid for a year and then depart for its second destination in July 2012 - the dwarf planet Ceres.
Vesta and Ceres are two of the largest surviving protoplanets - rocky bodies that nearly had enough mass to become full-fledged planets - in solar system.
With its iron core and possible lava flows, scientists believe Vesta is more similar to Earth or the moon than most of its other asteroid neighbours.
Several spacecraft have flown by asteroids before, including NASA's Galileo probe, which encountered three asteroids on its way to Jupiter.
Japan's Hayabusa spacecraft touched down on a near-Earth asteroid named 25143 Itokawa to pluck a few grains off its surface and return the sample to Earth.