The Dawn space probe is closing in on its first target, the massive asteroid Vesta, almost four years after its journey began in September 2007, NASA says.
Dawn is on a long-haul mission to unlock the secrets of the solar system by studying the two largest asteroids orbiting the sun, Vesta and Ceres - a trip that will carry it five billion
kilometres by the time it is over.
It will be about three more months before the spacecraft gets close enough to begin orbiting the huge proto-planet, but NASA said it is eager to get to work.
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"We feel a little like Columbus approaching the shores of the New World," said Christopher Russell, Dawn principal investigator, based at the University of California in Los Angeles.
"The Dawn team can't wait to start mapping this Terra Incognita."
After orbiting Vesta for about a year, the unmanned Dawn spacecraft will then carry on to Ceres, an even larger asteroid, in 2015, NASA said.
Mission to probe solar system creation
The deep-space probe is carrying photographic and science instruments to study the surface of the asteroids and analyse their gravitational pull.
Dawn's mission is to learn about the first moments of the solar system's creation 4.6 billion years ago by gathering information about Ceres and Vesta, including what kinds of elements form such terrestrial planets, among them Earth, Mars and Mercury.
It also plans to explain why and how Vesta and Ceres followed a different evolutionary and formative path, particularly the role that water may have played in their development.
Icy crust, rocky core
Ceres, discovered in 1801, has a spherical shape and a diameter of about 960 km. Scientists believe it may have a layer of thick ice under its crust, covering a rocky core.
Ceres was classified in 2006 as a "dwarf planet", according to a new definition by astronomers to describe asteroids in the solar system.
The decision by the International Astronomical Union was the result of a debate about the status of Pluto, which is now classified as a dwarf planet along with Ceres and another celestial body, Eris.
Rocky surface, hot inside
Vesta, discovered in 1807, is smaller than Ceres but is the third largest asteroid in the solar system. With a diameter of 520 kilometers, Vesta has a rocky surface without a trace of water and a hot interior.
Scientists are especially interested in the enormous crater on the south pole of Vesta, 460 km wide and 13 km deep, which is believed to be the result of a major collision.
After having cancelled the Dawn project previously, NASA revived the mission in 2006 after an investment of $US449 million ($A411 million).
"After more than three and a half years of interplanetary travel, we are finally closing in on our first destination," said Marc Rayman, Dawn's chief engineer, at NASA's Jet Propulsion Laboratory in Pasadena, California.
