NASA's Dawn satellite has reached the asteroid belt. First stop: Vesta.

After four years and 1.7 billion miles – including two swings around the sun – NASA's Dawn spacecraft is on final approach to Vesta, an object expected to provide an unprecedented glimpse into the early years of the solar system.

Dawn, with its futuristic ion-drive engine, is just over 100,000 miles from its target – closing in at the stately pace of about 260 miles an hour, relative to Vesta's orbital speed.

The craft is scheduled to take up its orbit July 16 and spend a year at Vesta. Then it heads for a year-long visit to Ceres, slated to begin in 2015, said Robert Mase, Dawn's project manager, during a final-approach briefing Thursday.

Both objects call the main asteroid belt home. The belt encircles the sun between the orbits of Mars and Jupiter.

Dawn is the first NASA mission designed to visit and orbit two objects with the same spacecraft.

As planetary scientists crafted their to-do list for a visit to Vesta, it became clear that "this tiny world has huge importance," says Carol Raymond, the mission's deputy lead scientist and a researcher at NASA's Jet Propulsion Laboratory in Pasadena, Calif. "Vesta is a window into the early origins of our solar system and the terrestrial planets."

In some ways, the notion of looking at asteroids and their tailed cousins, comets, as "windows on the early solar system" has become something of a cliché – an early hope that has been fulfilled in some cases, but not in others, as researchers have learned that some of these objects have been heavily altered by their billions of years orbiting the sun.

But researchers say they have strong reason to believe Vesta and its larger sibling, Ceres, will live up to their billing. Both represent objects that could have become planets in their own right, had Jupiter's rapid growth and quickly changing gravitational field not interfered.

Each represents an intriguing snapshot of planet formation taken at different stages of growth, holding clues to the processes that built planets – and stymied the formation of others – during the first 10 million years of the solar system's 4.6 billion-year history.

What is Vesta?

Vesta was discovered in 1807. Although its shape isn't uniform, the object averages about 329 miles across. It's the second most massive object in the main asteroid belt, after Ceres.

Ground-based and Hubble Space Telescope observations have uncovered what appears to be a vast crater some 290 miles across, near Vesta's south pole. The rim rises from 2.5 to 7.5 miles above the surrounding landscape, while the floor lies some 8 miles below the rest of the landscape and hosts a central peak that vaults 11 miles above the crater floor.

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