Around 10 p.m. Pacific time tonight, a 1,600-pound spacecraft called Dawn is scheduled to begin orbiting the asteroid Vesta – the second-largest asteroid in a belt of cosmic rubble that orbits the sun between Mars and Jupiter.

This is the first mission aimed at orbiting an asteroid, as well as the first designed to orbit two objects as part of the same project.

Vesta, thought to have formed within the first 30 million years after the sun ignited some 4.6 billion years ago, represents a case of planetary arrested development, researchers say. Thus, the object, large enough to cover about half the state of California, is expected to open a unique window on processes that led to the formation of the rocky, inner planets – Mars, Earth, Venus, and Mercury.

"We're very excited," says Carle Pieters, a planetary scientist at Brown University in Providence, R.I., and a member of Dawn's science team. "We've been working with the data that's been pouring down" during the spacecraft's final approach to the asteroid, "and we're just thrilled with some of the initial images. Everything is working perfectly."

Although Vesta has long been characterized as an asteroid, over the years, scientists have come to appreciate it as a "protoplanet" – an object that was on its way to joining the ranks of planets until something stunted its growth.

Ground and space-based observations have indicated that Vesta's surface is made up largely of solidified lava, dubbed basalt. This evidence for early volcanic activity on Vesta has led to the notion that early on, it was heated by the radioactive decay of a handful of elements. These radioactive elements, created when a nearby star exploded in a supernova, found their way into the cloud of dust and gas from which the sun and planets formed.

The volcanism stopped because the elements involved – radioactive varieties of aluminum and iron – have relatively short lives in radioactive form, compared with the elements that continue to toast Earth's core, Dr. Pieters explains.

Still, the volcanic activity went on long enough for the material making up Vesta to separate, or differentiate, with the heavier elements gathering at Vesta's center and the lighter ones making their way toward the surface.

Indeed, this differentiation is what has earned Vesta the title "protoplanet."

Data from Dawn will provide a reality check on this scenario.

And while Vesta appears to be essentially solid rock, the object also could host tiny amounts of water – as thin layers of water molecules on rock surfaces.

Pieters, who has studied the moon's surface for decades, notes that on the moon, hydrogen – from the solar wind that strikes the lunar surface – combines with oxygen in the silicate grains to form detectable levels of water all across the moon.

Finding all that water "was a total shock," recalls Pieters, who was a member of the science team for India's Chanadrayan 1 moon mission, which orbited the moon for about 10 months between 2008 and 2009.

Since Vesta's surface also is covered with silicate-bearing basalts, Pieters says she will be scanning Dawn's data for evidence that Vesta might also be covered with water – although in lesser amounts, given the protoplanet's greater distance from the sun.

Another puzzle Vesta presents is its relatively pristine-looking surface minerals, as seen via the object's spectra. Over billions of years, exposure to the solar wind and to galactic cosmic rays has changed the relative abundance of various minerals on the surface of other bodies. That doesn't seem to be the case for Vesta, and researchers want to figure out why.

Dawn arrives at Vesta: Then what?

When Vesta's gravity captures Dawn Friday night, the event will mark a significant milestone in a nearly four-year odyssey.

The asteroid orbits the sun some 127 million miles farther from the sun than Earth does. Powered by three revolutionary ion-drive engines, Dawn has reached Vesta after following a path that involved nearly two full orbits around the sun, and aided by a boost from Mars' gravity. Overall, Dawn has traveled 1.7 billion miles.

Once Vesta captures Dawn, the craft will trace an elliptical orbital path around the asteroid. Using the craft's course-correction thrusters, mission controllers at the NASA's Jet Propulsion Laboratory in Pasadena, Calif., will change the orbit's shape, shifting from elliptical to circular some 1,700 miles above Vesta.

Dawn will begin mapping Vesta's surface and scanning the exposed crust with spectrometers, to assess the types of minerals the object hosts. Importantly, Dawn also will be measuring Vesta's gravity field in great detail. Given Vesta's irregular shape, this information will allow controllers to figure out how to safely bring Dawn closer to Vesta's surface during future portions of the craft's year-long visit.

Scientists want to take measurements at altitudes of 420 miles and again at 110 miles – in effect, slowly zooming in on Vesta's surface to look at features in ever-finer detail.

Once Dawn finishes with Vesta, it will move on to Ceres, the largest asteroid in the main asteroid belt. If all goes well, Dawn will arrive there in February 2015 for a year-long study of that dwarf planet.