A spacecraft set to launch early next month will explore the two largest residents of the asteroid belt to look for clues to our solar system?s beginnings.

NASA?s Dawn mission will help scientists understand the first stages of planet formation by studying Vesta and Ceres, two rocky bodies whose growth stalled 4.5 billion years ago when they failed to gather enough raw materials to become planets themselves.

?We believe they?re the building blocks of planets,? said Dawn mission co-investigator Lucy McFadden from the University of Maryland, ?which is one reason why we want to go there and learn what we can about them. It will tell us about the early part of the solar system, sort of like looking back in time.?

By targeting both worlds, Dawn will be the first spacecraft to orbit and study the two objects after leaving Earth.

A study in contrasts

Vesta is a bone-dry asteroid, while Ceres is a dwarf planet suspected of harboring a layer of ice up to 75 miles (120 kilometers) deep or even liquid water beneath its dusty surface. Both are located in the solar system?s asteroid belt, a 130-million-mile wide rock-strewn corridor located between the orbits of Mars and Jupiter. Ceres is spherical and about as wide as the state of Texas. Vesta is more elongated, and has a diameter about the width of Arizona.

Vesta?s physical characteristics are like those of the inner terrestrial planets, whereas Ceres resembles the icy moons of the outer planets.

?They?re worlds in their own rights,? McFadden told SPACE.com. ?These are small bodies that did not grow into big giant planets, so we believe they hold information about what the planet forming processes were 4.5 billion years ago.?

Dawn will carry an infrared and visible camera to study Vesta and Ceres? surfaces, a gamma ray and neutron spectrometer to determine their composition, and instruments to measure their gravity fields and reveal details about their interiors.

Vesta

Dawn is scheduled to enter orbit around Vesta in October 2011. Past observations of features such as lava flows and magma oceans on the asteroid?s surface suggest it was partially molten early in its history. Studies of meteorites believed to have come from Vesta suggest the asteroid?s surface was once soft enough for heavy elements like iron to sink and form a dense core.

?That?s interesting, and a bit puzzling,? said Dawn principal investigator Chris Russell of the University of California, Los Angeles.

Melting requires a source of heat. One way to produce that heat is through the gravitational energy released when rocky particles collide to form an asteroid. But Vesta is so small that ?there would not be enough gravitational energy to melt the asteroid when it formed,? Russell said.

One possible explanation is that two supernovas believed to have exploded around the time of our solar system?s birth spattered Vesta with radioactive forms of elements like aluminum-26 and iron-60. The hot atoms could have provided the extra heat needed to melt Vesta, scientists speculate. Once the radioactive atoms decayed, the asteroid would have cooled to its present form.

What Dawn finds on Vesta could change how scientists think about planet formation.

?When I went to school, the thought was that the Earth got together, heated up, and the iron went to the center and the silicate floated on top, producing a core-forming event,? Russell said. This scenario assumes that rocky planets were formed by the collision and merger of smaller ?planetoids? that hadn?t yet formed their own iron cores. But if objects the size of Vesta could melt and form dense cores, ?it would affect the way the planets and their cores grew and evolved,? Russell said.

Ceres

After orbiting Vesta for seven months, Dawn will make its way towards Ceres. If Dawn used conventional rocket fuel, such a maneuver would be impossible because of the amount of fuel required.

?We would need one of the largest rockets the U.S. has to carry all the propellant,? said Marc Rayman, project system engineer for Dawn at NASA?s Jet Propulsion Laboratory.

Instead, Dawn uses an innovative ion propulsion engine that relies on accelerated charged particles to create the thrust required to accelerate the spacecraft. Rather than a burst of energy, the thrust is generated gradually, causing small changes in trajectory and speed over long periods of time.

Dawn is scheduled to arrive at Ceres by February 2015, a few months before NASA?s New Horizon probe reaches the dwarf planet Pluto. Scientists think Ceres could help answer some longstanding questions about the role water played in planetary evolution.

For example, why can some rocky worlds like Ceres and Earth hold on to large amounts of water, while others, like Vesta, are completely desiccated?

?Vesta will tell us about the earliest epoch, and Ceres will tell us about what happened later,? Russell said.