Missions to visit asteroids, search for planets

Artist's concept of the Kepler orbiting observatory

By Richard Stenger
CNN Sci-Tech

(CNN) – NASA selected two missions this week that promise to expand the horizons of science considerably, one that visits two of the largest asteroids in the solar system and another that scans the universe for planets like Earth.

The Dawn probe will study Ceres and Vesta, big boulders situated in the asteroid belt between Mars and Jupiter. The Kepler orbiting observatory will fix its gaze on a region of space with tens of thousands of stars, searching for perhaps hundreds of planets.

Both spacecraft could launch as early as 2006, NASA announced Friday. Each is expected to cost under $300 million.

"Dawn and Kepler are exactly the kind of missions NASA should be launching, missions that tackle some of the most important questions in science, yet do it for a very modest cost," said Ed Weiler, NASA deputy director.

The agency could use the savings. It has come under intense criticism in recent months for billions of dollars in cost overruns with the international space station.

Ceres and Vesta have little in common except their size. The largest known asteroid, Ceres possesses water-bearing minerals and has an older, more pristine surface, possibly with a weak atmosphere and frost.

The dry surface of Vesta, the brightest and third largest asteroid, has been reshaped by lava flows. Collisions with other asteroids have produced plentiful debris, including numerous meteorites recovered on Earth, according to space scientists.

Despite their differences, both could shed light on the formation of the planets during the infancy of the solar system.

Dawn will use a revolutionary ion-drive engine for the nine-year trip, which will take it as close as 62 miles (100 km) to the asteroids.

The Kepler space telescope will provide a major boost to the hunt for worlds beyond the solar system. Astronomers have identified dozens of so-called exoplanets, but only indirectly by observing the gravitational tug on their parent stars.

The method has limited the search to planets comparable or larger in mass to gas giants like Saturn and Jupiter.

In contrast, Kepler will watch a patch of 100,000 stars, focusing on the periodic dimming when planets pass in front of their parent stars, from the perspective of the telescope's line of sight.

The observatory could identify the orbit and size of perhaps 500 planets, some comparable in size to Earth, NASA said.

Dawn and Kepler were chosen from 26 proposals for NASA's Discovery program, a series of low-cost missions to study the solar system and beyond.

The first Discovery probe, the NEAR-Shoemaker, landed on the asteroid Eros in February after orbiting the space rock for a year.