Dawn spacecraft to aim toward two asteroids


LOS ANGELES (AP) – NASA this weekend is set to launch a spacecraft that will journey to the asteroid belt that lies between Mars and Jupiter, a mission that involves a rendezvous with two of the solar system’s largest asteroids.

Seeking clues about the birth of the solar system, the Dawn spacecraft will first encounter Vesta, the smaller of the two bodies, four years from now. In 2015, it will meet up with Ceres, which carries the status of both asteroid and, like Pluto, dwarf planet.

“We’re trying to go back in time as well as to go out there in space,” said planetary scientist Christopher Russell of University of California, Los Angeles, who is heading up the mission.

Weather permitting, Dawn is set to blast off Sunday afternoon from Cape Canaveral, Fla., on a Delta II rocket. The launch caps a tumultuous effort in which the $344 million mission was killed last year because of cost overruns and technical problems.

Ultimately, though, the Jet Propulsion Laboratory in Pasadena, which manages the spacecraft, appealed to NASA Administrator Michael Griffin and got the project revived.

Adding to the drama, Ceres briefly flirted with planethood during last summer’s scientific debate about whether Pluto is a planet. Both Pluto and Ceres were finally classified dwarf planets.

Vesta and Ceres are believed to have evolved in different parts of the solar system more than 4.5 billion years ago around the same time as the formation of the rocky planets including Mercury, Venus, Earth and Mars. Scientists believe the asteroids’ growth was stunted by Jupiter’s gravitational pull and never had the chance to become full-fledged planets.

Images by the Hubble Space Telescope show Vesta and Ceres as geologically diverse.

Mysteries abound: Why are Vesta and Ceres so different? How do size and water affect planet formation? What does the evolution of the asteroids say about Earth’s formation?

Vesta, which measures 326 miles across, is dry and pocked with a deep impact crater in its southern hemisphere. By contrast, Ceres, about twice as large as Vesta, has a dusty surface covered by what appears to be an ice shell and may even contain water inside.

When Dawn reaches each asteroid, first Vesta in 2011, it will orbit each body, photographing the surface and studying the asteroid’s interior makeup, density and magnetism. Pictures and data will be sent back to Earth.

Dawn will be powered by ion propulsion instead of conventional rocket fuel, making it more fuel-efficient and allowing it to cruise between the asteroids and lower itself to about 125 miles above the surface to study them in depth.

Although previous spacecraft have explored smaller asteroids, researchers hope Dawn will shed light on the solar system’s origins.

“If you want to understand the Earth, it’s important to understand how it came to be and that’s where asteroids come in. They’re the building blocks,” said Jay Melosh, a planetary geologist at the University of Arizona who has no role of the Dawn mission.

On the Net:

Dawn mission page: http://dawn.jpl.nasa.gov

University of California, Los Angeles: http://www.ucla.edu