Launch of probe to dwarf planets likely today

The weather in Florida looked promising today for a sunrise launch of NASA's Dawn mission to explore the mysterious dwarf planet Ceres and the giant asteroid Vesta.

Liftoff from the Cape Canaveral Air Force Station was expected as early as 7:20 a.m., just seven minutes after sunrise. The Delta II rocket and its nine strap-on boosters are to carry the unmanned spacecraft to a 100-mile-high Earth orbit in just nine minutes.


About an hour later, Dawn's third stage is to ignite to push the 1,600-pound spacecraft on its way toward the asteroid belt, between the orbits of Mars and Jupiter. Mission managers expected to get their first signals detailing the craft's health 1 1/2 to 3 1/2 hours after launch.

The eight-year, $449 million mission is the first designed to orbit two celestial bodies in succession and the first NASA science mission propelled by super-efficient, solar-electric ion propulsion. The engine is expected to accumulate more than five years of running time, carrying the craft almost 4 billion miles on just 72 gallons of xenon fuel.


Long delayed by cancellations, restarts and budget worries, Dawn came within hours of launch on July 7 when NASA officials postponed it again to avoid conflicts with the launches of a Mars mission and the space shuttle Endeavour.

If all goes well, Dawn is expected to reach the asteroid Vesta in October 2011, falling into orbit around the 359-mile-wide chunk of rock for six months to study its topography and elemental and mineral composition.

The spacecraft will then leave Vesta and fly on, orbiting Ceres by February 2015. The largest object in the asteroid belt and the first to be discovered, in 1801, Ceres is spherical in shape, about 600 miles in diameter and believed to be composed largely of ices. It was reclassified in 2006 as a "dwarf planet," along with Pluto.

Scientists believe these two very different objects hold clues to the conditions that prevailed in different parts of the solar system during its formation 4.6 billion years ago.

University of Maryland astronomer Lucy McFadden is a co-investigator and education director for the mission, which is managed by the California Institute of Technology's Jet Propulsion Laboratory in Pasadena, Calif.