NASA asteroid probe launched from Florida

CAPE CANAVERAL, Florida (Reuters) - A spacecraft that could serve as a model for interplanetary transport blasted off aboard an unmanned Delta rocket on Thursday on a mission to explore two giant asteroids between Mars and Jupiter.

The probe, called Dawn, will be the first to travel to and orbit two different celestial bodies, a maneuver that previously would have required far too much fuel to be possible.

"We're extremely happy with today's launch," said Chris Russell, the project's lead scientist. "It's been a long time that the science team has been waiting to get that mission in space."

Instead of traditional chemical-burning rocket thrusters, Dawn is powered by a trio of solar-powered electric engines that ionize and expel xenon gas, emitting a glowing blue after-burn in the process.

The first step in Dawn's journey was as routine as it comes, with a beefed-up Delta 2 rocket, built by United Launch Alliance, lifting off at 7:34 a.m. EDT from Cape Canaveral Air Force Station in Florida.

Launch was delayed seven minutes to allow time for a boat that had strayed beneath the flight path to move. The Delta rocket, outfitted with nine strap-on motors for extra power, drops its spent boosters into the ocean.

The U.S. Coast Guard notifies mariners to steer clear of the area.

BUILDING UP SPEED

After reaching orbit around Earth, Dawn stretched out its solar wing panels to begin collecting energy to power its ion engines, which work by electrically charging xenon gas and passing it through a magnetic field.

The gas is expelled at about 89,000 mph (143,200 kph), which pushes the spacecraft forward in the opposite direction.

The pressure is very gentle, about equal to the force of a piece of notebook paper on the palm of a human hand, but builds up over time.

Dawn will take four days to speed up to 60 mph (97 kph), but within a year will be zipping along at more than 5,500 mph (8,850 kph). During that time, it will use only about 15 gallons of fuel.

Dawn's targets are two of the largest objects in the asteroid belt, the rocky body Vesta, which is similar to Earth's moon, and the icy dwarf planet Ceres.

Although the objects are relatively close to each other, they formed under vastly different conditions.

Scientists are eager to compare the two worlds to learn more about the original materials and processes of the solar system's creation.

Dawn is expected to reach Vesta in four years and spend about six months in orbit before continuing on to Ceres.