NASA Gears Up for Big Asteroid Encounter

NASA has begun the countdown for its Dawn spacecraft's encounter with the giant asteroid Vesta, slated for less than a year from now.

Beginning next July, Dawn will orbit Vesta for a year, conducting a detailed study and becoming the first spacecraft ever to orbit a body in the asteroid belt. Previous missions have shown us a handful of asteroids, but Vesta will be special, scientists say.

"Vesta is going to amaze us," said Marc Rayman, Dawn's chief engineer at NASA's Jet Propulsion Laboratory.

At 350 miles (565 km) across, Vesta is nearly a world unto itself. It's the second-largest body in the asteroid belt, containing almost 10 percent of the entire belt's mass. The asteroid Ceres, so large it is considered a dwarf planet, is the only belt object bigger than Vesta.

"It's a big, rocky, terrestrial-type body ? more likely similar to the moon and Mercury than to the little chips of rocks we've flown by in the past," Rayman said of Vesta. "For example, there's a large crater at Vesta's south pole, and inside the crater is a mountain bigger than asteroid Eros."

Researchers hope Dawn's mission will help them understand how planets form. Astronomers think, after all, that Vesta was in the process of becoming a full-fledged planet when Jupiter interrupted its growth. The gas giant's gravity stirred up the material in the asteroid belt so objects there could no longer coalesce.

Dawn, which recently set a record for the all-time biggest speed boost for a spacecraft engine, will start its Vesta surveys out slow. Its first orbits will be high and leisurely, and the craft will take days to loop around Vesta at altitudes of about 1,700 miles (2,735 km).

After taking pictures and gathering data from up high, Dawn will spiral down to lower and lower orbits, eventually settling in a little more than 100 miles (nearly 161 km) high ? lower than satellites orbiting Earth.

The great Vesta view Dawn gets won't be restricted to astronomers: Researchers will quickly combine the images the probe captures into a movie, allowing the mission team and public all to vicariously ride along.

After a year orbiting Vesta, Dawn will move on to take a look at the asteroid belt's largest body: the dwarf planet Ceres.

Dawn's mission follows on the heels of several others, as astronomers have become increasingly interested lately in near-Earth objects.

For example, the European Space Agency's Rosetta spacecraft just beamed back pictures of Lutetia, which at 62 miles (100 km) wide is the largest asteroid yet visited by a spacecraft.

And in June, the Japanese space agency's Hayabusa probe returned to Earth after a seven-year journey to the asteroid Itokawa. Hayabusa's goal was to return samples of an asteroid to Earth, which would be a first ? and it may have done so, though the jury is still out.

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