Asteroid Probe Set to "Collide" With Earth

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The system predicted the time the asteroid entered the atmosphere to within 0.5 second and pinpointed the location of its entry to within 8 miles (13 kilometers).

Hayabusa's return to Earth will be even easier to track, the scientists said, because they will have months of advance warning and plenty of information on the craft's exact size and flight path.

The event therefore provides the team with a chance to fine-tune their asteroid-tracking calculations.

"It is very important that we develop accurate ways to predict where asteroids are going to strike, because even small ones can cause a great deal of damage," Hashimoto said.

For example, he noted, a comet or meteorite a mere 197 feet (60 meters) wide is thought to have caused the Tunguska event of 1908.

That blast, which flattened large swaths of Siberian forest, was a thousand times more powerful than the atomic bomb dropped on Hiroshima in 1945.

By contrast, Itokawa is about 984 feet (300 meters) wide. If that rock hit Earth, it would cause an explosion about 150,000 times as powerful as the Hiroshima bomb.

"Pristine" Recovery

The Japanese agency will get international help tracking Hayabusa's re-entry, noted Donald K. Yeomans, manager of NASA's Near-Earth Object Program based in Pasadena, California.

"The entrance of the Hayabusa spacecraft into the Earth's atmosphere will be tracked before entry by ground-based optical telescopes in an effort to verify the software that has recently been developed by JAXA," he said.

In addition, Yeomans said, ground-based telescopes around the world will watch for the sample-return capsule to help ensure its safe recovery.

Any samples from Itokawa would be a huge boon to science, JAXA's Hashimoto said, because they would be the first "pristine" pieces of asteroid ever recovered.

"We are not sure what we will learn until we have had a chance to analyze it," Hashimoto said, "but at the very least it will teach us many things about the makeup of Itokawa."

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