Photograph courtesy JAXA/ISIS

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The Hayabusa sample-return capsule landed in South Australia last summer.

Photograph courtesy JAXA/ISIS

Huge Asteroid Impact Formed "Rubble Pile" Space Rock

Discovery among first results from Hayabusa spacecraft's dust sample.

An asteroid larger than the Empire State Building is made from the guts of an even bigger space rock that broke apart in a cataclysmic impact, a new study finds.

The discovery is among the first scientific results based on dust samples collected by Hayabusa, a Japanese spacecraft that visited the asteroid 25143 Itokawa.

Itokawa is what's known as a rubble-pile asteroid—a body with such low density that the object can't be a solid hunk of rock and instead is most likely made from bits of debris held together by gravity. (See "Shaking Asteroid Sorts, Instead of Sheds, Its Rubble.")

Launched in 2003 by the Japan Aerospace Exploration Agency (JAXA), Hayabusa arrived at Itokawa in 2005. The craft returned to Earth last June, sending its sample-collection capsule to land in the Australian outback while the main body was destroyed in the fiery reentry.

Scientists collected more than 1,500 grains of dust from the sample chamber, and an international team set to work analyzing the particles. (See "First Asteroid Dust Brought to Earth Holds Clues to Planet Birth.")

Tomoki Nakamura, a planetary scientist at Tohoku University in Japan, and colleagues found mineral signatures that indicate Itokawa's rock had once been heated to more than 1,472 degrees Fahrenheit (800 degrees Celsius) for an extended period of time.

In general, asteroids are heated by the decay of radioactive atoms scattered throughout their insides. Asteroids smaller than 12.4 miles (20 kilometers) lose heat too quickly through their surfaces to sustain such a high temperature, Nakamura explained in an email.

Since Itokawa is just 0.2 mile (0.3 kilometer) long, the asteroid must be "made of reassembled pieces of the interior portions of a once larger asteroid," the team writes in their paper.

Itokawa's Asteroid Pieces Felt the Heat

Some of the rock samples from Itokawa also appear to have been partially melted during a high-speed impact—perhaps the same collision that shattered Itokawa's parent asteroid.

"We found evidence of several impact-melted portions in some particles" that had been exposed to temperatures exceeding 1,832 degrees Fahrenheit (1,000 degrees Celsius) for a very short period of time, Nakamura said.

The team has not yet determined how long ago the impact occurred, but they hope to date the impact-melted rocks soon, he added.

It's also unclear what type of object the impactor was, though it was "probably another asteroid," Nakamura said.

The finding may not be a total surprise, since scientists already think many of the solid, rocky asteroids that litter our solar system are broken pieces of larger objects that had collided and broke apart.

"Our finding suggests that low-density asteroids also might have formed through impact and [reassembly] processes," Nakamura said.

The asteroid-dust research is detailed in this week's issue of the journal Science.