Samples brought back from the Itokawa asteroid indicate that it's made up of the remnants of a much larger body.
The Hayabusa spacecraft was launched by the Japan Aerospace Exploration Agency (JAXA) in 2003, and in November 2005 it made two separate touchdowns on the surface of the Itokawa asteroid.
Although its primary sampler malfunctioned, the spacecraft was able to collect a tiny amount of dust particles - the first ever samples from an asteroid.
Itokawa is a rocky, S-type asteroid that looks rather like a rubble pile.
It's long been suspected that the most common meteorites found here on Earth, known as ordinary chondrites, are born from S-type, asteroids. However, the visible spectra of these asteroids have never precisely matched those of ordinary chondrites.
"Our study demonstrates that the rocky particles recovered from the S-type asteroid are identical to ordinary chondrites, which proves that asteroids are indeed very primitive solar system bodies," says Tomoki Nakamura from Tohoku University in Japan.
The researchers also noticed that Itokawa has gone through significant heating and impact shocks. Based on its size, they conclude that the asteroid is actually made up of small fragments of a much bigger asteroid.
"The particles recovered from the asteroid have experienced long-term heating at about 800 degrees Celsius," says Nakamura.
"But to reach 800 degrees, an asteroid would need to be about 12.4 miles in diameter. The current size of Itokawa is much smaller than that, so it must have first formed as a larger body, then been broken by an impact event and reassembled in its current form."
According to the researchers, the dust from Itokawa has been on the surface of the asteroid for less than eight million years. They suggest that material from such small asteroids might escape easily into space to become meteorites, traveling toward Earth.
"This dust from the surface of the Itokawa asteroid will become a sort of Rosetta Stone for astronomers to use," says Akira Tsuchiyama from Osaka University in Japan.
"Now that we understand the bulk mineral and chemical composition of the Hayabusa sample, we can compare them to meteorites that have struck the Earth and try to determine which asteroids the chondrites came from."