Asteroid Yields Its Secrets

New insights about how Earth and other planets formed to create our solar system are expected to follow from the first detailed analyses of particles snatched from the surface of an asteroid by a Japanese space mission. Chemical evidence is helping to confirm the link between asteroids and meteorites, space rocks that have fallen to Earth.

The Hayabusa mission, developed and managed by the Japanese Aerospace Exploration Agency, in 2003 sent a spacecraft to a near-Earth asteroid called 25143 Itokawa. Two years later the mission spacecraft reached the asteroid, which it then sampled. The craft finally returned to Earth in 2010, landing in South Australia and carrying with it some 1,500 particles from Itokawa. Several groups report in Science on their studies of the particles (DOI: 10.1126/science.1207758, 10.1126/science.1207776, 10.1126/science.1207865, 10.1126/science.1207794, 10.1126/science.1207807, 10.1126/science.1207785).

Itokawa is known as an S-type asteroid, a silicaceous, rocky variety common to the inner solar system. The chemistry of the Itokawa particles, report earth and planetary sciences professor Tomoki Nakamura of Tohoku University, in Miyagi, and colleagues, is identical to that of ubiquitous ancient meteorites known as chondrites—helping to confirm the long-held theory that chondrites come from S-type asteroids. Because chondrites are some of the most primitive objects known, their link to S-type asteroids “proves that asteroids are indeed very primitive solar system bodies,” Nakamura said in a statement.

In the past, scientists had to rely on lunar samples to make inferences about the nature of weathering on asteroids. But with the Itokawa particles, the researchers now have in hand direct evidence of asteroid weathering. They were able to show that weathering, caused by forces that include asteroid interactions with solar wind and micrometeorites, is different on Itokawa than on the moon. In fact, the researchers deduce, Itokawa is losing surface particles at a rate that will result in the asteroid’s disappearance in about a billion years.

The Hayabusa mission demonstrates that “solar system samples returned for study in terrestrial laboratories are crucial in understanding the origin and evolution of the solar system,” notes University of Hawaii researcher Alexander N. Krot in an essay accompanying the papers.

Sample return missions are still relatively rare because of the difficulty of safely navigating craft back to Earth. There have been only two other sample return missions to destinations other than the moon: NASA’s Genesis, which returned samples of solar wind in 2004, and NASA’s Stardust mission, which returned samples of the comet Wild 2 in 2006. Indeed, both Genesis and Hayabusa encountered problems. Genesis crashed to Earth, leaving only small amounts of uncontaminated material, and Hayabusa had difficulties with its sampling mechanism.