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THANKS to a solar flare, astronomers have made the first ever measurements of the composition of an asteroid. Their findings link the asteroid to the primordial Universe, and should help researchers decode a wealth of information from meteorites they have collected on Earth.

Meteorites often originate as asteroids, and by analysing them researchers can tell what elements the parent asteroid contained. But this doesn’t reveal how abundant the elements originally were, because it is impossible to tell precisely where the fragment came from. Earlier this year, however, the Near Earth Asteroid Rendezvous (NEAR) spacecraft reached 433 Eros, an asteroid roughly 34 kilometres long that orbits more than 150 million kilometres farther out than the Earth.

NEAR is equipped with instruments that measure solar X-rays reflecting off Eros. These measurements reveal which elements are present and how abundant they are. Under normal conditions, the reflected X-rays are very weak, so it can take months to collect enough X-rays to make an accurate reading-just as a camera shutter has to be kept open for a longer period in poor light. However, during a solar flare the Sun gives off an extra-large dose of X-rays, acting like a flashbulb. By good fortune, such an event happened on 4 May, allowing researchers to accurately determine within the space of 30 minutes the composition of a section of Eros about 6 kilometres across.

If the asteroid had been altered by heat since its formation-perhaps during the Universe’s infancy-melting would have caused lighter materials such as oxygen to accumulate on the surface. Instead, researchers found heavy materials such as iron evenly mixed with lighter ones. This suggests that Eros has not undergone this type of change. “It’s reasonable to guess that Eros is a leftover building block from the very first stage of Solar System formation,” says Steven Squyres, an astronomer at Cornell University in Ithaca, New York, who is a member of the NEAR X-ray team.

Squyres says the findings are already helping the team find meteorites with similar origins. Richard Binzel, an astronomer at the Massachusetts Institute of Technology who studies asteroids, says making such connections is important. “As we begin to learn where in the Solar System meteorites come from, we’ll be able to use meteorites as detailed snapshots of what our Solar System was like at the time it formed,” he says.

Unfortunately, the solar flare only helped reveal a small portion of Eros. The remainder is now being studied using normal daylight, which is likely to take months.


Author: Mark Schrope

New Scientist issue: 10th June 2000