Resource Topics
	Earth, Moon, and the Sun  The Solar System  Space Probes and Human Exploration  Stars and Nebulae  Astronomers and Observatories  Galaxies and Cosmology  Stargazing and Star Lore
	Resources
	Astronomy Gift Shop
	2009 Sky Almanac McDonald Observatory logo cap

Studies of Meteorites and Asteroids May Help Protect Earth from Impacts
(From the November/December 2008 issue of StarDate magazine)

A long-term study of the chemical make-up of asteroids by MIT's Richard Binzel may help prevent catastrophic Earth impacts, and has solved a puzzle on the origin of most meteorites. The findings appeared in a recent issue of the journal Nature.

Binzel's studies of the chemistry of near-Earth asteroids, whose orbits pose the greatest impact risk, have identified the type of asteroid most likely to threaten Earth.

His team compared the chemical compositions of near-Earth asteroids to that of thousands of meteorites — small asteroid fragments that already have smashed into Earth. They found that two-thirds of near-Earth asteroids match the chemical composition of stony meteorites called LL chondrites, which contain large amounts of the mineral olivine and not much iron.

"Odds are, an object we might have to deal with would be like an LL chondrite, and thanks to our samples in the laboratory, we can measure its properties in detail," Binzel says. "It's the first step toward ‘know thy enemy.'"

Knowing the properties of a potential Earth-impacting asteroid will help scientists figure out how to deflect or destroy it.
LL chondrites make up only eight percent of meteorites. That means most meteorites are not chips off of near-Earth asteroids. Rather, their compositions more closely match asteroids orbiting in the main belt between Mars and Jupiter.

"Why do we see a difference between the objects hitting the ground and the big objects whizzing by?" Binzel asks. The answer lies in the power of the Sun's heat.

Asteroids orbiting in the main belt absorb the Sun's heat on one side, and re-radiate it as they rotate, causing an imbalance called the "Yarkovsky effect." Building up over time, it can change an asteroid's orbit. The effect is strong on small asteroids, sending them careening into the inner solar system to crash to Earth as meteorites. This effect is weak on the more massive, larger main-belt asteroids that might be a threat to Earth. — Rebecca Johnson