[meteorite-list] Composition of Apophis Matches LL Chondrite Meteorites

[Ron Baalke]

MIT: Asteroid is "practice case" for potential hazards

For Immediate Release       FRIDAY, OCT. 12, 2007

Contact:
Elizabeth A. Thomson, MIT News Office
Phone: 1-617-258-5402     Email:  thomson at mit.edu

CAMBRIDGE, Mass.--In research that could aid decisions about future 
asteroids on a collision course with Earth, MIT researchers have for 
the first time determined the composition of a near-Earth asteroid 
that has a very slight possibility of someday hitting our planet.

That information could be useful in planning any future space mission to 
explore the asteroid, called Apophis. And if the time ever were to 
come when this object or another turned out to be on its way toward an 
impact on Earth, knowing what it's made of could be one important factor 
in deciding what to do about it.

"Basic characterization is the first line of defense," says Richard P. 
Binzel, Professor of Planetary Sciences in the Department of Earth, 
Atmospheric, and Planetary Sciences (EAPS). "We've got to know the enemy."

Binzel presented the new findings this week at the annual meeting of the
Division for Planetary Sciences of the American Astronomical Society.

Studying the composition of Apophis has been a useful "practice case," 
Binzel says, because "you never know when the real one will come along" 
that is on a collision with the Earth. For determining the composition 
of a threatening asteroid, Binzel says, "We don't know when the real 
test will come, but we're ready."

On April 13, 2029, Apophis will come relatively close to Earth (it will 
miss us by about 22,000 miles). But when it comes by again in 2036, 
there is a very small possibility - about one chance in 45,000 - that 
it could be on a collision course.

So Binzel, working with EAPS graduate students Cristina Thomas and Francesca 
DeMeo and others, has been using telescopes on Earth to find
out as much as possible about the nature of Apophis and other asteroids. 
Short of putting together a space mission that would take years and cost 
hundreds of millions of dollars, such observations are the best way to 
find out as much as possible about any space rock that might someday be 
coming our way, Binzel says.

Using the MIT Magellan telescope in Chile and NASA's Infrared Telescope 
Facility in Hawaii, they have now been able to figure out exactly what 
Apophis is made of. "The composition, I think, is really nailed," he says.

The key to understanding the mineral makeup of an asteroid is to compare it 
with samples of asteroidal material that have been delivered, free of 
charge, to the Earth, in the form of the many thousands of meteorites 
that have been collected over the years.

Spectral analysis - measuring how the meteorites reflect light of different
wavelengths - can be used to determine their exact mineral constituents. 
Similarly, a spectral analysis of the light reflected from a distant
asteroid shows the same telltale lines that reveal its composition. By 
comparing the two kinds of spectra, an asteroid that is just a faraway 
pinprick of light can be correlated with a piece of a space rock in the 
laboratory.

Binzel and his students were able to use both visible-light and infrared
spectroscopy to show that Apophis is "a good match" for a rare type of
meteorite, known as a type LL chondrite. These represent just 7 percent of 
the known meteorite falls on Earth, and are rich in the minerals pyroxene 
and olivine, which are also common on Earth.

"The beauty of having found a meteorite match for Apophis is that because we
have laboratory measurements for the density and strength of these 
meteorites, we can infer many of the same properties for the asteroid 
Apophis itself," Binzel says.

An object the size of Apophis (about 270 meters across) could devastate a 
region as large as France, or produce tsunamis over a wide area if it 
struck at sea. Many ideas have been proposed for how to deal with
such a threat, ranging from using bombs, lasers or spacecraft to nudge it 
out of the way to blowing it to pieces while it is still far away. The 
selection of the best course of action may depend of the physical 
characteristics of the object, including its mineral composition.

--END-

Written by David Chandler, MIT News Office