Asteroid hues intrigue UH


POSTED: Friday, February 05, 2010

University of Hawaii astronomers and colleagues are trying to understand the changing colors of asteroids, knowledge they say might help come up with methods to avoid catastrophic collisions with Earth.

“;Someday we are likely to find an object that could pose a threat, and hopefully we will have enough time to do something about it—either blow it up, push it out of the way or somehow get rid of the threat,”; said Schelte Bus, an associate astron-omer at the UH Institute for Astronomy in Hilo.

“;Impact hazard mitigation”; will depend heavily on the internal makeup of the object, which is related to the color, he said.

Bus and UH astronomer Alan Tokunaga are participating in a survey with Massachusetts Institute of Technology scientists using the NASA Infrared Telescope Facility on Mauna Kea to understand the composition of near-earth asteroids. Tokunaga, facility director, said the survey has been going on for about nine years.

“;We are also studying potential targets for landed missions to asteroids,”; he said, noting NASA is thinking about sending astronauts to a near-Earth asteroid.

He said the team is characterizing near-Earth asteroids—their size and surface composition—so NASA can decide what type of asteroid it wants to visit.

The latest findings of the survey were published in the Jan. 21 issue of the journal Nature, with Richard Binzel of MIT as lead author.

Bus, a major team member from MIT who has been with the IFA in Hilo for nine years, said the asteroids, or “;rubble piles,”; are believed to have been created over time from collisions in the main asteroid belt between Mars and Jupiter.

The resulting small asteroids eventually become near-Earth asteroids, and they also form meteorites or rocks that fall to Earth, he said.

“;We believe these things are very heavily fractured,”; he said, as opposed to one monolithic chunk of rock. The rocks and boulders are believed loosely held together by their own gravity, he said.

These objects can be easily jostled when they come close to another body with a strong gravitational field, such as Earth, and the team found those that passed particularly close to Earth in recent times have a pale color, Bus said.

“;We associate their pale color with a relatively fresh or younger surface,”; he said. As surface material is exposed to sunlight or other particles floating in space, either from radiation from the sun or other sources, the material starts to darken and is usually redder, he said.

“;The fresher material that comes to the top of the object through a gravitational encounter or jostling around would be paler in color ... and typically brighter”; because it would reflect sunlight better, he said.

Looking at the objects with a very large telescope, he said, “;All you see is a dot. It looks like a moving star, a small point of light that is moving.”;

One way of analyzing the light is to look at colors to see how objects reflect sunlight at different wavelengths, Bus said.

The color difference reveals the chemical composition of the object, and this information can help determine the method of dealing with an asteroid headed this way, he said.

“;If, in fact, these are solid, monolithic, un-fractured pieces of rock that are internally strong, you would want to treat it in a different way”; from a fragmented rubble pile,”; he said.

“;There are situations where you might not want to use a large amount of explosives. This could actually create more hazard to Earth.”;