Tuesday, December 18, 2012

Asteroid Home Movies

Last week, the asteroid 4179 Toutatis made its quadrennial encounter with Earth. This year marks Toutatis's closet flyby, coming about 4.3 million miles away (about 18 times the distance to the moon) on December 12, 2012. As the 3-mile-long asteroid whizzed by, NASA's 230-foot Deep Space Network antenna in Goldstone, California, captured radar images of the oblong asteroid tumbling through space.

In radar imaging, scientists send pulses of light at microwave frequencies towards the distant asteroid and measure the strength and round-trip time of the returning (reflected) signal. Since the microwaves travel at the speed of light, it's relatively straightforward to calculate signal's round-trip time.  The strength of the signal depends on how rough the asteroid surface is. Brighter areas on the image indicate rougher areas, with more of the scattered light reflecting back towards the detector antenna. Smooth surfaces reflect very little light back towards the detector and appear dark in radar images.

Light reflects differently on different surfaces. The detector collects light that scatters off the surface in the same direction it entered (back to the detector). Image Credit: JPL/NASA
The radar images of Toutatis illuminate the three-dimensional shape of the irregularly shaped asteroid. With image resolutions of 12 feet per pixel, the scientists are able to identify surface features of the asteroid, including what could be surface boulders.

Using the images to study the motion of the asteroid, the scientists report that the asteroid rotates about its long axis every 5.4 days and wobbles, "like a badly thrown football,"  every 7.4 days. Data on the asteroid's rotational motion will help the scientists better understand the interior composition of the asteroid.

NASA's Near-Earth Object Observations program, aka "Spaceguard," tracks asteroids and comets whose pass close to earth and identifies whether their orbits could at some point crash into earth. This is an interesting pursuit since most scientists believe that dinosaurs and many other plant and animal life across the globe were wiped out about 65 million years ago when an asteroid 6-miles wide slammed into earth. But, Toutatis's orbit is well understood; the next time the asteroid will pass this close to earth will be in 2069, and NASA's analysis indicates that there is a zero probability of Toutatis's hitting earth anytime in at least the next four hundred years.

Meanwhile, China also captured images of Toutatis, snapping photos only two miles away from the hurdling asteroid. In 2010, China launched the Chang'e-2 to explore potential landing sites for a Chinese moonlander. When that mission was successfully checked off, the scientists decided to send the spacecraft to fly by the Toutatis asteroid. On December 13, Chang'e-2 reportedly came within 2 miles of the asteroid and acquired detailed images of the asteroid. Scientists have yet to compare these with NASA's radar images.

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