[meteorite-list] Radar Echoes Reveal Asteroid Details

[Ron Baalke]

http://www.astronomy.com/Content/Dynamic/Articles/000/000/001/733pkmrh.asp

Radar echoes reveal asteroid details 

Bouncing radio beams give scientists a surprising glimpse of 
near-Earth asteroids. 

by Andrew Fazekas 
astronomy.com
May 21, 2004

Using powerful radar observations to produce strikingly detailed 
images, scientists are just beginning to reveal the menagerie of 
potentially hazardous near-Earth asteroids (NEAs). A team led by 
Steven Ostro of the Jet Propulsion Laboratory in Pasadena, 
California, combined the radar muscle of the Arecibo telescope in 
Puerto Rico with NASA's Deep Space Network antenna in California. 
The asteroid echoes the group has captured reveal surface details 
as small as tens of meters. To obtain a similar resolution with 
optical telescopes would require mirrors hundreds of meters across. 

Snapping a series of ghostly images over time allowed the 
researchers to determine the spin rate of each asteroid. This 
technique also enabled them to produce geologically detailed 
three-dimensional models of each object. Over 230 asteroids have 
been studied with radar so far, and the emerging picture shows a
surprisingly diverse set of objects. Their sizes and rotation 
periods vary by a factor of 10,000, while their composition ranges 
from stony to metallic, and shapes run from "nearly featureless 
spheroids" to highly elongated bodies. At the smallest scales, say 
the scientists, NEAs can be smoother than the powdery surface of 
our Moon or rougher than the rockiest terrain on Mars. 

The group's technique allows them to analyze how the radar echoes 
are distributed in range and frequency, instead of their angular
distribution, as in normal optical pictures. As a consequence, in 
addition to generating detailed images, the JPL team also can 
measure the distance and velocity of the asteroids with high 
precision, thus allowing scientists to predict the objects' 
future orbits accurately.

More precise than optical measurements alone, calculations of 
refined trajectories and collision-probability estimates are now
possible, according to Ostro. "These radar-based astrometric 
measurements can offer us as much as one-third more warning time 
for any asteroid collision," he says. That could mean decades or 
even centuries of advance notice before any close approaches. 

This detailed radar reconnaissance not only will be used as an 
early detection system for Earth-orbit crossers, but also will 
benefit directly any robotic or manned missions to asteroids 
planned for the near future. The team presented its findings at 
the 2004 Joint Assembly of the American and Canadian Geophysical 
Unions, held this week in Montreal, Canada.