[meteorite-list] Radar Echoes Reveal Asteroid Details

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Fri May 21 11:58:09 2004
Message-ID: <200405211558.IAA02007_at_zagami.jpl.nasa.gov>

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.
Received on Fri 21 May 2004 11:57:58 AM PDT