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Radar Investigation of NEAs
Steve Ostro* - Copyright Tumbling Stone 2001
Go out and buy 10,000 100-watt light bulbs and arrange them in 100 rows of 100 bulbs each. Then
plug them in and turn them on. The resulting megawatt of radiated power is close to what the 305meter Arecibo telescope transmits in an asteroid radar experiment. The 70-meter Goldstone
antenna transmits half of that, but it can look at the entire sky while Arecibo can point only within
20 degrees from zenith. These two complementary instruments, which have obtained radar echoes
from dozens of NEAs, are remarkably sensitive: the power that they can detect is less than what is
expended by an ant that climbs up the wall of your room in a time longer than the age of the
universe.
When we do an asteroid radar experiment, we measure the time it takes the echo to return with a
resolution as fine as a tenth of a microsecond, which tells the asteroid's distance with a resolution as
fine as a decameter. We also measure the echo's Doppler frequency shift, which tells us the
asteroid's line-of-sight speed with a resolution that can be as small as the speed of the tip of a
clock's minute hand. The fractional precision of radar astrometry is typically between 10-6 and 108
, making it very powerful in refining asteroid orbits.
The distribution of an asteroid's echo power in time delay and Doppler frequency forms a twodimensional image whose geometry can be visualized as follows. The radar observation cuts the
asteroid into rectangular cells using two sets of parallel planes, one set parallel to the plane of the
sky and one set parallel to both the line of sight and the asteroid's rotation pole. Within any given
cell, there can be surface regions in both the northern and southern "hemispheres" that give echo, so
the image contains ambiguities that do not afflict normal optical pictures.
Fortunately, if a sequence of radar images can be obtained with the asteroid in a lot of orientations,
then one can solve for both the shape and the spin state. Scott Hudson first figured out how to do
this, and the result has been publication of computer models for the shapes of several asteroids,
most of which also are available as replicas that you can hold in your hand.
Radar observations have revealed a variety of exotic NEAs, including a two-kilometer-wide piece
of metal ( 6178 (1986 DA)), a world shaped like a paramecium ( Geographos), asteroids in nonprincipal-axis spin states ( Toutatis and 1999 JM8), contact-binary shapes (e.g., Castalia) and
binary systems (e.g., 1999 KW4), and one tiny, rapidly rotating, spheroidal, carbonaceous monolith
that is a prime candidate for people to visit and eventually assimilate ( 1998 KY26).
*Steve Ostro
Jet Propulsion Laboratory
California Institute of Technology