Radar Observations of Asteroids


RADAR stands for RAdio Detection And Ranging.  Basically you transmit a radio wave towards some object, the wave reflects from the object, and some of the reflected wave makes its way back to be received at the radar antenna.  By receiving the reflected wave you've performed detection, in other words you've detected that something is out there.  By measuring how long it takes the wave to go to the object and back - the time delay - you can figure out how far away the object is.  That's the ranging part.  Additionally, if the object is moving then the frequency of the radio wave changes.  Measuring this Doppler shift and combining that with the delay information allows us to form two dimensional delay-Doppler images.  These are one of the most powerful sources of information we can obtain about the physical properties of an asteroid.

A radar telescope is essentially a radio telescope with the addition of a transmitter.  Although there are many radio telescopes, there are pretty much only two radar telescopes in the world today that can produce useful delay-Doppler images of asteroids.  They are ...
 
The Goldstone Solar System Radar is part of NASA's Deep Space Network.  Located near Barstow California, the largest dish is the fully-steerable, 70 meter DSS-14 (shown foreground at right).  This system can transmit 500 kW at 8510 MHz (3.5 cm).  Reception can be performed at DSS-14 or at DSS-13 (shown background at right).  This allows bistatic operation where transmission (at DSS-14) and reception (at DSS-13) is continuous.
 
photo courtesy NASA

 
The Arecibo Radar Telescope is operated by the NAIC.  This fixed-dish, 300 meter antenna is located near Arecibo, Puerto Rico.  It can transmit 1 MW at 2380 MHz (12.6 cm).  It has primarily operated in a monostatic mode (switching between transmit and receive) but bistatic operation with the NRAO's Greenbank antenna is possible.
Photo courtesy NAIC

Radar studies of asteroids began with the detection of 1566 Icarus in 1968, but delay-Doppler imaging with enough resolution to perform detailed physical modeling first occurred in 1989 when a team led by Steven Ostro of the Jet Propulsion Laboratory used the Arecibo radar to image 4769 Castalia.

Delay Doppler Images

Here's a brief description of the process of radar delay-Doppler imaging.  It glosses over lots of things, but the essential ideas are here.

Think of a radar transmitter as shooting out a bunch of green balls. These balls are chucks of radio-wave energy called photons. We're going to show just a few photons but a real planetary radar system sends out a HUGE number - as many as a million trillion trillion photons every second!  The balls travel millions of miles through space until they encounter the asteroid. Different balls will hit the asteroid in different places. Since some parts of the asteroid are nearer to the radar than other parts, the balls will get reflected at different times.Since the asteroid is spinning, some parts of the surface are moving towards the radar and some parts are moving away. Balls that hit the parts moving away get stretched a little (shown as turning red in the animation) while those that hit the parts moving towards the radar get scrunched (turn blue).  The balls, with different delays and different colors return to the antenna.