Earth-based radars are powerful tools for the investigation of asteroids and comets. The radar systems on the recently upgraded Arecibo telescope, operated by Cornell for the National Science Foundation, and the NASA/Deep Space Network 70-meter Goldstone antenna in California have the capacity to image Earth-approaching asteroids or comets with resolutions as small as 15 meters (50 feet) and to provide measurements of their distance and velocity, allowing their future orbits to be very accurately predicted. Measurements of radar reflection properties provide information about asteroids' surface bulk density, surface roughness and rotation state.
Radar is also one of the few means to investigate cometary nuclei directly. Seven have been detected to date by the Arecibo and Goldstone telescopes. Given the recent improvements in Arecibo's sensitivity, researchers are awaiting the next close approach of a comet to the Earth to obtain high-resolution radar images of its nucleus using a powerful tool called delay-Doppler radar mapping. Ten short-period comets are candidates for detection with radar during the next two decades, but it also is anticipated that several as yet undiscovered comets will be observed passing through the solar system.