Ostro will describe the increasingly important use of radar for imaging when he moderates one of the daily press conferences (Monday, July 26, 10 a.m., 305 Ives Hall) during the ACM. Ostro will describe how radar is being used to construct geologically detailed three-dimensional models of distant bodies, including 37 main-belt asteroids and 50 near-Earth asteroids.

High-resolution imaging has been obtained for a number of near-Earth objects (NEOs) with the most impressive results being obtained for asteroid 1998 KY26, asteroid Toutatis and Asteroid 1992 SK.

Donald Campbell, professor of astronomy at Cornell, and Ostro also will describe plans for the use of the Very Long Baseline Array (VLBA), in conjunction with Arecibo Observatory and JPL/Goldstone radars, to image near-Earth asteroids and comets. The VLBA will be able, for the first time, to obtain images with resolutions of about 100 meters.

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.

In recent years there has been an increase in the number of active meteor radar studies ranging in sophistication from systems that use existing commercial television broadcasts to special multifrequency transmissions.

High-power, low-frequency radar studies of meteor trails forming in the Earth's atmosphere have been carried out for many years at the AMOR radar in the Southern Hemisphere, but recently several studies incorporating the powerful UHF radars at Arecibo and the European Incoherent Scatter facility have routinely shown the behavior of the dense electron clouds, known as head echoes, that occur around even tiny high-speed particles, called micrometeors.

While many of these and other facilities have been used to study the famous Leonid shower, much progress is being made on the nonuniform structure of other meteoroid streams. The first inference of the masses and sizes of micrometeors using observations of deceleration using the Arecibo radar and the detection of extremely high velocity, and therefore apparently interstellar, micrometeors using the AMOR and Arecibo radars represent two of the more unusual radar meteor topics being discussed at ACM.