Image of asteroid 1998JM8 taken from data collected by the Arecibo Observatory on August 5, 1999.

Using the radar systems at the National Science Foundation's recently upgraded radio/radar telescope at Arecibo, Puerto Rico, and at NASA's Goldstone Solar System Radar in California, astronomers have obtained the most-detailed pictures yet of an asteroid which passed within 5.3 million miles of Earth earlier this month.

The radar images of this Earth orbit-crossing asteroid, known as 1999 JM8, reveal a several-mile-wide object with a peculiar shape and an unusually slow and possibly complex spin state, says Lance Benner, of NASA's Jet Propulsion Laboratory (JPL), Pasadena, Calif., who led the team of astronomers.

"The photographs of this asteroid are phenomenal," says Donald Campbell, Cornell University professor of astronomy and associate director of the National Astronomy and Ionosphere Center (NAIC) at Cornell, which manages the Arecibo Observatory, for the National Science Foundation. "This is one of the clear firsts for the telescope and radar upgrade."

Scott Hudson, of Washington State University, an expert in using radar images to determine the shapes of asteroids, added that at this stage of the analysis, the resemblance of 1999 JM8 to Toutatis, a similar sized, slowly rotating object also studied in detail with radar, is striking. Why these asteroids and perhaps others rotate so slowly is not understood. "Although collisions between asteroids are thought to be the primary process that determines asteroid-spin states, we don't know how the slow, complex states come about," he says.

The images show impact craters with diameters as small as 100 meters and a few as large as one kilometer. "The density of craters suggest that the surface is geologically old, and is not simply a 'chip' off of a parent asteroid," said Michael Nolan, a staff scientist at the Arecibo Observatory. "We also see a concavity that is about half as wide as the asteroid itself, although we're not sure yet whether or not it's an impact crater."

The pictures may not have been possible before Arecibo's upgrade, financed by the NSF and the National Aeronautics and Space Administration (NASA). Fitted with a powerful new 1 million-watt radar transmitter, it is now possible for the Arecibo radar system to capture detailed images of these kinds of objects at greater distances than in the past.

Provided

Aerial view of Arecibo Observatory in Puerto Rico.

Originally, the object was found in 1990 by Eleanor Helene of NASA's Jet Propulsion Laboratory, who used the Palomar Observatory on Palomar Mountain, Calif. It was dubbed 1990 HD1, and subsequently it went unseen until last spring. On May 13, 1999, the Massachusetts Institute of Technology/Lincoln Labs Near-Earth Asteroid (LINEAR) search program re-discovered the object and the Minor Planet Center in Cambridge, Mass., designated it 1999 JM8.

Realizing that 1999 JM8 would make a good radar telescope target, astronomers Lance Benner and Steve Ostro of JPL organized observations and radar data collection with the Arecibo and Goldstone telescopes. Ostro, who has led dozens of asteroid radar experiments, noted that radar has revealed a stunning array of exotically shaped worlds.

In early August, Benner and Arecibo astronomers Nolan and Jean-Luc Margot assisted by observatory staff took images of the asteroid with the Arecibo radar system at a resolution of 15 meters (50 feet). At its 5 million-mile distance it took the radar signal about one minute to travel to the asteroid and back to the earth. Arecibo's huge receiving dish captured the echo and stored the data. In all, about 70 to 100 gigabytes of data were collected requiring many days of computer processing to generate the images.

"This is the first good opportunity for radar imaging an asteroid in a very long time," says Margot. "You don't get these kinds of objects passing near the Earth everyday."

In addition to Benner, Campbell, Hudson, Margot, Nolan and Ostro, the radar team included Jon D. Giorgini, Raymond F. Jurgens, Donald K. Yeomans and Martin A. Slade, who are all from JPL.

The radar observations were supported by NASA's Office of Space Science in Washington. The Arecibo Observatory, in Puerto Rico, is part of the National Astronomy and Ionosphere Center, which is operated by the Cornell University under a cooperative agreement with the National Science Foundation and with support from NASA. The Goldstone Solar System Radar is part of NASA's Deep Space Network. JPL is operated by the California Institute of Technology, Pasadena, Calif.