Radar Images Capture Big, Slowly Tumbling Asteroid

With an average diameter of about 3.5 kilometers (2 miles), 1999 JM8 is the largest near-Earth asteroid ever studied in detail. Although this object can pass fairly close to Earth in celestial terms, astronomers concur that an actual encounter with Earth is not of concern in the next few centuries.

The new images, obtained with NASA's Goldstone Solar System Radar in California and the Arecibo Observatory in Puerto Rico, reveal that 1999 JM8 is a several-kilometer-wide object with a peculiar shape and an unusually slow and possibly complex spin state, said Dr. Lance Benner of NASA's Jet Propulsion Laboratory, Pasadena, CA, who led the team of astronomers. The images are available online at http://photojournal.jpl.nasa.gov or http://echo.jpl.nasa.gov/~lance/1999JM8.html.

"It will take much more data analysis to determine the object's shape and exact rotation state," Benner said. "But just from looking at the images we can see that this nearby world is extremely peculiar. At this point we do not understand what some of the features in the images are, much less how they originated." The asteroid was discovered on May 13, 1999, at a U.S. Air Force telescope in New Mexico that is part of the Lincoln Near Earth Asteroid Research Project, managed by the Lincoln Laboratories of the Massachusetts Institute of Technology. The discovery provided adequate notice for radar observations to be scheduled at Goldstone from July 18 to August 8 and at Arecibo from August 1-9 during the asteroid's close approach to 8.5 million kilometers (5.3 million miles), the equivalent of 22 Earth-Moon distances.

"Although Arecibo is the more sensitive telescope, Goldstone is more fully steerable, and we took advantage of the complementary capabilities of the two antennas," noted Benner. "The discovery of this object weeks before its closest approach was a stroke of luck," he said. "The asteroid won't come this close again for more than a thousand years."

Asteroid 1999 JM8 bears a striking resemblance to Toutatis, a similar-sized, slowly rotating object also studied in detail with radar, said Dr. Scott Hudson of Washington State University, who is an expert in using radar images to determine the shapes of asteroids.

"The fact that both these several-kilometer-wide asteroids are in extremely slow spin states suggests that slow rotators are fairly common among near-Earth asteroids," he said. "However, although collisions are thought to be the primary process that determines asteroid spin states, we don't know how the slow, complex states come about."

The radar imaging technique uses transmissions of sophisticated coded waveforms and computer determinations of how echoes are distributed in range and frequency, instead of their angular distribution, as in normal optical pictures. "Our finest resolution is 15 meters (49 feet) per pixel, which is finer than that obtained for any other asteroid, even for spacecraft" said Dr. Jean-Luc Margot, one of the team members from Arecibo Observatory. "To get that kind of resolution with an optical telescope, you'd need a mirror several hundred meters across. Radar certainly is the least expensive way of imaging Earth- approaching objects."

The images show impact craters with diameters as small as 100 meters (330 feet) -- about the length of a football field -- and a few as large as 1 kilometer (0.6 miles). "The density of craters suggest that the surface is geologically old, and is not simply a chip off of a parent asteroid," said Dr. Michael Nolan, a staff scientist at the Arecibo Observatory. "We also see a concavity that is about half as wide as the asteroid itself, but we're not sure yet whether or not it's an impact crater."

This is hardly the first time that radar has revealed a near-Earth asteroid with peculiar characteristics, said Dr. Steven Ostro of JPL, who has led dozens of asteroid radar experiments. Radar studies have revealed a stunning array of exotically shaped worlds with compositions ranging from solid metal to low-density carbonaceous rock and rotation periods ranging from 11 minutes to more than a week. "These are very, very strange places," he said. "I really envy the coming generations of space explorers who will visit them."

In addition to Benner, Hudson, Margot, Nolan and Ostro, the radar team included Drs. Jon D. Giorgini, Raymond F. Jurgens, Donald K. Yeomans and Martin A. Slade from JPL, and Donald B. Campbell from Cornell University, Ithaca, NY.

The radar observations were supported by NASA's Office of Space Science, Washington, DC. The Goldstone Solar System Radar is part of NASA's Deep Space Network. 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. JPL is a division of the California Institute of Technology, Pasadena, CA.