Mile Long Asteroid Looks Like Hippopotamus, Passes Safely by Earth, Will Return Closer in 2070

Image credit: NASA/JPL-Caltech/GSSR/NSF/GBO

The Green Bank Telescope was the receiver for the powerful microwave signals transmitted by either Goldstone or the NASA-funded Arecibo planetary radar in what is known as a “bistatic radar configuration.” Using one telescope to transmit and another to receive can yield considerably more detail than would one telescope, and it is an invaluable technique to obtain radar images of closely approaching, slowly rotating asteroids like this one.

“The radar images achieve an unprecedented level of detail and are comparable to those obtained from a spacecraft flyby,” said Lance Benner of the Jet Propulsion Laboratory in Pasadena, California, and the scientist leading the observations from Goldstone. “The most conspicuous surface feature is a prominent ridge that appears to wrap partway around the asteroid near one end. The ridge extends about 330 feet [100 meters] above the surrounding terrain. Numerous small bright spots are visible in the data and may be reflections from boulders. The images also show a cluster of dark, circular features near the right edge that may be craters.”

The images confirm what was seen in earlier “light curve” measurements of sunlight reflected from the asteroid and from earlier radar images by Arecibo: 2003 SD220 has an extremely slow rotation period of roughly 12 days. It also has what seems to be a complex rotation somewhat analogous to a poorly thrown football. Known as “non-principal axis” rotation, it is uncommon among near-Earth asteroids, most of which spin about their shortest axis.

With resolutions as fine as 12 feet (3.7 meters) per pixel, the detail of these images is 20 times finer than that obtained during the asteroid’s previous close approach to Earth three years ago, which was at a greater distance. The new radar data will provide important constraints on the density distribution of the asteroid’s interior – information that is available on very few near-Earth asteroids.

“This year, with our knowledge about 2003 SD220′s slow rotation, we were able to plan out a great sequence of radar images using the largest single-dish radio telescopes in the nation,” said Patrick Taylor, senior scientist with Universities Space Research Association (USRA) at the Lunar and Planetary Institute (LPI) in Houston.

These two radar images of near-Earth asteroid 2003 SD220 were obtained on Dec. 18 and 19 by coordinating observations with the Arecibo Observatory’s 1,000-foot (305-meter) antenna in Puerto Rico and the National Science Foundation’s (NSF) 330-foot (100-meter) Green Bank Telescope in West Virginia. The radar images reveal the asteroid is at least one mile (1.6 kilometers) long.