The proximity of December 2018 to the great asteroid 2003 SD220, near planet Earth, provided astronomers with an excellent opportunity to obtain detailed radar images of the surface and shape of the object and to improve the understanding of its orbit.
The asteroid will fly safely beyond Earth on Saturday, Dec. 22, at a distance of about 2.9 million kilometers. This will be the closest approach to the asteroid in more than 400 years and the closest to 2070, when the asteroid will safely approach Earth a little closer.
Radar images reveal an asteroid with a length of at least 1.6 km and a shape similar to that of the exposed portion of a hippopotamus crossing a river. They were obtained between December 15 and 17 coordinating the observations with the 70-foot antenna of the Goldstone Deep Space Communications Complex in California, the 330-foot (100-meter) National Science Foundation Green Bank Telescope in the West. Virginia and the 300-foot (305-meter) antenna at the Arecibo Observatory in Puerto Rico.
The Green Bank Telescope was the receiver of the powerful microwave signals transmitted by Goldstone or NASA-funded Arecibo planetary radar in what is known as "bi-static radar configuration." Using a telescope to transmit and one to receive can yield considerably more detail than a telescope, and is an invaluable technique for obtaining asteroid radar images that approach slowly, slowly rotating like this.
"Radar images reach an unprecedented level of detail and are comparable to those obtained on spacecraft flies," said Lance Benner of the Jet Propulsion Laboratory in Pasadena, Calif., And the scientist who conducted Goldstone's observations. "The most visible surface feature is a prominent ridge that seems to partially envelop the asteroid near one end. The ridge extends for about 330 feet [100 meters] above the surrounding terrain. Numerous small bright spots are visible in the data and can be reflections of boulders. The images also show a set of dark, circular features near the right edge, which may be craters. "
The images confirm what was seen in previous measurements of the "light curve" of the asteroid's reflected light and previous radar images by Arecibo: 2003 The SD220 has an extremely slow rotation period of approximately 12 days. It also has what appears to be a complex spin, something analogous to poorly played football. Known as "non-main axis" rotation, it is unusual among asteroids close to Earth, most of which revolve around their shortest axis.
With resolutions as good as 12 feet (3.7 meters) per pixel, the detail of these images is 20 times better than that obtained during the previous approach of the asteroid with Earth three years ago, which was at a greater distance. The new radar data will provide important restrictions on the distribution of the asteroid's interior density – information that is available in very few asteroids close to Earth.
"This year, with our knowledge of the slow rotation of the 2003 SD220, we were able to plan a large sequence of radar imagery using the largest single-plate radio telescopes in the country," said Patrick Taylor, senior scientist at the University Association for Space Research (USRA) at the Lunar and Planetary Institute (LPI) in Houston.
"The new details we've discovered, down to the 2003 SD220 geology, will allow us to rebuild its shape and rotation, as was done with Bennu, the target of the OSIRIS-REx mission," said Edgard Rivera-Valentin, USRA scientist at LPI. "The detailed reconstruction of the shape allows us to better understand how these small bodies have formed and evolved over time."
Patrick Taylor conducted the bi-static radar observations with the Green Bank Observatory, home to the Green Bank Telescope, the largest fully orientable radio telescope in the world. Rivera-Valentín will lead the shape reconstruction of the 2003 SD220 and lead the observations of the Arecibo Observatory.
The Asteroid 2003 SD220 was discovered on September 29, 2003 by astronomers from the Lowell Near-Earth-Object Search Observatory (LONEOS) in Flagstaff, Arizona – a NASA-supported Early Earth Object (NEO) research project that is no longer in operation. It is classified as being a "potentially dangerous asteroid" because of its size and proximity to Earth's orbit. However, these radar measurements further refine the understanding of the 2003 SD220 orbit, confirming that it does not pose a future threat to the impact on the planet.
The Arecibo, Goldstone and USRA planetary radar projects are funded by NASA's Own Object Observation Program within the Office of Planetary Defense Coordination (PDCO), which manages the Agency's Planetary Defense Program. The Arecibo Observatory is a facility of the National Science Foundation operated under a cooperative agreement between the University of Central Florida, Yang Enterprises and the Metropolitan University. GBO is a facility of the National Science Foundation, operated under a cooperative agreement with Associated Universities, Inc.
JPL hosts the Near-Earth Object Studies Center (CNEOS) for NASA's Own Object Observation Program.
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