The approach of large asteroids near the SD220 2003 provides a good opportunity to get detailed radar images of their shapes and surfaces and to improve their understanding of their orbits.
Asteroids reach a maximum approach of 2.9 million kilometers on December 22, the closest in more than 400 years and until 2070, when something else approaches but is without risk to our planet.
Radar images reveal an asteroid at least 1.6 kilometers long and a shape similar to the exposed portion of hippopotamus that crosses the river.
They were obtained from December 15-17 by coordinating observations with NASA's 70-meter antenna at Goldstone, the 100-meter Bank Green Telescope. The National Science Foundation, and antenna 305 meters from the Arecibo Observatory in Puerto Rico.
The Green Bank Telescope is the recipient of strong microwave signals transmitted by NASA's Arecibo-funded planetary radar or Goldstone, in what is known as a "bistatic radar configuration". The use of telescopes to transmit and others to receive can produce far more detail than telescopes, and it is a valuable technique to get radar images of asteroids that approach and rotate slowly like this.
"Radar images reach unprecedented levels of detail and can be compared to those obtained by excessive spacecraft flight," said Lance Benner, Jet Propulsion Laboratory in Pasadena, California, who was responsible for observing Goldstone.
"The most visible surface feature is a prominent ridge that seems to wrap a portion of the asteroid near one end. The ridge extends about 100 meters above the surrounding area. Many bright little spots visible in the data and can be reflections of the Rock also show a group of dark circular features near the right edge that can be a crater. "
The images confirm what was seen in previous measurements of sunlight reflected by asteroids and from previous Arecibo radar images: 2003 SD220 has a very slow rotation period of around 12 days. It also has what appears to be a complex rotation which is somewhat analogous to badly launched football. Known as the "non-main axis" rotation, it rarely occurs between near-Earth asteroids, which mostly revolve around its shortest axis.
With a resolution as smooth as 3.7 meters per pixel, the details of these images are 20 times finer than those obtained during the asteroid approach to Earth three years ago, which are at a greater distance. New radar data will provide important limitations in the distribution of interior density of asteroids, information available on very few asteroids near the Earth.
"The new details that we found, up to the geology of SD220 2003, will enable us to reconstruct the shape and rotation, as was done with Bennu, the mission objectives of OSIRIS-REx," said Edgar Rivera-Valentin, USRA scientist at LPI. "Detailed reconstruction of forms allows us to better understand how this small body is formed and develops over time."
The SD220 2003 asteroid was discovered on September 29, 2003 by astronomers at Lowell Observatory. It is classified as a "potentially dangerous asteroid" because of its size and close to Earth's orbit. However, this radar measurement further enhances the understanding of the SD220 2003 orbit, which confirms that it does not pose a threat to future impacts on Earth.