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Peanut-shaped asteroid ‘1999 JD6’ zoomed past Earth last weekend

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An asteroid – appearing to be a contact binary – zoomed past Earth last weekend at a distance of about 4.5 million miles (7.2 million kilometers) – near enough for NASA to capture images of the asteroid.

NASA astronomers used the 230-foot-wide (70-meter) Deep Space Network antenna at Goldstone, California and the 330-foot (100-meter) National Science Foundation Green Bank Telescope in West Virginia and bounced off radar signals off the passing asteroid to capture and produce images of peanut-shaped space rock.

A contact binary is a type of asteroid that has two lobes stuck together – similar to the comet 67P which the European Space Agency is investigating through its Rosetta mission.

Astronomers used the Goldstone antenna to beam radar signals at the asteroid, named 1999 JD6, and received the reflections using Green Bank. This particular technique, called the bistatic observation, dramatically improves the amount of detail that can be seen in radar images. The new views obtained with the technique show features as small as about 25 feet (7.5 meters) wide.

The individual images used in the movie were generated from data collected on July 25. They show the asteroid is highly elongated, with a length of approximately 1.2 miles (2 kilometers) on its long axis. The movie spans a period of about seven hours, 40 minutes.

Radar images of asteroid 1999 JD6 were obtained on July 25, 2015. The asteroid is between 660 - 980 feet (200 - 300 meters) in diameter. Credits: NASA/JPL-Caltech/GSSR

Radar images of asteroid 1999 JD6 were obtained on July 25, 2015. The asteroid is between 660 – 980 feet (200 – 300 meters) in diameter.
Credits: NASA/JPL-Caltech/GSSR

Last week’s flyby was the closest approach the asteroid will make to Earth for about the next 40 years. The next time it will approach Earth this closely is in 2054, at approximately the same distance of this week’s flyby.

“Radar imaging has shown that about 15 percent of near-Earth asteroids larger than 600 feet [about 180 meters], including 1999 JD6, have this sort of lobed, peanut shape,” said Lance Benner of NASA’s Jet Propulsion Laboratory in Pasadena, California, who leads NASA’s asteroid radar research program.

Despite the uncertainty about its size, asteroid 1999 JD6 has been studied extensively and many of its physical properties, as well as its trajectory, are well known. It rotates in just over seven-and-a-half hours and is thought to be a relatively dark object. Asteroid 1999 JD6 was discovered on May 12, 1999, by the Lowell Observatory Near-Earth-Object Search, located in Flagstaff, Arizona.

Radar is a powerful technique for studying an asteroid’s size, shape, rotation, surface features and surface roughness, and for improving the calculation of asteroid orbits. Radar measurements of asteroid distances and velocities often enable computation of asteroid orbits much further into the future than would be possible otherwise.