A mile-long asteroid that raced past Earth July 25 at about 45,000 miles per hour – at a safe distance of 4.5 million miles – was imaged by radar telescopes so that astronomers like Cornell’s Sean Marshall could discern its precise orbit and physical shape.

Using NASA’s Deep Space Network antenna at Goldstone, California, and the National Science Foundation’s Green Bank Telescope in West Virginia, the space agency’s scientists bounced radar signals off the passing asteroid – named 1999 JD6 – to produce images of the peanut-shaped celestial body as it zipped by Earth. The Goldstone radar signal was transmitted toward the asteroid, and the Green Bank Telescope received the radar echoes.

Marshall then used the National Science Foundation’s Arecibo Telescope in Puerto Rico on July 29 to re-examine the asteroid in further detail.

“I’m interested in this particular asteroid because estimates of its size from previous observations – at infrared wavelengths – have not agreed,” said Marshall, a Cornell doctoral student in the field of astronomy. “The radar data will allow us to conclusively resolve the mystery of its size to better understand this interesting little world.”

Jason Koski/University File Photo

Sean Marshall, a doctoral student in astronomy, demonstrates the celestial mechanics of the solar system, during a break at the Carl Sagan Institute's inaugural symposium May 9.

Radar images of asteroids – gathered from millions of miles away in some cases – have resolutions as small as 12 feet, with 1999 JD6 at about 30 feet.

“All of these images will allow us to determine its size and shape and measure its rotation rate – no other Earth-based observations can match the resolution of the radar,” said Marshall. “The only way to get a better shape model would be with a spacecraft flyby.”

NASA places a priority on protecting Earth by detecting and tracking asteroids. In fact, the robust detection program has discovered more than 90 percent of the large near-Earth asteroids.

The orbit of asteroid 1999 JD6 swings past Earth, Venus and Mercury. It comes within 4.4 million miles of Earth – about 18.6 times the distance of the moon from the Earth. The next time the asteroid gets this close to Earth will be in July 2054.