Scientists and amateur astronomers are tracking a 1.7-mile-wide asteroid on course to sail past Earth as it makes a wide U-turn on the way back toward the asteroid belt between Mars and Jupiter.
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At its closest point to the Earth, asteroid 1998 QE2 will be about 3.6 million miles away -- about 15 times the distance between the Earth and moon. But that comfortable distance -- scientists expected the pass to occur Friday at 1:59 p.m. PT -- is close enough for astronomers to get a good look at the space rock.
"This is one of the big ones," said Paul Chodas, of NASA's Near-Earth Object Program at Jet Propulsion Laboratory in Pasadena. "It's certainly one to keep an eye on."
On Wednesday night, astronomers discovered a smaller, 2,000-feet-wide moon asteroid circling the larger rock, The Associated Press reported.
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Friday's approach marks the closest the asteroid will get to Earth for at least the next two centuries. The fly-by allows researchers studying potential threats from space to better understand the asteroid's surface, rotation and other features by using radar (image sequence, right).
Researchers discovered the asteroid about 15 years ago and obtained their first sequence of radar images Thursday, when the asteroid was about 3.75 million miles from Earth.
Nicola Loaring, of the South African Astronomical Society, used JPL's web site to find the asteroid's coordinates Wednesday night. Astronomers around the world provide observations that JPL researchers use to predict the orbit.
"It looked like a star moving across the field," Loaring said. "It was moving very slowly. We knew it was the asteroid because it was moving against the background of the stationary stars."
QE2 -- the name is the product of a standard naming convention and has no connection to the ocean liner -- will not be visible without a powerful telescope. Scientists are using radar facilities in California's Mojave Desert and Puerto Rico to get the best views.
Twitter users can monitor hashtags #asteroidQE2 and #1998QE2 for updates.
The fly-by comes after a 150-foot asteroid -- 2012 DA14 -- passed within 17,200 miles of Earth in April. Another space rock lit up the sky over Russia earlier this year, causing a explosion and sonic boom that shattered glass windows.