A large asteroid that initially had scientists worried has only a small chance of striking the Earth in 2040, says NASA.
Asteroid 2011 AG5, discovered in January 2011 and measuring around 460 feet across, has less than a one percent chance of impact, they say.
"While there is general consensus there is only a very small chance that we could be dealing with a real impact scenario for this object, we will still be watchful and ready to take further action if additional observations indicate it is warranted," says Lindley Johnson, program executive for the Near-Earth Object Observation Program at NASA.
It was first discovered by the NASA-supported Catalina Sky Survey operated by the University of Arizona in Tucson, and monitored for nine months before it moved too far away to observe. It's now beyond the orbit of Mars and in the daytime sky on the other side of the sun.
In fall 2013, though, conditions will improve to allow space- and ground-based telescopes to better track the asteroid's path. It will still be 91 million miles from Earth, but well-positioned for observations in the late evening sky.
The crucial moment will arrive in 2023, when the asteroid is around 1.1 million miles from Earth. If 2011 AG5 passes through a particular 227-mile-wide region in space called a keyhole in early February of that year, the Earth's gravitational pull could affect it just enough to bring it back for an impact on February 5, 2040.
"Given our current understanding of this asteroid's orbit, there is only a very remote chance of this keyhole passage even occurring," says Johnson.
A few years ago another asteroid, Apophis, was thought to pose a similar impact threat in 2036. However, additional observations made between 2005 and 2008 showed the risk was much less than feared.
"Any time we're able to observe an asteroid and obtain new location data, we're able to refine our calculations of the asteroid's future path," says Don Yeomans, manager of NASA's NEO Program Office.
"When few observations exist, our initial orbit calculation will include a wider swath to account for uncertainties. With more data points, the knowledge of the potential positions of the asteroid improves and the swath becomes smaller - typically eliminating the risk of an impact."