Looming asteroid under close watch

US astronomers are keeping close watch on a newly discovered 2km-wide asteroid but do not expect it to collide with Earth.

There is a slight chance that the asteroid, dubbed 2002 NT7, could smack the Earth on February 1, 2019, causing a global catastrophe but astronomers said the odds were one in 250,000 and further calculation would probably show it will miss the planet entirely.

It now heads the list of asteroids and comets monitored by NASA, but as scientists learn more it isn't expected to stay there. "One way or another, this thing is coming off the risk page," said Donald Yeomans, manager of NASA's near-Earth object programme.

Astronomers with the Lincoln Near Earth Asteroid Research Project used a New Mexico telescope to discover the space rock on July 9, when it was about 135 million km from Earth. It is in orbit around the sun.

Follow-up observations allowed astronomers to calculate six other potential impact dates in 2044, 2053, 2060 and 2078. The asteroid will remain in the sights of astronomers for another year at least, allowing them to refine their estimates of its trajectory on its 837-day orbit.

"At that point, if it's still a threat, I'd start to get a little concerned, but not before then," said Gareth Williams, associate director of the Minor Planet Centre in Massachusetts.

Were the asteroid to hit Earth, it would enter the atmosphere at nearly 103,000 km/h and strike with the explosive energy of 1.2 million megatons of TNT.