"In fact, the threat is minimal. One in 250,000 is a very small number," JPL's Near-Earth Object Program manager Don Yeomans said in an interview on NASA's web page.

The scientist was referring to last week's media frenzy over an asteroid two to four kilometers (1.2-2.5 miles) in diameter spotted July 5 by astronomers in New Mexico that scientists at first said had a one in 60,000 chance of hitting earth.

Massachusetts Institute of Technology astronomers later reduced the odds to one in 250,000, but that did not stop predictions that the space rock could wipe out an entire continent and, even more frightening, the entire human race.

Not to worry, said Yeomans and his colleagues at JPL in Pasadena, California.

"We've been tracking 2002 NT7 for a very short time -- only 17 days so far," implying that predictions based on so brief an observation are highly inaccurate.

"As far as the public is concerned," said Jon Giorgini of JPL's Solar System Dynamics Group, "it just isn't worth getting worked up about an object with a couple weeks of data showing a possible Earth encounter many years from now.

"Additional measurements will shrink the uncertainty by a large amount, and Earth will (almost certainly) fall out of the risk zone," he added.

Giorgini said initial calculations on an asteroid's position are very uncertain: "We deal with probabilities, not absolute answers, because the measurements contain errors."

The end product of the calculations are "finite impact probabilities," he said.

"A finite probability, however, is not really a prediction of impact," he cautioned, "but a statement that one is possible."

2002 NT7 is traveling at 28 kilometers (17 miles) per second and orbits the sun at a 42 degree tilt to the plane of the planets, bringing it within the inner solar system not far from Earth's orbit every 2.29 years, NASA said.

Asteroids are often described as the rubble left over from the building of the solar system.

A football-pitch-sized asteroid capable of razing a major city came within a whisker of hitting the Earth on June 14.

Asteroid 2002 MN, estimated at up to 120 meters (yards) long, hurtled by the Earth at a distance of 120,000 kilometers (75,000 miles), well within the orbit of the Moon and just a hair's breadth from Earth in galactic terms.

It is the closest recorded near-miss by any asteroid, with the exception of a 10-metre (33-feet) rock, 1994 XM1, which approached within 105,000 kilometers on December 9, 1994.

SPACE.WIRE