An asteroid with a diameter of 45 metres will pass so close to Earth in 10 days time that it will travel between the planet and some satellites.
Nasa is reassuring the world that the path of the asteroid - 2012 DA14 - is understood well enough for it to be able to say there is no chance of a collision with the Earth.
But if a similar-sized asteroid to 2012 DA14, estimated to weigh 130,000 tonnes, were to hit Earth it would be expected to cause regional devastation, releasing about 2.5 megatons of energy in the atmosphere, Nasa said.
By comparison, a slightly smaller asteroid about 30 to 40 metres across was believed to have flattened about 1200sqkm of forest in Tuguska, Siberia in 1908.
The approach of 2012 DA14 to within 27,700km of the Earth's surface on Saturday, February 16 (NZT) would be the closest ever predicted approach to the planet for an object so large, Nasa said.
At its closest, the asteroid would only be about twice the Earth's diameter away from the planet.
While it was confident the asteroid was not on a collision course with the Earth, Nasa was less comforting with the observation that there were believed to be about 500,000 near-Earth asteroids of a similar size, less than one per cent of which had been discovered.
Scientists at Nasa's Near-Earth Object Program Office estimated an asteroid the size of 2012 DA14 passed as close to the Earth as it would be doing every 40 years on average, and that one would impact the Earth about every 1200 years on average.
Travelling about 28,100kmh, or 7.82km per second, asteroid 2012 DA14 would spend about 33 hours within the Earth/Moon system - within the distance of the Moon from the Earth, an average of about 384,000km.
It would be so close to the Earth that it would pass inside the ring of geosynchronous weather and communications satellites, although outside the large concentration of satellites orbiting much closer to Earth.
"Almost no" satellites were orbiting at the distance at which the asteroid would pass, and there was "very little" chance it would hit a satellite or spacecraft, Nasa said.
The flyby would provide a unique opportunity to study a near-Earth object up close and efforts would be made to determine its spin rate and composition.
During the closest approach, the asteroid could be visible from parts of Europe, Africa and Asia.
A good pair of binoculars, at least, would be needed as the asteroid was expected to be too dim to be visible to the naked eye.
The asteroid would be at its closest around 8.24am on February 16 (NZT). At that time it would be over the eastern Indian Ocean, off Sumatra.
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