A 164ft (50m) long space rock with the destructive power of an H-bomb will narrowly miss the Earth next year, scientists predict.
Asteroid 2012 DA14 is due to whizz past the Earth on February 15 2013, at a distance of just 14,913 miles (24,000km) - closer than many commercial satellites.
Scientists say there is no chance of an impact, but despite tiny odds such a possibility cannot be completely ruled out in years to come.
If it did enter the Earth's atmosphere and explode, the force would be enough to destroy an area the size of Greater London.
The asteroid was spotted last month by a team operating from the La Sagra Sky Survey observatory near Granada in Spain.
The observatory uses automated telescopes to track small asteroids and comets.
2012 DA14 was discovered after the astronomers decided to search areas of the sky where asteroids are not usually seen.
Dr Gerhard Drolshagen, a near-Earth object observer from the European Space Agency's Space Situational Awareness (SSA) office, said: "The object is roughly 50 metres across and at that size it could do some damage if it exploded over an inhabited area. It would have the force of the biggest nuclear weapon."
In 1908 an asteroid estimated to be 131ft (40m) across exploded over Tunguska in Siberia, flattening 772 square miles (2,000 sq km) of forest.
"That is an area the size of Greater London," said Dr Drolshagen. "This asteroid is a little bigger."
He said there was "no chance" of the rock hitting the Earth next year. The asteroid is expected to make its closest approach shortly after 6pm, UK time.
"Next year it will be nice to watch through a pair of binoculars, but there is nothing to worry about," said Dr Drolshagen. "In future times the possibility of a collision cannot be completely excluded. It is highly unlikely, but the chance is greater than zero."
An estimated 500,000 near-Earth objects measuring up to 98ft (30m) are believed to be undiscovered.
Dr Detlef Koschny, also from the SSA, said: "We are developing a system of automated optical telescopes that can detect asteroids just like this one, with the goal of being able to spot them at least three weeks before closest approach to Earth."