An asteroid half the size of a football pitch and with the destructive power of an H-bomb will narrowly miss striking Earth on February 15, scientists say.
The 164ft (50m) long space rock – named 2012 DA14 – will whizz past our planet at a distance of just 14,913 miles (24,000km) - closer than many commercial satellites.
Don Yeomans of Nasa’s Near Earth Object Program at the Jet Propulsion Laboratory, describes it as a “record-setting close approach”.
He added: “2012 DA14 will definitely not hit Earth. The orbit of the asteroid is known well enough to rule out an impact."
If it did enter the Earth's atmosphere and explode, the force would be enough to destroy an area the size of Greater London.
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 Gerhard Drolshagen. a near-Earth object observer from the European Space Agency's Space Situational Awareness (SSA) office. "This asteroid is a little bigger."
Dr Yeomans said: "Since regular sky surveys began in the 1990s, we've never seen an object this big get so close to Earth."
The rock in question is believed to be made of stone, rather than metal or ice and Yeomans estimates an asteroid of that sort flies past Earth on average every 40 years, but is only likely to strike the planet ever 1,200 years or so.
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."