An asteroid the size of a house will be watched carefully by astrophysicists as it hurtles "damn close" to Earth today.
The large space rock, named 2012 TC4, was first spotted five years ago by the Pan-STARRS telescope at the Haleakala Observatory, in Hawaii, before disappearing as it orbited the Sun. It then re-emerged in July on a trajectory well inside our lunar orbit.
Scientists have said the asteroid is on course to pass safely by, just south of Australia, and poses no threat.
However, it presents space agencies with a rare opportunity to test the planet's space defences and wargame what they would do if a larger, more threatening asteroid was detected heading straight for Earth.
HTC4 is between 50 to 100ft in diameter and travelling through space at roughly 26,000kmh - 7km a second. It is predicted to start to pass Earth from around 7am GMT today and will be about 43,000km from our atmosphere.
This may sound like a long way away, yet it's a short distance in planetary terms and around one eighth of the distance between the Earth and the Moon.
Rolf Densing, who heads the European Space Operations Centre in Darmstadt, Germany said: "It's damn close. The farthest satellites are 36,000km out, so this is a close miss."
Earth has been struck by asteroids and meteors repeatedly over its 4.5 billion-year lifespan.
The most famous event was the 95km-wide asteroid that hit the coast of Mexico around 65 million years ago, leading to the extinction of the dinosaurs. But there have been more recent, less dramatic examples.
The largest recorded asteroid incident is the 1908 Tunguska event in Russia.
A fireball believed to be between 160ft and 330ft wide incinerated around 19,00sqkm of forest in an area of remote Siberian forest near the Podkamennaya Tunguska river. Residents in the nearest town, 55km away, reportedly felt heat from the blast.
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