Asteroid 2012 DA14 set for record-breaking Earth pass
15 February 2013 Last updated at 06:17 GMT
An asteroid as large as an Olympic swimming pool will race past the Earth on Friday at a distance of just 27,700km (17,200mi) - the closest ever predicted for an object of that size.
It will pass closer even than the geosynchronous satellites that orbit the Earth, but there is no risk of impacts or collisions.
Its closest approach will be 19:25 GMT.
Rebecca Morelle reports.