An asteroid the size of a large house will pass by Earth at a distance of around 27,000 miles (44,000km) on Thursday.
2012 TC4 is estimated by NASA to measure 45 to 100ft (15 to 30 metres) and, even though in space terms it will narrowly miss our planet, scientists say there is no risk of collision.
Mike Kelly, who is leading the agency’s project to track it, said there was “no danger. Not even for satellites.”
He said: “We’ve now been observing TC4 for two months, so we have very accurate position information on it, which in turn allows very precise calculations of its orbit, which will not cross that of Earth nor its satellites.”
Its closest point of contact with earth will be over Antarctica at around 6:40am, GMT.
The space rock has a 1-in-750 chance of crashing into Earth, placing it at number 13 on the list of objects posing even a remote risk of collision.
As its name suggests, 2012 TC4 was first discovered five years ago, and was spotted most recently in July by the European Space Agency and the European Southern Observatory.
It is roughly the same size as the meteor that caused widespread damage when it exploded in the atmosphere over Chelyabinsk in central Russia in 2013.
That blast, which had 30 times the kinetic energy of the atomic bomb dropped on Hiroshima, left more than 1,200 people injured and blew out windows in around 5,000 buildings.
TC4 is being used as a test for a global asteroid pre-warning system, supported by a network of observatories, universities and laboratories around the world, to see if such rocks can be accurately tracked.
“For us this is a test case,” said Detlef Koschny of the European Space Agency’s Near-Earth Object programme.
“We are practising for a real serious case,” he added.
TC4, which orbits the Sun every 609 days and will return to Earth in 2050 and 2079, is one of thousands of space rocks whose location is known.
There are millions in the galaxy whose whereabouts are unknown.