An asteroid called 2012 DA14 will narrowly miss Earth as it flies past at 7pm tonight.
The 45 metre diameter rock is big enough to destroy London.
It has no chance of hitting, but it will enter the orbit of more than 100 satellites.
It is expected to fly 17,200 miles (27,000 kilometres) above Earth's surface at around 7pm GMT.
The flyby of 2012 DA14 is the closest-ever predicted approach to Earth for an object this large.