The asteroids would cause catastrophic damage, on a city-sized scale or larger. Research from the University of Southhampton has identified the countries most likely to be worst hit.
They have identified for the first time those which will suffer catastrophic loss of life or be so crippled it will be almost impossible for them to recover.
Nasa's Wide Field Infrared Survey Explorer has sampled 107 'potentially hazardous' asteroids near Earth - 330ft wide or larger - to make estimates about how many are out there - and the figure is a terrifying 47,000.
The WISE survey now estimates there are 47,000 'potentially hazardous' asteroids.

The PHAs - potentially hazardous asteroids - have the closest orbits to Earth's, coming within five million miles and they are big enough to survive passing through Earth's atmosphere and cause damage on a regional, or greater, scale.

The new results come from the asteroid-hunting portion of the WISE mission, called NEOWISE.
The project sampled 107 PHAs to make predictions about the entire population as a whole.
Findings indicate there are roughly 4,700 PHAs, plus or minus 1,500, with diameters larger than 330 feet . So far, an estimated 20 to 30 percent of these objects have been found.
Scientists have also drawn up a league table of the countries which will be worst affected in the event of an asteroid strike.
They have identified for the first time those which will suffer catastrophic loss of life or be so crippled it will be almost impossible for them to recover.
The list has been compiled by researchers from the University of Southampton using software called called NEOimpactor, short for NASA's ‘NEO’ or Near Earth Object programme.
Overall the top ten countries most at risk are: China, Indonesia, India, Japan, the U.S, the Philippines, Italy, the U.K, Brazil and Nigeria.

'The NEOWISE analysis shows us we've made a good start at finding those objects that truly represent an impact hazard to Earth,' said Lindley Johnson, program executive for the Near-Earth Object Observation Program at NASA Headquarters in Washington. 'But we've many more to find, and it will take a concerted effort during the next couple of decades to find all of them that could do serious damage or be a mission destination in the future.'

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