Asked by: Jon Linbergh, Chester

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A potentially hazardous asteroid (PHA) is an asteroid whose orbit comes nearer than 0.05AU (about 7.5 million km) to the Earth and whose brightness implies a size of the order of about 100m across or more. Such objects would have devastating consequences if they were to impact the Earth.

At the time of writing, the International Astronomical Union lists a total of 1,489 potential hazardous asteroids. This does not mean all these objects will eventually hit the Earth, just that they have the potential to do so. Of course, this number represents only the PHAs we know about. A survey by NASA’s WISE satellite suggested there are at least 4,700 such objects.

Although we are in no immediate danger, asteroids like this that are big enough to cause major destruction, particularly in heavily populated areas, have hit Earth every 200 to 300 years on average.

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