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 .