An asteroid described as "the most dangerous one we've found so far" may be on course for a 2028 AD collision with earth and certainly will pass closer than any such object in modern times, astronomers said. "The chance of an actual collision is small, but one is not entirely out of the question," according to a notice filed by the International Astronomical Union. But asteroid specialist Jack G Hills said on Wednesday the speeding space rock, called Asteroid 1997 XF11, poses a real danger to earth.