Earth-bound asteroids are often the bane of human societies calmly minding their own business in many Sci-Fi films. They lurk in the outer solar system awaiting an opportune moment to fall towards the sun, only to find a blue-green planet with an oddly large moon in their way.
The WISE mission at NASA has recently released a set of obserations that suggest there are fewer of these asteroids than previously thought. Most of the larger asteroids have already been discovered and tracked, but the smaller mid-sized asteroids up to one kilometer across are less well known. Analysis of the observations suggest there are only 19,000 of these rather than the previously expected number 35,000.
Although fewer are now expected only 5,000 of these have been discovered. So the JPL at NASA still has some important work left to do.
See the press release at NASA: http://www.nasa.gov/home/hqnews/2011/sep/HQ_11-333_NEOWISE.html