This chart shows how data from WISE has led to revisions in the estimated population of near-Earth asteroids as part of project called NEOWISE. NEOWISE observed more than 500 objects larger than 100 meters wide -- what can be thought of as medium to large-size asteroids. Near-Earth asteroids smaller than this size range were not studied, and near-Earth comets will be analyzed at a later time. Asteroid sizes are not drawn to scale in the chart.
Each asteroid image represents about 100 actual objects. Near-Earth asteroids that have already been found are filled in and appear brown. An entire row of asteroid images through the blue outlines shows how many total objects were thought to exist before the NEOWISE survey. The green outlines show the reduced new estimates based on the NEOWISE data. As the graphic reveals, only a small difference was observed in the estimated total numbers of the largest asteroids. For the medium-sized asteroids, new estimates predict fewer space rocks than previously thought. Details are listed below.
For the largest asteroids, larger than 1,000 meters, NEOWISE data revises the total population down to 981 from a prior estimate of about 1,000. While this is not a dramatic difference, the findings show that NASA has met an initial near-Earth asteroid goal agreed to with Congress in 1998, calling for at least 90 percent of the largest objects to be found. There are an estimated 911 objects of this size range known, which means that NASA has found 93 percent. That leaves roughly 70 of these bodies left to find.
The NEOWISE data reveals an approximately 44 percent decline in the estimated numbers of medium-sized asteroids, which are defined as those objects between 100 meters and 1,000 meters. Estimates now indicate about 19,500, where as 35,000 were thought to exist before.
The study does not apply to objects smaller than 100 meters (330 feet), but it is estimated that there are more than a million in this size range, based on previous studies.