New observations by NASA’s Wide-field Infrared Survey Explorer, or WISE, show there are significantly fewer near-Earth asteroids in the mid-size range than previously thought. The findings also indicate NASA has found more than 90 percent of the largest near-Earth asteroids, meeting a goal agreed to with Congress in 1998. According to astronomers there are roughly 19,500 not 35,000 mid-size near-Earth asteroids and this improved understanding of the population may indicate the hazard to Earth could be somewhat less than previously thought. However, the majority of these mid-size asteroids remain to be discovered. More research also is needed to determine if fewer mid-size objects (between 330 and 3,300-feet wide) also mean fewer potentially hazardous asteroids, those that come closest to Earth.
The WISE space telescope mapped the entire sky twice between January 2010 and February 2011 during its mission, which was aimed at mapping near-Earth asteroids, brown dwarfs, galaxies and other deep space objects. For its near-Earth asteroid search, the space observatory scanned for space rocks that orbited within 120 million miles (195 million kilometers) of the sun. The Earth is about 93 million miles (150 million km) from the sun. The telescope’s NEOWISE mission discovered more than 100,000 previously unknown asteroids in the asteroid belt between the orbits of Mars and Jupiter. It spotted 585 asteroids in orbits that brought them near Earth.The WISE asteroid survey also lowered the estimated number of giant asteroids, space rocks the size of a mountain, from 1,000 to 981, with about 911 of those already known. According to Tim Spahr, the director of the Minor Planet Center at the Harvard Smithsonian Center for Astrophysics in Cambridge, Mass., the risk of a really large asteroid impacting the Earth before we could find and warn of it has been substantially reduced.