Observations from NASA's Wide-field Infrared Survey Explorer (WISE) indicate there are about twice as many potentially hazardous asteroids (PHAs) in lower-inclination orbits than previously believed.
PHAs orbit within five million miles of Earth and are big enough to survive passing through the atmosphere, causing damage on at least a regional scale.
By sampling 107 PHAs, the NEOWISE team was able to to make predictions about the entire population as a whole, concluding thatthere are roughly 4,700 PHAs, plus or minus 1,500, with diameters larger than 330 feet. So far, around 20 to 30 percent of these objects have been found.
"The NEOWISE analysis shows us we've made a good start at finding those objects that truly represent an impact hazard to Earth," says Lindley Johnson, program executive for the Near-Earth Object Observation
"But we've many more to find, and it will take a concerted effort during the next couple of decades to find all of them that could do serious damage or be a mission destination in the future."
The new analysis suggests that about twice as many PHAs as previously thought are likely to reside in lower-inclination orbits - those which are more aligned with the plane of Earth's orbit. These asteroids appear to be somewhat brighter and smaller than the other near-Earth asteroids that spend more time far away from Earth.
The team suggests that this may be because they originated from a collision between two asteroids in the main belt lying between Mars and Jupiter. A larger body with a low-inclination orbit could have broken up in the main belt, causing some of the fragments to drift into orbits closer to Earth.
Asteroids with lower-inclination orbits would be more likely to encounter Earth and would be easier to reach. The results therefore suggest more near-Earth objects might be available for future robotic or human missions.
"Because they will tend to make more close approaches to Earth, these targets can provide the best opportunities for the next generation of human and robotic exploration," says Amy Mainzer, NEOWISE principal investigator.
The discovery that many PHAs tend to be bright indicates that they're likely to be either stony or metallic - important in assessing their potential hazards to Earth.