A new survey has revealed there are many more potentially Earth-threatening asteroids whizzing around the solar system close to Earth than was previously thought.
The study, by NASA’s Wide-field Infrared Survey Explorer (WISE), has revealed new information about their numbers, origins and the possible threats they may pose.
Potentially hazardous asteroids (PHAs) are near-Earth asteroids that have the closest orbit to Earth – often passing within five million miles (eight million km) – and are large enough to survive the trip through Earth’s atmosphere and cause significant damage.
The craft’s asteroid hunting segment, NEOWISE, looked at 107 PHAs and extrapolated that information to make predictions about the population as a whole.
"The NEOWISE analysis shows us we've made a good start at finding those objects that truly represent an impact hazard to Earth," said Lindley Johnson, program executive for NASA’s Near-Earth Object Observation Programme.
"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 findings suggest that, although 20 to 30 per cent have actually been found, there are about 4,700 PHAs that measure in at more than 330ft (100m).
The results suggest that there about twice as many such asteroids in so-called “lower-inclination” orbits – orbits that are more aligned with the plane of Earth’s orbit – than previously thought.
Such an orbit would make these asteroids more likely to strike the Earth, but also makes them easier to reach by future robotic or human missions.
Visiting asteroids can provide information about their origins and fate but could also provide valuable data about how quickly they might burn up in Earth’s atmosphere and how much damage they would cause if an encounter did take place.
Another finding that is intriguing the researchers is that PHAs which occupy a lower orbital plane seem to be brighter and smaller than other near-Earth asteroids. One possible explanation for this is that they may have originated from a collision between two asteroids in the main asteroid belt that lies between Mars and Jupiter. It could also tell us something about their composition – suggesting they are more likely to be stony (like granite) or metallic.
NEOWISE detects the infrared light (or heat) emitted by asteroids, which, when combined with visible light observations, means it is able to make accurate measurements of the asteroids’ diameters.
The WISE spacecraft took images of about 600 near-Earth asteroids, about 135 of were new discoveries.
The mission is managed by NASA's Jet Propulsion Laboratory (JPL).