How many scary asteroids out there? A lot, says NASA

The asteroid was between six and 19 metres in diameter. Photo / Thinkstock

A new NASA project says there are roughly 4,700 "potentially hazardous asteroids" - give or take 1,500 - with orbits close to Earth.

The figures, which come from observations from NASA's Wide-field Infrared Survey Explorer's NEOWISE project, are the best assessment yet of our solar system's population of potentially hazardous asteroids (PHAs), according to NASA.

Potentially hazardous asteroids are a subset of the larger group of near-Earth asteroids with the closest orbits to Earth's own orbit.

PHAs come within eight million kilometres of earth, and are big enough to survive passing through Earth's atmosphere and potentially cause widespread damage.

The NEOWISE project sampled 107 PHAs to make predictions about the entire population as a whole, estimating there are roughly 4,700 PHAs, plus or minus 1,500, with diameters larger than 100 metres.

So far, about 20 to 30 percent of these objects have been found.

NASA said previous estimates of the asteroids were similar, however this latest project provided more credible estimates on numbers and size.

Lindley Johnson, program executive for the Near-Earth Object Observation Program at NASA Headquarters in Washington, said the project's analysis was a good start in finding out those objects which "that truly represent an impact hazard to Earth".

"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 NEOWISE analysis also suggested there were about twice as many PHAs residing in the "lower-inclination" orbit than previously thought. These asteroids have an orbit more aligned with the plane of Earth's orbit. It was found the lower-inclination orbit asteroids were somewhat smaller and brighter than the other near-Earth asteroids that spend more time far away from Earth.

Asteroids with lower-inclination orbits would be more likely to encounter Earth but would be easier to reach in future robotic or human missions.

The discovery that many of the PHAs tend to be bright may be due to their composition - they are more likely to be either stony, like granite, or metallic.

NASA said this information is important in assessing the space rocks' potential hazards to Earth, as their composition would affect how quickly they might burn up in our atmosphere if one were to strike Earth.

Amy Mainzer, NEOWISE principal investigator at NASA's Jet Propulsion Laboratory in Pasadena, California, said the project was not originally planned as part of the Wide-field Infrared Survey Explorer operations, but has turned out to be a "huge bonus".

"Everything we can learn about these objects helps us understand their origins and fate.

"Our team was surprised to find the overabundance of low-inclination PHAs.

"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."