A new research led by NASA's Wide-Field Infrared Survey Explorer, WISE, reveals that there are significantly fewer near-Earth asteroids in the mid-size range than previously imagined.

The research also indicates NASA has found more than 90 per cent of the largest near-Earth asteroids, meeting the "Spaceguard" goal, agreed to with Congress in 1998.

Astronomers now estimate there are roughly 19,500 -- not 35,000 -- mid-size near-Earth asteroids. Scientists say this improved understanding of the population may indicate that the hazard to Earth could be somewhat less than previously imagined.

However, the majority of these mid-size asteroids remain to be discovered. More research 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 results come from the most accurate census to date of near-Earth asteroids, the space rocks that orbit within 120 million miles (195 million kilometers) of the sun into Earth's orbital vicinity. WISE observed infrared light from those in the middle to large-size category. The survey project, called NEOWISE, is the asteroid-hunting portion of the WISE mission.

"NEOWISE allowed us to take a look at a more representative slice of the near-Earth asteroid numbers and make better estimates about the whole population. It is like a population census, where you poll a small group of people to draw conclusions about the entire country," said Amy Mainzer, lead author and principal investigator for the NEOWISE project at NASA's Jet Propulsion Laboratory (JPL) .

Though the WISE data reveal only a small decline in the estimated numbers for the largest near-Earth asteroids, which are 3,300 feet (1 kilometer) and larger, they show that 93 per cent of the estimated population have been found. This fulfills the initial "Spaceguard" goal agreed to with Congress. These large asteroids are about the size of a small mountain and would have global consequences if they were to strike Earth.

The new data revise their total numbers from about 1,000 down to 981, of which 911 already have been found. None of them represents a threat to Earth in the next few centuries. It is believed that all near-Earth asteroids approximately 6 miles (10 kilometers) across, as big as the one thought to have wiped out the dinosaurs, have been found.

"The risk of a really large asteroid impacting the Earth before we could find and warn of it has been substantially reduced," said Tim Spahr, the director of the Minor Planet Center at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass.

Commenting about the research, Program Executive for the Near Earth Object (NEO) Observation Program at NASA, Lindley Johnson, said: "NEOWISE was just the latest asset NASA has used to find Earth's nearest neighbors. The results complement ground-based observer efforts over the past 12 years. These observers continue to track these objects and find even more."

NASA