The U.S. space agency, NASA, says there are slightly fewer massive, planet-buster asteroids and far fewer mid-sized, city-buster asteroids than previously thought in near-Earth orbit. The findings were the latest from NASA's asteroid-hunter, the Wide-field Infrared Survey Explorer, known as WISE.
Imagine a census of the inner solar system. Specifically, imagine a count of asteroids that orbit within 195 million kilometers of the Sun into Earth's orbital vicinity.
This is a population that fascinates Amy Mainzer of NASA's Jet Propulsion Laboratory in California. She is the principal investigator for the near-Earth Orbit Wide-field Infrared Survey Explorer, or NEOWISE.
"As one of my colleagues at the Jet Propulsion Laboratory likes to say, 'the best three ways of dealing with the potential of an asteroid impact are to find them early, find them early and find them early,'" Mainzer said.
Asteroids, space-rock scourge of the Earth in some apocalyptic thriller films. But it's not just science fiction. Scientists believe it was an asteroid strike that killed off the dinosaurs.
Mainzer says that is a reason to carry out a survey such as this one.
"If you can find near-Earth asteroids when they're far away, it would take far less energy to mitigate a potentially threatening objec," Mainzer said.
So researchers put WISE to work. WISE is a space-based telescope, small enough to carry under your arm, that faces away from the Earth. Between January 2010 and February 2011, it scanned the entire celestial sky twice in infrared light.
But why infrared? Imagine, for a moment, that you're looking at objects in the distance. A bright object would look bigger than a dim, faint one. Well, a standard visible light telescope would have the same problem as your eye. But NEOWISE measures heat being emitted from the asteroids, so it can see bright and dim objects equally. An asteroid's reflective properties won't skew the results.
NASA's Mainzer says by looking at a subset of the asteroid population, scientists can make estimates about the whole asteroid population.
"If we look at the very largest asteroids -- these are one kilometer and larger objects -- these are the planet busters. These are the things that are like the one that is thought to have caused the extinction of the dinosaurs. The good news here is that with NEOWISE we've been able to confirm that the worldwide community of astronomers, both amateur and professional, all over the place, have now found more than 90-percent of all of these really big asteroids," Mainzer said.
Scientists previously believed about 1,000 such asteroids existed, but, given the new data, they were able to revise downward. They now believe 981 such asteroids exist, and 911 of them have already been discovered.
This means researchers found at least 90 percent of all the one kilometer or larger asteroids, meeting a "Spaceguard" goal NASA agreed to with the U.S. Congress.
"By virtue of the fact that we know these objects, and we know their orbits, we can predict that they are no longer hazardous to Earth in the sense that we can follow them, and we know that there are none that pose any imminent risk of an impact," Mainzer said.
At least, says NASA, they don't pose a threat to Earth in the next few centuries.
That's one part of the asteroid population.
Using WISE data, astronomers now estimate there are roughly 20,000 mid-size near-Earth asteroids. The previous estimate was 35,000. Mid-sized asteroids are about 100 meters or larger, and they could destroy a city if they struck.
So far, NASA says, the Spaceguard effort is tracking more than 5,200 asteroids of that size. NASA estimates there are more than 15,000 left to discover.