Good news on potential asteroid troubles for tender-hearted Earth. NASA announced yesterday that a recent study of the sky by the Wide-field Infrared Survey Explorer (WISE) determined that there are significantly fewer mid-sized asteroids in Earth’s neighborhood by a factor of almost a half. Previously, scientists had estimated the number of space rocks with sizes ranging from 330 to 3,330 feet within 120 million miles of our planet at 35,000. That’s now been revised downward to about 19,500. Fewer near-Earth asteroids means fewer potentially hazardous bodies causing trouble for our planet in the future.
WISE scanned the entire sky twice during the asteroid-hunting portion of its program called NEOWISE (Near Earth Objects) between January 2010 and February 2011 taking pictures of everything from galaxies to comets to asteroids in the light of infrared. Infrared light, which lies just beyond the red end of the rainbow spectrum, is sensed by us as heat. When you boil a pot of water for tea, your teaport becomes a powerful emitter of infrared light. Telescopes can be designed to gather infrared light just like ordinary visible light, but their detectors must be chilled to near absolute zero, so the images aren’t overwhelmed by the heat of the instrument. The WISE detector was surrounded by frozen hydrogen in an “ice chest” at -430 F !
NEOWISE observed more than 100,000 asteroids in the main belt between Mars and Jupiter in addition to 585 near Earth. Observing in infrared light allowed WISE to make the most accurate asteroid population census ever. In visible light, darker-colored asteroids are hard to detect because they reflect so little light and therefore are exceedingly faint. Asteroids covered in lighter materials are much easier. With an ordinary telescope, these small, chalky-bright asteroids appear similar in brightness to much larger, charcoal-toned ones. Telling size becomes a tricky business. Infrared light has no bias. If an asteroid’s small, it radiates less heat than a bigger one, making it easier to determine accurate sizes and potential threats.
Not only did WISE update the mid-size asteroid count, but its findings indicate that NASA, through a variety of asteroid detection programs both satellite and ground-based, has detected more than 93% of the largest near-Earth asteroids, defined as those 1 kilometer (0.62 mile) and larger. These mountain-sized rocks would cause significant destruction and climate change if one were to collide with our planet.
“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 across, as big as the one thought to have wiped out the dinosaurs, have been found,” according to the agency.
Don’t get too comfortable just yet. NASA’s Spaceguard NEO Survey currently tracks 5,200 mid-sized and larger asteroids with an estimated 15,000 left to discover. A mid-sized asteroid would destroy a city in a direct hit. As for near-Earth bodies under 330 feet, scientists believe more than a million are buzzing around within 120 million miles of the planet. Some are potentially capable of damage.
What I find most interesting about the survey results is the picture they paint of what seems to be a very busy neighborhood. However the fact that asteroid impacts are extremely rare tells us that there’s still a lot more empty space out there than rocks.