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| Space on Sep 13, 2012

How many killer asteroids are out there?

NASA survey suggests roughly 4,700 potentially hazardous asteroids. So far, an estimated 20 to 30 percent of these objects have been found.

Results from a NASA survey released in 2012 suggest there are roughly 4,700 potentially hazardous asteroids in our solar system. The results reveal new information about their total numbers, origins and the possible dangers they may pose.

Potentially hazardous asteroids, or PHAs, are a subset of the larger group of near-Earth asteroids. The PHAs have the closest orbits to Earth’s, coming within five million miles (about eight million kilometers), and they are big enough to survive passing through Earth’s atmosphere and cause damage on a regional, or greater, scale.

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The new results come from the asteroid-hunting portion of the Wide-field Infrared Survey Explorer (WISE) mission, called NEOWISE. The project sampled 107 PHAs to make predictions about the entire population as a whole. Findings indicate there are roughly 4,700 PHAs, plus or minus 1,500, with diameters larger than 330 feet (about 100 meters). So far, an estimated 20 to 30 percent of these objects have been found.

Results from NASA's NEOWISE survey - released in June 2012 - find that more potentially hazardous asteroids, or PHAs, are closely aligned with the plane of our solar system than previous models suggested. This diagram shows an edge-on view of our solar system. The dots represent a snapshot of the population of NEAs and PHAs that scientists think are likely to exist based on the NEOWISE survey. Positions of a simulated population of PHAs on a typical day are shown in bright orange, and the simulated NEAs are blue. Earth's orbit is green. Image credit: NASA/JPL-Caltech

While previous estimates of PHAs predicted similar numbers, they were rough approximations. NEOWISE has generated a more credible estimate of the objects’ total numbers and sizes.

The new analysis also suggests that about twice as many PHAs as previously thought are likely to reside in “lower-inclination” orbits, which are more aligned with the plane of Earth’s orbit. In addition, these lower-inclination objects appear to be somewhat brighter and smaller than the other near-Earth asteroids that spend more time far away from Earth. A possible explanation is that many of the PHAs may have originated from a collision between two asteroids in the main belt lying between Mars and Jupiter. A larger body with a low-inclination orbit may have broken up in the main belt, causing some of the fragments to drift into orbits closer to Earth and eventually become PHAs.

This diagram illustrates the differences between orbits of a typical near-Earth asteroid (blue) and a potentially hazardous asteroid, or PHA (orange). The sun sits at the center, while the orbits of the planets Mercury, Venus and Mars are shown in grey. Earth's orbit stands out in green between Venus and Mars. As the diagram indicates, the PHAs tend to have more Earth-like orbits than the rest of the NEAs. The asteroid orbits are simulations of what a typical object's path around the sun might look like. The blue and orange dots in the background represent a simulation of the population of near-Earth asteroids and the PHAs, respectively, which are larger than 330 feet (100 meters). Image credit: NASA/JPL-Caltech

Asteroids with lower-inclination orbits would be more likely to encounter Earth and would be easier to reach. The results therefore suggest more near-Earth objects might be available for future robotic or human missions.

The discovery that many PHAs tend to be bright says something about their composition; they are more likely to be either stony, like granite, or metallic. This type of information is important in assessing the space rocks’ potential hazards to Earth. The composition of the bodies would affect how quickly they might burn up in our atmosphere if an encounter were to take place.

The WISE spacecraft scanned the sky twice in infrared light before entering hibernation mode in early 2011. It catalogued hundreds of millions of objects, including super-luminous galaxies, stellar nurseries and closer-to-home asteroids. The NEOWISE project snapped images of about 600 near-Earth asteroids, about 135 of which were new discoveries. Because the telescope detected the infrared light, or heat, of asteroids, it was able to pick up both light and dark objects, resulting in a more representative look at the entire population. The infrared data allowed astronomers to make good measurements of the asteroids’ diameters and, when combined with visible light observations, how much sunlight they reflect.

Bottom line: Results from the asteroid-hunting portion of NASA’s Wide-field Infrared Survey Explorer (WISE) mission, called NEOWISE – released in June 2012 – suggest there are roughly 4,700 potentially hazardous asteroids in our solar system. The results reveal new information about their total numbers, origins and the possible dangers they may pose.

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