Potentially hazardous asteroids (PHAs) are a subset of near-Earth asteroids (NEAs) that have the potential to come within five million miles (eight million kilometers) of Earth, and are of a size large enough to make it through Earth’s atmosphere to cause significant damage on a regional, or greater, scale. NASA’s asteroid-hunting NEOWISE mission has now provided the best estimate yet of the number of PHAs in our solar system, along with their origins and the potential dangers they might pose.

While all NEAs have an orbit that brings them within close proximity to Earth, only some of them have orbits that intersect with Earth’s and are of a large enough size to be classified as PHAs. After its initial 10-month astronomical survey mission to create infrared images of 99 percent of the sky, NASA’s Wide-field Infrared Survey Explorer (WISE) was put into hibernation mode in early 2011. To search for small solar system bodies close to Earth’s orbit, NEOWISE was approved as an additional four-month mission extension after the NASA Planetary division stepped up to the plate with funding.

Diagram showing the differences between orbits of a typical near-Earth asteroid (blue) and a potentially hazardous asteroid, or PHA (orange) (Image: NASA/JPL-Caltech)

To make predictions about the entire population as a whole, NEOWISE snapped images of about 600 NEAs and sampled 107 PHAs, allowing the project team to extrapolate that there are around 4,700 PHAs – give or take 1,500 – with diameters larger than 330 ft (100 m). It is estimated that around 20 to 30 percent of these PHAs have been found. NASA points out that previous estimates on PHA numbers reached similar conclusions, but NEOWISE’s observations provide a more credible estimate of their number and size.

Analysis of NEOWISE’s new observations also suggests that around twice as many PHAs are likely to reside in “lower-inclination” orbits, which are more aligned with Earth’s orbit, than previously thought. As these asteroids are more likely to encounter Earth and would be much easier to reach, they could become potential targets for future robotic or human missions.


The new observations also show that the lower-inclination objects appear to be brighter and smaller than many of the PHAs that spend more time far away from Earth. Their brightness indicates they are more likely to be either stony, like granite, or metallic. This kind of composition Information is important, as it gives an indication of how quickly they might burn up in our atmosphere.


Source: NASA WISE mission page