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A New Count of Potentially Hazardous Asteroids

Observations from NASA’s Wide-field Infrared Survey Explorer (WISE) have led to the best assessment yet of our solar system’s population of potentially hazardous asteroids. Also known as “PHAs,” these asteroids have orbits that come within five million miles (about eight million kilometers) of Earth, and they are big enough to survive passing through Earth’s atmosphere and cause damage on a regional, or greater, scale.

The asteroid-hunting portion of the WISE mission, called NEOWISE, sampled 107 PHAs to make predictions about the 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.

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. Because the WISE space 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 NEOWISE analysis shows us we’ve made a good start at finding those objects that truly represent an impact hazard to Earth," said Lindley Johnson, program executive for the Near-Earth Object Observation Program at NASA Headquarters. "But we’ve many more to find, and it will take a concerted effort during the next couple of decades to find all of them that could do serious damage or be a mission destination in the future."

The new analysis suggests that about twice as many PHAs as previously thought reside in low-inclination orbits, which are roughly aligned with the plane of Earth’s orbit.

"Our team was surprised to find the overabundance of low-inclination PHAs," said Amy Mainzer, NEOWISE principal investigator, at NASA’s Jet Propulsion Laboratory. "Because they will tend to make more close approaches to Earth, these targets can provide the best opportunities for the next generation of human and robotic exploration."

The NEOWISE analysis suggests a possible origin for the low-inclinaton PHAs: Many of them could 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.

The lower-inclination PHAs appear to be somewhat brighter and smaller than other near-Earth asteroids. The discovery that 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 NEOWISE project, which wasn’t originally planned as part of WISE, has turned out to be a huge bonus," said Mainzer. "Everything we can learn about these objects helps us understand their origins and fate."

The NEOWISE results have been accepted for publication in the Astrophysical Journal.

IMAGE…New results from NASA’s NEOWISE survey find that more potentially hazardous asteroids, or PHAs, are closely aligned with the plane of our solar system than previous models suggested. PHAs are the subset of near-Earth asteroids (NEAs) with the closest orbits to Earth’s orbit, coming within 5 million miles (about 8 million kilometers). They are also defined as being large enough to survive passage through Earth’s atmosphere and cause damage on a regional, or greater, scale.

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.

The diagram shows that the orbits of the PHAs tend to be more closely aligned with the plane of our solar system, or less tilted above and below the plane, than the NEAs. This characteristic of PHAs was known before the NEOWISE survey. Now, NEOWISE has found the PHAs to be about twice as likely to have these “lower-inclination” orbits than previously thought.

To make these estimates, the NEOWISE project observed a small portion of the total PHA and NEA populations. The survey not only looked at the objects’ orbits but also their total numbers and physical properties such as size. The latest results provide the best count yet of the total PHA population, finding about 4,700 plus or minus 1,500, with diameters larger than 330 feet (about 100 meters). These numbers are in loose agreement with prior, rougher predictions. The NEOWISE team estimates that about 20 to 30 percent of the PHAs thought to exist have actually been discovered to date.

In an earlier study, NEOWISE estimated that there are about 20,500 near-Earth asteroids larger than 330 feet, fewer than previous estimates.

NEOWISE is the asteroid-hunting portion of NASA’s Wide-field Infrared Survey Explorer, or WISE, mission, which scanned the entire sky twice in infrared light before entering hibernation mode in 2011.

Image credit: NASA/JPL-Caltech

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