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Tally of most hazardous asteroids doubles

A snapshot of the population of near-Earth objects and PHAs that scientists think are likely to exist.


NASA’s efforts to scan the skies for asteroids that could pose a danger to Earth have found twice as many sizeable chunks of space rubble making close approaches to the planet than previously predicted.

The US space agency announced on Wednesday that its asteroid-hunting mission, NEOWISE, performed by the Wide-Field Infrared Survey Explorer satellite telescope, had made its “best assessment yet” of the Solar System’s population of potentially hazardous asteroids, or PHAs.

PHAs are the subset of  near-Earth objects that have the closest orbits to that of our planet — coming within 8 million kilometres — and are also large enough to survive hurtling through the atmosphere to cause damage on a regional or greater scale. An earlier analysis by NEOWISE pegged the number of near-Earth objects at 20,500, fewer than had been predicted.

NEOWISE looked at 107 PHAs to make predictions about the wider population. The new estimates indicate the existence of some 4,700 PHAs, plus or minus 1,500, with a diameter wider than 100 metres. An asteroid of that size could destroy a metropolitan area or cause a major tsunami.

Roughly 20–30% of the PHAs out there have been found. A total of 843 planet-killers have been discovered, but just 152 are classed as PHAs. Similar estimates have been made in the past, but NASA describes them as just “rough approximations”. “Neowise has generated a more credible estimate of the objects’ total numbers and sizes,” the agency says. “The results reveal new information about their total numbers, origins and the possible dangers they may pose.” The data behind the analysis were collected before WISE was put into hibernation last year.

Although there are fewer near-Earth objects than earlier believed, the projections also indicate that there are about twice as many PHAs as previously thought that have “lower-inclination” orbits, which are more aligned with the plane of Earth’s orbit and so are more likely to hit the planet. According to NASA, this may mean that this subgroup of near-Earth objects is slightly more hazardous than had been thought.

It’s not all bad news for planetary defence, however. The results also mean it may be easier to access these asteroids in future robotic or human missions. “The NEOWISE analysis shows us we’ve made a good start at finding those objects that truly represent an impact hazard to Earth,” says Lindley Johnson, programme executive for the Near-Earth Object Observation Program at NASA headquarters in Washington DC

“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.”


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