There are roughly 4,700 asteroids — give or take 1,500 — that come close enough to Earth to pose a hazard, new estimates from NASA reveal. According to the space agency, potentially hazardous asteroids, or PHAs, are large enough to pass through Earth’s atmosphere without being burnt to a crisp and cause widespread damage.
These space rocks are bigger than 330 feet across (that’s slightly less than the length of a football field ) and come within 5 million miles of Earth.
About 20 to 30 per cent of the 4,700 PHAs have been found, NASA says.
The observations were made by the agency’s WISE spacecraft, which scans the sky with infrared light to detect the heat of asteroids. Unlike your run-of-the-mill telescope where different size asteroids appear as uniform tiny dots, WISE is able to sense the difference between objects that are small and very reflective and objects that are large and very dark. This helps astronomers determine the composition and size of asteroids, which is key to assessing their potential risk to Earth.
The new figure suggests that about twice as many PHAs are closely aligned with the plane of Earth’s orbit than previously thought. Asteroids with “lower-inclination” orbits have two implications: they would be more likely to approach our planet, but are easier for spacecraft to reach for future exploration missions.
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