Number and Classification of Near-Earth Objects
While orbiting the Sun, most potential impactors can be classified as meteoroids, asteroids, or comets depending on size and composition. Asteroids can also be members of an asteroid family, and comets can leave debris in their orbits.
As of May 2012, 8,971 NEOs have been discovered: 91 near-Earth comets and 8,880 near-Earth Asteroids. Of those there are 704 Aten asteroids, 3,339 Amor asteroids, and 4,826 Apollo asteroids. There are 1,304 NEOs that are classified as potentially hazardous asteroids (PHAs). Currently, 152 PHAs and 843 NEAs have an absolute magnitude of 17.75 or brighter, which roughly corresponds to at least 1 km in size.
As of May 2012, there are 401 NEAs on the Sentry impact risk page at the NASA website. A significant number of these NEAs – 215 as of May 2010 – are equal to or smaller than 50 meters in diameter and none of the listed objects are placed even in the "yellow zone" (Torino Scale 2), meaning that none warrant the attention of general public. As of November 2011, only asteroids 2007 VK184 and 2011 AG5 are listed as having a Torino Scale of 1.
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