NASA’s Wide-Field Infrared Explorer (WISE) spacecraft was used to conduct a survey of Near-Earth-Objects, some of which could pose a threat to the Earth. Image Credit: NASA

The Menace of the Asteroids. Part 1: The Risks

By David Darling

Of the several hundred thousand asteroids known, about 9,000 are of the near-Earth variety, and of these more than 1,300 fall into the category known as potentially hazardous asteroids (PHAs). To qualify as a PHA, an asteroid must be able to come closer to the Earth than 7.5 million kilometers in its orbit and be larger than 100 to 150 meters in diameter. An asteroid of this size, if it impacted in a sensitive area, could cause significant regional devastation, laying waste to a major city or even an entire state.

Unless steps are taken to do something about it, the Earth will be hit hard again in the future. Collisions such as the one that happened 65 million years ago, involving an asteroid roughly 10 kilometers across which slammed into what is now Mexico’s Yucatan Peninsula and contributed to a mass extinction, are extremely rare. However, smaller but still potentially devastating impacts happen much more often. As recently as 1908 an object roughly 50 meters across—either an asteroid or part of a comet nucleus—exploded high above the Tuguska region of Siberia, flattening 80 million trees radially outward from the epicenter of the blast. A similar encounter over a highly populated area would cause massive disruption and loss of life.

Read more: The Menace of the Asteroids. Part 1: The Risks « AmericaSpace.

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