An Earth impact by a large comet or asteroid could knock out human civilization with a single blow, as most people are now aware thanks to recent Hollywood movies and public outreach by planetary scientists. Since 1998, when NASA initiated its Spaceguard program to find comets and asteroids 1 km in diameter and larger, researchers have made some crucial inventories of the risky space rocks with orbits that come into close proximity of Earth. For instance, there are almost 1,000 of these so-called near-Earth objects with diameters of 1-kilometer or more.
However disconcerting this might seem, we can rest assured that none will make it here in our lifetimes. “We can say with a very good deal of certainty that no asteroid or comet large enough to threaten life as we know it will hit Earth in the next 100 years,” says Donald Yeomans. At NASA’s Jet Propulsion Laboratory, Yeomans is a senior research scientist and manager of the Near-Earth Object Program Office. He has spent his career studying the physical and dynamical modeling of near-Earth objects, as well as tracking them down.
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