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Some, called long-period comets, are in elliptical orbits of the Sun that take them far out beyond the planets and back. Scientists believe that gravitational perturbations periodically jar comets out of this population, setting these “dirty snowballs” on orbital courses that bring them closer to the Sun. Comets are thought to originate from a region beyond the orbits of the outermost planets. From the beginning of life on Earth to the recent spectacular impact of Comet Shoemaker-Levy 9 with Jupiter, these so-called “small bodies” play a key role in many of the fundamental processes that have shaped the planetary neighborhood in which we live.Ĭomets are bodies of ice, rock, and organic compounds that can be several miles in diameter. What is a Potentially Hazardous Asteroid (PHA)?Īsteroids and comets are believed to be ancient remnants of the earliest years of the formation of our solar system more than four billion years ago.NEAs are further divided into the following groups according to their perihelion distance ( q),Īphelion distance ( Q) and their semi-major axes ( a): GroupĮarth-crossing NEAs with semi-major axes smaller than Earth’s (named after asteroid 2062 Aten).Ī > 1.0 au, q 1.0 au, 1.017 < q < 1.3 auĮarth-approaching NEAs with orbits exterior to Earth’s but interior to Mars’ (named after asteroid 1221 Amor). The vast majority of NEOs are asteroids, referred to as Near-Earth Asteroids (NEAs). In terms of orbital elements, NEOs are asteroids and comets with
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