214 J. Johnson et al. Fig. 22.1 Prograde orbits: unstable trajectories with similar initial conditions that quickly diverge from each other. One escapes the system (red), while the other crashes (yellow) Fig. 22.2 Retrograde orbits: stable past 1 month while staying close to one another 22.2 Background Near-Earth Objects are asteroids or comets in our Solar System that approach Earth’s orbit. More specifically, their perihelion distance is less than 1.3 Astronomical Units (AU) [2]. The perihelion distance is the body’s distance to the Sun at its closest point to the Sun it its orbit (Fig. 22.3). One AU is the average distance between the Sun and the Earth, and is used as a standard of measurement ( 1.4960 1011 m). Therefore, an asteroid or comet is considered an NEO if the closest point in its orbit around the Sun is less than 1.3 times the average distance between the Sun and the Earth. Figure 22.4 shows NEO types recognized by NASA which has a team of scientists dedicated to discovering and tracking NEOs in the Solar System. There is a great interest in NEOs since (1) they are studied by the planetary science community, and provide clues for topics in this discipline such as the formation of our Solar System, (2) they posses raw materials that can be mined and used to collect in-situ resources for manned and unmanned spaceflight, and (3) they have the potential to collide with Earth, with an immense amount of kinetic energy, causing catastrophic destruction. At the time of this writing, there are over 1,500 Potentially Hazardous Asteroids (PHAs) that are being tracked and monitored by NASA [3]. The term potentially hazardous does not mean that an impact with Earth is imminent, only that the orbit and size of the object are cause for concern. As new observations of these PHAs become available, scientists can better predict their orbit and asses the likelihood of a future close approach to Earth.
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