Near-Earth object

A near-Earth object (NEO) is any small Solar System body whose orbit brings it to proximity with Earth. By convention, a Solar System body is an NEO if its closest approach to the Sun (perihelion) is less than 1.3 astronomical units (AU). If an NEO's orbit crosses the Earth's, and the object is larger than 140 meters (460 ft) across, it is considered a potentially hazardous object (PHO). Most known PHOs and NEOs are asteroids, but a small fraction are comets. There are over 20,000 known near-Earth asteroids (NEAs), over a hundred short-period near-Earth comets (NECs), and a number of solar-orbiting spacecraft and meteoroids large enough to be tracked in space before striking the Earth. It is now widely accepted that collisions in the past have had a significant role in shaping the geological and biological history of the Earth. NEOs have become of increased interest since the 1980s because of greater awareness of the potential danger. Asteroids as small as 20 m can damage the local environment and populations. Larger asteroids penetrate the atmosphere to the surface of the Earth, producing craters or tsunamis. Asteroid impact avoidance by deflection is possible in principle, and methods of mitigation are being researched. Based on how well constrained the orbit calculations of identified NEOs are, two scales, the Torino scale and the more complex Palermo scale, rate a risk. Some NEOs have had temporarily positive Torino or Palermo scale ratings after their discovery, but as of March 2018, more precise calculations based on longer observation arcs led to a reduction of the rating to or below 0 in all cases. Since 1998, the United States, the European Union, and other nations are scanning for NEOs in an effort called Spaceguard. The initial US Congress mandate to NASA of cataloging at least 90% of NEOs that are at least 1 kilometre (0.62 mi) in diameter, which could cause a global catastrophe, had been met by 2011. In later years, the survey effort has been expanded to smaller objects having the potential for large-scale, though not global, damage. NEOs have low surface gravity, and many have Earth-like orbits making them easy targets for spacecraft. As of January 2019, five near-Earth comets and five near-Earth asteroids have been visited by spacecraft. A small sample was returned to Earth in 2010, and similar missions are in progress. Preliminary plans for commercial asteroid mining have been drafted by private companies. The major technical astronomical definition for Near-Earth objects (NEOs) are small Solar System bodies with orbits around the Sun that by definition lie partly between 0.983 (perihelion) and 1.3 (aphelion) astronomical units (AU; Sun–Earth distance) away from the Sun. Thus, NEOs are not necessarily currently near the Earth, but they can potentially approach the Earth relatively closely. However, the term is also used more flexibly sometimes, for example for objects in orbit around the Earth or for quasi-satellites, which have a more complex orbital relationship with the Earth. When a NEO is detected, like all other small Solar System bodies, it is submitted to the International Astronomical Union's (IAU's) Minor Planet Center (MPC) for cataloging. MPC maintains separate lists of confirmed NEOs and potential NEOs. The orbits of some NEOs intersect that of the Earth, so they pose a collision danger. These are considered potentially hazardous objects (PHOs) if their estimated diameter is above 140 meters. For the asteroids among PHOs, the potentially hazardous asteroids (PHAs), MPC maintains a separate list. NEOs are also catalogued by two separate units of the Jet Propulsion Laboratory (JPL) of the National Aeronautics and Space Administration (NASA): the Center for Near Earth Object Studies (CNEOS) and the Solar System Dynamics Group. PHAs are currently defined based on parameters relating to their potential to approach the Earth dangerously closely. Mostly objects with an Earth minimum orbit intersection distance (MOID) of 0.05 AU or less and an absolute magnitude of 22.0 or brighter (a rough indicator of large size) are considered PHAs. Objects that cannot approach closer to the Earth (i.e. MOID) than 0.05 AU (7,500,000 km; 4,600,000 mi), or are smaller than about 140 m (460 ft) in diameter (i.e. H = 22.0 with assumed albedo of 14%), are not considered PHAs. NASA's catalog of near-Earth objects also includes the approach distances of asteroids and comets (expressed in lunar distances).