claim: A near-Earth object is a small Solar System body.
evidence: A near-Earth object -LRB- NEO -RRB- is any small Solar System body whose orbit brings it into proximity with Earth .. small Solar System body. small Solar System body. Earth. Earth. By definition , a solar system body is a NEO if its closest approach to the Sun -LRB- perihelion -RRB- is less than 1.3 astronomical unit -LRB- AU -RRB- .. perihelion. Apsis. astronomical unit. astronomical unit. NEOs include more than fourteen thousand near-Earth asteroids -LRB- NEAs -RRB- , more than one hundred near-Earth comets -LRB- NECs -RRB- , and a number of solar-orbiting spacecraft and meteoroids , large enough to be tracked in space before striking the Earth .. Earth. Earth. spacecraft. spacecraft. 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 .. Earth. Earth. NEOs have become of increased interest since the 1980s because of increased awareness of the potential danger some of the asteroids or comets pose , and mitigations are being researched .. In January 2016 , NASA announced the Planetary Defense Coordination Office to track NEOs larger than 30 to 50 meters in diameter and coordinate an effective threat response and mitigation effort .. Planetary Defense Coordination Office. Planetary Defense Coordination Office. NASA. NASA. NEAs have orbits that lie partly between 0.983 and 1.3 AU away from the Sun .. When a NEA is detected it is submitted to the IAU 's Minor Planet Center for cataloging .. IAU. IAU. Minor Planet Center. Minor Planet Center. Some NEAs orbits intersect that of Earth 's so they pose a collision danger .. Earth. Earth. The United States , European Union , and other nations are currently scanning for NEOs in an effort called Spaceguard .. United States. United States. European Union. European Union. Spaceguard. Spaceguard. In the United States and since 1998 , NASA has a congressional mandate to catalogue all NEOs that are at least 1 kilometer wide , as the impact of such an object would be globally catastrophic .. United States. United States. NASA. NASA. congressional. United States Congress. In 2006 , it was estimated that 20 % of the mandated objects had not yet been found .. In 2011 , largely as a result of NEOWISE , it was estimated that 93 % of the NEAs larger than 1 km had been found and that only about 70 remained to be discovered .. NEOWISE. NEOWISE. , there have been 875 NEAs larger than 1 km discovered , of which 157 are potentially hazardous .. The inventory is much less complete for smaller objects , which still have potential for large scale , though not global , damage .. Potentially hazardous objects -LRB- PHOs -RRB- are currently defined based on parameters that measure the object 's potential to make threatening close approaches to the Earth .. Earth. Earth. Mostly objects with an Earth minimum orbit intersection distance -LRB- MOID -RRB- of 0.05 AU or less and an absolute magnitude -LRB- H -RRB- of 22.0 or brighter -LRB- a rough indicator of large size -RRB- are considered PHOs .. Earth. Earth. absolute magnitude. absolute magnitude. Objects that can not approach closer to the Earth -LRB- i.e. MOID -RRB- than 0.05 AU , or are smaller than about 150 m -LRB- 500 ft -RRB- in diameter -LRB- i.e. H = 22.0 with assumed albedo of 13 % -RRB- , are not considered PHOs .. Earth. Earth. albedo. albedo. NASA 's catalog for near-Earth objects also includes the approach distances of asteroids and comets measured in lunar distances , and this usage has become a common unit of measure used by the news media in discussing these objects .. Earth. Earth. NASA. NASA. lunar distances. Lunar distance ( astronomy ). Some NEOs are of special interest because they can be physically explored with lower mission velocity than for even the Moon , due to their combination of low velocity with respect to Earth -LRB- ΔV -RRB- and weak gravity , so they may present interesting scientific opportunities both for direct geochemical and astronomical investigation , and as potentially economical sources of extraterrestrial materials for human exploitation .. Earth. Earth. mission velocity. mission velocity. delta-V. This makes them an attractive target for exploration .. As of 2016 , three near-Earth objects have been visited by spacecraft .. Earth. Earth. spacecraft. spacecraft