Query: A near-Earth object is a tiny Solar System entity.

Relevant passages:
Near-Earth object <=SEP=> A near-Earth object ( NEO ) is any small Solar System body whose orbit brings it into proximity with Earth . By definition , a solar system body is a NEO if its closest approach to the Sun ( perihelion ) is less than 1.3 astronomical unit ( AU ) . NEOs include more than fourteen thousand near-Earth asteroids ( NEAs ) , more than one hundred 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 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 .   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 . Some NEAs orbits intersect that of Earth 's so they pose a collision danger . The United States , European Union , and other nations are currently scanning for NEOs in an effort called 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 . 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 . , 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 ( PHOs ) are currently defined based on parameters that measure the object 's potential to make threatening close approaches to the Earth . Mostly objects with an Earth minimum orbit intersection distance ( MOID ) of 0.05 AU or less and an absolute magnitude ( H ) of 22.0 or brighter ( a rough indicator of large size ) are considered PHOs . Objects that can not approach closer to the Earth ( i.e. MOID ) than 0.05 AU , or are smaller than about 150 m ( 500 ft ) in diameter ( i.e. H = 22.0 with assumed albedo of 13 % ) , are not considered PHOs . 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 .   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 ( ΔV ) 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 . This makes them an attractive target for exploration . As of 2016 , three near-Earth objects have been visited by spacecraft .