Earth-approaching comets (comets with orbits that approach the Sun at distances less than 1.3 astronomical units [AU] and have periods less than 200 years), of which 151 are currently known. These represent a class of objects probably doomed to be perpetually only partly known, as they are not likely to be detected in advance of a close Earth encounter. These objects, after the completion of exhaustive searches for NEOs, could dominate the impact threat to humanity.

Thus, assessing the completeness of the NEO surveys is subject to uncertainties: Some groups of NEOs are particularly difficult to detect. Asteroids and comets are continually lost from the NEO population because they impact the Sun or a planet, or because they are ejected from the solar system. Some asteroids have collisions that change their sizes or orbits. New objects are introduced into the NEO population from more distant reservoirs over hundreds of thousands to millions of years. The undiscovered NEOs could include large objects like 2009 HC82 as well as objects that will be discovered only months or less before Earth impact (“imminent impactors”). Hence, even though 85 percent of NEOs larger than 1 kilometer in diameter might already have been discovered, and eventually more than 90 percent of NEOs larger than 140 meters in diameter will be discovered, NEO surveys should nevertheless continue, because objects not yet discovered pose a statistical risk: Humanity must be constantly vigilant.

Finding: Despite progress toward or completion of any survey of near-Earth objects, it is impossible to identify all of these objects because objects’ orbits can change, for example due to collisions.

Recommendation: Once a near-Earth object survey has reached its mandated goal, the search for NEOs should not stop. Searching should continue to identify as many of the remaining objects and objects newly injected into the NEO population as possible, especially imminent impactors.


Recognizing that impacts from near-Earth objects represent a hazard to humanity, the United States, the European Union, Japan, and other countries cooperatively organized to identify, track, and study NEOs in an effort termed “Spaceguard.” From this organization, a nonprofit group named the Spaceguard Foundation was created to coordinate NEO detection and studies; it is currently located at the European Space Agency’s (ESA’s) Centre for Earth Observation (ESRIN) in Frascati, Italy. The United States input to this collective effort comprises three aspects: telescopic search efforts to find NEOs, the Minor Planet Center (MPC) at the Harvard-Smithsonian Center for Astrophysics, and the NASA NEO Program Office at the Jet Propulsion Laboratory. Existing, retired, and proposed telescopic systems for the U.S. NEO searches are detailed below. Other telescopic survey, detection, and characterization efforts are conducted worldwide and work synergistically with U.S. telescopic searches (e.g., Asiago-DLR Asteroid Survey, jointly operated by the University of Padua and the German Aerospace Center [DLR], Campo Imperatore Near-Earth Object Survey at Rome Observatory; and the Bisei Spaceguard Center of the Japanese Spaceguard Association). To date, the U.S. search effort has been the major contributor to the number of known NEOs. The functions of the two U.S. data- and information-gathering offices, the MPC and the NEO Program Office, are complementary. A European data- and information-gathering office, the Near-Earth Objects Dynamic Site (NEODyS) is maintained at the University of Pisa in Italy, with a mirror site at the University of Valladolid in Spain. These three services are described below.

Minor Planet Center

The MPC serves as the clearinghouse for positional information from the observers of minor planets (including all asteroids) from all observatories around the world. The MPC is charged with processing and publishing every single positional measurement made, worldwide, of asteroids, comets, and outer satellites of the Jovian planets. Its efforts are sanctioned by the International Astronomical Union (IAU), the international professional society for astronomers. The IAU provides guidance but currently only minor financial support for the MPC. Current MPC efforts are supported mostly by NASA’s Near-Earth Object Program, with a much smaller contribution from the Smithsonian Institution.

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