Asteroids are discovered by telescopes which repeatedly survey large areas of sky. Efforts which concentrate on discovering NEOs are considered part of the "Spaceguard Survey," regardless of which organization they are affiliated with.
The United Kingdom hosts the self-styled The SpaceGuard Centre which conducts astrometric research (MPC code J26) and is open to the general public daily, but it is not affiliated with or supported by any public body.
Arthur C. Clarke coined the term in his novel Rendezvous with Rama (1972) where "Project Spaceguard" was the name of an early warning system created following a fictional catastrophic asteroid impact. This name was later adopted by a number of real life efforts to discover and study near-Earth objects. The name was used for the Survey "with the permission and encouragement of Clarke."
A 1992 US Congressional study produced a "Spaceguard Survey Report" which led to a mandate that NASA locate 90% of near-Earth asteroids larger than 1 km within 10 years. This is often referred to as the "Spaceguard Goal." A number of efforts which receive money through NASA are all considered to be working on the "Spaceguard Project."
The effect of the impact of Comet Shoemaker–Levy 9 to Jupiter in July 1994 created a greater perception of importance to the detection of near Earth objects. As David Levy stated in an interview "The giggle factor disappeared after Shoemaker-Levy 9." He was referring to the contemporary attitude that extinction level events were so improbable that those advocating for research for detection and possible deflection methods were only paranoid alarmists. The impact of one of its fragments created a giant dark spot on Jupiter over 12,000 km across, and was estimated to have released an energy equivalent to 6,000,000 megatons of TNT (600 times the world's nuclear arsenal). After the impact of Comet Shoemaker-Levy 9, asteroid detection programs all over the world received greater funding.
The Working Group on Near-Earth Objects (WGNEO) of the International Astronomical Union held a workshop in 1995 entitled Beginning the Spaceguard Survey which led to an international organization called the Spaceguard Foundation. Subsequently, there have been Spaceguard associations or foundations formed in countries around the world to support the ideas of discovering and studying near-Earth objects. Generally, the Spaceguard organizations formed within individual countries are associated with the international foundation or with the NASA efforts only by name, common interests, and similar goals.
The initial Spaceguard Goal was achieved, although in slightly longer than 10 years. An extension to the project gave NASA the mandate of reducing the minimum size at which more than 90% of near-Earth asteroids are known to 140 m.
New survey projects, such as the Asteroid Terrestrial-impact Last Alert System (ATLAS) program operated by the University of Hawaii, aim to greatly increase the number of small (down to approximately 10 m) impactors that are discovered before atmospheric entry—typically with days to weeks of warning, enabling evacuations of the affected areas and damage mitigation planning. This is in contrast to other surveys which focus on finding much larger (greater than 100 m) objects years to decades before any potential impacts, at times when they could potentially still be deflected away from Earth.
Another short-term warning system is the NASA Scout program that came into operation in 2016.
According to Dr. Michael F. A'Hearn, a typical mission would take too long from approval to launch if there was an emergency:
REP. STEWART: ... are we technologically capable of launching something that could intercept [an asteroid]? ... DR. A'HEARN: No. If we had spacecraft plans on the books already, that would take a year ... I mean a typical small mission ... takes four years from approval to start to launch ...
^Levy, David H. (2010-09-09). Cosmic Discoveries: The Wonders of Astronomy. Prometheus Books. p. 169. ISBN 978-1-61592-566-7. In January 1993, [Levy] attended a meeting about hazards that could be caused from comets and asteroids. One of the biggest concerns that attending scientists had was what to do about the "giggle factor." Whenever anyone suggested that comets could post a hazard, the press responded with sarcasm. In those early days of 1993, it was difficult to make anyone take the threat seriously. [...] How could anyone know that at that meeting three of its participants would discover, within a few weeks, a comet that would spotlight the consequences of impacts very effectively?
^Beginning the Spaceguard Survey, Vulcano Workshop (1995), IAU Working Group on Near-Earth Objects. (accessed on 2010-06-26)
^Harris, Alan. What Spaceguard did, Nature, Vol. 453, pp. 1178–1179, June 26, 2008, doi:10.1038/4531178a; Published online 25 June 2008 (subscription required).
^Asteroid Terrestrial-impact Last Alert System Project (ATLAS), FallingStar.com website, last revised on March 29, 2013.
^Tonry, John L. An Early Warning System for Asteroid Impact (thesis), Cornell University Library, submitted on November 3, 2010.
^NASA's New 'Intruder Alert' System Spots An Incoming Asteroid
^Incoming! New Warning System Tracks Potentially Dangerous Asteroids
^"ESO Observations Show First Interstellar Asteroid is Like Nothing Seen Before". European Southern Observatory. Archived from the original on 15 May 2018. Retrieved 15 May 2018.
^U.S.Congress (Spring 2013). "Threats From Space: a Review of U.S. Government Efforts to Track and mitigate Asteroids and Meteors (Part I and Part II) - Hearing Before the Committee on Science, Space, and Technology House of Representatives One Hundred Thirteenth Congress First Session" (PDF). United States Congress (Hearings held 19 March 2013 and 10 April 2013). p. 147. Retrieved 3 May 2014.
^Stefan Lovgren (2004-03-08), "Asteroid False Alarm Shows Limits of Alert Systems, National Geographic News. (accessed on 2010-06-26)
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Spaceguard Central Node
NASA's Near-Earth Office Program
NEO News: Spaceguard Survey of Near Earth Asteroids David Morrison Tuesday, August 1, 2000 (Discovery statistics by 2000)
Progress in the Spaceguard Survey Saturday, February 9, 2002 (Status report by 2002)