The Earth orbits the Sun in a sort of cosmic shooting gallery, subject to impacts from comets and asteroids. It is only fairly recently that we have come to appreciate that these impacts by asteroids and comets (often called Near Earth Objects, or NEOs) pose a significant hazard to life and property. Most of the hazard is from asteroid impacts; comets make only a minor contribution. Although the annual probability of the Earth being struck by a large asteroid or comet is extremely small, the consequences of such a collision are so catastrophic that it is prudent to assess the nature of the threat and prepare to deal with it.

Studies have shown that the risk from cosmic impact increases with the size of the projectile. The greatest risk is associated with objects large enough to perturb the Earth's climate on a global scale by injecting large quantities of dust into the stratosphere. Such an event could depress temperatures around the globe, leading to massive loss of food crops and possible breakdown of society. Such global catastrophes are qualitatively different from other more common hazards that we face (excepting nuclear war), because of their potential effect on the entire planet and its population. Various studies have suggested that the minimum mass impacting body to produce such global consequences is several tens of billions of tons, resulting in a groundburst explosion with energy in the vicinity of a million megatons of TNT. The corresponding threshold diameter for an asteroid is between 1 and 2 km. Smaller objects (down to tens of meters diameter) can cause severe local damage but pose no global threat.

For the past 15 years we have addressed this impact hazard by carrying out a comprehensive telescopic search for potentially hazardous near-Earth asteroids (NEAs). This program is called the Spaceguard Survey, and it has resulted in the discovery of approximately ten thousand NEAs, including more than 90% of those larger than 1 km (which was the goal of this survey). The U.S. Congress has now mandated that this survey be extended to 90% completeness at a diameter of 140 meters. The are several proposals to meet this goal, using a combination of larger ground-based survey telescopes and space-based infrared surveys. There is also increased interest in possible impacts at small sizes, such as the Tunguska airburst in Siberia in 1908 or the airburst over Chelyabinsk, Russia, on February 15, 2013.

Today no known asteroid is on a collision course with the Earth, and the Spaceguard Survey does not expect to find any large asteroid that directly threatens us. If, however, such a rock is discovered on a collision course, then we anticipate that we would apply appropriate technology to deflect it before it hits. Asteroid impacts are the only natural hazard that we can, in principle, eliminate entirely.

HISTORICAL TIMELINE FOR IMPACT HAZARD 

1758: Recovery of Comet Halley confirms comets have periodic orbits
1801: G. Piazzi at Palermo discovers first asteroid (Ceres)
1803: French Academy concludes meteorites have extraterrestrial origin
1819: Non-gravitational forces are identified altering orbit of Comet Encke
1893: G.K.Gilbert suggests that lunar craters are the result of impacts
1898: Discovery of Eros, first NEA
1908: Tunguska impact (5-10 Mt) in Russian Siberia
1932: Discovery of Apollo, first Earth-crossing NEA
1947: IAU Minor Planet Center established
1947: Sikhote-Alin iron meteorite strike in western Russia
1949: Ralph Baldwin’s The Face of the Moon discusses Earth impacts
1952: E. Opik “Collisions with heavenly bodies” estimates impact risk
1959: Pribram meteorite fall traced to asteroid belt
1960: E. Shoemaker shows Meteor Crater to be impact feature
1961: E. Shoemaker identifies impact origin of Ries Crater
1964: R. Dietz identifies Vredefort & Sudbury as impact structures
1968: Close encounter with Icarus; first NEA radar detection
1969: MIT study of deflection technology “Project Icarus”
1969: Allende and Murchison carbonaceous meteorite falls
1969: Discovery by Japanese scientists of first Antarctic meteorites
1971: First IAU Colloquium on Physical Studies of Minor Planets
1972: Daylight Fireball over western U.S. and Canada: Earth-grazing asteroid
1973: Shoemakers begin long-term Palomar NEO photo search
1977: Novel Lucifer’s Hammer (Niven & Pournelle) depicts comet impact
1979: Hollywood film Meteor depicts joint US/USSR NEA deflection
1980: Identification of KT mass extinction with cosmic impact (L. Alvarez)
1981: NASA workshop “Collision of Asteroids and Comets with the Earth”
1981: E. Shoemaker makes first modern estimate of impact hazard
1981: First Snowbird conference on “Geology of Large Body Impacts”
1981: Chicxulub 180km crater discovered in Yucatan (Camargo & Penfield)
1983: First international ACM conference on “Asteroids, Comets, Meteors”
1984: Spacewatch (Gehrels & McMillan) first CCD discovery of NEAs
1989: Discovery of Toutatis, largest known hazardous asteroid (PHA)
1989: First radar image of NEA (Castalia), Ostro & Hudson, using Arecibo
1989: NEA Asclepius close pass (684,000 km) is major news story
1989: Chapman & Morrison propose that greatest risk is from NEAs >1km.
1990: AIAA recommends impact study to U.S. Congress
1991: U.S. House bill directs NASA to study impact risk and defense
1991: NASA International NEO Detection workshops (D. Morrison chair)
1991: IAU establishes Working Group on NEAs
1991: Chicxulub impact crater tied to KT mass extinction (A. Hildebrand)
1992: NEO Interception Workshop, Los Alamos NM
1992: Peekskill fireball, good orbit determined, meteorite struck car
1993: L. Johnson coins term “planetary defense”
1993: Major workshop “Hazards Due to Comets & Asteroids” (Tucson)
1993: First U.S. Congress hearing on “The Threat of Large NEAs”
1993: Workshop on Impact Hazard (Erice) (P. Worden chair)
1994: Marshall Islands fireball & airburst, estimated at 100 kT
1994: Collision of Comet Shoemaker-Levy 9 with Jupiter
1994: IAU establishes Working Group on NEOs (A. Carusi chair)
1994: International Conference “Space Protection of Earth” (Snezhinsk)
1994: Publication of Hazards Due to Comets & Asteroids (T. Gehrels ed.)
1995: Report of NASA NEO Survey Working Group (E. Shoemaker chair)
1995: U.N. conference on NEOs, New York
1995: Start of NASA/JPL Near Earth Asteroid Tracking (NEAT) program
1996: Council of Europe resolution on detection of asteroids and comets
1995: Foundation of Spaceguard Foundation in Italy
1996: Foundation of Japanese Spaceguard Association
1996: Foundation of Space Shield Foundation in Russia
1996: Foundation of Spaceguard UK
1997: Start Lincoln NEA Research (LINEAR) using U.S. Air Force telescopes
1997: British TV documentary “The Day the Earth Got Hit”
1997: Great Comet of 1997 (Hale-Bopp) visible to naked eye for >1 yr
1998: Start of Lowell Observatory NEO Search (LONEOS)
1998: B. Marsden warns media of possible impact by NEA 1997XF11
1998: Start of Catalina Sky Survey
1998: NASA announces start of Spaceguard Survey to find 90% NEAs >1km
1998: IAU concludes detection of NEOs is an “international responsibility”
1998: Spectacular Leonid meteor shower
1998: Hollywood films Deep Impact and Armageddon released
1999: NASA NEO Program Office established at JPL (D. Yeomans manager)
1999: Threat from NEOs is debated in UK Parliament
1999: Orbit analysis of 1999AN10 indicates multiple resonant returns
1999: Fastest spinning NEA 1998KY26 (10 min) found at Arecibo
1999: NEO Dynamics (NEODyS) website established in Pisa
1999: Torino Impact Hazard scale adopted by IAU & NASA
2000: Tagish Lake (Canada) meteorite, exceptionally primitive object
2000: NASA/APL NEAR-Shoemaker spacecraft orbits NEA Eros
2000: Report of UK Task Group on Potentially Hazardous NEOs
2000: Spaceguard Survey half done; 900 NEAs known
2000: First binary NEA 2000DP107 found by radar
2001: NEAR spacecraft lands on Eros, operates for 10 days
2002: Sentry automatic orbital monitoring system at JPL
2002: NEA 1950DA has 2080 collision probability of 1 in 300
2002: NEA 2002MN reported by media as “near miss” at 120,000 km
2002: B612 Foundation formed to promote asteroid defense
2003: NASA mitigation workshop recommends extending survey down to 200m
2003: NASA Science Definition Team (G. Stokes) reports on survey options
2004: Radar studies of Golevka demonstrate Yarkovsky effect
2004: ESA plans Don Quijote mission to test kinetic impact deflection
2004: First biennial Planetary Defense Conference in California
2004: Apophis (2004NM4) briefly reaches Torino=4 with 2% impact probability
2005: ICSU workshop in Tenerife on Impacts and Human Society
2005: Congress mandates 90% NEA survey down to 140m
2005: 3000 NEAs discovered so far, 90% by Spaceguard Survey
2005: JAXA Hayabusa reaches Itokawa and collects samples
2005: NASA Deep Impact hits comet Tempel 1
2006: Apophis is focus of orbital dynamics studies of keyholes
2007: Carancas high-velocity meteorite impact in Peruvian Andes
2007: NASA Program Analysis Group proposes ground and space surveys
2008: Association of Space Explorers issues Call for Global Response
2008: Small NEA 2008TC3 tracked for 19 hrs before hit in Sudan
2008: Almahata Sitta meteorites recovered from 2006TC3
2009: UN forms Action Team 14 on impact defense (Comacho-Lara chair)
2009: ESA Space Situational Awareness program (SSA-NEO)
2010: Obama announces NASA goal of human mission to NEA by 2025
2010: NASA EPOXI high-res images of Comet Hartley 2
2010: Pan-STARRS (Hawaii) survey begins with USAF support
2010: NEOWISE space IR survey finds 129 NEAs, measures albedos
2010: JAXA Hayabusa returns samples to Earth
2011: Spaceguard Survey reaches of goal of 90% of NEAs >1km
2012: UN Action Team recommends integrated international effort
2013: New data eliminate possibility of Apophis impact in 2036
2013: Inauguration of ESA NEO Coordination Center
2013: Asteroid airburst with 0.5Mt energy over Cheyabinsk Russia
2013: NEA 2012DA14 (30-40m) flies by Earth at 28,000 km altitude
2013: Number of known NEAs passes 10,000

David Morrison, May 2013