Map Shows Frequency of Small Asteroid Impacts, Provides Clues on Larger Asteroid Population

December 17, 2014

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It happens all the time: small asteroids impact Earth's atmosphere

Small asteroids near Earth, with sizes of only about a meter, hit the atmosphere and disintegrate with surprising frequency - around every other week, new data show.

Data gathered by U.S. government sensors and released to NASA for use by the science community reveal that these small impact events are frequent and random. A map of these small impact events - known as fireballs or bolides - recently released by NASA shows the frequency and approximate energy released by bolide events detected from 1994 through 2013. It dwarfs a data-base of small impacts based on infra-sound detections released last fall, but it does not contain all fireballs - objects less than a meter in size - that impacted the Earth during this period.

Bolide Events 1994-2013

Bolide Events 1994-2013 (click for larger image)

Over this 20-year interval, U.S. Government assets recorded at least 556 bolide events of various energies. On this world map illustration, the size of the orange dots (daytime events) and blue dots (nighttime events) are proportional to the optical radiated energy of the impact event measured in billions of Joules (GJ) of energy. An approximate conversion between the measured optical radiant energy and the total impact energy can be made using an empirical relationship provided by Peter Brown and colleagues in 2002. For example the smallest dot on the map represents 1 billion Joules (1 GJ) of optical radiant energy, or when expressed in terms of a total impact energy the equivalent of about 5 tons of TNT explosives. Likewise, the dots representing 100, 10,000 and 1,000,000 Giga Joules of optical radiant energies correspond to impact energies of about 300 tons, 18,000 tons and one million tons of TNT explosives respectively.

The largest impact energy recorded during this 20-year interval was the recent daytime Chelyabinsk event (440,000 - 500,000 tons of TNT) recorded over central Russia on February 15, 2013. This small asteroid that exploded in the atmosphere near Chelyabinsk, Russia was about 20 meters in size before it hit the Earth. While that impact focused public attention on the potential hazards of NEO impacts with Earth, space scientists have long known that such events are just a part of Earth's geologic history.

NASA's Near Earth Object (NEO) Observations Program finds, tracks, and characterizes asteroids whose orbits bring them within approximately 50 million kilometers (31 million miles) of Earth's orbit about the sun.

"We now know that Earth's atmosphere does a great job of protecting Earth from small asteroids", said NASA NEO Observations Program Executive Lindley Johnson. The new data will be extrapolated to estimate more precisely the frequency of impacts by asteroids large enough to cause ground damage. "How big is the population of larger asteroids we really need to worry about? We need to better understand that." Johnson said.

While the new data emphasize that small asteroid impacts with Earth are not unusual, the risk of future impacts is not to be taken lightly. "The aim is to find potentially hazardous asteroids before they find us," said Donald Yeomans, manager of NASA's NEO Program Office at the Jet Propulsion Laboratory.

NASA's Asteroid Initiative features a Grand Challenge to the community "to create a plan to find all asteroid threats to human populations and know what to do about them."

The NEO Observations Program already has identified more than 96 percent of the estimated population of nearly one thousand one-kilometer or larger sized asteroids. The Program's current objective is to identify 90 percent or more of the far more numerous NEOs larger than 140-meters in diameter. It is estimated they may be as much as 25 times more numerous than 1 kilometer asteroids.

Every day, Earth is bombarded with more than 100 tons of dust and sand-sized particles from space. About once a year, an automobile-sized asteroid hits Earth's atmosphere, creating a spectacular fireball (bolide) event as the friction of the Earth's atmosphere causes them to disintegrate - sometimes explosively.

Studies of Earth's history indicate that about once every 5,000 years or so on average an object the size of a football field hits Earth and causes significant damage. Once every few million years on average an object large enough to cause regional or global disaster impacts Earth. Impact craters on Earth, the Moon and other planetary bodies are evidence of these occurrences.

Meteor Crater near Winslow, Arizona, is evidence of the impact with Earth's surface of a 50-meter asteroid about 50,000 years ago. Impact of the metal-rich object released energy equivalent to a 10 megaton explosion and formed a 1.2 kilometer-diameter crater. Scientists have identified several dozen impact craters in North America alone, most masked by erosion and vegetation.

Scientific assessments of the risk of, as well as the hazards posed by, future asteroid impacts with Earth vary. In a 2013 paper published in Nature, Peter Brown and his colleagues reported that "telescopic surveys have only discovered about 500 near-Earth asteroids that are 10-20 meters in diameter (comparable to the Chelyabinsk asteroid) of an estimated near-Earth asteroid population of around 2 x 10 7 [20 million], implying that a significant impactor population at these sizes could be present but not yet cataloged in the discovered near-Earth asteroid population."

"These newly released data will help NEO scientists construct a more complete picture of the frequency and scope of asteroid impacts with Earth," said Johnson.

In conducting its work, the NEO Observations Program collaborates with other U.S. government agencies, other national and international entities, and professional and amateur astronomers around the world. NASA works closely with the Federal Emergency Management Agency and other federal government departments and agencies on NEO impact warning, mitigation and response planning. The Program is responsible for facilitating communications between the astronomical community, the federal government and the public about NEO impact hazards and risks. The NEO Observations Program is a lead participant in a newly organized International Asteroid Warning Network.

For more information:

For a documented list of bolide events, see:


P.G. Brown et al., The flux of small near-Earth objects colliding with the Earth. Nature, vol. 420, 21 Nov. 2002, pp. 294-296

P.G. Brown et al., A 500-kiloton airburst over Chelyabinsk and an enhanced hazard from small impactors, Nature 503, 14 November 2013

Reprinted from Near Earth Orbit Program News.



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