[meteorite-list] Newly Released Map Data Shows Frequency of Small Asteroid Impacts, Provides Clues on Larger Asteroid Population

[Ron Baalke]

http://neo.jpl.nasa.gov/news/news186.html

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

Linda Billings & NASA/JPL Near-Earth Object Program Office
November 14, 2014

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.

[Graphic]
Map showing the bolide events from 1994 to 2013

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:

http://science.nasa.gov/planetary-science/near-earth-objects/

http://www.nasa.gov/mission_pages/asteroids/overview/fastfacts.html

For a documented list of bolide events, see:

http://neo.jpl.nasa.gov/fireball

References:

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