Every other week or so, a small asteroid impacts Earth’s atmosphere and disintegrates – creating a very bright meteor technically known as a “bolide,” though “fireball” works too. These events are apparently random and surprisingly frequent: “It happens all the time,” as NASA puts it.
The agency's Near Earth Object (NEO) Observations Program has just released a map (above) that diagrams data gathered by U.S. government sensors from 1994 to 2013 on small asteroids—between one and nearly 20 meters in size—impacting our atmosphere.
Over this 20-year period, at least 556 bolide events of various energies were detected. The size of the orange dots (daytime impacts) and blue dots (nighttime impacts) are proportional to the “optical radiated energy” of the impact event measured in billions of Joules (Giga Joules, GJ) of energy. One GJ of optical radiant energy (the smallest dot on the map) is equivalent to the total impact energy of five tons of TNT explosives.
The dots representing 100, 10,000 and 1,000,000 GJ of optical radiant energies correspond to impact energies of 300 tons, 18,000 tons, and one million tons of TNT explosives, respectively. The largest impact energy recorded during this interval was the Chelyabinsk event over central Russia on February 15, 2013. This small asteroid was about 20 meters in size before it hit the planet, releasing about 440,000 to 500,000 tons worth of TNT.
"We now know that Earth's atmosphere does a great job of protecting Earth from small asteroids," NEO’s Lindley Johnson says in a news release. They simply just burn up harmlessly. The new data will help astronomers more precisely estimate the frequency of impacts by asteroids large enough to cause ground damage—those are the ones we need to worry about. "The aim is to find potentially hazardous asteroids before they find us," NEO’s Donald Yeomans adds.
Every day, the planet is bombarded with more than 100 tons of dust and sand-sized particles from space, and once a year, a car-sized asteroid will hit the atmosphere and disintegrate (sometimes explosively). Every 5,000 years or so, something the size of a football field will smash into Earth, causing significant damage. And then there are the objects so big that they cause regional—and even global—disaster. These smash into the planet once every few million years or so on average.
The NEO program has identified more than 96 percent of the estimated population of nearly 1,000 one-kilometer or larger asteroids. And their current objective is to identify at least 90 percent of the near-earth objects that are larger than 140-meters in diameter—these are 25 times more numerous than one-kilometer ones.