Planetary Defense: Potential Mitigation Roles of the Department of Defense

By Garretson, Peter; Kaupa, Douglas | Air & Space Power Journal, Fall 2008 | Go to article overview

Planetary Defense: Potential Mitigation Roles of the Department of Defense


Garretson, Peter, Kaupa, Douglas, Air & Space Power Journal


EARTH'S ORBIT AROUND the sun is a hazardous location, and our collective safety so far has been purely a matter of luck. Despite the image of a pristine "harmony of spheres" that we inherited from the ancients, the solar system is a cosmic shooting gallery filled with leftover debris from planetary formation. This debris, including asteroids and comets, orbits the sun at relative velocities of 11-25 kilometers (km) per second or 10 times faster than a speeding bullet.1 As our planet transits this dangerous ocean, we have established no worldwide security network to warn of or mitigate collisions with space debris.

Both a position paper by the American Institute for Aeronautics and Astronautics entitled "Protecting Earth from Asteroids and Comets" (2004) and a 2007 planetary-defense conference in Washington, DC, examined the issue of finding a home in government for asteroid defense, designating it a top priority.2 This article advocates establishing a lead agency, such as US Strategic Command (STRATCOM), for handling mitigation procedures, creating lines of communication, and defining planetary-defense policy for the United States and perhaps for the United Nations.

Background Data

According to the National Aeronautics and Space Administration (NASA), "Every day, Earth is bombarded with about 25 tons of dust and sand-sized particles. About once a year, an automobile-sized asteroid hits Earth's atmosphere, creating] an impressive fireball."3 US missile-warning satellites annually record as many as 30 bolides (meteoroids that detonate in the atmosphere, otherwise known as fireballs), often releasing as much energy as a nuclear blast (see fig. 1, which includes several years of data superimposed over Earth's surface) .4 Composed of ice-rock mixtures, these bolides range in size from a few meters in diameter up to 50-60 meters. It is important to emphasize that objects smaller than 50-60 meters seldom penetrate the entire depth of the atmosphere to create impact disasters.5 However, more massive objects occasionally do so, causing greater concern.

We shouldn't become complacent because even larger objects intersect Earth's orbit. The surfaces of the moon, Mercury, and Mars show that debris has hit with relative frequency. Unlike these heavenly bodies, Earth is an active planet with tectonic and erosion forces that largely obscure impact craters. Nevertheless, geologists have now confirmed that asteroids or comets have scarred Earth with 160 craters (fig. 2), and they discover more each year. Although we have found impact craters mostly on land (fig. 2), bolides can occur anywhere on our home planet (fig. 1).

This article divides potential Earth-impacting asteroids into four categories. Generally, asteroids with a density less than or equal to that of rock and less than .5 km across can cause "local damage," defined as destruction of an area equivalent to a moderate-sized city, such as Kansas City, Missouri. These "city-killers" would reduce most houses and buildings to rubble, and any combustible material within 8 to 16 km of the impact would burn. Debris would scatter for tens of kilometers, possibly causing widespread fires. If the asteroid fell into the ocean, it could produce tsunamis more powerful than the Indian Ocean earthquake of 2004, leaving thousands dead. Based on lunar-cratering studies, local-damage asteroids collide with Earth every 200 to 300 years, on average.6 (Other studies indicate every few thousand years. A denned planetary defense would refine such estimates of the danger of impact.)7 A city-killing asteroid hit Tunguska, Siberia, in 1908, missing Moscow, Russia, by only three hours.8 This atmospheric explosion flattened a forested area three times as large as the District of Columbia/' Definitive research published in Nature magazine indicates that the Tunguska bolide had asteroid origins and detonated approximately 10 km above the ground with a force of 10 to 20 megatons of TNT, making it over 1,000 times more powerful than the first atomic weapons. …

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