Asteroid and Comet Collisions with Earth

Key Investigators: Owen B. Toon, Kevin Zahnle, David Morrison

Kilometer-sized asteroids and comets cause global scale disasters when they hit Earth. Ames' researchers found that the Chicxulub crater in the Yucatan Peninsula is the site of an impact 65 million years ago that killed the dinosaurs. Known as the K-T impact, it led to massive extinctions throughout the biosphere, while it paved the way for the ascent of mammals and the rise of humans. Recently, Ames' astronomers have pointed out that future collisions are inevitable. If we wish to avoid the fate of the dinosaurs we need to be alert for colliding objects.

Large numbers of asteroids and comets could collide with Earth, making the task of astronomers trying to find and keep track of all of them very difficult. To make this problem more tractable, we have used theoretical models of collisions as well as data from the K-T impact, from volcanic eruptions, and from nuclear weapons tests to determine which objects are large enough to do significant damage and therefore should be tracked.

Asteroids and comets smaller than a football field in size do little damage. Our studies suggest that during the next 500 years more than one thousand of these small objects, with energies greater than that of the nuclear weapons dropped on Japan in World War II, will hit Earth. For example, in 1908 an area of about 2000 km2 of forest was knocked down and partially burned in the Tunguska region of Siberia. The Tunguska event represents an extreme example of the fate of small impactors. Most small impactors simply break up in the upper atmosphere and do little surface damage, so astronomers need not keep track of them (Figure 1).

Many objects with sizes ranging up to about a kilometer will hit the Earth in the next one thousand to ten million years. These impacts could produce damaging blast waves, earthquakes, fires, and tidal waves, but the area affected will be less than the size of the U.S. Except for a direct hit on a major population center, the greatest damage may be done by tidal waves sweeping entire ocean coastlines (Figure 2). These objects could kill millionsof people, but the damage is not greater than that from other natural events such as earthquakes.

At time periods approaching one hundred million years, mountain-sized comets and asteroids with diameters near 10 km will hit Earth. When a 10 km sized object hits the surface it creates an enormous ejecta cloud which explodes outward into space, and reenters the atmosphere as countless shooting stars. The sky would be converted from its normal transparent blue to a brilliant red sheet of glowing lava. The red sky would cool over the next hour or so, leaving the world in total darkness as the shooting star remnants blot out the sun, and vast billows of smoke fill the sky from the continental scale fires ignited by the glowing lava-filled sky (Figure 3). Smoke from fires, dust from pulverized ejecta, and sulfate from within the objects would cause so much light loss that photosynthesis would cease for several months, a likely mechanism for causing extinctions in the ocean. Land surface temperatures would also plummet. Pity the poor dinosaurs, to whom this happened.

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