Comets: How Big A Threat To Earth?

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Consider this example. An asteroid 0.6 mile (1 kilometer) wide with a density of 187 pounds per cubic foot (3,000 kilograms per cubic meter) traveling at 12 miles per second (20 kilometers per second) would impact Earth with a force approximately 15 times greater than the world's total nuclear arsenal. A comet of just over half the size and one-third the mass traveling at 37 miles (60 kilometers) per second could achieve an impact of similar force if it were to strike Earth. "Size matters," said Mazanek. "But so does density and speed."


Some astronomers are working to safeguard the Earth from potential impact by comets or other near-Earth objects in orbit around the Sun. The Near-Earth Object Program at NASA's Jet Propulsion Laboratory in Pasadena, California, coordinates the study of these objects.

As near-Earth objects are detected, scientists perform calculations on their orbits to determine if or when they pose a threat to impact Earth. The hope is that astronomers can detect all near-Earth objects decades before they would potentially impact Earth.

Meanwhile, other scientists are busy trying to figure out how to throw such threatening objects off course, thus mitigating the pending doom.

Long-period objects like comets, however, are not easily detected until they enter the solar system.

"A long-period object by definition may not have any records of sightings in written history," said Mazanek. "If it came back into the solar system and it was on [an Earth-bound trajectory], we would not have much warning."

Mazanek leads NASA's Comet/Asteroid Protect System, a program that would expand on the Near-Earth Object Program to include the detection of long-period comets, as well as small asteroids and short-period comets that pose an Earth impact threat. The space-based system, not to be in place for at least 25 years, would provide constant monitoring and a system to divert and modify the orbits of threatening objects.

Confirmation of a long-period object on an impact trajectory would be possible at least a year before impact, allowing more time to take defensive action than current detection systems allow.

The problem is that not much could be done if a long-period object on an Earth-bound trajectory were detected today, said Durda.

"The worst scenario I can think of is a multi-kilometer-diameter, long-period comet discovered several months out on an impact trajectory as it is entering the inner solar system," he said. "There is absolutely nothing we could do about it at this point in time. Nothing."

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