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One Comet Strike Equals 6 Trillion Tons of TNT

July 19, 1994 GMT

GREENBELT, Md. (AP) _ The energy of explosions caused when fragments of comet Shoemaker-Levy 9 hit Jupiter is almost impossible for humans to imagine.

By some calculations, the two-mile-wide Shoemaker-Levy 7 fragment G exploded in the upper atmosphere of Jupiter with the force of 6 million megatons of TNT.

This is many times the explosive force of all of the nuclear weapons ever stockpiled by the United States and the former Soviet Union, according to Stan Norris of the Natural Resources Defense Council.

At the peak of its weapons collection, the United States had 20,000 megatons of nuclear explosive power. The Soviet Union is thought to have had about 60,000 megatons.

The atomic bomb exploded over Hiroshima, Japan, in World War II was 15 kilotons.

A Department of Defense study once calculated the probable effects of a 25- megaton bomb exploded over Detroit, estimating that no habitable structure would be left for up to 30 miles from the explosion center.

Comets or asteroid fragments hitting Earth can release energy more powerful than atomic weapons. A rock just 30 feet across, hitting Earth from outer space, releases an energy equal to about five Hiroshima-sized bombs.

Some 50,000 years ago, a meteorite 180 feet wide smashed into northern Arizona and dug a crater 4,000 feet wide and 600 feet deep. The energy released has been estimated at 15 million tons of TNT.

About 15 million years ago, a 5,000-foot asteroid hit in Bavaria. More than a trillion tons of soil and rock were splashed into the air and scattered all over Europe.

A 300-foot meteorite struck in the Tunguska region of Siberia, in 1908. It leveled trees for miles.

And about 65 million years ago, a six-mile-wide boulder crashed into an area around the Yucatan peninsula and the Gulf of Mexico, creating a 112-mile- wide crater known as Chicxulub. Some scientists believe this event killed more than 80 percent of all animals on Earth and led to the extinction of the dinosaurs.

Such impacts are thought to occur about once every 100 million years.