Phenomena, Comment & Notes
While Comet Shoemaker-Levy 9 is history, space scientists are just beginning to piece together the details of its pyrotechnic encounter with Jupiter last July
- By John P. Wiley jr.
- Smithsonian magazine, January 1995, Subscribe
(Page 2 of 3)
As we all know, life on Earth is nowhere near so violent. That's because Jupiter is 11 times as wide and 318 times as massive as our planet. Jupiter is not only a much larger target than Earth; its much stronger gravitational field pulls in comets and asteroids that would go speeding by Earth. Those same poll respondents thought our chances of avoiding catastrophe are a million times better. Yet we know Earth has been hit hard in the not so distant (geologically speaking) past; craters are still visible (Smithsonian, September 1989). While 12 percent thought three-mile-wide objects might strike Earth once every 100,000 years, 60 percent thought 100 million years was closer to the truth.
David Morrison of the NASA Ames Research Center supplied perspective. The postulated object that 65 million years ago caused a mass extinction, including most species of our beloved dinosaurs, would have been six or seven miles across. Comets or asteroids the size of the fragments that hit Jupiter, one or two miles across, would not cause mass extinction but would trigger global ecological catastrophe. Exploding with the force of 100,000 to 1 million megatons of TNT, just one such object would dig a crater ten miles across and blast enough dust into the atmo-sphere to block 99 percent of the sunlight for a month. Even in the Tropics temperatures would drop below freezing. Most natural ecosystems would survive, but crops would die, leading to the deaths of up to a billion people.
(If such an object hit the ocean rather than land, we would not be as lucky as one might at first think. Jack Hills of Los Alamos National Laboratory said after the talk that a two-mile-wide object coming down in the middle of the Atlantic would set off tsunamis that would reach all the way to the Appalachians. Washington, D.C. would be under 500 feet of water.)
In Morrison's view, there is a 1-in-5,000 chance that such an event will occur during anyone's lifetime. Or, in the language of the poll, he estimates it happens once in 100,000 to 300,000 years. Very rare, he concedes, but when it does happen the results are apocalyptic. "It is the only natural hazard that we could-in principle-do something about," he continued. If we picked up one coming in, we could set off a nuclear bomb nearby, nudging it into a slightly different orbit, one that would miss Earth by a comfortable margin.
Smaller objects would do catastrophic damage locally. Chris Chyba of the White House Office of Science and Technology Policy said that the comet or asteroid that exploded over Siberia in 1908 was about 200 feet across and exploded with the force of 12 megatons of TNT set off six miles above the ground. It blew down all the trees within 850 square miles, an area about the size of Washington, D.C. and neighboring counties. The strength of shock waves required to blow down trees, Chyba pointed out, is that needed to explode frame houses. He estimated that such objects hit Earth about once every 300 years. Over land, he added, it happens once every thousand years.
Still smaller bits and pieces hit us more often. I had not known that in 1965 one exploded in the air near Revelstoke, British Columbia, with the force of a few tens of kilotons, the magnitude of the Hiroshima bomb. Trappers found meteorite fragments on top of a snowfield.
Morrison said that worldwide, only about 15 people or, as he put it, a work force about the size of one shift at a McDonald's, are trying to catalog all the objects that could hit Earth. They estimate there are 2,500 short-period comets or asteroids whose orbits cross Earth's. It could take 10 to 20 years to plot 95 percent of them, he said, "and a very long time" to get the last one. Gene Shoemaker heads a committee that will present its recommendations on the matter to Congress later this month.
Emphasizing that he was speaking personally, Chyba responded to the thinly veiled appeals for more money by noting that it will be difficult to find the funds for an expanded effort. Other scientists want comparable sums to study equally important global problems, he said, and there is not enough money to properly fund everything.
Subscribe now for more of Smithsonian's coverage on history, science and nature.
Related topics: Astronomy Comets
Tweet | Digg |
Comments