The chances of a large asteroid or meteor striking earth are extremely unlikely, but the impact would be devastating. Over the past year, experts and members of Congress have been discussing the potential threat posed by chunks of rocky debris from outer space.
The idea of so-called Near Earth Objects, or NEOs, striking earth is so compelling, it's been the subject of two recent Hollywood films. In the thriller Deep Impact, a killer comet is on a collision course with earth. In Armageddon, a deadly asteroid threatens mankind. Actor Stanley Anderson, plays the president of the United States, who gets the bad news from actor Billy Bob Thornton as the head of the U.S. space agency NASA.
"It's an asteroid, sir."
"How big are we talking?"
"It's the size of Texas, Mr. President."
"What kind of damage..."
"Damage? Total, sir. It's what we call a global killer, the end of mankind. It doesn't matter where it hits. Nothing would survive. Not even bacteria."
NEOs have hit earth before. The question is - how often? In recent times, the most devastating example occurred in 1908 when an asteroid, estimated to be 50-60 meters in diameter, exploded in the upper atmosphere over Siberia. The impact of the wind blast that followed flattened hundreds of square kilometers of trees in the Tunguska forest. Experts say the energy was up to 10 times that of the atomic bomb dropped on Hiroshima.
In 2002, two sizable asteroids came within the orbit of the moon, and early data indicated a third nearing earth's orbit might collide in about 20 years. But revised observations took earth out of the asteroid's path, according to Peter Brown, an assistant professor of physics and astronomy at the University of Western Ontario in Canada. "So, there have been several of events, several of these objects in the tens to sort of hundred meter size range that have come close to the earth, that the telescopic surveys have recently found. And it just underscores that these objects are in our neighborhood and they interact with the earth," he says.
Not only that, but Professor Brown and colleagues found, in a study published in November in the scientific journal Nature, that medium-size asteroids bombard the earth's atmosphere all the time.
Using U.S. Department of Defense and Department of Energy satellites that record light flashes on and above the earth's upper atmosphere, Professor Brown and colleagues discovered some 300 collisions over an eight and a half year period. "These objects don't pose a direct hazard at the size range we looked at, a few kilotons to tens of kilotons, because most of the energy is deposited in the upper atmosphere, and its really only - in some cases, the rocks that get to the ground, and the rocks aren't traveling very fast," he says.
The researchers also revised an estimate of when a massive asteroid or meteor might strike earth. Previous projections put the catastrophic event at every 200 to 400 years. The new observations suggest a killer NEO could hit our planet every 1,000 years.
"It's really good news for the earth that these things don't happen as often as we originally thought," says Robert Jedicke, a planetary scientist at the University of Arizona who looks for NEOs. But he says even small asteroids could be extremely damaging. In a commentary in the science journal Nature, Professor Jedicke notes current detection methods fall short. "There are a lot of these objects out there, and they're relatively small based on what we can see with telescopes and they're pretty hard to detect. So, because there' a lot of them and they're pretty hard to detect, the job to find them is going to take a very long time," he says. "And right now, we don't even have the technology to do it."
Joe Burns, an astronomer at Cornell University in New York, testified recently at a congressional hearing on the potential threat of asteroids and near earth objects. Professor Burns, who looks for asteroids in the one kilometer range, says little research is focused on what to do about an NEO hurdling faster than a speeding bullet toward earth. But he says you definitely don't want to blow it up. "Because then you'd just have rather than just one large thing coming in you'd have a multitude of small things that are going to have the same energy impact," he says.
Professor Burns says one option is to gently push a large asteroid out of the way with a propulsion system placed on its surface.
Astronomers can detect really big asteroids, the kind that can destroy civilization, some 50 years in advance, giving scientists time to come up with a strategy to take them out of earth's path. But it's the smaller asteroids, under a kilometer but larger than the one that flattened Tunguska, that could wreak significant damage to major population centers. In that case, according to Professor Burns, the only strategy for survival would be evacuation.
Part of VOA's yearend series