Bruce Willis blowing up a Texas-size asteroid to save the Earth in "Armageddon" may have been a great Hollywood moment. It was pure fiction.
An asteroid threatening to destroy life on Earth isn't.
Earth has been struck by asteroids through the eons, and not just the "big one" that is believed to have wiped out the dinosaurs.
Astronomers observing our solar system are certain an asteroid could hit the Earth again. The question is "where and when?" Observatories around the world, including Air Force satellite tracking facilities on the Haleakala summit, have been looking for objects in near space for decades, cataloging those that could become problems as they circle through the solar system.
But most observatories locate asteroids and comets in the course of doing other research.
For the new prototype Pan-STARRS PS1 facility on Haleakala, locating and tracking potentially hazardous objects in space will be a primary role.
The risk is difficult to quantify.
Writer Gregg Easterbrook in a recent article on the threat of an asteroid strike, "The Sky is Falling" (Atlantic Magazine, June 2008), suggested the probability could be as high as one in 10 this century.
Robert Jedicke, a researcher with the University of Hawaii Institute for Astronomy, said it might be one in 1,000 over the next century.
To determine the risk, it would help to know what's out there. That's being determined already by the National Atmospheric and Space Administration with its Near Earth Objects project, which gathers data and posts information on what's whizzing by at neo.jpl.nasa.gov/ca/.
Near Earth Objects, or NEO, are asteroids and comets that appear to be on tracks that carry them close to the Earth's orbit. Asteroids - chunks of rock or metal ores that weren't coalesced into planets or moons when the solar system formed - may collide or be nudged by the gravitational pull of nearby planets to spin onto new courses. Comets may be on orbits through the solar system that eventually carry them close to Earth.
Currently, NASA's program has catalogued over 900 potentially hazardous objects, asteroids or comets, that are on tracks that can bring them within a few thousand miles of Earth - close enough that a slight gravitational tug could draw them into a collision course.
A lot of material isn't hazardous, such as the bits of debris from comets that flash across the night sky as periodic meteor showers. The concern is the big pieces that won't just disintegrate in the atmosphere.
Jedicke, an IfA asteroid specialist, said there are key differences between a comet and an asteroid. A comet is a ball of gas, dust and ice, whereas an asteroid is much more like a giant piece of rock, generally less than a mile in diameter and usually irregular in shape.
Comets typically are in long, elliptical orbits carrying them through and even beyond the solar system. Periodically when they approach the sun, they glow from the solar energy and exhibit a "tail" illuminated by the sun. Possibly the best known is Halley's Comet, in a orbit that brings it into view of Earth every 75 years.
Asteroids typically are in regular orbits around the sun, with the bulk of them found in the "asteroid belt" between Mars and Jupiter.
Easterbrook described comets as mostly frozen water mixed with dirt that were likely formed in the cold outer planetary system, while asteroids were more likely formed in the warmer inner planetary system, closer to the sun.
NEOs impact the Earth frequently, although most are small pieces that burn up as meteors. When a space rock hits the ground, it's a meteorite.
One is known to have struck as recently as Sept. 17, 2007, when a crater was found in Carangas, Puno, Peru. It is believed to be the result of a meteorite strike.
Mike Maberry, associate director of the University of Hawaii Institute for Astronomy, noted that a crater in the Arizona desert 1,300 meters across and 170 meters deep is believed to have been the result of a meteorite impact that occurred 50,000 years ago. Researchers estimate the object that struck the ground was 50 meters across with a mass of 1 million tons.
The "big one" involves what is believed to have been a meteorite strike that blasted the massive Chicxulub Crater in what is now the Yucatan Peninsula of Mexico. The theory is that a chunk of rock six to nine miles in diameter had spun out of the asteroid belt after a collision broke up a larger asteroid. The massive explosion 65 million years ago and dust plume that resulted are believed to have caused the extinction of all of the large dinosaurs from the late Cretaceous Period, and laid the ground for the evolution of mammals in the early Cenozoic Period.
When such an object enters Earth's atmosphere, according to Jedicke, it is pulled by the Earth's gravity and travels at nearly 20 miles per second. When it impacts the Earth at a direct angle, the force generated is equal to that of a nuclear explosion.
The impact not only generates heat but spews massive amounts of dust and debris into the atmosphere blocking sunlight and bringing on a long-lasting "winter." The Chicxulub meteorite is believed to be responsible for what is know as the Cretaceous-Tertiary- a distinct layer in geological sites that marks the transition from dinosaurs to mammals.
Maberry said a space object doesn't need to hit land or sea to create a lot of damage. In 1908, an airburst that was equivalent to a 15-megaton bomb leveled 1,000 square kilometers in Tunguska, Siberia. The blast was believed to have come from an object with a mass of 100,000 tons that exploded shortly after entering the atmosphere.
Recent research done by NASA and other science institutions has shown that smaller asteroids can be just as lethal. Writer Easterbrook suggested that an asteroid only 300 meters across could hit the Earth with a force equivalent to 60,000 Hiroshima atomic bombs.
"An asteroid that's about one mile in diameter could potentially kill about 1 billion people," said Jedicke.
But a large asteroid will be easier to locate and track. The difficulty is locating smaller objects that still could pose a hazard - which is why Pan-STARRS was created.
Jedicke compared the process to someone protecting his house from burning up.
"The first thing you do is go around looking for loose wires around your house," he said, "And that's kind of what we're doing with Pan-STARRS. We're going out there and verifying that there are no asteroids that are actually incoming.
"It begins with looking for the big asteroids," the elephant in the room.
"You find the big objects to protect the Earth, then you start looking for the small objects that are a more frequent nuisance," he said.
If an object is found to be on a track to intercept Earth, the next step would be to try to prevent an impact, said Jedicke.
"If we have enough time, we have ideas for scenarios for how we might deflect the asteroid," he said.
One of the ideas is "gravitational deflection," launching a heavy spacecraft to intercept the asteroid with enough mass to affect its course.
"Just by having the spacecraft so close to the asteroid, you're causing the asteroid to be attracted to the spacecraft, and you can use that small attraction to change the course of the asteroid," he explained.
The process would take years to cause just a few degrees of change in an asteroid's track that still would move it away from a collision course.
Contrary to what "Armageddon" depicted, Jedicke said that blowing up a large asteroid is the last thing they want to do.
"You definitely do not want to do like they did in the movie 'Armageddon,' where you blow the asteroid up, because then you'd just create a bunch of smaller asteroids that hit the Earth."
On the Net:
NASA Near Earth Object program - neo.jpl.nasa.gov/neo.html
Near Earth Object Fact Sheet - nssdc.gsfc.nasa.gov/planetary/factsheet/neofact.html
American Meteor Society - www.amsmeteors.org
"The Sky is Falling," Greg Easterbrook - www.theatlantic.com/doc/200806/asteroids