Huntington Beach, CA--Deep Impact and Armageddon, two of this summer's blockbuster films, feature large rock chunks on collision courses with Earth. After a few stray boulders wipe out major cities and create towering tsunamis, the screen heroes use nuclear weapons to obliterate the primary threats.
Just how much of this is movie-making fantasy? Little doubt remains that the earth has seen its share of asteroid and comet impacts. The remnants of a 110- to 180-mile-wide crater near the Yucatan Peninsula, for instance, are believed to be evidence of the event that exterminated the dinosaurs. And as recently as 1908, an asteroid just 250 ft in diameter flattened 800 square miles of forest near the Tunguska river valley in Siberia.
But the questions remain. Can we detect threatening space debris in time to dispense with it? Could we dispense with it once detected? And what would happen if we failed?
Don't look to Hollywood for answers. The real heroes in asteroid detection, deflection, and damage analysis will be engineers and scientists, not a wildcatter from Texas resembling Bruce Willis.
Asteroid hunting. Tracking down potential killer asteroids is the job of researchers involved in several ground-based programs. Examples include the Jet Propulsion Laboratory's (JPL) fully automated Near Earth Asteroid Tracking (NEAT) system, the University of Arizona's Spacewatch program, the Lowell Observatory Near Earth Object Survey (LONEOS), and Lincoln Laboratory's LINEAR program.
NASA's overall work in asteroid detection will soon be coordinated by the new Near Earth Object (NEO) Program Office at JPL. The goal: to discover and chart within 10 years 90% of the estimated 2,000 comets and asteroids that are larger than one kilometer, pass within 30 million miles of Earth, and have Earth-crossing orbits. Fewer than 200 have so far been identified.
"Objects one kilometer or larger in size have the potential to cause global catastrophes," says Donald Yeomans, JPL program manager. "Those between 100 meters and a kilometer can cause significant localized damage, but the thinking is, let's concentrate first on those objects that have the potential of wiping out life on the planet."
It's challenging work. To achieve the 90%-in-10-years goal, each of the current wide-field telescopes searches the entire accessible sky every month down to a brightness level of twentieth magnitude.
Technology advancements have helped accelerate the process. Lincoln Lab's LINEAR, for example, uses a fast-readout CCD that has helped it discover nearly twice as many hazardous asteroids as the other programs combined. And a new computer system for JPL's NEAT uses four Sun Microsystems 300-MHz Ultra-SPARC processors to expedite processing of the 40 Gbytes of data generated each night.
The investment has already paid off. This August, NEAT discovered two mile-wide NEOs that, luckily, pose no near-term threat to Earth.
Interplanetary ping pong. Just for fun, though, let's imagine that one of these mile-wide Earth crossers was scheduled for an untimely rendezvous, say in two months. What could we do about it?
In truth, not much.
Eric Asphaug, a researcher at the University of California Santa Cruz, recently led a team of colleagues who discovered that blowing up asteroids isn't so easy. They ran computer simulations of a 16m-diameter, 6,000-ton asteroid projectile colliding at 5 km/sec with a 1.6-kilometer- long, 1-billion-ton asteroid called Castalia. Total impact energy equaled 17 kilotons, about the same as the Hiroshima atomic bomb.
With Castalia's composition unknown, the team modeled the asteroid three ways: as a homogeneous rock; two large boulders; and as a pile of rubble held together by gravity.
Each simulation run took about a month on a Silicon Graphics O2 model R10000 workstation. Recently the team acquired access to NASA Goddard's Cray T3E supercomputer, cutting execution to a matter of hours and allowing for more complex models. Software consists of a smooth particle hydrodynamics code developed by Asphaug and Dr. Willy Benz of the University of Bern in Switzerland.
In each case, the impactor had only little to moderate effect on Castalia's trajectory. "You don't really disrupt an asteroid so much as change its configuration," says Asphaug.
Though badly fractured, more than 90% of the asteroids modeled as either a homogenous rock or two large boulders remained intact; the maximum velocity change measured a paltry 7 cm/sec. And were Castalia a rubble pile, more than half of it would be accelerated beyond escape velocity, but the remaining 500 million tons of boulders would change speed by only 14 cm/sec.
"You would never be able to knock this out of the way with days, weeks, or even months of advance notice," says Asphaug. "You would have to know about this thing decades ahead of time."
Global devastation. Okay, so we can't easily deter a large asteroid from hitting Earth. What would happen if one did? That's the question researchers at Los Alamos National Laboratory (LANL) and Sandia National Laboratory (SNL) set out to answer using some sophisticated computer analysis.
In January, LANL astrophysicists Jack Hills and Charles Mader presented the results of a study examining the mid-Atlantic Ocean impact of asteroids ranging from 400 meters to 5 kilometers in diameter. The largest asteroid, they found, would generate a tsunami that would sweep across the upper East Coast of the United States to the Appalachian Mountains. Even the smallest impactor would cause localized devastation with tsunamis measuring up to 90 meters high and traveling hundreds of miles an hour.
Sandia researchers modeled a similar scenario in which a 1.4-kilometer-diameter asteroid strikes the Atlantic Ocean 25 miles south of Brooklyn, NY at a grazing angle of 15 degrees. The impactor and 300 to 500 cubic kilometers of seawater and earth would be vaporized instantaneously in an explosion equaling 300 gigatons of TNT.
You can forget about running. Just 2.4 seconds after impact, a fireball with temperatures up to 5000C would sweep across Long Island. After only 8.4 seconds, chunks of the Atlantic seabed would have reached orbital escape velocity and begun heading into space.
The simulation ran for 18 hours on the world's fastest supercomputer, the Intel TFLOPS highly massively parallel machine. For analysis, researchers leveraged the Sandia-developed virtual-reality system called Eigen/VR.
Researchers hope that their current work coupled with the exposure of the recent disaster movies will encourage more funding for asteroid study. Today, the sum total being spent on asteroid detection is said to be less than the cost to operate a single McDonald's franchise. "The potential destruction of the human species has a way of focusing your attention,"says JPL's Yeomans. "The best thing we can do is to find these things long before they become a problem."
What this means to you
- Advanced computers aid searches for killer asteroids
- Explosion-simulation software can model high-energy events
- Supercomputers enable more rapid analysis of massive, complex events
NASA research of asteroids and comets http://george.arc.nasa.gov/dx/basket/factsheets/astimpac.htmlSpacewatch http://www.lpl.arizona.edu/spacewatch/index.htmlLINEARproject http://www.ll.mit.edu/LINEAR/NEAT program http://huey.jpl.nasa.gov/~spravdo/neat.htmlAsteroid collision simulations at UCSChttp://www.es.ucsc.edu/~asphaug/Sandia impact simulationhttp://www.cs.sandia.gov/projects/comet.htmlLos Alamos impact simulationhttp://w4.lanl.gov/external/news/releases/archive/98-005.html
Chances of dying fromselected causes
|(USA)Motor vehicle accident 1 in 100
|Homicide 1 in 300
|Fire 1 in 800
|Firearms accident 1 in 2,500
|Electrocution 1 in 5,000
|Asteroid/comet impact 1 in 20,000
|Aircraft accident 1 in 20,000
|Flood 1 in 30,000
|Tornado 1 in 60,000
|Venomous bite 1 in 100,000
|Fireworks accident 1 in 1 million
|Botulism poisoning 1 in 3 million
|Drinking water 1 in 10 million
|with EPA limit of trichloroethylene
|Source: Cr. Chapman & D. Morrison, 1994, Nature 367, 33-40