This computer-generated image by Sandia National Laboratories' scientists shows the impact of a 1-km comet (or asteroid) hitting in the open ocean. The comet and 300 to 500 cubic kilometers of ocean water would be vaporized nearly instantaneously by the tremendous energy of the impact. The impact energy of about 300 gigatons of TNT would be equivalent to about 10 times the explosive power of all the nuclear weapons in existence in the 1960s at the height of the Cold War. Download 150dpi JPEG image, 'comehit1.jpg', 384K

Using virtual reality techniques, decades of experience in shock physics, advanced computer programs, and the world's fastest computer, the scientists have completed in recent weeks one of the largest hypervelocity impact physics calculations ever performed.

In the latest computing scenario, an asteroid 1.4 kilometers in diameter strikes the Atlantic Ocean 25 miles south of Brooklyn, N.Y. To model the event the scientists broke up a 120-square-mile space that roughly approximates the New York City metropolitan area, the air above, and the water and earth below, into 100 million separate cubes, or grids. Sandia's teraflops supercomputer then calculated what happened inside each cube as the asteroid splashed down. The cubes were reassembled to produce a three-dimensional moving picture of the collision. The teraflops, currently the world's fastest computer, performs more than one trillion mathematical operations per second.

The simulation is no video game; the calculations take into account the real-world laws of physics governing time, temperature, pressure, gravity, the densities of water and earth, and hundreds of other considerations to create an accurate prediction. What's more, the resulting computer simulation can be explored using interactive virtual reality techniques. For instance, scientists can "fly through" the 3-D movie to get a better idea of what's happening on Coney Island if they want.

The simulations in Quick Time format, other illustrations, and links to information about other Sandia comet modeling work are available at http://sherpa.sandia.gov/asteroid/ and http://www.cs.sandia.gov/projects/comet.html

The work supports Sandia's Department of Energy mission to use the world's highest-performance computers to develop computer codes that can one day model the extremely complex physics that occur during a nuclear weapon blast. In the absence of real-world nuclear testing, DOE and the weapons labs are developing continually more powerful supercomputers and computer codes to simulate the complex 3-D physics involved in nuclear-weapon performance and to accurately predict the degradation of nuclear weapon components as they age in the stockpile. Simulating comet impacts provides an opportunity to test and improve the codes.

Five seconds after a 1.4 kilometer-wide asteroid crashes into the Atlantic Ocean off the coast of New York, an impact plume containing superheated water, earth, and other debris blankets major portions of Long Island. The viewpoint is from orbital altitude from a location about 100 kilometers west of New York City looking east. Long Island trails off in the distance. Manhattan and Staten Islands are in the foreground. Download 150dpi JPEG image, 'comehit2.jpg', 512K

Eleven seconds after impact, Long Island and the New York shoreline are engulfed in debris and superheated steam, and much of the material in the upper portions of the impact plume is on suborbital trajectories. In both images, water is blue, land is brown, water vapor is white, and hot material (greater than 5,000 Celsius) is orange. Download 150dpi JPEG image, 'comehit3.jpg', 768K

An impact of this magnitude can be expected to occur on Earth about once every 300,000 years and approximates what scientists consider to be the "global catastrophe threshold," he says.

So how close is Spielberg's interpretation of the event to the teraflops' virtual predictions?

"The movie makers didn't have the benefit of the world's fastest computer, but they produced superior visuals that appear remarkably realistic," says Arthurine Breckenridge of Sandia's Computer Architectures Department.

In the movie preview, the comet strikes at an angle and raises a symmetrical steam cloud, she says, which probably wouldn't happen. "We now know that the vapor cloud produced by an impact is initially asymmetric, sending more material in the direction of the ricochet." The movie does realistically depict a tsunami that would surely follow an ocean impact, she says.

Spielberg's Deep Impact opens this Friday, May 8. Another movie, Armageddon, which depicts an asteroid "the size of Texas" threatening to strike Earth, opens July 1.

An experiment you would never want to do
The teraflops simulations employ "massively parallel computing," a computing approach pioneered by Sandia in the late 1980s. In massively parallel computing, thousands of discrete computing tasks are assigned to several hundred separate computing "processors" inside the supercomputer. The computing tasks are accomplished simultaneously and their results reassembled. All of today's high performance supercomputing employs a massively parallel approach.

In the most recent 100-million-cell calculation, the teraflops used 8,192 of its 9,000 processors. The entire calculation lasted 18 hours. Sandia has done similar calculations on its high performance computers, including a 54-million-cell simulation of a comet striking the ocean. In 1994, Crawford and Sandia scientist Mark Boslough accurately simulated what would happen when Comet Shoemaker-Levy 9 plunged into Jupiter's atmosphere. Months later, the world's astronomers watched the Sandia-predicted event unfold in real life through the Hubble space telescope.

"A lot of major breakthroughs in science are going to come from these kinds of calculations," Boslough says. He notes that the impact simulations are something that can't be done any other way. "It's almost like doing an experiment -- one you could never do. One you would never want to do."

Sandia's teraflops computer is a joint development of DOE, Sandia, and Intel. It represents the initial goal of DOE's Accelerated Strategic Computing Initiative, a ten-year program designed to move nuclear weapons design and maintenance from a test-based to simulation-based engineering approach.

Sandia is a multiprogram Department of Energy laboratory operated by Lockheed Martin Corp. With main facilities in Albuquerque, N.M., and Livermore, Calif., Sandia has research and development programs contributing to national defense, energy and environmental technologies, and economic competitiveness.

Arthurine Breckenridge (3-D computer visualization), arbreck@sandia.gov, (505) 845-8390

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