Real (not reel) deep impacts: Sandia scientists predict what an asteroid strike would look like, really

ALBUQUERQUE, N.M. — Steven Spielberg’s new blockbuster movie — with its computer-animated interpretation of a comet striking Earth — promises to be a big hit at the box office this summer. But computer scientists at Sandia National Laboratories are creating some big hits of their own these days that they think may better approximate a real asteroid catastrophe.

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.

How did Spielberg do?
So what would happen during such an impact, really? According to the simulation, the impact would vaporize the asteroid, deform the ocean floor, and eject hundreds of cubic miles of superheated water vapor, melted rock, and other debris into the upper atmosphere and back into space. Much of the debris would then rain down over the world for the next several hours and also form a high global cloud, says David Crawford of Sandia’s Computational Physics and Mechanics Department. The shock wave from the impact would level much of the New England region. The heat would incinerate cities and forests there instantaneously. The global cloud would then lower temperatures worldwide, and a global snowstorm likely would ensue and last several days to several weeks, initiating a “nuclear winter” that would create more hardships for earth’s inhabitants.

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.

Technical contacts:
David Crawford (hypervelocity impact physics), dacrawf@sandia.gov, (505) 845-8975

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