May 1, 1997
During the initial testing of the new computer, Gil Weigand, U.S. Department of Energy (DOE) Deputy Assistant Secretary for Strategic Computing and Simulation, requested that Sandia complete a simulation that would be of general interest to the scientific community. For this reason, and also to generate unclassified data to test innovative visualization techniques, Sandia scientist David Crawford has carried out a computational simulation of a major cosmic event of potential significance to all people on Earth: What would happen if a kilometer-wide comet struck the ocean?
A kilometer is about the size of the largest fragment of Comet Shoemaker-Levy 9 that crashed into Jupiter in 1994 -- an event that was also the subject of highly praised computational simulations by Crawford and colleague Mark Boslough. The close correspondence between those predictions of a visible plume rising above the rim of Jupiter and the actual plume as observed by astronomers lent even more confidence to the accuracy of the Sandia simulation codes.
The new calculation again used Sandia's CTH "bang and splat" shock physics code, but this time the simulation was run on 1,500 processors of the new Intel Teraflops computer being installed at the Labs. That's only one-sixth of the expected final 9,000-processor configuration.
The calculation assumed a 1-kilometer-diameter comet (weighing about a billion tons) traveling 60 kilometers per second and impacting Earth's atmosphere at about a 45¡ angle. This is small as far as comets go (the massive Comet Hale-Bopp weighs about ten trillion tons).
The problem was divided into 54 million zones and ran for 48 hours.
The results, although dramatic, pretty much confirm earlier predictions about a comet impact, but they do so with much finer resolution in three dimensions than has ever before been possible.
The fully-resolved three-dimensional resolution is extraordinary.
"It's like an astronomer getting a new, more powerful telescope," says Boslough. "I think it's a major step forward in science." He said the capability raises computational simulations to the status of a third branch of scientific inquiry equal to, and complementary to, experimentation and theorizing.
"It really is a revolution in science," Crawford says. "A lot of major breakthroughs in science are going to come from these kinds of supercomputers." He notes that the comet-impact simulation is 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."
The simulation starts with the comet 30 kilometers above the surface. The comet produces a strong luminescent bow shock in the atmosphere as it speeds downwards. Seven-tenths of a second later it hits the ocean with an impact energy of 300 gigatons of TNT -- about 10 times the explosive power of all the nuclear weapons in existence in the 1960s at the height of the Cold War -- forming a large transient cavity in the ocean and a dent in the ocean floor. The comet itself is almost instantaneously vaporized, along with 300 to 500 cubic kilometers of ocean. This high-pressure steam explosion rises into the atmosphere. Comet vapor and water vapor are ejected into ballistic trajectories that will take it around the globe, with some of it even achieving escape velocity.
Low-lying areas like Florida would indeed be washed over, but Crawford says the event is very close to the size threshold at which impact experts expect that a global catastrophe could occur, by screening out much sunlight for long periods of time and disrupting agriculture, among other effects. "Simulations of this kind can help pin down that energy threshold and help answer the question: Is it a regional or global catastrophe?"
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 (ASCI), a ten-year program designed to move nuclear weapons design and maintenance from a test-based to simulation-based approach. DOE announced last December, when the machine was then still at Intel, that the one-trillion-operations-per-second breakthrough had been achieved.
The full machine is expected to have a peak performance capability of 1.8 teraflops, or 1.8 trillion mathematical operations per second. DOE and the weapons labs are developing continually more powerful supercomputers 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.
The comet-simulation was essentially a test of the teraflops machine's capabilities. "This is an exercise for the computer," says Boslough (who notes that he's also using it for a weapon-component simulation), "but we wanted to do something that people would be interested in."
Sandia is a multiprogram DOE laboratory, operated by a subsidiary of Lockheed Martin Corp. With main facilities in Albuquerque, N.M., and Livermore, Calif., Sandia has major research and development responsibilities in national security, energy, and environmental technologies and economic competitiveness.
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