The Defense Department has picked International Business Machines Corp. to build the U.S. military's fastest supercomputer and the fourth-fastest in the world.

The supercomputer, which IBM said will cost less than $100 million, will be used to produce short-term weather forecasts for Navy fleets at sea. The Pentagon said the supercomputer's immense power will allow military scientists to model atmosphere and ocean dynamics for the entire surface of the Earth. The computer also will be able to analyze aircraft material at a molecular level to produce wings less likely to crack and to examine the flow of water around submarine hulls to improve their design.

The machine will be able to complete 20 trillion operations per second, about three times the capacity of systems at the Naval Oceanographic Office at Stennis Space Center in Mississippi, where it will be housed.

One reason the military, which was legally required to tap a domestic producer, selected IBM was because it proved its machine would "fail gracefully," shifting to backup computing power seamlessly in case of a problem, said Stephen Adamec, director of the supercomputing center at Stennis Space Center.

The world's fastest computer, the Earth Simulator, can execute 35 trillion operations per second. The machine, built by Tokyo-based NEC Corp. and housed in a Japanese government laboratory, models long-term climate change. The second- and third-fastest computers are at the Lawrence Livermore and Los Alamos national laboratories.

IBM is working on a prototype of a new supercomputer called Blue Gene/L that it expects to be the fastest in the world within the next few years. Blue Gene/L will rely on thousands of standard microprocessors, unlike the custom chips that power the Earth Simulator. The use of standard microprocessors means a less expensive supercomputer.

The military's new supercomputer falls somewhere between the Earth Simulator and Blue Gene/L because its chips will be partly customized, said Debra Goldfarb, a vice president in IBM's deep computing division. The processors in the military's supercomputer cost a fraction of the highly specialized chips in the Earth Simulator, and they are produced in the tens of thousands, not the hundreds.

The Blue Gene/L project was initially conceived in 1999 to study protein folding, a mysterious process that leads to diseases such as Alzheimer's when it goes awry. But the effort eventually became a way for the company to explore expansion of the supercomputer market beyond governments and universities to businesses.

"The people who used supercomputers lived in ivory towers," said Goldfarb. "That has limited the marketplace and stunted innovation."

The military's supercomputer is made up of 368 of IBM's high-end corporate servers, the same ones used by Fortune 500 companies to generate the best drilling plans for oil fields or analyze the market for complicated financial securities. The processors are part of the family of chips that powers Apple's G5 Macintosh desktops and Nintendo's video game consoles. The supercomputer will comprise 30 to 40 3,000-pound "cabinets" the size of large vending machines.