China's planned exascale computer threatens Summit's position at the top.

618 9 FEBRUARY 2018 • VOL 359 ISSUE 6376 sciencemag.org SCIENCE C R E D IT S : (G R A P H IC ) K . S U T L IF F/ S C IE N C E ; (D A T A ) T O P 5 0 0 dustry,” Chang says. DOE changed tacks. It scrapped plans for Aurora, and replaced it with A21, a machine five times bigger. That pushed the launch date back to 2021, but because it was to be the first U.S. exascale machine, it also effectively pushed up the U.S. timeline by 2 years. Skipping the intermediate step of Aurora is risky, says Kenneth Jansen, an aerospace engineer at the University of Colorado in Boulder. “It means one of the stepping stones is not going to be there.” Still, others say it’s a risk worth taking. “This is the right way to do it,” says Thom Dunning, a computational chemist at the University of Washington in Seattle. Details of A21’s architecture remain closely guarded to protect proprietary technology. But scientists writing software for the new machine will be given detailed briefings on the new architecture after they sign nondisclosure agreements. Some of the first briefings are taking place this week in Knoxville at the second annual Exascale Computing Project meeting. Researchers already familiar with the plans say the machine is unlike any they’ve ever seen before. “A21 is a very different architecture,” Chang says. In general terms, he says, the design focuses on decreasing the need to move data long distances between processors, an energetically expensive process. He says the new machine will likely require 25 to 30 megawatts of power, only about twice that of Summit. Asked whether he thinks Intel will be able to pull off the new architecture, Chang says, “I am confident they will.” One outside challenge could be money. Congress has yet to pass the fiscal 2018 budget, and instead has funded the government through a series of continuing resolutions that keep funding levels the same as the prior year while forbidding the launch of new projects, such as building the A21 machine. For now, that’s not a problem, because DOE is still able to support the underlying scientific developments as part of its existing Exascale Computing Project, says Jack Dongarra, a supercomputing expert at the University of Tennessee in Knoxville. But soon it will be time to start fabricating chips for A21, which is expected to cost between $300 million and $600 million, according to market research firm Hyperion Research. “In 2021 will the budget be there to do this?” asks Horst Simon, a supercomputing expert and deputy director of the Lawrence Berkeley National Laboratory in Berkeley, California. “I don’t know.” j