American researchers have lost contact with an experimental aircraft designed glide down from the upper atmosphere at 20 times the speed of sound.
The unmanned hypersonic glider was on its second test flight when engineers lost track of it, a Pentagon agency said.
The first part of the test apparently went according to plan. The Falcon HTV-2 launched from Vandenberg Air Force Base in California on a rocket that carried it into the earth's upper atmosphere, and successfully detached from it.
However contact was lost soon after the aircraft was said to be entering its glide phase, when it was expected to hit speeds of about 20,000 km/h, flying above the Pacific.
America's Defense Advanced Research Projects Agency (DARPA) is trying to come up with a system able to strike anywhere in the world in an hour or less, with conventional or nuclear warheads.
The agency says the test aircraft was designed to ditch itself in the ocean if something went wrong.