Sophistication of the technology
Establishing the viability of this new technology, NASA's solar-powered aircraft Pathfinder, with a 100-foot wingspan,
reached an altitude of 71,500 feet altitude in 1997, and its fourth-generation successor the "Helios" flew at 97,000 feet with a 247-foot wingspan in 2001, two miles higher than any previous aircraft.
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For the next tier above the Earth, in the upper stratosphere, DARPA and the Air Force are
collaborating in the development of the Falcon Hypersonic Cruise Vehicle. Flying at an altitude of 20 miles, it is expected to "deliver 12,000 pounds of payload at a distance of 9,000 nautical miles from the continental United States in less than two hours".
Although the first test launches in April 2010 and August 2011 crashed midflight, they did
reach an
amazing 13,000 miles per hour, 22 times the speed of sound and
sent back "unique data" that should help resolve remaining aerodynamic problems.