The company's Pegasus XL rocket was flown to a launch altitude of about 39, 000 feet by a specially configured L-1011 "Stargazer" jetliner taking off from the Kwajalein Atoll in the Marshall Islands in the Pacific, near the equator about halfway between Hawaii and Australia.

WSJ: NASA NuStar Telescope to Hunt for Black Holes

One problem was that the rocket used as a booster (a type known as Pegasus, that is designed to be used as a satellite launcher) is usually dropped from its launch aircraft at an altitude of 12, 200 metres.

ECONOMIST: Hypersonic aviation