Peter Siebold is a member of the Scaled Composites astronaut team. He is their Director of Flight Operations, and was one of the test pilots for SpaceShipOne and SpaceShipTwo, the experimental spaceplanes developed by the company. On April 8, 2004, Siebold piloted the second powered test flight of SpaceShipOne, flight 13P, which reached a top speed of Mach 1.6 and an altitude of 32.0 kilometres (105,000 ft). On October 31, 2014, Siebold and Michael Alsbury were piloting the SpaceShipTwo VSS Enterprise on flight PF04, when the craft came apart in mid-air and then crashed, killing Alsbury and injuring Siebold.

SpaceShipOne is an experimental air-launched rocket-powered aircraft with sub-orbital spaceflight capability at speeds of up to 3,000 ft/s (2,000 mph) / 910 m/s (3,300 km/h) using a hybrid rocket motor. The design features a unique "feathering" atmospheric reentry system where the rear half of the wing and the twin tail booms folds 70 degrees upward along a hinge running the length of the wing; this increases drag while retaining stability. SpaceShipOne completed the first crewed private spaceflight in 2004. That same year, it won the US$10 million Ansari X Prize and was immediately retired from active service. Its mother ship was named "White Knight". Both craft were developed and flown by Mojave Aerospace Ventures, which was a joint venture between Paul Allen and Scaled Composites, Burt Rutan's aviation company. Allen provided the funding of approximately US$25 million.