[1] He is their Director of Flight Operations,[2] and was one of the test pilots for SpaceShipOne and SpaceShipTwo, the experimental spaceplanes developed by the company.

Siebold holds a degree in aerospace engineering from California Polytechnic University at San Luis Obispo, from 2001.

[7] Although Siebold flew SpaceShipOne to an altitude of 32 kilometres (20 mi), he did not cross the 100 km Kármán line—the international standard for reaching space.

For his contribution to the SpaceShipOne project, Siebold, along with Mike Melvill and Brian Binnie, received the 2004 Iven C. Kincheloe Award presented by the Society of Experimental Test Pilots.

Thrown clear of the Enterprise when it broke up in mid-air, Siebold survived a descent from about 50,000 feet (15,000 m) at Mach 1 speed with just a flightsuit.

He was diagnosed with corneal scratches and doctors removed a piece of fiberglass from his left eye during a hospital stay.

The ophthalmologist removed some foreign matter from his left eyelid and a "silver sliver" from his right cornea.