But past administrations have not been shy about pulling the financial plug on previous projects aimed at replacing the space shuttles – notably McDonnell Douglas's Delta Clipper and Lockheed-Martin's X-33/Venture Star, notes Howard McCurdy, a historian at American University in Washington. Both were canceled well into their development phases due to mishaps and technological hurdles that threatened to push up costs.
A commission is currently reviewing NASA's human spaceflight program, and it is not necessarily bound to hold onto Constellation, according to its chairman, Norman Augustine. "We will be looking at different architectures, as well as the existing architecture, and I am not in a position to make any predictions," he said during a briefing earlier this month. "We have been asked to provide options."
One important political factor that could keep Constellation on track: Without it, US astronauts would have to rely on Russian Soyuz launches to reach the International Space Station far longer than they would otherwise would. An approach other than Constellation would take years to develop. It is a potential gap in US capabilities that former NASA administrator Michael Griffin called "unseemly."