Satisfying safety concerns is a challenge for both of NASA’s commercial crew contractors as the flexibility and speed that has characterized private-sector development of space shuttle replacements runs into the engineering realities of human spaceflight. That is true not only for the capsules being built to carry astronauts into space from U.S. soil for the first time since 2011 but also for the launch vehicles that will take them there. The United Launch Alliance Atlas V and SpaceX ...


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