Air & Space Magazine asked NASA's Commercial Crew Program head Ed Mango about the safety requirements private companies must guarantee to land a contract to taxi future NASA astronauts between Earth and the International Space Station. (Above is the concept design for Sierra Nevada Corp.'s "Dream Chaser" transport.) From Air & Space:
The probability of "loss of crew" has to be better than 1 in 1000?
Mango: Yes and no. What we've done is we've separated those into what you need for ascent and what you need for entry. For ascent it's 1 in 500, and independently for entry it's 1 in 500. We don't want industry … to [interpret the 1-in-1,000 requirement] to say, "We've got a great ascent; we don't need as much descent protection." In reality we've got to protect the life of the crew all the time.
Now [the probability for] the mission itself is 1 in 270. That is an overall number. That's loss of crew for the entire mission profile, including ascent, on-orbit, and entry. The thing that drives the 1 in 270 is really micrometeorites and orbital debris … whatever things that are in space that you can collide with. So that's what drops that number down, because you've got to look at the 210 days, the fact that your heat shield or something might be exposed to whatever that debris is for that period of time. NASA looks at Loss of Vehicle the same as Loss of Crew. If the vehicle is damaged and it may not be detected prior to de-orbit, then you have loss of crew.