Atlas, Delta, And Manned Space Flight

The current generation of expendable boosters has not been rated for manned space flight.

There must be enough confidence in the performance of certain flight systems, and there must be redundancy of certain features. The Shuttle is redundant, for example, in that any one of its three main engines can fail without causing an unrecoverable disaster, within limits. The Shuttle can lose any single main engine and still abort and land successfully at various locations, depending on when the main engine fails. The Shuttle can lose even more than one of its main engines without necessarily requiring it to be ditched.

Qualification of a version of the current Atlas or Delta system for use for manned flight is not ruled out, but appears unlikely at the moment.