. . . and the question, as always, is: "Who cares?"
To presume that any actor should maintain the persona of a fictional character when the cameras quit rolling is silly. It is true that certain members of the cast of the original series lent their presence in support of NASA - significantly, Nichelle Nicholls - but neither Stewart nor anyone else is required (by anything but the mistaken expectations of whining juveniles of all ages) to support manned space exploration - or anything else - simply because he played a space explorer on TV.