He admitted it.
"Oefelein told police he had an affair with Nowak, a 43-year-old married mother of three, that lasted "for some time."
He actually is quite replaceable. NASA has far more astronauts than it needs or who will ever pilot or fly aboard the shuttle, and competition is fierce among them to get space missions. Do you think there is not a waiting list a mile long of top-notch military pilots to join the shuttle program? Of course there is.
I dont think they are accepting new astronauts for the flying brick. The new ones will be trained for the new vehicle.