Flying astronauts has always been risky. No wonder NASA and SpaceX are on edge.

CAPE CANAVERAL, Fla. — Spaceflight is a dangerous enterprise. That unrelenting reality underlies every decision as NASA and SpaceX near the hour when they’ll strap two veteran astronauts into a spacecraft that will mark not only the first human launch to orbit from U.S. soil in nearly a decade but also the first time a private company has performed the feat.