An anonymous reader writes: An article at Ars Technica tells the story of Shuttle-Centaur, a NASA project during the mid-1980s to carry a Centaur rocket to orbit within the cargo bay of a space shuttle. As you might expect, shuttle launches became vastly more complex with such heavy yet delicate cargo. Still, officials saw it as an easy way to send probes further into the solar system. They developed a plan to launch Challenger and Atlantis within 5 days of each other in mid-1986 to bring the Ulysses and Galileo probes to orbit, each with its own Shuttle-Centaur. Though popular opinion at the time was that the shuttle program was "unstoppable," individuals within NASA were beginning to push back against slipping safety standards. "While a host of unknowns remained concerning launching a volatile, liquid-fueled rocket stage on the back of a space shuttle armed with a liquid-filled tank and two solid rocket boosters, NASA and its contractors galloped full speed toward a May 1986 launch deadline for both spacecraft." The destruction of Challenger in January, 1986 put Shuttle-Centaur on hold. The safety investigation that ensued quickly came to the conclusion that it presented unacceptable risks, and the project was canceled that June.