The Centaur upper stage, the first hydrolox rocket stage ever flown and (in its highly-evolved forms) still one of the most-used, as well as one of the highest-performance (if not the highest-performance) upper stages available, was used from the 1960s onward, as part of the Atlas-Centaur, to launch heavy probes to the Moon and to other planets, and to launch very heavy satellites into Earth orbit; in the 1970s, it was also used, as part of the Titan-Centaur, to launch six even heavier probes1 away from the Earth.
In the early 1980s, with the advent of the space shuttle, a new version of the Centaur was developed to launch heavy spacecraft from a shuttle orbiter’s payload bay; known as the Shuttle-Centaur, or, technically, as the Centaur G (for the Air Force) and Centaur G-prime (for NASA), its first missions were to be to launch the Galileo and Ulysses probes (to study Jupiter and the polar regions of the sun, respectively) in May 1986, with the Magellan Venus orbiter following at a later date. The shuttle’s first G-series Centaur was rolled out in August 1985, followed by a second shortly afterwards, and, by late January 1986, the two Centaurs and the two probes were at KSC and already mated to their payload adapters in preparation for being mounted in their orbiters’ payload bays.
However, when the shuttle orbiter OV-099 Challenger was destroyed in a fatal launch failure on the 28th of that month, it sounded the death knell for the Shuttle-Centaur programme, as the high-profile deaths of the seven crewmembers onboard caused NASA and its astronauts to suddenly become far more risk-averse, eliminating any realistic chance of a Centaur flying on board a shuttle (notably, following the loss of Challenger, a large fraction of the corps of shuttle astronauts flat-out refused to fly with a Centaur under any circumstances whatsoever) even once the shuttle’s grounding was lifted;2 although the Shuttle-Centaur was initially able to linger on, work towards the Galileo and Ulysses launches was ordered stopped in February, and the entire Shuttle-Centaur programme was cancelled in June.
At the time of the Challenger launch failure, there was still a ready supply of Atlas-Centaurs3 which could have been used to launch Galileo and Ulysses, potentially even still being able to make the May launch windows had the two probes immediately been transferred from Shuttle-Centaur to Atlas-Centaur. Instead, however, Galileo and Ulysses, which remained slated for shuttle launches, had to wait until the shuttles’ return to flight, and, without the Centaur, when they finally did fly (in October 1989 and October 1990, respectively), they had to make do with the anemic Inertial Upper Stage (IUS; actually two stages), cobbled together from two solid-fuel kickmotors originally designed to propel satellites no further than GTO.4
This was especially bad for Galileo; as even the two-stage IUS had less than two-fifths the oomph of a Centaur, the spacecraft had to get three gravity assists from Venus and Earth before it could get to Jupiter, taking a total of six years to arrive at its destination, whereas the Centaur would have been easily capable of boosting it directly to Jupiter in only two years.5 Moreover, in the three years Galileo sat warehoused to await its ride (and exacerbated by its two coast-to-coast journeys as it was shipped back to JPL in 1986 and then once again to KSC in 1989, both times by truck), the lubricant for its high-gain antenna deployment mechanism broke down, with the result that, when ground control tried to extend the antenna in 1991, it didn’t; switching it to a Centaur on an expendable rocket when the shuttles were grounded and launching it during the original May 1986 launch window would have eliminated the wait and cross-country travel that together allowed the lubricant to deteriorate, in addition to getting Galileo to Jupiter seven years earlier (in 1988 rather than 1995).
Why were Galileo and Ulysses made to wait for the shuttles to return to flight and then be launched from the orbiters using a mediocre propulsion system, rather than simply transferring them from a Shuttle-Centaur to an Atlas-Centaur?
1: Two each of the Helios, Viking, and Voyager probes, to be specific.
2: Which, as it turned out, took the better part of two years.
3: For that matter, there still is a large ready supply, albeit of a much-evolved and much-more-capable version from which the Centaur is practically the only part still recognisable.
4: Magellan also used an IUS to throw it at Venus in May 1989; ironically, what had originally been planned to be the third Shuttle-deployed interplanetary probe ended up actually being the first to launch!
5: Ulysses, which also had to go to Jupiter (for gravity-assist purposes), instead stuck yet another kickmotor on top of the IUS.