KENNEDY SPACE CENTER, Fla. — As 1985 faded into 1986, NASA was racing the clock to perfect a powerful hydrogen-fueled upper stage to propel the Ulysses and Galileo probes to Jupiter. Both nuclear-powered spacecraft were to be deployed from two space shuttles launched just five days apart in May 1986.

The General Dynamics-built Centaur-G rockets, mounted in the cargo bays of Challenger and Atlantis, would be loaded with 20,000 kilograms of supercold liquid-oxygen and volatile liquid-hydrogen propellant in tanks thin enough to dent with a gloved finger.

Complex plumbing was required for prelaunch fueling and to permit an emergency overboard propellant dump if tank pressures threatened to collapse internal bulkheads or if a shuttle main-engine failure triggered a return-to-launch-site abort.

The payloads were so heavy the shuttle’s main engines would have had to operate at 109 percent normal power, so heavy only four-man crews could be launched, prompting jokes about swapping underwear instead of carrying enough to go around.

But it was no joke, even though Atlantis commander David Walker privately referred to the Centaur as the “death star.”

“We thought that they were the most dangerous missions that we were to have flown to that date,” said former astronaut Norman Thagard, a member of the Galileo mission’s four-man crew. “We really thought if we lost a shuttle, it would probably be one of those two missions.”

Concern was so serious, Thagard said, that he and astronaut David Hilmers, a fellow Marine, close friend and member of the Ulysses crew, used to share a Vietnam-era joke: “If you guys blow up, can I have your stereo?” Thagard’s job during launch was to monitor a display showing Centaur tank pressures. The hydrogen and oxygen tanks shared a common bulkhead that used pressurization to maintain structural integrity.

“It was a tank within a tank with a single bulkhead separating the oxidizer and the propellant,” he said. “And if the pressures ever reversed so that the hydrogen tank pressure got higher, then you would basically cave in the bulkhead, it would rupture, the two fuels would mix and then you’d be in a lot of trouble.”

Larry Bourgeois, the flight director in charge of Centaur planning, agreed the missions would have been the most dangerous NASA ever flew, “no question about it.”

“There were too many hazards,” he said. “With that hydrogen and oxygen in the payload bay, having to maintain the pressure in those tanks and then having to be able to dump that stuff, it was downright scary.”

It became a moot point on Jan. 28, 1986, when Challenger was lost in a catastrophic failure. In the aftermath, the shuttle Centaur program was canceled and the Ulysses and Galileo spacecraft eventually were launched using less-powerful solid-fuel boosters and velocity-boosting planetary flybys.

“We were actually in some kind of flight rules meeting the day the Challenger exploded,” Hilmers recalled in an interview. “We kind of put our meeting on pause while we watched the launch. And of course, after that happened, everybody just kind of walked out. We knew things were going to be different.”

Challenger also resulted in the loss of two other high-profile missions planned for 1986 — launch of Discovery that September from Vandenberg Air Force Base, Calif., on a classified flight around Earth’s poles; and launch of Challenger in October on a satellite-deployment mission featuring the first journalist in space.

Robert Crippen, pilot of the first shuttle mission in 1981, was commander of the classified Vandenberg mission, known as STS-62A. His crewmates included Edward “Pete” Aldridge, secretary of the Air Force.

“If I have one flying regret, it was I didn’t get that mission out of Vandenberg,” he told Space News. “I kind of lobbied for the first flight.

“I think the Air Force had somewhat of a problem with a Navy guy commanding the first flight. But they did end up naming me and we trained for a flight.”

Launching heavy military payloads out of Vandenberg into polar orbits required more power to make up for losing the benefit of Earth’s rotation and like the Centaur missions, Crippen’s flight faced technical challenges.

Unlike the steel booster segments used for shuttle flights out of Florida, Atlantis would have needed lightweight carbon composite filament-wound cases.

“There was lots of concern back at that time with regard to could you, through nondestructive testing, really tell there wasn’t a flaw in the case?” Crippen said. “We wrestled with that somewhat.”

Other issues included the effects of the sonic shock wave produced by the shuttle’s boosters on the futuristic-looking launch pad and ice formation.

Jet engines were installed at the pad to help prevent dangerous icing.

But in the aftermath of Challenger, after spending some $3 billion on Space Launch Complex 6 at Vandenberg, the Air Force and NASA called off plans for launching manned missions from the West Coast.

“We were at Los Alamos going through one of our payloads when we lost Challenger,” Crippen said. “We were all out there together. I remember, Pete Aldridge asked me, ‘Crip, do you think this is going to delay our flight?’” Crippen smiled at the memory, adding his answer: “You bet it will.”

The planned journalist-in-space flight aboard Challenger, mission STS-61I, would have launched later that fall. The primary goal of the mission was to deploy a communications satellite.

Plans to fly a journalist were announced in October 1985, amid Christa McAuliffe’s training to become the first teacher in space. In an ironic twist, McAulilffe’s pilot, Mike Smith, also was named to serve as pilot for the journalist-in-space mission.

After the Challenger disaster, the Association of Schools of Journalism and Mass Communication went ahead and narrowed down the list of candidates to 40. One of them was Rob Navias, then a radio correspondent with United Press International and now a NASA public affairs officer.

Asked if he saw a conflict working as a professional journalist on a NASA shuttle flight, Navias said, “I must tell you, at the time when a lot of us were caught up in the romance of it all, what I would call the more innocent days where a tragedy like the Challenger was not something we could have ever dreamt of, I thought it was an honor and an opportunity for those of us who loved human spaceflight.”

But it was not to be.