The precedent

On April 12, 1981, astronauts John Young and Bob Crippen climbed aboard space shuttle Columbia and blasted off on a 2-day shakedown cruise. Beyond a series of glide tests, the shuttle had never flown.

I contacted Crippen to ask about the mission, and get his perspective on the risks involved in flying astronauts on a new vehicle. He declined to be interviewed, saying he preferred to let NASA conduct its feasibility study first. He did, however, tell me he thought flying SLS and Orion without a crew first seemed like "a good idea."

Prior to Columbia, NASA flew its rockets without people first for a very simple reason: they used to blow up a lot more.

The first two Mercury flights of Alan Shepard and Gus Grissom in 1961 were brief suborbital jaunts atop the Army's Redstone booster. For John Glenn's orbital flight, NASA switched to the more powerful Atlas rocket. The very first time Glenn showed up to watch an Atlas test flight, the mission ended in disaster shortly after liftoff.

"That wasn't a confidence builder," Glenn later said.

By the time the shuttle flew, NASA had a better track record, and computer simulations were able to more accurately predict a vehicle's performance, giving engineers a higher certainty things would go right on launch day.

Mike Neufeld, a senior curator in the space history department at the Smithsonian National Air and Space Museum, said it was decided relatively early in the shuttle's development cycle that a pilot would have to be in the cockpit to fly the shuttle during landing. And there was no shortage of astronauts ready to give it a try.

"NASA picked test pilots for Mercury, and soon after they arrived, they said, 'Hey, we're not going to just be passengers on these things,'" he told me. "So that's really embedded in the history of the U.S. human spaceflight program."

This was a stark contrast, he said, with the Soviet Union's space program.

"The first class of cosmonauts were these 25-year-old kids who just ordinary jet pilots taken from the Soviet Air Force," said Neufeld. Soviet spacecraft were highly automated, and this mentality extended all the way to Buran, the legendary Soviet shuttle clone that only made a single test flight—an automated one.

NASA astronauts continue to play a large role in the development and operations of their spacecraft. But since the shuttle days, the agency has returned to its automated roots. Orion can fly without a crew; as can upcoming commercial vehicles like Boeing's Starliner and SpaceX's Crew Dragon.

Heritage technologies

If NASA does put a crew on the first SLS mission, it would probably be a lot safer than Columbia's flight.

Most SLS components are shuttle-derived. The core stage is essentially a shuttle external fuel tank with four shuttle main engines mounted at the bottom. All four of the engines slated for the first SLS flight have already carried shuttles into space. The SLS side-mounted solid rocket boosters are effectively shuttle boosters with extra propellant segments.

Orion has already flown once. In December 2014, a United Launch Alliance Delta IV Heavy rocket blasted an Orion capsule to an altitude of 5,800 kilometers, subjecting it to a high-velocity atmospheric reentry meant to simulate a lunar return. Orion's European-built service module is based on the Automated Transfer Vehicle, which is used by the European Space Agency to ferry cargo to the International Space Station.

The biggest question mark may be the rocket's upper stage.

The first SLS flight will use a Delta IV upper stage—the same used for the 2014 Orion test flight—called the Interim Cryogenic Propulsion Stage, or, ICPS.

The second SLS flight—meant to be the first crewed flight, no earlier than 2021—will use the under-construction Exploration Upper Stage, or EUS.

Both stages use Aerojet Rocketdyne RL-10 engines. But the ICPS only has one engine; the EUS will have four.

This presents NASA with a bit of a paradox: the ICPS has flown; the EUS has not. But the ICPS was never meant to carry humans, whereas the EUS is being built with humans in mind from the start. Deciding which coniguration is safer, then, is hard to judge. (In 2015, NASA said the ICPS could be human-rated at a cost of $150 million.)

If anything goes wrong during the initial climb to orbit, Orion is equipped with a traditional escape tower that would pull the capsule away from SLS. The space shuttle, on the other hand, relied on a risky return-to-launch-site abort scenario that involved ditching the solid rocket boosters and external fuel tank, turning the shuttle around, and gliding back to Kennedy Space Center.

"There's a very limited set of circumstances in which that would have worked," Neufeld said. "And obviously, the orbiter had to stay intact."

NASA actually considered testing this abort mode on Columbia's first flight, before ultimately deciding flying all the way to orbit was safer.