The Orion that will fly atop SLS for EM-1 is already coming together at the Michoud Assembly Facility in New Orleans, Louisiana. On Saturday, September 5, engineers welded together the capsule's tunnel and forward bulkhead—the first of seven welds required to assemble the vehicle's primary structure.

Gerstenmaier said that though the flights of EM-1 and EM-2 are important prerequisites needed to prepare the vehicle for operational missions, a delay for one mission does not necessarily mean a delay for the other.

"If we just wanted to launch EM-1 by some date, we could move test requirements off of EM-1 and move them later in the first portion of EM-2, and maybe down-scope EM-2," he said. "And that's not what we want to go do. We want to make sure we get all the right testing done on EM-1, and we can actually move forward with EM-2."

Even if EM-2 slips to 2023, he said, it would not likely be the mission NASA uses as part of the Asteroid Redirect Mission—a crewed visit to a captured asteroid in Earth orbit.

"In general, you would really like to get the vehicle fully checked out and spend some time doing some unique things with the vehicle, and make sure crew interactions with the vehicle really work they way you intended," Gerstenmaier said. "Then when you go to that operational mission like the Asteroid Redirect Mission, where your focus now is to go do an EVA and [de-pressurize] the Orion capsule and go out and grab a sample off the asteroid, you make sure you can really focus on all of those [objectives] first."