Latest estimate for Starship launches required to support a Moon landing? 22.

NASA does not have, nor want, nuclear weapons. But when it comes to a particular metric that is part of the Artemis Program to land people on the Moon, NASA guards a particular number like they are trying to prevent outsiders from cracking nuclear codes.

I am speaking of the number of Starship launches required to fuel a Starship-based lunar lander in low-Earth orbit such that it can fly out to the Moon, land there, and return to lunar orbit.

This number has been a moving target over the years. The general response from NASA has typically been, ‘We don’t know for sure, so we’re not going to say.’ In 2023, for example, the then-director of NASA’s Human Landing System, Lisa Watson-Morgan, told Ars, “We have a general idea, but I’m reluctant to say exactly what that is.”

Later, she said the number was in the “high single digits to the low double digits.” That suggests about a dozen launches, which is a lot. SpaceX would need to launch a “depot” Starship, and then 11 “tanker” Starships to fully load it with liquid oxygen and methane propellant for a lunar voyage. Other officials have said “high teens.” The hope has always been that SpaceX would dramatically improve the lift capacity of Starship to bring down the number of tanker flights.

But now there are concerns among some NASA officials that this might not be happening, or at least not happening soon enough to allow SpaceX’s Starship rocket to play a meaningful role in returning humans to the Moon for NASA as part of Artemis this decade.

A new estimate emerges

On Thursday, the NASA Aerospace Safety Advisory Panel held its quarterly meeting, during which panel member Charlie Precourt (a former NASA astronaut) spoke about risks associated with the Artemis Program and landing humans on the Moon.

“With respect to Artemis IV, however, we’ve got a large number of firsts that reduce the likelihood of mission success,” Precourt said. “We need to keep our eyes on numerous required fuelings for the lander. I think the last estimate is around 22 flights.”

Precourt did not name Starship, but he could only be referring to the SpaceX vehicle. NASA’s other lunar lander for human missions, Blue Origin’s Blue Moon, will not be using orbital refueling as part of its initial mission design. The safety panel almost certainly would have obtained this estimated number of refuelings, 22, from discussions with NASA officials as part of its deliberations during quarterly meetings.

After I posted Precourt’s estimate on the social media site X, SpaceX founder Elon Musk replied that NASA’s safety panel estimate was not accurate. “That’s way too high,” Musk said. “Actual number will be more like 6 to 8.”

Seeking to bring down barriers

To be fair to NASA and SpaceX, there remains a lot of uncertainty in how much cryogenic methane and liquid oxygen will boil off in orbit and how efficient the process of transferring propellants from one ship to another will be. But NASA can take the current performance of the lift capacity of Starship and determine the mass of propellant needed to support a lunar landing. For now, the space agency’s answer to the Starship question appears to be something on the order of two dozen Starship launches.

SpaceX, of course, is working hard to increase the flight rate of Starship. But even if SpaceX achieves “twice weekly” launches by the end of 2027, the company would still need about three months to fuel a depot for a Starship landing.

Precourt said the problem is a challenging one for NASA, and he praised the space agency chief, Jared Isaacman, and his team for trying to find ways to help SpaceX and Blue Origin be successful in their development of landers. NASA is still working toward a human lunar landing in 2028.

“With respect to schedule, I personally really appreciate the focus the administrator and the leadership team is putting on how to press schedule by bringing in resources to enable it to look for places where there are barriers,” he said.