NASA’s upcoming Artemis III will be the tiebreaker between two potential lunar landers for future crewed missions to the moon’s surface, the space agency’s head Jared Isaacman said on Friday. But both SpaceX and Blue Origin are planning to touch down on the moon in 2028 no matter what.

Only one will take part in Artemis IV, the 2028 mission to return humans to the lunar surface—NASA will wait as long as possible before making a selection.

Artemis program manager Jeremy Parsons said at a press briefing on Friday that both companies “are planning for lunar landing missions in 2028.”

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Before that, they must get through Artemis III. In that mission, NASA’s Orion crew capsule will attempt to dock with both Blue Origin’s Blue Moon and a version of SpaceX’s Starship. Though no exact launch window was announced, Artemis III is expected to lift off in the summer of 2027, Isaacman said.

Artemis III was previously slated to be a crewed mission to the moon’s surface but was reconfigured earlier this year to instead allow for a complex, two-week-long test run of both landers in orbit.

Artemis III requires several rockets to be in space at once. First, Blue Moon will launch onboard a Blue Origin New Glenn rocket. Once in orbit, tests will be conducted to ensure the lander is in good shape.

Meanwhile NASA’s Orion will already be on the launchpad atop a Space Launch System (SLS) rocket. “We’re going to do everything we can to ensure that that vehicle is ready to go as quickly as we can after that first Blue Origin launch,” Parsons said.

Because the SLS only needs to put Orion into orbit, the rocket has been modified to remove its upper stage.

“When we pivoted to a low-Earth orbit mission, the SLS team quickly came back and said, ‘We don’t need the upper stage for that. Core stage has plenty of performance to drop us in the orbit we need,’” Parsons said.

Three days after Blue Moon launches, Orion will attempt to dock with the spacecraft, after which there will be two days of testing its guidance, navigation and communications systems.

On day six, SpaceX’s Starship will launch, and Orion will detach from Blue Moon, adjust its orbit and attempt a second docking with Starship. The Artemis III crew will not enter Starship during the operation, however, and the test will last just two hours.

“Once Starship operations are complete, Orion and the crew have the remaining days available for additional test objectives before needing to come home,” Parsons said.

Hurdles have to be overcome before any of this can happen: Blue Origin’s New Glenn rocket has not launched since one of the spacecraft exploded spectacularly in May. Parsons said the company expects to resume launches in November and that, right now, NASA has no contingency plans to send Blue Moon to space onboard an alternate vehicle.

“Their design for the Artemis III lander test article is in good shape,” he said. “We have gone through design reviews. We’ve been participating with them, and they have parts coming together from across the United States.”

Meanwhile SpaceX is aiming to conduct another test of Starship next month, which will include upgrades to the craft’s Raptor engines. Starship has seen a number of hiccups, including several explosions during test flights. But in its most recent demonstration, the vehicle was able to enter orbit for the first time—a major milestone on the way to being ready for Artemis III.

Artemis III’s optimal flight timeframe is 14 days, but the mission could be expanded to 17 days if necessary. While the attempted dockings with the landers are key, the mission will also involve collecting vital data on the health of the astronauts.

“Every day of this mission is built around specific tests that reduce risk, give us valuable science and allow us to test out our capabilities before we go and undertake the lunar landing,” Parsons said.

And looking ahead, work has already begun on assembling and testing the Orion crew and service modules that will be used on Artemis IV, Parsons added.

“We are hardly in a position to ‘set it and forget it,’” Isaacman quipped at the event, referring to future Artemis missions planning. “We will never stop listening to data and adapt as circumstances require.”