SpaceX has signed its first customer to fly on the company’s huge new rocket, the BFR, the company says. The passenger will fly on the monster ship around the Moon, though there are no details yet regarding when the trip will happen. SpaceX says it will announce who is flying — and why — on Monday, September 17th.

The BFR, or the Big Falcon Rocket, is the giant rocket that SpaceX is currently developing to send humans to the Moon and Mars. The BFR design, presented by CEO Elon Musk last year, consists of a combined rocket and spaceship, called the BFS for Big Falcon Spaceship. The main rocket will have 31 main Raptor engines and be capable of sending up 150 tons to low Earth orbit, according to that presentation.

In February 2017, SpaceX announced plans to send two passengers around the Moon on the company’s Falcon Heavy rocket, claiming that the flight would happen at the end of 2018. SpaceX never named the passengers, and, ultimately, Musk admitted during the inaugural flight of the Falcon Heavy that the trip probably wasn’t going to happen. “We’re sort of debating whether to do that on Falcon Heavy or BFR,” Musk told The Verge before the launch in February of this year. “It will sort of depend on how well BFR development is going as to whether we focus on BFR for deep-space human flight or whether we do that on Falcon Heavy.”

It’s unclear if this new passenger being announced is one of the two original passengers from the Falcon Heavy flight or a new customer altogether. Musk hinted on Twitter that the customer might be from Japan. SpaceX says it will give more details on Monday, and the company has set up a live stream for the announcement.

Musk gave a detailed presentation about the design for the BFR last September during the annual International Astronautical Congress. The presentation called for a slimmed-down version of the vehicle, using 31 main engines, compared to the version Musk had pitched the year before, which would have had 42. However, Musk confirmed on Twitter that the rendering of the BFR posted by SpaceX for the announcement was a new version of the vehicle.

Both the BFR and BFS are reusable and designed to use their engines to lower themselves to the ground, much like how the Falcon 9s land. Musk envisions using the BFR for setting up a Moon base on the lunar surface, as well as starting a human settlement on the surface of Mars. However, Musk also noted that the BFR could be used to launch satellites, and he even proposed the idea of using the vehicle to do point-to-point travel. Conceivably, passengers on Earth could ride the rocket to distant cities, with travel times lasting just 30 minutes for destinations on the other side of the planet.

Musk noted last year that his ultimate goal is to transition SpaceX’s focus from the company’s current line of vehicles — the Falcon 9, the Falcon Heavy, and the Dragon spacecraft — to the BFR. “All our resources will turn toward building BFR,” Musk said last year. “And we believe we can do this with the revenue we receive from launching satellites and servicing the space station.”

It’s still uncertain when the BFR will be complete, though. The company has already started working on the rocket near SpaceX’s headquarters in Hawthorne, California, and it also received approval to develop the vehicle at a new facility in the Port of Los Angeles. Additionally, SpaceX has started doing tests on subscale versions of the Raptor engine that will power the BFR. SpaceX president Gwynne Shotwell recently said she believes that the BFS will do short hop tests sometime by the end of 2019. These tests will be similar to SpaceX’s initial Grasshopper tests of the Falcon 9, where the rocket climbs to a certain height above Earth and then attempts to land gently back on the ground.

Musk has predicted that the BFR’s first trip to Mars could happen in 2022, which has been considered to be an extremely optimistic goal. The development cost of the BFR is expected to be around $10 billion, and it’s still unclear if SpaceX’s profits will be enough to cover that. Plus, Musk is known for setting unrealistic timelines. The Falcon Heavy was supposed to launch as early as 2013 or 2014, but it didn’t get off the ground until this year. Seeing as how the BFR is a much bigger and more complex vehicle, it seems reasonable that its development will take longer than expected, too.

Update Friday, August 14th, 9AM ET: This article was updated to provide more context on the BFR timeline.