Elon Musk’s SpaceX is delaying trips around the moon for tourists.

SpaceX was originally planning to send two private tourists on a trip around the moon sometime this year. However, it now appears that this won’t happen until at least 2019, but probably not until later.

The exact reason for the delay is unknown, but it likely has to do with a lack of interest in the Falcon Heavy rocket that would have been used in the mission. Upgrades to the Falcon 9 rocket have seen it getting more of a use and focus lately.

The Falcon 9 rocket is seeing more use due to upgrades that give it more thrust. This has it carrying larger satellites into orbit. There are also some concerns about the Falcon Heavy rocket, which hasn’t been under as much testing as the Falcon 9 rocket.

SpaceX was hoping to conduct additional tests of the Falcon Heavy rocket with the help of NASA. It was seeking to transport astronauts to and from the International Space Station. However, a delay in these plans means it won’t start doing this until the end of December, reports The Wall Street Journal.

SpaceX says that all hope isn’t lost. The company is expecting both the U.S. and Europe to turn to the Falcon Heavy in the next couple of years for certain satellite launches. This is due to expectations that they will both retire their current rockets for the task in the next few years.

Sending tourists around the moon is all well and good, but that’s not Elon Musk’s true goal for SpaceX. The company founder is working toward his goal of colonizing Mars. Musk was originally shooting for 2026 as the year his company will have people on Mars, but it’s possible these plans will also face a delay.

As of this writing, William White did not hold a position in any of the aforementioned securities.