SpaceX will launch its first uncrewed Starships to Mars in two years when the next Earth-Mars transfer window opens, CEO Elon Musk said in a post on X.

“These will be uncrewed to test the reliability of landing intact on Mars,” Musk said on Saturday, adding if those landings go well, his space company will launch its first crewed flights to Mars in four years.

“Flight rate will grow exponentially from there, with the goal of building a self-sustaining city in about 20 years,” the billionaire said.

In April, Musk, who founded SpaceX in 2002, said the first uncrewed starship to land on Mars would be within five years, with the first people landing on Mars within seven years.

In June, a Starship rocket survived a fiery, hypersonic return from space and achieved a breakthrough landing demonstration in the Indian Ocean, completing a full test mission around the globe on the rocket’s fourth try.

Musk is counting on Starship to fulfil his goal of producing a large, multipurpose next-generation spacecraft capable of sending people and cargo to the moon later this decade, and ultimately flying to Mars.