(Reuters) – SpaceX plans to launch about five uncrewed Starships to Mars in two years, Chief Executive Officer Elon Musk said in a post on social media platform X on Sunday.

“If those all land safely, then crewed missions are possible in four years,” he said, adding that in case of any challenges the crewed missions will be postponed another two years.

“No matter what happens with landing success, SpaceX will increase the number of spaceships traveling to Mars exponentially with every transit opportunity,” the SpaceX founder said.

Musk mentioned the launch of the uncrewed Starships to Mars earlier this month in a post on X, saying “first Starships to Mars will launch in 2 years when the next Earth-Mars transfer window opens.”