A SpaceX Starship is shown at its launchpad in Boca Chica, Texas. Markel Lee Simmons/Getty Images SpaceX CEO Elon Musk announced on his social-media site X on Sunday that the company plans to send five uncrewed Starships to Mars within the next couple of years. Musk said the company has to wait for the next Earth-Mars launch window before sending the missions. These windows occur when Mars and the Earth are lined up in such a way that flights between them take the least amount of energy and time. The next window is in 2026. Should SpaceX miss the deadline, the next launch window is late 2028 into early 2029. Should the uncrewed ships land safely, Musk anticipates sending crewed missions to Mars during the 2028-29 launch window. If the tests don't succeed, the company will try uncrewed missions again in the 2028 launch window and push the crewed missions back to the launch window after that. SpaceX has yet to land the Starship, its biggest ever vehicle, on solid ground. On its latest mission in June it landed for the first time in the Indian Ocean. "No matter what happens with landing success, SpaceX will increase the number of spaceships traveling to Mars exponentially with every transit opportunity," Musk said on X. The ultimate goal, according to Musk, is the building of a self-sustaining Martian city in "about 20 years." SpaceX President Gwynne Shotwell echoed those claims two years ago in an interview with CNBC. The Starship's max payload is 150 metric tons. A representative for SpaceX did not respond to a request for comment. SpaceX has challenges ahead