SpaceX launched astronauts for the first time barely a week ago but CEO Elon Musk does not want the company resting on its laurels, CNBC reports.

Instead, Musk urged SpaceX employees to accelerate progress on its next-generation Starship rocket “dramatically and immediately,” writing Saturday in a company-wide email seen by CNBC.

“Please consider the top SpaceX priority (apart from anything that could reduce Dragon return risk) to be Starship,” Musk wrote in the email.

His space company launched a pair of NASA astronauts on May 30, marking a historic first for SpaceX and a crucial step forward for the U.S. space program. A SpaceX Falcon 9 rocket and Crew Dragon capsule carried the astronauts to orbit and on to the International Space Station. That mission is ongoing, as Musk noted, with the spacecraft set to return the astronauts to Earth in the next couple months.

Musk last year unveiled the Starship prototype, built of stainless steel and dwarfing the company’s existing spacecraft. SpaceX is developing Starship with the goal of launching as many as 100 people at a time on missions to the moon and Mars.