When NASA’s next big rocket launches for the first time, chances are good it won’t have people on board.

For the last two months, the space agency has been studying what it would take to fly a crew of two on the maiden flight of the Space Launch System, or SLS — the monster rocket that NASA has been developing to take people into deep space and on to Mars. Specifically, NASA wanted to know if such a crewed flight could be done safely by 2019. But after figuring out the costs and challenges associated with putting astronauts on that inaugural mission, called EM-1, NASA says doing a crewed flight first wouldn’t be the best way to go.

“After evaluating cost, risk, and technical factors in a project of this magnitude, it is difficult to accommodate changes needed for a crewed EM-1 mission at this time,” NASA’s acting administrator, Robert Lightfoot, sent in an email to agency employees that was obtained by The Verge.

So for now, it seems that NASA will stick with its original strategy for debuting the SLS: doing an uncrewed flight first, followed by a crewed mission. However, the vehicle’s targeted launch dates are probably going to slip a bit. Originally, NASA had planned to launch the first SLS mission in November 2018, sending an uncrewed spacecraft called Orion on a three-week trip around the Moon. That mission plan still stands, but technical challenges and limited budgets are forcing NASA to push back the launch date to 2019. An exact date will be decided upon in the coming months. A follow-up flight with crew inside Orion is tentatively scheduled for no earlier than 2021, but will probably be pushed back as well.

NASA only started thinking about changing the mission plan earlier this year, after Lightfoot sent out a memo to employees saying NASA would study the feasibility of putting people on the first fight of the SLS. NASA later admitted that appointees for the Trump administration had directed the agency to look into that possibility, according to Space News.

Lightfoot said in press conference today that putting a crew on the first flight of the SLS was technically feasible. However, a lot of technologies would have to be accelerated, such as a life support system to keep the crew alive and abort system that could save the crew in case of an emergency during flight. Plus it would have cost a lot more money, approximately between $600 million and $900 million extra, and NASA would have needed additional time to pull it off. Lightfoot said that a 2020 flight was more likely if people needed to be on board. So NASA conferred with the White House and agreed that the best route would be stick with its original plan all along.

Putting crew on the first flight of a new vehicle isn’t usually how NASA does things, though. Typically, the agency does an uncrewed test flight first to determine if a vehicle is safe. The Saturn V rocket, for instance, did a mission without passengers before crew rode on board, and the future space taxis that SpaceX and Boeing are developing to take NASA astronauts to and from the International Space Station will fly empty first to make sure they are safe for crew. However, one notable exception to this rule was the first flight of the Space Shuttle, which carried a crew of two into orbit around Earth. It was a bold move at the time, but NASA needed a pilot in the Shuttle to control the vehicle’s landing.

It’s been bad news for the SLS program in recent weeks. NASA admitted that part of a propellant tank needed for testing out the rocket was damaged at the agency’s Michoud Assembly Facility in New Orleans. And one of NASA’s associate administrators, Bill Gerstenmaier, acknowledged to the Government Accountability Office two weeks ago that the first flight of SLS would likely slip to 2019.