NASA has reaffirmed that it will not launch humans into space again until 2021, after the Trump administration asked them to look into bringing it forward to 2019.

The agency is currently developing its huge new Space Launch System (SLS) rocket, which will be used to one day launch astronauts to Mars.

In 2019, NASA is planning to launch the rocket for the first time with an unmanned Orion spacecraft on top, known as Exploration Mission 1 (EM-1), on a three-week mission around the Moon and back. A manned flight is scheduled to follow in 2021, called Exploration Mission 2 (EM-2).

However, the Trump administration had asked if NASA would consider putting a crew on the first flight – which would put it in Trump's first term. NASA looked into this, and has come back with a firm “no”.

“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, told employees in an email seen by The Verge.

In a press conference on Friday, Lightfoot said it would cost between $600 million and $900 million to put a two-person crew on EM-1. That’s because the spacecraft would need to be upgraded with life support systems, a launch abort system, and more.

“It really reaffirmed the baseline plan we had in place was the best way for us to go,” he said. “Leave EM-1 uncrewed.”

Trump had previously joked about the possibility of sending humans to Mars by 2024, so that it would take place in his potential second term (shudder). That has not really been taken seriously though, with the idea very much off the table.

“They have not asked us to go to Mars by 2024,” Lightfoot said in the press conference.

EM-1 has had a few problems so far, having been pushed back from 2018 to 2019 recently. And part of the rocket was damaged last week when it was accidentally dropped.

Once it does get up and running, though, the SLS will be the most powerful rocket in operation. And it will eventually eclipse the most powerful rocket of all time, the Saturn V.

This first launch is known as Block 1, and has a lifting capability of 70,000 kilograms (154,000 pounds) to low Earth orbit. Block 2, a bigger version of the SLS, will be able to carry 130,000 kilograms (287,000 pounds) to low Earth orbit.

Of course, this is not the only heavy-lift rocket in development. SpaceX is developing its Falcon Heavy rocket, which it intends to use to send humans around the Moon in 2018 and also for unmanned missions to Mars starting in 2020.

And SpaceX and Boeing will start launching astronauts to space, some for NASA, as early as next year.