NASA’s new heavy-lift rocket will not get off the ground until December 2019 at the earliest, and its maiden flight could easily slip to the middle of 2020, the space agency announced on Wednesday.

The rocket known as the Space Launch System would succeed the Saturn 5 that took astronauts to the moon more than four decades ago. NASA says it plans to use the vehicle to take astronauts not only to the moon — one of the goals of the Trump administration — but someday Mars.

Earlier this year, NASA acknowledged it would not be able to make the previously announced launch date of November 2018 for the first flight, which will not carry any astronauts.

The completed review indicates a launch date of June 2020, but NASA said it might be possible to move up the launch date by six months.