Nasa is aiming to land the first woman on the moon by 2024 as the US gears up for a new $28 billion (£22 billion) space programme.
The space agency will send a woman and a man to the moon as part of the Artemis scheme, which will mark the first human trip to the moon since 1972.
The timeline is dependent on Congress releasing $3.2 billion (£2.5 billion) for a moon landing system.
Nasa administrator Jim Bridenstine said: "The $28 billion represents the costs associated for the next four years in the Artemis programme to land on the Moon. [Space launch system] funding, Orion funding, the human landing system and of course the spacesuits - all of those things that are part of the Artemis programme are included."
But he explained: "The budget request that we have before the House and the Senate right now includes $3.2 billion for 2021 for the human landing system. It is critically important that we get that $3.2 billion."
Congress has already given $600 million (£471 million) for the landing system but Nasa says it needs more.
"I want to be clear, we are exceptionally grateful to the House of Representatives that, in a bipartisan way, they have determined that funding a human landing system is important - that's what that $600 million represents. It is also true that we are asking for the full $3.2 billion."
Mr Bridenstine told CNN in 2019 that the first woman astronaut to walk on the Moon in 2024 would be someone "who has been proven, somebody who has flown, somebody who has been on the International Space Station already".
He added that he wanted to choose the astronauts at least two years before the first mission.
"I think it's important we start identifying the Artemis team earlier than not... primarily because I think it will serve as a source of inspiration," he said.
Nasa is first planning an unmanned month-long test flight around the moon in autumn 2021. This would be followed by a trip around the moon with a crew.
The US' renewed moon programme is seen as an attempt to reassert US dominance in the field of space exploration, after China became the first country to land a robot on the far side of the moon in 2019.
