The US space agency (Nasa) has formally outlined its $28bn (£22bn) plan to return to the Moon by 2024.
As part of a programme called Artemis, Nasa will send a man and a woman to the lunar surface in the first landing with humans since 1972.
But the agency’s timeline is contingent on Congress releasing $3.2bn for building a landing system.
Astronauts will travel in an Apollo-like capsule called Orion that will launch on a powerful rocket called SLS.
Speaking on Monday afternoon (US time), Nasa administrator Jim Bridenstine said: “The $28bn represents the costs associated for the next four years in the Artemis programme to land on the Moon. SLS 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 $3.2bn for 2021 for the human landing system. It is critically important that we get that $3.2bn.”
Artemis: To the Moon and Beyond
The US House of Representatives has already passed a Bill allocating $600m towards the lunar lander. But Nasa will need more funds to develop the vehicle in full.
Mr Bridenstine added: “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 $600m represents. It is also true that we are asking for the full $3.2bn.”
The new document outlines Phase 1 of the plan, which includes an uncrewed test flight around the Moon – called Artemis-1 – in the autumn of 2021.
Nasa’s human spaceflight chief Kathy Lueders said that Artemis-1 would last for about a month to test out all the critical systems.
She said that demonstration flight would reduce the risk for Artemis-2, which will repeat the trip around the Moon with astronauts.
A new test has been added to this mission – a proximity operations demonstration. Shortly after Orion separates from the upper-stage of the SLS rocket – known as the interim cryogenic propulsion stage – astronauts will manually pilot the spacecraft as they approach and back away from the stage.
This will assess Orion’s handling qualities, along with the performance of the spacecraft’s hardware and software.
Artemis-3 will become the first mission to send astronauts to the lunar surface since Apollo 17 some 48 years ago.
Nasa has provided $967m (£763m) to several companies to work on designs for the landing vehicle that will take them there.
Later in the decade, the plan calls for Nasa to establish a base for humans, called Artemis Base Camp, that would include the infrastructure needed for long-term exploration of the Moon.
Scientists would like to extract water-ice from the lunar South Pole, because it could potentially be used to make rocket fuel on the Moon, at a lower cost than carrying it from Earth.
By comparison with Artemis, the Apollo programme in the 1960s and 70s cost upwards of $250bn in inflation-adjusted US dollars.
However, the $28bn for this new plan does not include money already spent developing the Orion spacecraft and Space Launch System (SLS) rocket.
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