Nasa has chosen the companies that will develop landers to send astronauts to the Moon's surface in the 2020s.
The White House wants to send the next man and the first woman to the Moon in 2024, to be followed by other missions.
Jeff Bezos' Blue Origin, Elon Musk's SpaceX and Alabama-based Dynetics were selected to work on landers under the space agency's Artemis programme.
The 2024 mission will see astronauts walk on the Moon's surface for the first time since 1972.
Combined, the contracts are worth $967m (£763m; €877m) and will run for a "base period" of 10 months.
"With these contract awards, America is moving forward with the final step needed to land astronauts on the Moon by 2024, including the incredible moment when we will see the first woman set foot on the lunar surface," said Nasa's administrator Jim Bridenstine.
"This is the first time since the Apollo era that Nasa has direct funding for a human landing system, and now we have companies on contract to do the work for the Artemis programme."
The winning lander concepts take different approaches to the challenge of setting humans down on the lunar surface.
Blue Origin, founded by Amazon president and CEO Jeff Bezos, is partnering with Lockheed Martin, Northrop Grumman and Draper to develop its lander. Bezos' firm is the prime contractor in the partnership which has been dubbed the "National Team".
The design is split into three parts: a transfer element that will carry astronauts from a higher lunar orbit to a lower one; a descent stage that will take them from low lunar orbit to the surface, and an ascent stage that will blast the crew back off the surface at the end of their mission.
The concept is designed to be launched on Blue Origin's New Glenn and ULA's Vulcan rocket systems.
Dynetics' concept is a single module that can launch on the Vulcan rocket. According to Lisa Watson-Morgan, the human landing system program manager at Nasa's Marshall Space Flight Center in Huntsville, Alabama, this design features a "unique, low-slung crew module, putting the crew very close to the lunar surface for transfer and access".
SpaceX's design is the Starship, which will use the company's Super Heavy rocket for launch. A prototype of the craft has been tested at the company's Boca Chica site in southern Texas.
Other key elements in Nasa's Moon plan are further along in their development. Astronauts will launch from Earth in a capsule called Orion atop a powerful rocket known as the Space Launch System (SLS). Nasa also plans to build a small space station in lunar orbit, known as Gateway, where Orion could dock and different lander stages could be assembled before the journey to the lunar surface.
However, it was reported last month that Gateway had been removed from the "critical path" for 2024's mission. But during a news conference on Thursday, Mr Bridenstine re-stated the agency's support for Gateway. It's not likely to be used by the Artemis-3 mission in four years, but will be important for later stages in the programme: "We absolutely need a Gateway," he said.
Doug Loverro, associate administrator for Nasa's Human Explorations and Operations Mission Directorate, commented: "With these awards we begin an exciting partnership with the best of industry to accomplish the nation's goals. We have much work ahead, especially over these next critical 10 months."
Support within Congress will be crucial for securing the funds Nasa requires for the programme, which is projected to cost $35bn over the next four years.
In the 1960s and 70s, Nasa launched seven missions intended to land on the Moon under the Apollo programme. Apollo 13 did not touch down after an oxygen tank exploded. The last crewed mission was Apollo 17, which explored the Moon's Taurus-Littrow valley in December 1972.
This time, Nasa wants to establish a long-term presence.
"We're not going back to the Moon to leave flags and footprints and then not go back for another 50 years," Jim Bridenstine said last year. "We're going to go sustainably - to stay - with landers and robots and rovers and humans."
The agency is acting on a space policy directive signed by President Donald Trump in 2017. It instructs Nasa to return American astronauts to the Moon, and on to "other destinations".
The Moon could be used as a potential testing ground for capabilities that would help humans perform a landing on Mars.
SpaceX included plans for an uncrewed test landing on the Moon in its proposal to Nasa. Dynetics says it will perform a demonstration flight to test key capabilities for its lander system prior to a mission to the lunar surface.
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