The US and Europe have cemented their plan to work together on the Americans' next-generation capsule system to take humans beyond Earth.
The Orion vehicle is being built to carry astronauts to the Moon, asteroids and Mars, but it will need a means to propel itself through space.
Europe has now formally agreed to provide this technology.
Space agency executives have just signed an "implementing agreement" to cover the legal aspects of the work.
The first flight of Orion with its European-built "service module" will take place in 2017.
This demonstration will be unmanned and will see Orion go around the back of the Moon before returning to Earth for an ocean splashdown.
If all goes well, a crew is expected to repeat the feat in about 2021. The venture would echo the famous Apollo 8 mission of 1968.
The service module is the unit that sits directly behind the capsule and provides the propulsion, power, temperature control, as well as holding the astronauts’ supplies of water and air.
"This is a new page in the transatlantic co-operation," said Thomas Reiter, the director of human spaceflight and operations at the European Space Agency (Esa).
"This is the first time that Esa is involved in the critical path for a human transportation system. It is a fantastic perspective for the future, taking humans beyond low-Earth orbit to new destinations for exploration," he told BBC News.
The current plan calls for Europe to build the prototype module for 2017 and a number of components that would be needed for the second vehicle in 2021, although a formal go-ahead to complete this additional model is some years off.
A clear subtext, of course, is that Europe hopes this initial deal will develop into a long-term relationship, and that this will increase the chances of its astronauts joining their American colleagues on missions into deep space.
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