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.