The US and European space agencies have signed the "letter of intent" that ties together their Mars programmes.
The agreement, which was penned in Washington DC, gives the green light to scientists and engineers to begin the joint planning of Red Planet missions.
The union will start with a European-led orbiter in 2016, and continue with surface rovers in 2018, and then perhaps a network of landers in 2018.
The ultimate aim is a mission to return Mars rock and soils to Earth labs.
Their broad vision would encompass the following launch opportunities:
In 2016: A European-led orbiter to study trace gases, including methane, in Mars' atmosphere. The mission would also put a static meteorological station on the surface. Critically, Europe would handle the entry, descent and landing (EDL) of this station - a capability it has yet to demonstrate.
In 2018: European and American rovers would be dispatched to Mars. The US would do the EDL.
In 2020: "Under consideration" is a network of landers focused on geophysics and the environment.
The U.S. Congress wants an amendment to the 'Letter Of Intent' that states no American funding for these space flights will be used on aborted missions.