A joint NASA/ESA mission would happen well before NASA's manned mission to Mars, which is loosely slated for the mid-2030s. It would go down something like this: Two rovers would set off for Mars in 2018, one to drill into the surface, the other to collect surface samples. A few years later, their ride home would set off to pick them up. Again, two spacecraft would be needed, one to head to the surface and one to fall into orbit around Mars. The surface craft would collect the samples and launch them up to the orbiting ship, which would then return make the 50 million mile return trip to Earth.