The European Space Agency's (ESA) Earth Return Orbiter (ERO) would be the biggest spacecraft to ever orbit Mars. The spacecraft would also be the first interplanetary spacecraft to rendezvous and capture hardware launched from another planet and return it to the Earth's surface, making a full round trip to Mars and back. ERO would be a multi-stage modular spacecraft equipped with both chemical and solar electric propulsion. The electric propulsion system would be the most powerful ever flown on any previous planetary mission.

ERO would carry a radiation monitor to measure the total radiation dose experienced by the spacecraft throughout the entire mission, which in addition to monitoring the health of the ERO, should provide important information on how to design systems for future human explorers. Launch is planned in 2027, entering into Mars orbit in 2029.

The Earth Return Orbiter is part of the multi-mission Mars Sample Return campaign being planned by NASA and the European Space Agency (ESA).