Mars rover to regain contact. Mars 2, Mars 3 were physically tethered probes; "Sojourner" was dependent on the Mars Pathfinder base station for communication with Earth; MER-A & B and "Curiosity" were on their own. Of these "Curiosity" is still active, and "Spirit", "Opportunity" and "Sojourner" completed their missions before losing contact. Seven rovers have been dispatched to Mars: "Examples Landed instrumentation" (Rovers) Mars rovers in development include: One experimental design, not proposed for any actual mission, is: NASA distinguishes between "mission" objectives and "science" objectives. Mission objectives are related to progress in space technology and development processes. Science objectives are met
Mars sample-return mission complexity and technical problems. One mission concept (provisionally named simply Mars Sample-Return) was originally considered by NASA's Mars Exploration Program to return samples by 2008, but was cancelled following a review of the program. A NASA-ESA concept mission was aborted in 2012. The United States' Mars Exploration Program, formed after "Mars Observer" failure in September 1992, supported a Mars sample-return. One example of a mission architecture was the "Groundbreaking Mars Sample-Return" by MacPhereson in the early 2000s. In early 2011, the National Research Council's Planetary Science Decadal Survey, which laid out mission planning priorities for the period 2013–2022 at the