[1] The selection of Meridiani Planum and Gusev crater as the Mars Exploration Rover landing sites took over 2 years, involved broad participation of the science community via four open workshops, and narrowed an initial $155 potential sites (80-300 Â 30 km) to four finalists based on science and safety. Engineering constraints important to the selection included (1) latitude (10°N-15°S) for maximum solar power, (2) elevation (less than À1.3 km) for sufficient atmosphere to slow the lander, (3) low horizontal winds, shear, and turbulence in the last few kilometers to minimize horizontal velocity, (4) low 10-m-scale slopes to reduce airbag spin-up and bounce, (5) moderate rock abundance to reduce abrasion or strokeout of the airbags, and (6) a radar-reflective, load-bearing, and trafficable surface safe for landing and roving that is not dominated by fine-grained dust. The evaluation of sites utilized existing as well as targeted orbital information acquired from the Mars Global Surveyor and Mars Odyssey. Three of the final four landing sites show strong evidence for surface processes involving water and appear capable of addressing the science objectives of the missions, which are to determine the aqueous, climatic, and geologic history of sites on Mars where conditions may have been favorable to the preservation of evidence of possible prebiotic or biotic processes. The evaluation of science criteria placed Meridiani and Gusev as the highest-priority sites. The evaluation of the three most critical safety criteria (10-m-scale slopes, rocks, and winds) and landing simulation results indicated that Meridiani and Elysium Planitia are the safest sites, followed by Gusev and Isidis Planitia.

The Gravity Recovery and Interior Laboratory (GRAIL) Mission -a NASA DiscoveryProgram mission currently in development and scheduled to launch in September 2011 -will make a high resolution determination of the lunar gravity field in an effort to understand the internal structure and thermal evolution of the Moon "from crust to core" and will extend that knowledge to other terrestrial planets within the inner solar system. This will be accomplished by placing two nearly identical orbiters, flying in tandem, in a low-altitude, near-polar orbit around the Moon. Precise measurements of the relative velocity between the two orbiters combined with measurements of the absolute position of the orbiters about the Moon as determined via Earth-based tracking will allow the global lunar gravity field to be mapped to unprecedented accuracy and resolution. The development of the baseline strategy to achieve the objectives of this mission involves the integration of a variety of design elements into a coherent mission plan, including: trans-lunar trajectory and navigation design, in-flight calibration and checkout planning, lunar orbit insertion design, design of the maneuvers for orbit circularization and for the establishment of the formation necessary for gravity science data collection, and gravity science data collection design. These mission design elements are described in chronological order as the mission progresses through its seven mission phases:

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