The construction of a high-resolution map of the lunar gravity field would be very useful for studies of the lunar interior, and would be invaluable for accurately planning future lunar orbiter missions. Previous gravity-mapping missions have tracked the gravitational perturbations of lunar satellite orbits from Earth to construct nearside gravity maps, but have only been able to provide extrapolated measurements of the far side gravity field due to the lack of tracking data while the satellite's orbit is occluded by the Moon. Gravity-mapping payloads utilizing satellite-to-satellite range-rate tracking between a pair of lunar orbiters have been proposed on previous lunar missions, but have not yet flown. The University of Toronto Space Flight Laboratory, using expertise and design heritage from the CanX nanosatellite program, is in the process of developing a payload for the European Student Moon Orbiter (ESMO) called "Lunette," a gravity-mapping nanosatellite that will separate from a parent spacecraft and fly along track in a 100 km altitude circular polar lunar orbit. The Lunette nanosatellite is based on SFL's Generic Nanosatellite Bus and includes a coherent S-band radio transponder, three-axis attitude determination and control, and a 100 m/s propulsion system, allowing it to maintain an along-track orbital formation and measure the range-rate between itself and the parent spacecraft using Doppler tracking. These range-rate measurements will be used to construct a full-sphere lunar gravity map with an accuracy of 20 mGal or better, comparable to the current best-accuracy nearside gravity map from the Lunar Prospector mission data. Lunette has been selected as a payload for the ESMO project under the Student Space Exploration and Technology Initiative (SSETI) program of the European Space Agency. ESMO is currently in Phase A study, and is targeting a launch in 2011.
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