![]() The decadal survey process necessarily requires compromise among different science communities, and the interdisciplinary nature and relatively small size of the planetary science community has made this compromise possible thus far. Finally, small-sized (“Discovery”) missions are neither identified nor ranked, but the themes and science questions identified in the decadal survey are used in proposals for Discovery missions by advocates. In the US system, the highest-priority large (“flagship”) missions are recommended in the decadal survey report, while a set of high-priority medium-sized (“New Frontiers”) missions are identified but not ranked. These themes and questions lead to identifications of specific high-priority missions and investigations for a 10-year period (thus the “decadal survey” name). The need to prioritize various science goals and to effectively utilize the available resources led to the development of NASA-requested and community-led decadal surveys, where members of the planetary science community at large identify large-scale themes and smaller groups identify important specific science questions about solar system object types. At the same time, the success of the Apollo program led to a vast reduction of the resources made available for planetary exploration by the United States. As the initial reconnaissance of an object was completed, more focused and directed science questions and goals could be formulated. The first wave of lunar exploration focused on providing the information necessary for landing humans on the Moon and returning them safely to Earth, while the initial exploration of other objects was geared toward simple reconnaissance of unknown surfaces. Rivkin, in Airless Bodies of the Inner Solar System, 2019 Community-Defined Science Goals The Chang'e 2 spacecraft at the end of its mission was transferred successfully from lunar orbit to the Earth–Sun L2 Lagrangian point, the first satellite successfully to make this maneuver, and was subsequently sent on an extended mission to rendezvous with a near-Earth asteroid. The Chang'e 1 mission concluded on March 1, 2009, with a controlled crash on the lunar nearside in Mare Fecunditatis (1° 30′ S, 52° 22′ E). Publication of the first global microwave maps of the lunar surface. The derivation of global major element maps. Chang'e 2 operated in a lower 100-km orbit and included similar instrument suite as Chang'e 1, but with improved spatial resolution to perform detailed analysis of future landing sites for the planned future landers and investigations of lunar resources. Chang'e 1 orbited in a 200-km orbit, conducting scientific experiments to image and map the composition of the lunar surface and space environment, including a microwave radiometer to map global temperatures at different depths down to several meters. The two spacecraft served as the first phase of an ambitious Chinese lunar exploration program to move from an orbiter series, to soft landers to robotic sample return, with the long-term goal of sending people to the Moon. A follow-up mission, Chang'e 2, followed on October 1, 2010. The Chinese Lunar Exploration Program (also known as the Chang'e series) was initiated on October 24, 2007, with the launch of the Chang'e 1 spacecraft.
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