Icarus 273 (2016) 2–24 Contents lists available at ScienceDirect Icarus journal homepage: www.elsevier.com/locate/icarus The Lunar Reconnaissance Orbiter Mission –Six years of science and exploration at the Moon ∗ J.W. Keller , N.E. Petro, R.R. Vondrak , the LRO team NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA a r t i c l e i n f o a b s t r a c t Article history: Since entering lunar orbit on June 23, 2009 the Lunar Reconnaissance Orbiter (LRO) has made compre- Received 30 June 2015 hensive measurements of the Moon and its environment. The seven LRO instruments use a variety of Revised 17 November 2015 primarily remote sensing techniques to obtain a unique set of observations. These measurements provide Accepted 21 November 2015 new information regarding the physical properties of the lunar surface, the lunar environment, and the Available online 17 December 2015 location of volatiles and other resources. Scientific interpretation of these observations improves our un- Keywords: derstanding of the geologic history of the Moon, its current state, and what its history can tell us about Moon the evolution of the Solar System. Scientific results from LRO observations overturned existing paradigms Moon, surface and deepened our appreciation of the complex nature of our nearest neighbor. This paper summarizes Geological processes the capabilities, measurements, and some of the science and exploration results of the first six years of the LRO mission. Published by Elsevier Inc. 1. Introduction team successfully defined entirely new mission goals in biannual senior reviews that resulted in extension of the LRO Science Mis- 1.1. LRO history sion, which is now scheduled to continue until at least September 2016. The history of lunar science and exploration and the need for The dual role of LRO as an exploration and science mission advanced remote sensing missions, such as LRO, have been sum- has proven to be very successful. Here we highlight important marized by Mendell (2010) . LRO was initiated in 2004 as an Explo- advances in both realms, focusing on contributions across LRO’s ration Mission with the goal of identifying desirable safe landing evolving objectives without attempting to be fully comprehensive. sites for the return of humans to the Moon or future robotic mis- As the mission has progressed, the LRO spacecraft has been op- sions as part of NASA’s Exploration Systems Mission Directorate. In erated robustly to maximize its value to NASA. It is still working addition, LRO’s objectives included the search for surface resources well with a long lifetime ahead. The LRO mission has successfully and the measurement of the lunar radiation environment. The in- accomplished all its objectives during each mission phase and has struments were selected competitively to accomplish these focused yielded an incredible data return of more than 625 terabytes from objectives ( Vondrak et al., 2010 ). its data releases. These data and data products are a durable legacy LRO was launched on June 18, 2009 and entered lunar orbit on for future explorers and scientists. June 23, 2009. After spacecraft commissioning in lunar orbit, the Exploration Mission began on September 15, 2009 and was com- 1.2. Importance of LRO pleted on September 15, 2010 when operational responsibility for LRO was transferred to NASA’s Science Mission Directorate for a LRO has provided technical innovations and made surprising two-year Science Mission with a new set of science goals. Follow- discoveries that have changed our view of the Moon. The science ing successful completion of the initial Science Mission, the LRO and exploration measurements will be a legacy of LRO that will be extremely useful to generations of lunar scientists and explorers. Some of the LRO technical innovations are: –First deep space precision orbit determination by laser ranging ∗ Corresponding author at: Code 691, NASA Goddard Space Flight Center, 8800 from Earth. Greenbelt Road, Greenbelt, MD 20771, USA. –First global thermal mapping of a planetary body covering a full E-mail address: [email protected] (J.W. Keller). range of local times and seasons. http://dx.doi.org/10.1016/j.icarus.2015.11.024 0019-1035/Published by Elsevier Inc. J.W. Keller et al. / Icarus 273 (2016) 2–24 3 –First bi-static radar imaging measurements from Earth to a –Lunar Orbiter Laser Altimeter (LOLA), PI, David Smith (NASA planetary orbiter. Goddard Space Flight Center, Greenbelt, MD): a system that –First multi-beam laser altimeter system in space. splits a single laser pulse into five laser spots at 28 times per – More than five years of laser altimetric measurements yielding second to measure topography, slopes and roughness ( Smith 8 billion topographic points, better than any other object in the et al., 2010 ). Solar System. –Lunar Reconnaissance Orbiter Camera (LROC), PI, Mark Robin- –First collimated epithermal neutron detectors in space. son (Arizona State University, Tempe, Arizona): consisting of –First use of tissue-equivalent-plastic (TEP) in deep space radia- two narrow-angle cameras (NAC) with a spatial resolution of tion detectors. 50 cm from an altitude of 50 km and an ultraviolet/visible wide-angle camera (WAC) for global imaging in seven color Some of the LRO exploration and science results are: bands with 100 m resolution ( Robinson et al., 2010 ). –Lunar Exploration Neutron Detector (LEND), PI, Igor Mitro- –In polar shadowed regions found the coldest spots measured fanov (Institute for Space Research, and Federal Space Agency, (below 30 K) in the Solar System. Moscow): neutron albedo measurements in three energy bands –Discovered significant subsurface hydrogen deposits in regions for detection of subsurface hydrogen ( Mitrofanov et al., 2010a ). cold enough for water ice to survive, as well as in additional – Diviner Lunar Radiometer Experiment (Diviner), PI, David Paige hydrogen deposits in warmer areas where surface water ice is (University of California, Los Angeles, California): a nine- not thermally stable. channel infrared radiometer to measure thermal state, rock – Measured surprising amounts of several volatiles (e.g., CO, H2 , abundance, and regolith composition ( Paige et al., 2010a ). and Hg) in the gaseous cloud released from Cabeus by the –Lyman Alpha Mapping Project (LAMP), PI, Kurt Retherford LCROSS impact. (Southwest Research Institute, San Antonio, Texas): a far ultra- –New ( < 5 years old) impact craters and were found to be violet imaging spectrometer to measure water frost in perma- widespread across the lunar surface, with a surprising abun- nently shadowed regions and the components of the lunar ex- dance of related surface changes. osphere ( Gladstone et al., 2010b ). – Developed an improved catalogue of lunar craters larger than – Cosmic Ray Telescope for the Effects of Radiation (CRaTER), 20 km in diameter, thus providing constraints on the ancient PI, Nathan Schwadron (University of New Hampshire, Durham, impactor population that affected the inner Solar System. New Hampshire): an energetic particle detector system to mea- –First radar measurements of the lunar farside. sure galactic cosmic rays and solar energetic particle events –Improved the age dating of small landforms by using crater ( Spence et al., 2010 ). counts from the new high-resolution images. – Miniature Radio-Frequency Technology Demonstration (Mini- –Discovered that the Moon is in a general state of relatively re- RF), P.I. Wes Patterson (Applied Physics Laboratory, Laurel, cent ( < 1 Ga) contraction. Maryland): a synthetic aperture radar to measure regolith prop- – Characterized relatively young volcanic complexes, such as Ina, erties and search for subsurface ice ( Nozette et al., 2010 ). and revealed first direct evidence of the presence of highly sili- cic volcanic rocks on the Moon. The instruments are shown and described in Fig. 1 and Table – Measured galactic cosmic ray interactions with the Moon dur- 1 . Each LRO instrument has independent capabilities for provid- ing a period with the largest cosmic ray intensities observed ing both scientific and engineering-enabling measurements of the during the space age. lunar surface and environment. Additionally, a key feature is that – Mapped in detail the temperatures, UV reflectance, and near- the instrument payload set makes complementary measurements surface hydrogen abundance of the Moon’s polar cold traps. that reinforce the discoveries of any individual instrument, so as –Created the first cosmic ray albedo proton map of the Moon. to reduce ambiguity and to make as comprehensive a set of ob- –Made high-resolution images of robotic and human exploration servations as possible ( Vondrak et al., 2010 ). For example, surface sites that showed hardware, the tracks of the astronauts, and features and hazards are measured by LROC, Mini-RF, and LOLA, surface disturbances from landing and ascent. while Diviner infers rock abundance from its temperature data. The search for water ice is accomplished by: LAMP and LOLA, which can measure surface frost; Mini-RF, which can detect near-surface 1.3. Purpose and organization of this paper rocky ice; and LEND, which is sensitive to hydrogen within a me- ter of the surface. As yet another example, the interiors of po- Here we summarize the capabilities of the LRO spacecraft and lar shadowed regions are revealed by LAMP (which uses both UV science results of the LRO mission to this point. Instrument and starlight and interplanetary Lyman-alpha sky-glow), LOLA (which spacecraft capabilities are described in Section 2 . Section 3 de- makes high resolution topographic maps), Diviner (which makes scribes the LRO operations and evolution of its orbit. The LRO infrared emission images), LROC (which uses secondary solar illu- exploration measurements are described in Section 4 . Highlights mination), and Mini-RF (which makes radar images). of the first six years of science accomplishments are summarized in Section 5 , support for future landed missions is described in Section 6 , and conclusions are summarized in Section 7 . 2.2. The LRO spacecraft The LRO orbiter is a 3-axis stabilized spacecraft that is generally 2.