An Earth-based observer cannot view the largest and oldest giant impact basin because it is out of view on the moon's hidden Farside. South Pole-Aitken (SPA) Basin is about 2500 kilometers wide, making it perhaps the largest impact scar in the Solar System. Lunar Orbiter data revealed its existence in the 1960s, though little was known of it until the 1990s, when the U.S. Clementine and Lunar Prospector polar orbiters mapped surface chemistry over the entire moon. Their data showed that the basin floor probably includes material excavated from the moon's lower crust and upper mantle. In the first decade of the 21st century, laser altimeters on the U.S. Lunar Reconnaissance Orbiter (LRO) and Japanese Kaguya spacecraft confirmed that SPA includes the lowest places on the moon.
In 2002, however, the National Research Council's planetary science Decadal Survey declared SPA sample return to be a high scientific priority and, at the same time, proposed a new class of competitively selected medium-cost missions. The latter marked the genesis of NASA's New Frontiers Program, which originally had a cost cap per mission of $700 million.
The New Horizons Pluto/Kuiper Belt Object flyby mission was already under development when NASA created the New Frontiers Program. NASA gave New Frontiers a highly visible first mission by adopting New Horizons into the program. Selection of the Pluto/KBO mission came to be regarded as the first New Frontiers proposal cycle, though it included no competition. NASA had taken a similar approach when it made Mars Pathfinder its first Discovery Program mission in 1992.
|Geologist Michael Duke in 2004. Image credit: NASA|
Duke argued that the SPA sample-return mission could collect ancient deep crust and mantle rocks without a costly rover. Clementine and Lunar Prospector had shown that at least half of the surface material in the central part of SPA was native to the basin, so stood a good chance of having originated deep within the moon.
Furthermore, Apollo demonstrated that any lunar site is likely to yield a wide assortment of samples because the moon's low gravity and surface vacuum enable asteroid impacts to widely scatter rock fragments. The Apollo 11 mission to Mare Tranquillitatis, for example, found and returned to Earth rocks blasted from the moon's light-hued Highlands. Duke proposed that the SPA sample-return lander sift about 100 kilograms of lunar dirt to gather a one-kilogram sample consisting of thousands of small rock fragments. These would have many origins, but a large percentage would be likely to have originated in the moon's deep crust and mantle.
A $700-million budget would also enable a relay satellite "more competent" than its bare-bones Discovery predecessor. It might be placed in a halo orbit around the Earth-moon L2 point, 64,500 kilometers behind the moon as viewed from Earth. From that position, the satellite would permit continuous radio contact between Earth and the landers. A satellite in lunar orbit could remain in line-of-sight contact with both the landers and Earth for only brief periods.
NASA had argued that a single day on the moon provided too little time to modify the SPA Discovery mission if it suffered difficulties. The SPA New Frontiers mission would, therefore, remain on the moon for longer. Duke noted, however, that stay-time would probably be limited to the length of the lunar daylight period (14 Earth days) because designing the twin landers to withstand the frigid lunar night would boost their cost.
Juno's selection did not end proposals for SPA Basin sample-return, though it did mark the end of Duke's involvement. In the third New Frontiers proposal cycle, which began in 2009, a Jet Propulsion Laboratory/Lockheed Martin/Washington University in St. Louis team proposed a SPA Basin mission called MoonRise. In 2011, the SPA sample-return mission was again selected as a New Frontiers finalist, but it lost out in the final selection to the OSIRIS-Rex asteroid sample-return mission.
"Sample Return from the Lunar South Pole-Aitken Basin," M. Duke, Advances in Space Research, Volume 31, Number 11, June 2003, pp. 2347-2352
"NASA Selects Two 'New Frontiers' Mission Concepts for Further Study," D. Savage, NASA Press Release 04-222, NASA Headquarters, 16 July 2004
NASA Facts: MoonRise - A Sample-Return Mission From the Moon's South Pole-Aitken Basin, NASA Facts, JPL 400-1408, June 2010
"MoonRise: Sample Return from the South Pole-Aitken Basin," L. Akalai, B. Jolliff, and D. Papanastassiou; presentation to the International Planetary Probe Workshop, Barcelona, Spain, 17 June 2010
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