Some scientists believe that the time has come for a robotic sample-return mission to the moon's largest impact structure, the South Pole-Aitken Basin. Such a mission has twice been a finalist in the competitive selection process for NASA's New Frontiers-class exploration missions. Beyond Apollo blogger David S. F. Portree looks at Moonrise, for the past decade the leading candidate for a South Pole-Aitken sample-return mission.
A CASUAL GLANCE at the full moon reveals signs of ancient violence. Nearside, the lunar hemisphere we can see from Earth, is marked by gray areas set against white. Some are noticeably circular. The Apollo expeditions revealed that these relatively smooth basalt plains are scars left by large asteroids that pummeled the moon between 3.85 and 3.95 billion years ago.
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 moon's entire surface. 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.
Michael Duke, a retired NASA geologist with the Colorado School of Mines, participated in both Apollo Era and 1990s lunar exploration. In 1999, Duke led a team that proposed a robotic SPA sample-return mission for NASA's low-cost Discovery Program. To fit under Discovery's $300-million-per-mission cost cap, Duke's team had proposed "the simplest-possible mission" - a single lander with no sample-collecting rover, a lunar-surface stay-time of just 24 hours, and a low-capability lunar-orbiting radio-relay satellite (needed because Farside is out of radio line-of-sight with Earth). Believing that these limitations added up to a high risk of mission failure, NASA rejected the 1999 proposal.
In 2002, however, the National Research Council's planetary science Decadal Survey declared SPA sample return to be a 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 had a cost cap per mission of $700 million. Duke's team immediately began to upgrade its SPA proposal for New Frontiers.
In October 2002, Duke described the new SPA mission design at the 53rd International Astronautical Federation Congress (the Second World Space Congress) in Houston, Texas. To avoid helping competing New Frontiers proposers, his paper provided only limited technical details.
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 from deep within the moon.
Furthermore, Apollo had 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 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.
NASA had rejected the SPA Discovery mission in part out of concern for lander safety. Duke noted that, with New Frontiers' $700-million cost cap, the SPA sample-return mission could include two landers. This would provide a backup in case one crashed. He pointed out, however, that automated Surveyor spacecraft of the 1960s had found the moon to be a relatively easy place on which to land even without the benefits of 21st-century hazard-avoidance technology. Two landers would also increase the already good chance that the mission could collect samples representative of the basin's earliest history.
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.
In February 2004, Duke's mission - catchily named Moonrise - became one of two SPA sample-return missions proposed for New Frontiers. In July 2004, NASA awarded Moonrise and a Jupiter polar orbiter called Juno $1.2 million each for additional study. In May 2005, the space agency selected Juno for full development.
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