The Value of the Moon

by Paul D. Spudis

  Before Clementine, good topographic maps existed only for the near-equatorial areas under ground tracks of the orbital Apollo spacecraft. From Clementine’s laser ranging data, we obtained our first global topographic map of the Moon. It revealed the vast extent and superb preservation state of the SPA basin and confirmed many large-scale features, mapped or inferred, from only a few clues provided by isolated landforms. Correlated with gravity information derived from radio tracking, we produced a map of crustal thickness, thereby showing that the lunar crust thins out under the floors of the largest impact basins.

  As a result of this mapping, scientists could place the results of studies of the Apollo samples into a regional, and ultimately, a global context. Clementine collected special data products, including broadband thermal, high resolution and star tracker images for a variety of special studies. In 1996, after our paper was published in Science, a press conference was held at the Pentagon to announce the results of the bistatic experiment: the discovery of ice at the south pole of the Moon. In addition to discovering new knowledge of lunar processes and history, this mission led a strong wave of renewed interest in the processes and history of the Moon, an interest that spurred a commitment to return there with both machines and people. By peeking into the Moon’s dark polar areas, we now stood on the edge of a revolution in lunar science.

  This renewed interest in the Moon led to the selection of Lunar Prospector (LP) as the first of NASA’s new, low-cost Discovery series of planetary probes. This mission found enhanced concentrations of hydrogen at both poles, again suggesting that water ice was probably present there. Buttressed by this new information, the Moon once again became an attractive destination for robotic and human missions. With direct evidence for significant amounts of hydrogen (regardless of form) on the surface, there now was a known resource that would support long-term human presence. Lunar Prospector’s hydrogen discovery was complemented by the identification in Clementine images of several areas near the pole that remain sunlit for substantial fractions of the year—not quite the “peaks of eternal light” anticipated by the astronomers Beer and Mädler in 1837, but something very close to it.30 The availability of material and energy resources, the two most pressing necessities for permanent human presence on the Moon, was confirmed in one pass. These two missions certified the possibility of using lunar resources to provision ourselves in space, thus permanently establishing the Moon as an enabling asset for continued human spaceflight. A remaining task was to verify and extend the radar results from Clementine and to map the ice deposits of the poles.

  Missions flown over the last twenty years show how significantly Clementine’s programmatic template has influenced spaceflight. The Europeans flew the SMART-1 spacecraft to the Moon in 2002, largely as a technology demonstration mission with goals very similar to those of Clementine. NASA directed the Applied Physics Laboratory (APL) to fly the Near-Earth Asteroid Rendezvous (NEAR) spacecraft to the asteroid Eros in 1995 as a Discovery mission, to attain the asteroid exploration opportunity missed when control of the Clementine spacecraft was lost after leaving the Moon; the mission was renamed NEAR-Shoemaker after Gene’s tragic death in an automobile accident in Australia in 1997. India’s Chandrayaan-1 had a size and payload scope similar to Clementine. The selection of the LCROSS impactor as a low-cost, fast-tracked, limited objectives mission further extended use of the Clementine paradigm.

  The Faster-Better-Cheaper mission model, once panned by some in the spaceflight community, is now recognized as a valid mode of operations, absent the emotional baggage of that name.31 A limited-objectives mission that flies is more desirable than a gold-plated one that sits forever on the drawing board. While some missions do require significant levels of fiscal and technical resources to attain their objectives, an important lesson of Clementine is that for most scientific and exploration goals, “better” is the enemy of “good enough.” Space missions require smart, lean management; they should not be charge codes for feeding the beast of organizational overhead. Clementine was lean and fast; perhaps we would have made fewer mistakes had the pace been a bit slower, but despite its shortcomings, the mission gave us a large, high-quality dataset, one still used extensively to this day. In recognition of its substantial accomplishments, the Naval Research Laboratory transferred the Clementine engineering model to the Smithsonian in 2002, where it was put on display in the National Air and Space Museum, suspended above the display of the Apollo Lunar Module.

  It is probably not too much of an exaggeration to say that Clementine changed the direction of the American space program. After the failure of SEI in 1990–92, NASA was left with no long-term strategic direction. For the first time in its history (but alas, not the last), the agency had no follow-on program to the shuttle/station, despite attempts by Dan Goldin and others to secure approval for a human mission to Mars, an insurmountable challenge both technically and financially. This programmatic stasis continued until 2003, when the tragic loss of the space shuttle Columbia led to a top-down review of US space goals. Because Clementine had documented the strategic value of the Moon, the lunar surface once again became an attractive destination for future robotic and human missions. The resulting Vision for Space Exploration (VSE) in 2004 made the Moon the centerpiece of a new American effort beyond low Earth orbit. Though Mars was declared as an eventual (not ultimate) space objective, specific activities to be done on the Moon were detailed in the VSE, particularly with regard to the use of its material and energy resources to build a sustainable program. Regrettably, as I will detail, various factors combined to subvert the Vision, thereby ending any strategic direction for America’s civil space program.

  Clementine was a watershed, a hinge point that forever changed the nature of space policy debates. We now recognize a fundamentally different way forward in space—one of extensibility, sustainability, and permanence. Once an outlandish idea found in science fiction, we now know that lunar resources can be used to create new capabilities in space, a welcome genie that cannot be put back in the bottle. Americans need to ask why their national space program was diverted from such a sustainable path. We cannot afford to remain behind while others plan and fly missions to understand and exploit the Moon’s resources. Our path forward into the universe is clear. In order to remain a world leader in space, and a participant in and beneficiary of a new cislunar economy, the United States must again direct its sights and energies toward the Moon.

  4

  Another Run at the Moon

  With the completion of the successful Department of Defense Clementine mission, the Moon was again viewed as a destination of value. Both Lawrence Livermore and the Naval Research Laboratory prepared for a Clementine II asteroid flyby mission, the unmet objective of the first mission due to a technical failure with Clementine’s thrusters after it departed the Moon. Some on the study team proposed a mission profile that would mirror the plan of the first Clementine, an asteroid flyby followed by insertion into lunar orbit. The objective would be to map the Moon at greater resolution with additional instruments and follow up on the discoveries made by the first Clementine. This renewed lunar attention was not without detractors, who questioned what Clementine had found. Congress appropriated funds for the mission in 1997, but plans to fly it were scuttled when President Bill Clinton used his newly acquired line-item veto to zero out funds for Clementine II. The Supreme Court subsequently declared the line-item veto unconstitutional, but too late to save the Clementine II mission.

  Around this time, NASA called for proposals to the Discovery program, a new series of small planetary missions, cost-capped at $150 million.1 This mission series was NASA’s attempt to emulate the Faster–Cheaper–Better paradigm that Clementine encapsulated. The new NASA administrator Dan Goldin was renowned for his advocacy of the FBC mode of business. The Discovery program received dozens of mission proposals. A single planetary scientist, called the principal investigator (PI), led each proposal. In 1995, NASA picked Lunar Pros
pector (LP), led by Alan Binder, as the first Discovery mission, deeming it the least expensive, least risky mission proposal it had received, requiring only a small operations team. Its selection also avoided having the Clementine team again set foot on what NASA thought to be its turf, since a second NRL Clementine multiple asteroid flyby of comparable cost was also proposed.

  The particle and geochemical sensors of LP perfectly complemented the multispectral images and laser altimetry data obtained by Clementine. Combined, these two missions gave us our first global look at lunar mineral and chemical compositions, surface topography and gravity, and regional geology and produced the data sets of the never-flown Lunar Polar Orbiter, the mission scientists had long desired. For example, we found that high concentrations of radioactive elements in the lunar crust are localized in the Procellarum topographic depression of the western near side, an unusual global asymmetry that is still unexplained. More importantly, LP’s neutron spectrometer found high concentrations of hydrogen at both poles in roughly equal quantities. The neutron experiment only measures the concentration of elemental hydrogen, not its physical state—that is, whether it is present in the form of water ice or excess solar wind gas in the cool polar regolith. From this information, as well the results on lunar polar lighting and the bistatic radar from Clementine, the evidence continued to mount that something very interesting was present at both poles of the Moon.2

  The Moon’s spin axis is inclined 1.5° from the normal to the ecliptic plane, a nearly perpendicular orientation; this means that the Sun always hovers near the horizon at the poles of the Moon. The apparent angular width of the Sun at the Earth-Moon distance is about 0.5° of arc, so sometimes the Sun could be above the horizon and at other times, below it. But because the Moon’s surface is rough and irregular, with large craters and basins, there are areas near the poles that in theory could see either permanent sunlight or permanent darkness. Clementine spent only seventy-one days in lunar orbit during the southern winter solstice, so it provided illumination data for only part of the lunar year. Nonetheless, analysis showed that small areas near the crater Shackleton, located near the south pole of the Moon, were sunlit more than 70 percent of the southern winter day. Three areas near the north pole were illuminated 100 percent of the day (northern summer). Data for the opposite seasons were not obtained.

  An intense scientific debate over the existence of ice at the lunar poles spanned most of the decade around the turn of the millennium. The lunar ice controversy stemmed from the ambiguity of radar CPR as an indicator of both surface physical properties and composition. Because the data were nondeterminative, they proved fertile ground for intense argument. A paper presented at the 1995 Lunar Science Conference in Houston described the results of high resolution imaging of the lunar south pole by the large dish antenna at Arecibo, Puerto Rico. These images were able to peek into the totally sunless areas near the pole. Interestingly, small regions of high diffuse backscatter were seen.3 This high diffuse backscatter, called circular polarization ratio, or CPR, is consistent with a surface composed of water ice, a high concentration of angular blocks, or both. Based on our result from the bistatic experiment, the Clementine team preferred the water ice interpretation, while others in the radar planetary science community argued for an origin from surface roughness.

  The new LP neutron data clearly showed an excess of hydrogen at the poles, but the surface resolution of its hydrogen concentration maps was very low and we could not be certain whether the signal was caused by a large area of relatively low concentration—that is, solar wind gases implanted in the regolith—or by small, isolated zones of very high concentration such as ice in permanently dark areas. This controversy raged on as we tried to design, build, and fly a small imaging radar to the Moon to follow up on the Clementine and LP discoveries. Despite proposals for small NASA robotic missions, European Space Agency interest, and even some proposed commercial missions, no flight opportunities were to arise until late in 2003.

  The greatest remaining unknowns were about the poles, those areas where we had found permanent darkness, possible permanent sunlight, and enhancement of hydrogen concentration, possibly indicating the presence of water ice in the dark regions. All of these new insights showed that the Moon was more complex and interesting than we had thought. A key discovery was the zones of extended sunlight. Finding areas on the surface that receive illumination for almost all of the lunar day removed one of the biggest hurdles to human habitation of the Moon: the need to provide a power source for electricity and heat during the fourteen-day nighttime. Nuclear power is best suited to the task, but the high costs of such power, both technical and societal, made lunar return unaffordable.

  In contrast, the discovery of areas where power could be generated constantly by solar arrays now made extended stays on the Moon by people feasible. In addition, illuminated terrain—even by sunlight at grazing incidence—makes the extremely cold lunar night tolerable. Areas of constant sunlight near deposits of water ice create “oases” near the poles of the Moon where human habitation is possible and perhaps even profitable.

  There was an interesting coda to the Lunar Prospector mission. After lowering its orbit to about 20 kilometers, as close as it is possible to orbit the Moon without running into some of its higher mountaintops, and collecting some high-resolution data from this close orbit, the spacecraft was deliberately crashed into a crater near the south pole on July 31, 1999. The objective of this effort was to kick up material from the impact to try to detect the polar water in the ejecta cloud with telescopes on Earth. Unfortunately, no ejecta were observed, so the debate over lunar polar water continued. (The same experiment was repeated ten years later during the LCROSS mission, with more productive results.) The LP spacecraft did carry an unusual cargo, however: some of Gene Shoemaker’s ashes.4 The urn and the LP spacecraft now rest in the floor of a crater near the south pole, subsequently given the name Shoemaker, a fitting tribute to Gene and his contributions to the study and exploration of the Moon.

  We believed that we had found water at the poles of the Moon. Now we needed a commitment to go back to verify the new findings.

  Mars Mania

  Robotic missions to Mars have dominated the last twenty years of planetary exploration, an emphasis stemming in part from the planetary science community’s efforts to fly a series of robotic missions that will eventually lead to the return of samples from Mars to Earth, an ambitious and very expensive proposition. With NASA’s robotic spaceflight program still under intense scrutiny after the failures of the initial Hubble Space Telescope mission and JPL’s 1993 Mars Observer spacecraft, Administrator Dan Goldin, a booster of both human Mars missions and the search for life, was unable to convince the Clinton administration or Congress to pony up enough money to fund an ambitious Mars sample return effort.5 Undeterred, Goldin applied the FBC paradigm to Mars missions and moved forward with the Mars Pathfinder mission, a small rover called Sojourner that landed on Mars using a parachute and airbags deployed for final impact. Sojourner took some images and made a rudimentary chemical analysis of the soil. Pathfinder, although technically successful, did not fundamentally advance our knowledge and understanding of Mars and its surface processes and history, thus missing out on the “better”—or even the “good enough”—part of FBC.

  An event that would concentrate everyone’s attention on Mars during the 1990s turned out not to be from any space mission but from a laboratory right here on Earth. We had known for some time that a rare group of meteorites, the Shergottite-Nahklaite-Chassignite group (SNC), had unusual chemical properties and relatively young ages of crystallization. Analysis of these rocks found trapped argon within them identical in composition to analysis of the martian atmosphere made in 1976 by the Viking spacecraft. Scientists concluded that these meteorites are pieces of the martian crust, thrown into space by an impact on Mars that eventually made their way to Earth. The composition of these meteorites seemed to fit what we had inferred fr
om Viking and remote sensing about the composition of Mars. These meteorites had ages much younger than most meteorites, nearly all of which formed 4.6 billion years ago, and lunar samples three to four billion years old, again congruent with our understanding of the extended geological evolution of the martian surface; crater density data suggested that ages of geological units on Mars spanned an estimated range from four billion to less than one billion years old. One rather unusual martian meteorite, ALH 84001, was found in Antarctica and determined to be relatively old: 4.5 billion years. Using a scanning electron microscope, tiny lifelike shapes were found inside the martian meteorite, forms resembling certain types of terrestrial bacteria, though much smaller and unique in detail. The authors of the ALH 84001 study suggested that these objects might be fossils of bacteria from an early epoch of Mars history. In other words, they asserted that traces of former extraterrestrial life had been discovered, a sensational claim that grabbed and held headlines.6

  A flood of media coverage followed, eventually leading to press conferences at NASA Headquarters and finally, a Presidential Rose Garden statement. With that explosion of publicity, Dan Goldin moved to leverage some high-level political backing for a permanent, sustained Mars exploration program.7 Although human missions to Mars remained beyond the reach of technology, a series of robotic probes leading up to that elusive sample return mission would keep the Mars scientists and NASA’s Jet Propulsion Laboratory busy. “The Quest for Life” scientific gravy train was born.

  By Saganizing the nation’s civil space program—that is, by enshrining the Quest for Life as NASA’s principal rationale for space exploration—Dan Goldin took the martian aspect of this new rationale and encapsulated it into the slogan, “Follow the Water.” The idea behind this messaging was that life as we know it requires the presence of liquid water.8 This dictum was followed by conducting a series of missions to areas on Mars where it was suspected that flowing water had occurred in the past. A cynical observer might notice that aside from the potential finding of extant or fossil martian life, no criteria for a programmatic exit from this exploratory path were defined. In essence, the new series of Mars missions took on a life of its own, becoming a permanent scientific and engineering entitlement program—a constant, uninterruptible cadence, elevated beyond the realm of peer-review selection pressure or second thoughts from planetary scientists, a situation viewed with dismay by the lunar community.