Lessons Learned From the Clementine Mission
Clementine and Lunar Science
Clementine should be judged by the science community in the context of the rules under which it was planned and conducted. First, it should be recognized that lunar science was a subordinate objective for Clementine, though ultimately an important one. Second, the mission's funding was tightly constrained: not only was the planned cost low, but the reserves were also tightly held. Third, the short schedule (less than 2 years) before launch limited the software development and testing, as well as reduced the prelaunch calibrations of instruments. Fourth, the mission was redirected approximately 12 months before launch from a test of technology, with some scientific participation in the form of an ad hoc science advisory committee, to one that included scientific goals and was open to the full participation of the planetary science community. Taken together, these factors meant that neither the ad hoc science advisory committee nor its successor, NASA's peer-selected science team, could affect the basic nature of the mission or influence the design and selection of Clementine's instruments in any fundamental way. Nevertheless, the mission scientists and BMDO's operational team collaborated very effectively; this cooperation allowed the scientists to make some modifications to the mission design (e.g., rotating the major axis of Clementine's orbit at the midpoint of the mapping mission and using a mix of nadir pointing and oblique coverage at high latitudes) and to participate extensively in prioritizing and planning the data acquisition sequences of the Moon.
The scientific payload on Clementine (see Table 2.1) consisted of four instruments (an ultraviolet-visible camera, a long-wave infrared camera, a combination laser-ranger/high-resolution camera, and a near-infrared camera); in addition, the spacecraft carried two star-trackers, which occasionally were used as wide-field cameras, and an S-band transponder, which provided information on the orbit and thereby allowed the Moon's gravitational field to be inferred. A summary of the instrument payload may be found elsewhere.1
TABLE 2.1 Characteristics of Clementine's Instruments
Before describing the achievements of Clementine, it is valuable to recall COMPLEX's primary objectives for lunar science achievable from orbit:2
1. Global composition and mineralogy;
2. Global topographic and gravitational field mapping;
3. Improved determination of the magnetic field; and
4. High-resolution imaging of selected areas.
Below COMPLEX assesses the extent to which the early analyses of Clementine data3-7 suggest that these objectives will ultimately be met by further research. In doing so, COMPLEX is cognizant of the fact that Clementine was not designed primarily to address any of the above goals. Thus the fact that it might not have been responsive to some or all of these priorities cannot be interpreted in any way as a failure on its part. COMPLEX notes that Clementine's achievements have been exaggerated in some quarters and denigrated in others. Thus, the spirit of this section is to place Clementine's preliminary scientific results in context and to outline areas where its achievements have motivated additional studies. COMPLEX's assessment covers three areas: geophysics and geodesy, geology and surface physics, and mineralogy.
GEOPHYSICS AND GEODESY
To date, the most thoroughly studied measurements from Clementine are those of the laser-ranging (Lidar) and gravitational-field experiments. Although the Lidar instrument was not designed for scientific studies of planetary topography, it provided a near-global topographic data set that is an important advance over our previous knowledge of lunar shape. The single-shot ranging precision of the Lidar was about 40 m and is comparable to the stated accuracy (100 m) of the spacecraft orbit with respect to the lunar center of mass. The high relief observed is unexpected and interesting. The verification of (previously proposed) degraded, ancient impact basins and the uniquely deep floors of Maria Crisium and Humboltium are other examples of the findings. The fact that the 2° by 2° (60 km by 60 km) grid is complete except at latitudes above 75° makes this set particularly valuable for geophysical studies across regional to global scales. However, because of the data's resolution and the instrument's inability to track over rough terrains (80% of all valid returns were obtained over maria, even though maria cover just 18% of the Moon's surface), the data set is generally not well suited for detailed regional modeling or short-to-medium-scale geophysical characterization of the lunar topography. Improvement in both the horizontal and vertical geodetic control of the Moon is, however, significant. Horizontal control, that is, the accuracy with which we now know the latitude and longitude of features on the Moon, particularly those
on the far side, has been improved by about an order of magnitude.8 Similarly, improvement in the geodetic control to about 100 to 200 m vertically from the Clementine observations will prove valuable for registration of various data sets. This knowledge will also be useful for targeting future robotic and human exploration missions.
The gravity results also improve our understanding of the Moon, though to a lesser degree. The lunar highlands are found to be nearly isostatically compensated, whereas impact basins display a wide range of compensation states that do not correlate simply with basin size or age. The lunar crust is apparently thinned under all resolvable basins. Thus the Moon's structure and thermal history are more complicated than was previously believed.
The data on the far-side gravity field contain useful new findings but are poorly constrained with respect to the precise magnitudes of the anomalies owing to the usual problems associated with the tracking of an intermittently obscured spacecraft. Accordingly, significant uncertainty remains about the lunar gravity field, especially the far-side values and high-spatial resolution data; a future mission (including a subsatellite to allow differential tracking) will be required to complete the global gravitational survey of the Moon. In addition, gravitational observations over an approximately 1-year period will be necessary to unambiguously resolve possible tidal signatures that could indicate the presence (or absence) of a molten deep interior.
GEOLOGY AND SURFACE PHYSICS
Clementine yielded global coverage of the surface with digital images of ~100- to 400-m resolution at all latitudes. When fully calibrated and mosaicked, these will form the first global digital image database for the Moon. Such data are appropriate for comparative geological studies of surface morphology, stratigraphy, and structure, although the high Sun-angles of some images severely limit their utility in discriminating the details necessary to allow such studies. This limitation was not due to inadequacies in Clementine's cameras or failures in mission planning but, rather, a consequence of the advisory committee's decision to concentrate on high-Sun-angle observations necessary for spectrophotometric mapping of the Moon's lithological units rather than low-Sun-angle observations necessary for morphological studies.
An exception to this is the data obtained by Clementine's high-resolution camera. It acquired one-color (750-nm) images in both hemispheres poleward of 500 in the form of narrow strips that overlap at latitudes greater than some 820 to provide continuous coverage of both lunar poles. These images, with naturally low Sun angles and a spatial resolution of ~20 m, will be valuable for geological analyses of these previously poorly imaged regions. The identification of perennially shadowed areas is a significant finding because these areas are potential depositories of lunar ice.
Processing of the high-resolution data is currently under way, and release of calibrated, monochromatic mosaics of both polar regions is scheduled for late 1997. Calibrated, monochromatic strip mosaics of the data taken between 500 and 820 north and south, four-color strip mosaics of selected areas, and a limited number of four-color maps constructed from pointed observations of areas of special interest are also scheduled for release in late 1997.
Overall, the restricted latitudinal range of the high-resolution, low-Sun-angle imagery and the complete lack of selective very-high-resolution (~1-m) imagery, cited in COMPLEX's previous recommendations,9 constrain the usefulness of Clementine's imaging to provide additional information about lunar geological processes and to provide context for interpreting other kinds of data.
The long-wave infrared camera obtained images of ~20% of the Moon at ~100-m resolution. Work on reducing these images is in progress. Algorithms to remove the large number of instrumental artifacts in these images have been developed and calibration of the images has begun. The complete data set should be available in late 1997. These data may ultimately be valuable for the determination of particle sizes and the evaluation of surface porosity.
Analyses of Clementine star-tracker images of the lunar limb are in a preliminary stage. Nonetheless they have the potential to provide additional constraints on the distribution of any electrostatically levitated lunar dust.
The reduction and calibration of the great bulk of the mission data, a vast archive of images in five visible and six near-infrared spectral channels, are currently in progress. Release of a global, one-color (750-nm) mosaic with a resolution of ~150 m is scheduled for release on compact disks in late 1997. The current rate of calibration and processing efforts suggests that a global data set, in five visible channels and with a resolution of some 150 to 250 m, will be available in early 1998.
The reduction and calibration of the images from Clementine's near-infrared camera have been hindered by instrumental effects such as spatial variations in the detector's dark current. These difficulties now seem to have been constrained, and first-order calibrated should be completed by late 1997. Calibration of the entire near-infrared data set should be completed in early 1998. The final product will be a six-channel global mosaic image of the Moon at a resolution of some 200 to 250 m.
Preliminary investigations with a small fraction of the available data indicate that it will be possible to map limited mineralogical information (i.e., major components) for at least near-side areas that can be calibrated through comparison with Apollo and Earth-based data sets.10 Initial results for the Aristarchus, Copernicus, Tycho, and Giordano Bruno areas show previously unresolved lithologic diversity and indicate that useful comparative results can be obtained, and possibly more. A global map of lunar iron concentration derived after much postprocessing from the multispectral images has yielded useful results supporting evidence from studies of lunar samples for a more mafic lower crust as excavated by impact basins.
The success of ongoing efforts to accurately calibrate Clementine's images will ultimately determine whether or not worthwhile additional mineralogical information can be obtained for regions on the Moon's far side. If current efforts are successful, the analysis of these various data would allow an important first step toward mapping the lithologic diversity of the lunar surface, one of the major objectives of lunar orbital science.
Clementine carried out a simple version of a bistatic-radar experiment in which the spacecraft's radio signals were scattered off the near-polar surface and then these reflected signals were ultimately received by NASA's Deep Space Network. While the observations are consistent with the presence of water ice in the permanently shadowed region of the Moon's southern polar region,11 such an interpretation requires confirmation with independent data obtained using a less ambiguous approach.
Even if one is optimistic about how much information on lunar composition might be gleaned from Clementine, a systematic global mineralogical and geochemical mapping of the lunar surface will still be necessary. Geochemical mapping is needed to identify concentrations of major elements to the degree that would indicate Mg/Fe ratios and abundances of radioactive elements (U, Th, and K). Another high priority is confirmation of the existence of possible polar ice deposits. Unambiguous mineral identification will require an optimized high-spectral-resolution spectrometer.
CLEMENTINE'S CONTRIBUTION TO LUNAR SCIENCE OBJECTIVES
Clementine measurements made significant contributions to recognized lunar science goals in geophysics and geodesy, demonstrated the potential for limited lithological mapping, and provided a valuable supplement to earlier imaging of the lunar surface. The spacecraft did not furnish information on surface geochemistry, the magnetic field, global heat flow, or deep internal structure.
In summary, most of the work in data reduction and analysis for Clementine is still in progress, but there is reason to hope for a yield of
important new understanding of the Moon. If this proves true, this constrained mission will have accomplished significant lunar science. The
fact that the majority of the fundamental scientific questions posed by COMPLEX and other groups such as NASA's Lunar Exploration Science Working Group12 may not be answered with data provided by the Clementine instrument complement is not relevant to an overall assessment of the Clementine mission. It was not designed to achieve these or any other scientific objectives, and so cannot be judged by these standards. To critically assess questions that deal with the origin and evolution of the Moon, further orbiters and landers seem necessary. COMPLEX notes, but does not discuss within the context of this report on Clementine, that NASA has selected Lunar Prospector as the third (but first competitively chosen) Discovery mission.
1. S. Nozette et al. "The Clementine Mission to the Moon: Scientific Overview," Science 266:1835-1839, 1994.
2. Space Studies Board, National Research Council, 1990 Update to Strategy for Exploration of the Inner Planets, National Academy Press, Washington, D.C., 1990, page 18.
3. S. Nozette et al., "The Clementine Mission to the Moon: Scientific Overview," Science 266:1835-1839, 1994.
4. P.G. Lucey, G.J. Taylor, and E. Malaret, "Abundance and Distribution of Iron on the Moon," Science 268:1150, 1995.
5. B.J. Buratti, J.K. Hillier, and M. Wang, "The Lunar Opposition Surge: Observations by Clementine," Icarus 124:490-499, 1996.
6. A.S. McEwen, and M.S. Robinson, "Mapping of the Moon by Clementine," Advances in Space Research, 1997, in press.
7. A.C. Cook et al. "Clementine Imagery: Selenographic Coverage for Cartographic and Scientific Use," Planetary and Space Science 44:1135-1148, 1996.
8. T.L. Becker et al., Bulletin of the American Astronomical Society 28:17.01, 1996.
9. Space Studies Board, National Research Council, 1990 Update to Strategy for Exploration of the Inner Planets, National Academy Press, Washington, D.C., 1990, page 18.
10. P.G. Lucey, G.J. Taylor, and E. Malaret, "Abundance and Distribution of Iron on the Moon," Science 268:1150, 1995.
11. S. Nozette et al., "The Clementine Bistatic Radar Experiment," Science 274:1495, 1996.
12. Lunar Exploration Science Working Group, A Planetary Science Strategy for the Moon, JSC-25920, NASA Solar System Exploration Division, Houston, Texas, 1992.
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