by Jeff Gillis
Here in the suburbs of Alexandria, Virginia, I have been fortunate to experience the meaning behind "faster, cheaper, better" as applied to planetary exploration. My expectations of Clementine were shaped largely by two things. My first impression was formed by word of mouth. I had been told by some that "faster, cheaper, better" gives you science that is boring, second rate, and not any better than we already have. My second impression was formed when I saw the outside of the mission control center for the first time. It is an old brick warehouse that sits across the street from an undistinguished row housing complex. In the movie Batman, the corrupt darkness of Gotham City contrasts with the high-tech power inside the Batcave; hence the nickname "Batcave" for Clementine's mission control. I am more than happy to report that as dissimilar as the earthy surroundings outside are to the extraterrestrial pictures on display screens inside, so is truth separated from the concept that "faster, cheaper, better" is not good enough.
Mission control here in Alexandria looks similar to mission control at the NASA Johnson Space Center, except that it's much smaller and has a staff of only 55 people. It has the same quality of high- tech equipment and bright engineers, just a smaller quantity. The people who put Clementine together for the two years prior to launch are now flying her. They definitely have pride and excitement in what they are doing, because I don't know what else would carry them through their 12-hour-plus workdays, and they never tire in their efforts to better the spacecraft's performance. Mission control is always compliant to special requests by the science team to perform unscheduled maneuvers. In fact, they often query the scientists for any input on how a particular maneuver can be optimized to maximize scientific return.
Many members of the science team have worked on mission studies in the past. Clementine, by contrast, has provided them an opportunity to work on a flying spacecraft. Even though they are "fancy Ph.D. scientists," as Lt. Col. Pedro Rustan, the Ballistic Missile Defense Organization program manager, has called them, it is remarkable to see the degree of enthusiasm that lights up these scientists when they are looking at Clementine data and contemplating information they've never had before. The richness of the data can be understood when you consider that the team has examined only 1% of the total dataset and have seen many new things emerge from this drop in the bucket.
It is truly a labor of love, this sifting through the data to figure out what it is telling us about Clementine and her lunar mapping mission. I have seen Dr. Paul Spudis, with childlike glee, sit and paste strips of images onto a globe. With each step toward completion he grows more excited as he can read what the information is telling him. The bug has even bitten Dr. Alfred McEwen, who is reported to be happy only 10 seconds a day. He is delighted to show his work on Aristarchus Plateau and give an in-depth explanation of the information revealed in the color mosaic.
A big thrill for me, that rivals the fact that this is the first time in my life I can experience the United States at the Moon, is the chance to work with the father of planetary geology, Dr. Eugene Shoemaker, and some of the top planetary scientists in the world. Many of us only get to read about the great people who make major advances in their fields, but I have been fortunate enough to work with them, learn from them, and get to know them personally. For instance, I have seen Dr. Shoemaker sit on the floor staring at a 7 x 7-foot south polar mosaic, seemingly entranced by it. I can see him figuring, calculating, and resolving problems that my untrained eyes have not even picked up.
As the scientists and mission control staff prepare for Clementine to leave lunar orbit on May 3, it is evident that her mission at the Moon has been (nothing but) a complete success. Although Clementine's scientific return is a by-product of her military mission, it has given scientists a rich harvest of more than two million images collected over two months of systematic global mapping that will greatly increase our knowledge of the Moon. Clementine has digitally imaged 100% of the lunar surface under constant geometry and lighting conditions and in 11 different wavelengths. This has never been done before, for the Moon or for any other planet.
After a mapping orbit is completed, from 90 degrees south to 90 degrees north on the periselene side of the Moon, the spacecraft rotates using inertial forces generated by spinning wheels within the craft. This maneuver points the high-gain antenna toward Earth so that Clementine can download the data stored in its solid- state memory recorder. During the aposelene part of the orbit, the satellite downloads data gathered on that orbit to one of four groundstations that make up the Deep Space Tracking Network back on Earth. Each location, Madrid, Spain, Canberra, Australia, Goldstone, California, and Pomonkey, Maryland, has a 26-34-meter dish that receives the spacecraft's signal. The receiving groundstation sends the data to the Jet Propulsion Lab in California via underground lines. JPL in turn sends it to Goddard Space Flight Center in Maryland, which relays it to the Batcave in Arlington.
In order for the images from Clementine to be useful to the scientists, they have to be mosaicked together. A computer program called ISIS is used to calculate how each frame fits with its neighbors, using information about spacecraft altitude, camera angle, and position of the Moon at the time the image was taken, and to place the image into a specified cartographic projection. The south pole mosaic put together by Eric Eliason of the U.S. Geological Survey in Flagstaff, for instance, covers 90 degrees to 70 degrees south latitude and contains 1500 UV/VIS images from one color filter. This mosaic gives us our first look at features of the south pole and has revealed what appears to be a major depression near the pole, evident from extensive shadows around the pole. This depression is probably an ancient basin formed by impact of an asteroid or comet. A significant fraction of the dark area near the pole may be in permanent shadow and would be sufficiently cold to preserve water of cometary origin as ice.
Alfred McEwen, also from U.S.G.S. Flagstaff, has processed a mosaic of 500 images acquired through three spectral filters (415, 750, and 100 nm) and has combined them into a multispectral mosaic of the Aristarchus Plateau region. Color ratios serve to cancel out the dominant brightness variations in the scene, which are caused by albedo variations and topographic shading, thus isolating the color differences related to composition or mineralogy. The Aristarchus mosaic covers only 0.4% of the lunar surface and is only 0.1% of the entire Clementine image database. Such data will be invaluable for mapping the geology of the Moon and planning future exploration and utilization of lunar resources.
One of the fascinating products made with Clementine data so far is a preliminary map of global mineral distributions put together by Paul Lucey of the University of Hawaii and Eric Malaret of ACT Corporation. They have extracted the average brightness from each image and plotted it as an individual pixel on a cylindrical map projection, enabling the reconstruction of low-resolution global color ratio images. Using the same color principles to determine mineralogy as in the Aristarchus mosaic, we have found new heterogeneities in the composition of the lunar crust. The more we can find out about compositional variations, the better we can piece together the genesis and evolution of the early lunar crust.
Another important return is from the laser altimeter (LIDAR). From this we will be able to constrain the global shape of the Moon. Three-dimensional plots of the altimetry data have been used by Paul Spudis to identify old impact basins whose existence was previously uncertain and, in one instance, previously unknown. The altimetry data has allowed us to measure the depth of many old basins. The amount of topographic relief is surprising: the South Pole-Aitken Basin is up to 12 km deep. Altimetry data combined with gravity data should shed light on crustal geology and isostasy.
Clementine UV/VIS camera image of the lunar crater Plato.
Clementine has also performed a bistatic radar test in which the spacecraft sends a radar signal from its transmitter to the Moon's pole and the reflection is received by Earth-based groundstations. Scientists are looking for a high backscattering of the reflected signal that is characteristic of an icy surface. Ice would be a major discovery and could provide water for lunar bases.
Working on the Clementine mission is a dream come true for me. Where else can you work and every four out of five hours see images live from the Moon? As I watch a new orbit of images being dumped from Clementine, I realize that buried somewhere deep in this enormous, scientifically rich dataset exists my Ph.D. thesis. With the Clementine dataset, we are redefining scientific frontiers that were thought to be settled. I hope the lesson that space exploration can be affordable and provide first-rate science will be learned from Clementine before the lesson is lost and gone forever.
(Jeff Gillis worked on structural and stratigraphic mapping of the Moon's nearside under the direction of Dr. Paul Spudis in the LPI Summer Intern Program in 1992. He is currently a Visiting Graduate Fellow at LPI, where he has worked with Dr. Spudis on selecting geologically interesting features for Clementine's HIRES camera to target. He will pursue graduate study at Rice University. For the last two months, Jeff has been a resident of the Batcave, processing Clementine images and data.)