The Galileo spacecraft, on its way to Jupiter, captured some new images of the Moon. These images were useful, both to provide more detail about parts of the Moon that had not been mapped before, but also to test the instruments aboard Galileo. In this sense, the Moon is a "reference planet'' - an accessible, basic example that can be observed from space and compared with what is already known from studying the lunar samples.
A lot is known about a few places on the Moon, such as the Apollo and Lunokhod landing sites. However, very little is known about the rest of the Moon - for example, the polar regions and the far side. Thus, it is likely that future missions will emphasize global, rather than local, studies, and for this reason, lunar orbiter missions will be important. Galileo is a good example of the usefulness of global imaging missions, as illustrated by Galileo's images of the South Pole-Aitken Basin on the far side. Galileo images suggest that the area has volcanic deposits that have been covered over by later ejecta deposits, creating "cryptomare". This is an important discovery, because it reveals that the Moon was subject to much more widespread volcanic activity than was originally thought. While Galileo's images of the Moon had 5.5 km resolution, future lunar orbiters will have much better resolution for global coverage, and that will allow even more discoveries to be made.
The Clementine mission: In 1994 the U.S. Department of Defense sponsored the Clementine mission to the Moon21 (Figure 1.14). This was a partnership with NASA in which both groups gained important information. The Department of Defense wanted to know how well their new space-based detection instruments could survive the space environment, particularly the harsh radiation found in the Van Allen belts and beyond. NASA was able to use those instruments to explore the Moon in much greater detail than had been done previously. The Clementine spacecraft was placed in a lunar polar orbit for two months. During that time, it obtained detailed images of almost the entire surface of the Moon, in eleven spectral bands.22 Scientists have been using these images to learn more about the chemical composition of the entire Moon, and to perform detailed studies of many interesting areas that could not be examined previously, because of a lack of information.
21 Visit http:jjwww.lpi.usra.edujpubjexpmoonjclementinejclementine.html for more images and further details on the Clementine mission.
22 A spectroscope acts like a prism, in the sense that it shows us the different components of the light that would otherwise reach our eyes as a single "color". When light passes through a prism, there are at least seven colors that the human eye can clearly pick out. A spectroscope, however, can see in much more detail, and can pick out millions of tiny variations in color. When rocks and minerals are studied in the laboratory, the spectroscope provides a pattern of intensities of the various "colors" that are unique to each mineral that we observe. The known spectrum of the mineral olivine, for example, is identical to spectra taken of the central peak of the crater Copernicus, so it is known that there is some olivine on the Moon. A great deal of work must still be done to sort out the signatures of various minerals from each other to understand the composition of the lunar surface.
By studying various rocks and minerals, it has been learned that there are certain wavelengths (colors) that are characteristic of certain substances. Cameras cannot "un-blend" colors, but they can be made to pick up only the "colors" (spectral bands) that give the most information. Eleven particular spectral bands are known to provide a great deal of information about the composition of rocks and soils, and these color filters were used on the Clementine mission cameras.
The Clementine spacecraft was built and operated by the Naval Research Lab. It was the first of a new class of small, low-cost spacecraft that use very lightweight instruments and hardware. During its two months in lunar orbit, Clementine mapped 38 million square kilometers of the Moon, taking nearly two million images. It also carried a laser ranging instrument to collect accurate data on the topography of the Moon. It was learned that the surface of the Moon is even rougher than had been previously thought.
One of the most newsworthy discoveries from the Clementine mission was evidence for possible water-ice at the lunar poles.23 If this discovery is verified, it would mean that future lunar manned exploration and eventual colonization will be much easier and sooner than previously estimated.
Lunar Prospector: The "Lunar Prospector" satellite, an inexpensive, long-awaited scientific mission to the Moon, was launched on 6 January 1998, and placed in a 100-kilometer orbit around the Moon (Figure 1.15). It carried five science instruments,24 including a neutron spectrometer that was designed to look for evidence of water on the surface of the Moon.
Neutron spectrometers measure the energy of neutrons that emanate from the lunar surface as the result of cosmic ray bombardment. Hydrogen in the lunar
23 Visit http:jjnssdc.gsfc.nasa.govjplanetaryjicejice_moon.html for more information.
24 The five instruments are a magnetometer, electron reflectometer, gamma-ray spectrometer, neutron spectrometer, and alpha-particle (alpha particles are Helium-4 nuclei) spectrometer.
regolith decreases the energy of neutrons on the lunar surface in a characteristic manner. When a concentration of hydrogen is present on the Moon, a spectrometer in lunar orbit will detect that concentration by identifying the characteristic decrease of neutron energy.
The neutron spectrometer on the Lunar Prospector did, in fact, record characteristic dips in neutron energy over the polar regions of the Moon, indicating concentrations of hydrogen in those locations. Because elemental hydrogen is a gas, concentrations of hydrogen on the Moon would have to be present in the form of a stable molecule, and the most likely molecule, based upon physical, chemical, and temperature factors, is water-ice in the permanently shadowed floors of craters.
Initial data from Prospector indicated that from 10 to 300 million tons of waterice are present in the polar regions of the Moon, with a greater amount of water (hydrogen) in the north polar region than in the south. More recent data now indicate the presence of between 1 and 10 billion tons of water. Because the spectrometer is only able to detect hydrogen to a depth of 0.5 meters, water may be present at depths much greater than those from which the Prospector derived its information. The significance of finding concentrations of hydrogen on the Moon is that hydrogen is a highly valuable element for biological systems, rocket fuel, and industrial purposes. With the hydrogen resources that are indicated by the Lunar Prospector, the timetable for the permanent return of humans to the Moon will be significantly shortened.
In addition to the discovery of hydrogen concentrations in the polar regions, the Lunar Prospector provided data on the lunar surface distribution of other elements such as potassium, rare-earth elements, phosphorus, and iron, and the interaction of
the Moon with the solar wind. Data is also being used to generate gravity maps of the Moon. Later in its mission, the orbit of the Prospector was lowered to 10 kilometers above the lunar surface so that more detailed information could be obtained.
When the spacecraft had exhausted its fuel, a final experiment was performed: the spacecraft was de-orbited into a permanently-shadowed region of the south pole, to determine if the impact would create water vapor that could be detected by Earth-based instruments or by the Hubble Space Telescope. Spectrometers were used to look for the signature expected from the hydroxyl (OH) molecules that should be a by-product of the impact. Several explanations are possible for the lack of detection of the hydroxyl molecules, so the experiment was not conclusive. Visits to the lunar polar regions by landers, equipped with geologic tools and sensors, will be required to learn if ice (or any other type of hydrogen-containing substance) exists there.
SMART-1: ESA's first spacecraft in its Small Missions for Advanced Research in Technology program (SMART-1) arrived in lunar orbit on 15 November 2004, using solar-electric propulsion and carrying a battery of miniaturized instruments (Figure 1.16). Originally scheduled to end in 2005, the success of the probe later warranted an extension of the mission until 3 September 2006 when the spacecraft impacted the lunar surface in the Lacus Excellentiae region.
The spacecraft's instruments included an ultra-compact electronic camera, AMIE, which surveyed the lunar surface in visible and a near-infrared light, and a near-infrared point-spectrometer (SIR) for mineralogical investigations. It was hoped that the SIR instrument could peer into permanently-shadowed craters using only the light that is reflected from nearby sunlit peaks. However, no definitive results have been reported to date.
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