GUEST COLUMN (continued from front page) elements of interest. An accumulated observation time of 100 hours is more desirable. In order to achieve observing times in this range in the high latitudes, spacecraft operating time in its final observing orbit should be at least one month. Several months is preferred since this not only improves resolution in the immediate vicinity of the poles but extends the total area of the Moon over which at least minimal observation is obtained. One should not overlook the importance of obtaining data from the Apollo, Surveyor, and Luna sites since we have samples and/or in-situ composition measurements to provide “ground truth” calibration to the orbital measurements. Based upon these considerations, the spacecraft must be designed to operate for at least six weeks to allow three days to reach the Moon and several days for calibration and checkout plus at least a month of observation. Since any spacecraft capable of operating for six weeks is quite likely to last for several months, the additional observation time can be obtained at no cost other than for tracking and ground operations. A full year of operation would provide at least minimal data for the entire Moon. A radar survey mission could complete a survey of the Moon much more rapidly. Placed in a polar orbit, the spacecraft would complete a full survey of the Lunar surface within two weeks of beginning operations. (The time required for the Moon to complete a 180 deg. rotation under the inertially fixed orbit.) Somewhat longer term operation would probably be desirable to improve resolution and to get a second look at interesting locations. The spacecraft for a single-instrument mission to look for water at the Lunar poles can be relatively simple and light in weight compared to the more elaborate scientific spacecraft commonly flown today. With one instrument there is no competition for field of view and data rates are modest. The instrument does impose some requirements or at least preferences, however. As noted earlier, because the spacecraft itself emits gamma rays, it is desirable to have the instrument mounted on boom which will place it a distance from the spacecraft approximating the major spacecraft dimension, farther is better. In addition, it is desirable that the instrument be extended toward the Moon and that orientation maintained. Since the instrument can detect radiation coming from any direction it can still obtain data even if the spacecraft is rotating it alternately toward and away from the Moon as a spin stabilized vehicle might. However the variation in bearing, shielding by the spacecraft bulk and other such factors can greatly complicate data analysis. Thus a nadir pointing configuration is desired. The modest data rate and relatively short range back to Earth render a high gain antenna unnecessary. A mid-gain antenna for downlink (spacecraft to Earth) and an omni for command reception and downlink during maneuvers will probably fulfill all requirements for the mission. In order to maintain a nadir pointing attitude, a Lunar horizon sensor will be required. On the day side of the Moon this presents no problem and an adaptation of an infrared based horizon sensor for Earth satellites will be quite satisfactory. The night side however presents a different problem. The lunar surface becomes so cold that it becomes very difficult (impossible for an ordinary sensor) to distinguish the Lunar surface from deep space. The ability to do this would require development of a highly sophisticated and expensive sensor. Fortunately, pointing accuracy requirements for the gamma-ray spectrometer are not especially tight. A drift of one or two degrees while the spacecraft is over the dark side would be acceptable. This can be met by a momentum bias system which uses a fly wheel to make the spacecraft rotate as it proceeds along its orbit at such a rate as to compensate for the motion and keep the spacecraft pointed toward the surface. Such systems require updates by horizon sensors to take out drift errors. In the case at hand, the horizon sensors keep the system updated while on the day side and the momentum bias system maintains control within adequate tolerance on the dark side. To the author’s knowledge, no studies have been conducted on a dedicated Lunar radar mission. One might hazard a guess that the spacecraft design would be somewhat more demanding since deployment of multiple antennas would probably be required. The data rate back to Earth would probably be higher than for a gamma-ray spectrometer mission. Pointing accuracy requirements are not clear. If very tight, it may be that good data can only be taken on the day side which would then imply that a month is required for complete mapping. The potential for a combined mission carrying both a gamma-ray spectrometer and a radar sounder is obvious. There is no obvious reason why the requirements for the two payloads would be in conflict to such a degree as to preclude flying them on the same spacecraft. Configuration and field-of-view constraints should present no more difficulty than many such spacecraft already flown. The greater complexity and higher data rate would make the project more costly than a singleinstrument concept but certainly less than flying two independent spacecraft. Candidate Spacecraft As will be discussed below under “Costs,” the concept of modifying existing Lunar Polar Probe showing boom for gamma-ray spectrometer. design Earth orbit spacecraft for the Lunar mission appears to offer considerable saving over new design. An in-house study at JPL (Ref. 8) identified an adaptation of the RCA Atmospheric Explorer/Dynamics Explorer series of spacecraft as quite viable for the gamma-ray spectrometer mission. The TIROS/DMSP weather satellite series is also a candidate but represents considerable “overkill” in terms of capability for a single instrument mission. Other potential candidates include the Boeing MESA low-cost spacecraft and possibly other similar concepts. This does not constitute a recommendation of any particular spacecraft but rather serves to define the class of vehicle which might be modified for the task. Spacecraft in this general size category can be placed on a Lunar transfer trajectory by a variety of launch vehicles including Delta, Ariane, and Shuttle with a suitable upper stage. Among the latter the PAM-D and PAM-DII seem to be the most logical choices. To allow for the propulsion required to enter Lunar orbit one may conservatively assume that 50% of the mass on the transfer trajectory can be delivered into Lunar orbit as useful spacecraft mass over and above propulsion inerts, maneuvering fuel, etc. Based upon these approximations, it appears that PAM-D could place a 300 to 350 kg spacecraft in Lunar orbit while PAM-DII could deliver a spacecraft mass of about 500 kg. Reference 8 states that a Lunar spacecraft based upon the AE/DE as discussed above would have a mass of about 280 kg while a new design optimized spacecraft might be as low as 180 kg. Either of these is clearly within the capability of PAM-D. Going to the PAM-DII would have the salutary effect of effectively (continued on page 4)
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