Lunar Polar Orbiter Research Articles

Lunar polar orbiter: A global survey of the Moon

Lunar data from previous automated and manned exploratory missions have now been analyzed to the point where it is possible to define objectives for new missions to the Moon. The most logical next step is a polar orbiter designed to measure the Moon's gravity field, figure, heat flow, magnetism and surface composition. NASA has commissioned a study of this mission at JPL with participation by lunar scientists from JSC, and also has tentatively selected a group of investigators who constitute a Science Working Team. This paper describes the mission objectives and reports progress in the mission-definition study. As presently visualized, the orbiter will be launched by a Delta vehicle in 1980. After insertion into an eccentric, polar lunar orbit it will deploy a small, spinning subsatellite whose purpose is to relay precise radio Doppler measurements from the orbiter on the Moon's far side. After subsatellite delivery, the orbiter will maneuver into a low, circular polar orbit whence the instrument fields of view will be continuously pointed to the nadir for a nominal mission time of one year. Tracking and data acquisition will be via 26-m ground antennas and the JPL Mission Control and Computing Center, with distributed computing elements used in both flight and ground systems to simplify the data interfaces. Significant design advances are intended to include: (1) all systems designed to cost, (2) advanced scientific sensors aboard the spacecraft, (3) ground operations systems designed for largely automated, routine operation, and (4) design and operations concepts applicable to a variety of low-cost orbital missions in the Solar System.

Epilogue

We end this review of the knowledge gained since the Luna spacecraft, Pioneer, Surveyor and Apollo missions brought first scientific instruments and then men to study the lunar surface by looking forward to a new era of lunar, and planetary, exploration. The complete solution of many of the fascinating questions which have been posed in these pages depends on completing the survey of the Moon which has been so well begun. In this task, the concepts developed in the Soviet Union - the automated return of samples as in the Luna 16 and 20 missions and the idea of automatic survey vehicles, such as Lunakhod - and the concept now being actively studied in the U.S.A. - the lunar polar orbiter — will be of major importance. From such an orbiter it is clear that a variety of physical and chemical properties can be studied: the magnetic and gravitational fields, the Moon’s shape and topography, the elemental abundances at the surface, the thermal gradient in the regolith and the internal electrical parameters (see the account by Runcorn & Coleman 1977). It seems appropriate to end by illustrating the possibilities of this new development by reproducing the surveys, covering as yet a small area o the Moon and not nearly so discriminating as they will become, of the distribution of key chemical elements in the lunar surface. Figures 1 and 2 are taken from recent lunar science conference proceedings, f We look forward to the mapping of the entire surface which will result from these new developments.

On deducing the magnetization of the lunar surface from orbital surveys

A combination of orbital photographic, selenochemical and magnetic surveys may elucidate the mechanism by which the lunar surface became magnetized and possibly yield an estimate of the intensity of the ancient magnetizing field and its time variation. The determination of the size and shape of the magnetized regions requires the measurement of the altitude dependence of field, especially at low altitudes (< 100 km) and with a high enough sampling rate to resolve the profile at the edges of magnetized bodies. The planned Lunar Polar Orbiter may well provide the necessary data.

Gravity gradient mapping from the lunar polar orbiter - a simulation study

Simulations of the gravity data to be expected from a Lunar Polar Orbiter spacecraft utilizing either a Doppler velocity tracking system or a gravity gradiometer instrument system are generated using a point mass model that gives an excellent representation of the types of gravity anomalies to be found on the Moon. If the state of the art in instrumentation of both systems remain at the level of ±1 mm/sec at 10 sec integration time for the Doppler velocity system accuracy and at ±1 Eotvos at 10 sec integration time for the gravity gradiometer system accuracy, inspection of the simulations indicates that a gravity gradiometer system will give science data with better resolution and higher amplitude-to-measurement noise ratio than the Doppler velocity system at altitudes below 100 km. The error model used in the study is one where the system errors are assumed to be dominated by the point measurement noise and data quantization noise. The effects of other, more controllable, systematic error sources are not considered in this simplified analysis. For example, both systems will be affected by errors in LPO orbital altitude and position knowledge, spacecraft maneuvers, and data reduction errors. In addition, a Doppler tracking system will be sensitive to errors produced by spacecraft acceleration (from outgassing or solar pressure) and poor relative position of the LPO, Relay Satellite and ground tracking station, while a gravity gradiometer system will be sensitive to errors from spacecraft attitude and angular rates. These preliminary study results now need to be verified by a more complete error analysis in which all the uncertainties of the data gathering process are formally mapped into uncertainties in the resulting gravity maps.

Lunar Polar Orbiter considered for 1980

Another lunar mission is being studied at NASA's Jet Propulsion Laboratory, the first US lunar mission since Apollo 17 in 1972. This one, to be launched in 1980, would be unmanned. It would consist of an instrumented polar‐orbiting spacecraft (at an altitude of 100 kilometers) and a smaller companion subsatellite (at an altitude of 5000 km), which would serve to track the orbiter for gravity sensing when it is hidden from the Earth by the Moon. The ensemble would be launched by a single Delta launch vehicle. The spacecraft would orbit the moon for one year, examining the Moon's surface with a variety of instruments. These would measure lunar gravity, shape, magnetism and heat flow, and allow the determination of the chemical and mineral composition of the Moon's surface.

Chemical Mapping of Planetary Surfaces

Two instruments, the gamma-ray spectrometer and the X-ray fluorescence spectrometer, are uniquely suited to the chemical mapping of planetary surfaces from orbit. Through their detection of characteristic line spectra they measure the concentrations of a suite of elements in each area overflown. Multielement chemical maps derived from these remote measurements are used in the construction of evolutionary models of planetary bodies and of the solar system as a whole. The NaI(T1) gamma-ray spectrometer and a gas proportional X-ray spectrometer were flown over 20 percent of the lunar surface during the Apollo 15 and 16 missions. These instruments measured chemical differences across the boundaries of known lunar provinces and revealed several new features of lunar-surface composition. Advanced spectrometers which are under development for future missions are able to educe much more information in a given time span than the Apollo instruments. They may be used in possible future missions such as Lunar Polar Orbiter, a Mars orbiter, a Mercury orbiter, outer planet satellite missions, rendezvous with asteroids and cometary nuclei, and surface-penetrating planetary probes.

Opportunity to participate in NASA LPO mission

The National Aeronautics and Space Administration (NASA) has issued an Announcement of Opportunity (AO) providing information and guidelines for prospective investigators who are interested in participating in the Lunar Polar Orbiter (LPO) mission, tentatively planned for a June 1980 launch.The LPO will be a cost‐limited geochemical, geophysical, and geological mission capable of extending to the entire moon the orbital and surface science results obtained from previous lunar missions, expecially Apollo. Because of the applicability of LPO objectives and concepts to the exploration of planetary bodies with limited atmospheres, this mission may serve as a scientific prototype for similar missions to Mars and to Mercury.

The moon after Apollo

The Apollo missions have gradually increased our knowledge of the Moon's chemistry, age, and mode of formation of its surface features and materials Apollo 11 and 12 landings proved that mare materials are volcanic rocks that were derived from deep-seated basaltic melts about 3.7 and 3.2 billion years ago, respectively. Later missions provided additional information on lunar mare basalts as well as the older, anorthositic, highland rocks. Data on the chemical make-up of returned samples were extended to larger areas of the Moon by orbiting geochemical experiments. These have also mapped inhomogeneities in lunar surface chemistry, including radioactive anomalies on both the near and far sides. Lunar samples and photographs indicate that the moon is a well-preserved museum of ancient impact scars. The crust of the Moon, which was formed about 4.6 billion years ago, was subjected to intensive metamorphism by large impacts. Although bombardment continues to the present day, the rate and size of impacting bodies were much greater in the first 0.7 billion years of the Moon's history. The last of the large, circular, multiringed basins occurred about 3.9 billion years ago. These basins, many of which show positive gravity anomalies (mascons), were flooded by volcanic basalts during a period of at least 600 million years. In addition to filling the circular basins, more so on the near side than on the far side, the basalts also covered lowlands and circum-basin troughs. Profiles of the outer lunar skin were constructed from the mapping camera system, including the laser altimeter, and the radar sounder data. Materials of the crust, according to the lunar seismic data, extend to the depth of about 65 km on the near side, probably more on the far side. The mantle which underlies the crust probably extends to about 1100 km depth. It is also probable that a molten or partially molten zone or core underlies the mantle, where interactions between both may cause the deep-seated moonquakes. The three basic theories of lunar origin—capture, fission, and binary accretion—are still competing for first place. The last seems to be the most popular of the three at this time; it requires the least number of assumptions in placing the Moon in Earth orbit, and simply accounts for the chemical differences between the two bodies. Although the question of origin has not yet been resolved, we are beginning to see the value of interdisciplinary synthesis of Apollo scientific returns. During the next few years we should begin to reap the fruits of attempts at this synthesis. Then, we may be fortunate enough to take another look at the Moon from the proposed Lunar Polar Orbit (LPO) mission in about 1979.