Lunar Hemisphere Research Articles

Lunar ejecta origin of near-Earth asteroid Kamo’oalewa is compatible with rare orbital pathways

Near-Earth asteroid, Kamo’oalewa (469219), is one of a small number of known quasi-satellites of Earth; it transitions between quasi-satellite and horseshoe orbital states on centennial timescales, maintaining this dynamics over megayears. The similarity of its reflectance spectrum to lunar silicates and its Earth-like orbit both suggest that it originated from the lunar surface. Here we carry out numerical simulations of the dynamical evolution of particles launched from different locations on the lunar surface with a range of ejection velocities in order to assess the hypothesis that Kamo‘oalewa originated as a debris-fragment from a meteoroidal impact with the lunar surface. As these ejecta escape the Earth-Moon environment, they face a dynamical barrier for entry into Earth’s co-orbital space. However, a small fraction of launch conditions yields outcomes that are compatible with Kamo‘oalewa’s orbit. The most favored conditions are launch velocities slightly above the escape velocity from the trailing lunar hemisphere.

Compositional and morphological analysis of Gassendi crater

Gassendi impact crater is located on the western side of the visible lunar hemisphere just north of Mare Humorum. It has a diameter of 110 km, and it is a Floor Fractured Crater (FFC) belonging to Class-3 type that exhibits a wide annular depression or moat. Is Gassendi crater formed due to igneous intrusion or viscous relaxation? To investigate this, the analysis explores the compositional and morphological dimensions of the crater. The area studied has been found to contain various minerals such as LCP (Low Calcium Pyroxene), HCP (High Calcium Pyroxene), olivine, plagioclase, and mixtures of pyroxene and plagioclase. To differentiate between similar spectral profiles, Integrated Band Depth map (IBD) and Band Depth Parameter (BD) maps are generated using the M3 data. Features such as central peak complex, isolated mound, fractures, rimae, wall terraces, wall slumps, etc., have been identified and marked in the morphological map. We have also studied the morphometrical aspects of all the morphological features. Another impact crater just on the northern rim of Gassendi, Gassendi A crater shows the presence of LCP and olivine mineral along with extensive slumping, melt pool, and distorted northern rim. We have mapped the three mare regions towards the south of the crater floor and estimated their ages by using the CSFD technique. The present appearance of the Gassendi crater is mostly due to past volcanic activities rather than the result of the impact event.

Fly me to the moon

Two robotic missions, one from China and one from India, will explore the southern lunar hemisphere, sending back a wealth of information about the surface and composition of the Moon, and perhaps even more.

Gravity anomalies and crustal structure of the Lunar far side highlands

The far side of the lunar hemisphere is dominated by a large number of impact craters having a smaller diameter and few Mare basins compared to the near side of the Moon. This study presents a detailed analyses of gravity anomalies derived from Gravity Recovery and Interior Laboratory (GRAIL) mission and topography over the central equatorial region of Lunar far side covering the major impact basins such as Dirichlet-Jackson, Korolev, Hertzsprung and adjoining regions of the highlands, to understand sub-surface structure. Bouguer gravity anomaly map of the region is prepared using the density of 2.7 g/cc, which is estimated using fractals based approach. The spectral analyses using Maximum Entropy Method of calculated Bouguer anomaly and topography provided an estimated of Effective Elastic Thickness (Te) as 19 km and also a cut-off wavelength of 270 km for separating Regional and residual Bouguer gravity anomalies. Further a crustal thickness of this region of Moon is estimated by inversion of regional Bouguer gravity anomalies, which varies from 20 to 71 km.An estimated crustal thickness of Dirichlet Jackson and Hertzpsrung, Korolev basins varies between 39 and 65 km, 20–65 km and 39–64 km, respectively. The residual gravity anomaly map shows symmetric gravity highs and lows in Dirichlet-Jackson, Hertzsprung and Korolev basins. These gravity highs can be attributed to high-density mare basalts within inner rims of these basins. The observed nature of gravity anomalies and crustal thickness map can be attributed to the buried impact crater under this region.

Orbit determination of the Lunar Reconnaissance Orbiter: Status after seven years

The Lunar Reconnaissance Orbiter (LRO) has been orbiting the Moon since 2009, obtaining unique and foundational datasets important to understanding the evolution of the Moon and the Solar System. The high-resolution data acquired by LRO benefit from precise orbit determination (OD), limiting the need for geolocation and co-registration tasks. The initial position knowledge requirement (50 m) was met with radio tracking from ground stations, after combination with LOLA altimetric crossovers. LRO-specific gravity field solutions were determined and allowed radio-only OD to perform at the level of 20 m, although secular inclination changes required frequent updates. The high-accuracy gravity fields from GRAIL, with <10 km spatial resolution, further improved the radio-only orbit reconstruction quality (<10 m). However, orbit reconstruction is in part limited by the 0.3–0.5 mm/s measurement noise level in S-band tracking. One-way tracking through Laser Ranging can supplement the tracking available for OD with 28-Hz ranges with 20-cm single-shot precision, but is available only on the nearside (the lunar hemisphere facing the Earth due to tidal locking). Here, we report on the status of the OD effort since the beginning of the mission, a period spanning more than seven years. We describe modeling improvements and the use of new measurements. In particular, the LOLA altimetric data give accurate, uniform, and independent information about LRO's orbit, with a different sensitivity and geometry which includes coverage over the lunar farside and is not tied to ground-based assets. With SLDEM2015 (a combination of the LOLA topographic profiles and the Kaguya Terrain Camera stereo images), another use of altimetry is possible for OD. We extend the ‘direct altimetry’ technique developed for the ICESat mission to perform OD and adjust spacecraft position to minimize discrepancies between LOLA tracks and SLDEM2015. Comparisons with the radio-only orbits are used to evaluate this new tracking type, of interest for the OD of future lunar orbiters carrying a laser altimeter. LROC NAC images also provide independent accuracy estimation, through the repeated views taken of anthropogenic features for instance.

Quantitative Study of Lunisolar Precession Mechanism

According to Xiaodong Song et al.’s study about difference rotation in the Earth’s inner core, a physical model of lunar motion changing the angular momentum of the Earth’s circles is presented in this work. The Lunar motion makes there be a huge gap existing in the core angular momentum between far lunar hemisphere and near lunar hemisphere. The gap results in the difference rota-tion in the Earth’s inner core, meanwhile, it increases the angular velocity of far lunar hemisphere and decreases the angular velocity of near lunar hemisphere. The Earth’s liquid outer core gene-rates an astro-geodynamical effect including the difference rotation among the Earth’s circles. It is found that when the moon moves into the apogee or the perigee of the lunar orbit, and the moon phase is the upper and lower chords (i.e. semi-diameter place), the true anomaly of the moon will change from 270-degree back (or forward) to 90-degree; this results in the mantle shell of the Earth westward, and forms lunisolar precession, the vernal equinox westward and Chandler polar motion.

Human space exploration and human spaceflight: Latency and the cognitive scale of the universe

The role of telerobotics for space exploration in placing human cognition on other worlds is limited almost entirely by the speed of light, and the consequent communications latency that results from large distances. This latency is the time delay between the human brain at one end, and the telerobotic effector and sensor at the other end. While telerobotics and virtual presence is a technology that is rapidly becoming more sophisticated, with strong commercial interest on the Earth, this time delay, along with the neurological timescale of a human being, quantitatively defines the cognitive horizon for any locale in space. That is, how distant can an operator be from a robot and not be significantly impacted by latency? We explore that cognitive timescale of the universe, and consider the implications for telerobotics, human spaceflight, and participation by larger numbers of people in space exploration. We conclude that, with advanced telepresence, sophisticated robots could be operated with high cognition throughout a lunar hemisphere by astronauts within a station at an Earth–Moon L1 or L2 venue. Likewise, complex telerobotic servicing of satellites in geosynchronous orbit can be carried out from suitable terrestrial stations.

Absolute Calibration of the Clementine UVVIS Data: Comparison with Ground-Based Observation of the Moon

Calibrated Clementine images of the Moon and data from Pieters's absolutized spectrophotometric catalog are compared. The scales of these two photometric systems are shown to differ greatly: the albedo of the lunar surface at a phase angle of 6° determined from the Clementine data turns out to be a factor of 2.5 higher than that inferred from ground-based photometry. The fact that the lunar-soil samples from the Apollo 16 landing site used for absolute calibration of the Clementine data are not representative may be responsible for the differences between the scales. Maps are constructed (for wavelengths of 0.42, 0.75, and 0.95 μm), which reproduce the distribution of the ratio of the brightnesses determined from telescopic images of the visible lunar hemisphere and from the corresponding Clementine mosaics. The comparison shows fairly good agreement in the details. On the average, the differences are about 10%. The streakiness of the Clementine mosaics are clearly seen in the maps, which is attributable to a low quality of the photometric joining of the data from two one-month imaging series.

Random processes as a cause of the lunar asymmetry

The offset of the center of mass of the Moon from its center of figure together with moment of inertia differences are explainable by a lunar crust of randomly varying thickness. The necessity of postulating a method of preferential material transport into a particular lunar hemisphere to explain the lunar asymmetry is eliminated.

The mapping of lunar radar scattering characteristics

This is the first of four articles describing a comprehensive series of radar maps of the entire visible lunar hemisphere carried out at wavelengths of 3.8 and 70 cm and analyzing the echoes in both orthogonal senses of circular polarization. In this paper, the basic techniques of delay-Doppler mapping by radar are developed, and the particular steps employed in mapping the Moon are outlined. Succeeding articles present the results obtained and discuss the way in which these results relate to other, nonradar measurements as well as to the actual lunar surface properties.

The lunar conductivity profile and the nonuniqueness of electromagnetic data inversion

A review of the theory for the electromagnetic functional used to date to determine the lunar conductivity profile from spectral analyses of lunar magnetometer data is presented. The “hard” boundary condition used by Sonett et al. (1971 a, b) and others appears to be a good approximation for the sunlit lunar hemisphere. The use of only the first spherical harmonic in the electromagnetic functional is not justified; further, there are certain classes of lunar models where the transverse magnetic modal response may not be neglected. The use of the spectral data in conjunction with the functional to find a least squares conductivity profile is examined from the point of view of the nonuniqueness of nonlinear estimation. An unsuccessful search was carried out to find models that were both geophysically reasonable and consistent with the data. It is asserted that the failure to find these models is due to the inadequacy of the functional. Of the models generated, those that best fit the data all have a conductivity peak at a depth of 240km similar to that of the model discussed by Sonett et al. (1971 a, b). However, all of these models are quite distinct elsewhere in the profile. It is shown how such models are dependent on both the nature of the functional as well as the initial guess in the least squares procedure. A correlation analysis shows that the only resolvable features from this type of model are a low conductivity crust, a high conductivity peak of limited radial extent, and a low conductivity zone beneath the peak. It is also concluded that the two-layer model of Kuckes (1971) is equally as valid as the peaked models, and the totality of “equally valid” models derived from the spectral data place no reasonable constraint on the electrical conductivity below 350 km depth.

A Lunar “New Jerusalem.”

A PAMPHLET has been received containing a series of lectures by the Rev. G. B. Berry on “The New Jerusalem,” with a preface by the Lord Bishop of Exeter. With the spiritual interpretation of the Apocalypse we are not concerned in these columns, but an astonishing speculation put forward in the last lecture demands a word of comment. Mr. Berry suggests that the invisible part of the moon has the same size and shape as the-mighty pyramid which, according to Revelation, forms the heavenly Jerusalem. Eventually the lunar hemisphere visible to us is to bury itself in the earth, and the pyramidal portion is to project above “the rack and ruin of the elements” caused by the catastrophe, and to be the Celestial City in which the faithful will pass eternity. As a vision, this picture may appeal to imaginative minds, but from the point of view of celestial mechanics it can scarcely be taken seriously. A pyramid of the dimensions; of that upon which Mr. Berry's New Jerusalem rises tier upon tier would be crushed by its own weight even if it were built of steel. As, however, the structure is visionary, we imagine that this material fact affords no valid objection to it. The changed moment of inertia of a moon with the invisible side of a pyramidal form would necessitate modification of the whole theory of the physical librations of our satellite; but perhaps Mr. Berry does not appreciate the force of this difficulty. He is certainly not familiar with the theory of tidal friction or with the fact that Laplace, who studied the physical librations, showed that one side of the moon always faces the earth because that position is one of dynamical stability. A fuller knowledge of celestial mechanics might have made Mr. Berry hesitate before erecting such a visionary structure as he describes upon so slender a foundation. His views would have pleased mediaeval schoolmen, but modern science demands that even the most fascinating hypothesis should be based upon results of observation capable of Being put to the test of inquiry rather than upon “revealed truth” to be accepted without criticism.