SELENE (Selenological and Engineering Explorer) / Kaguya
SELENE is a lunar mission of JAXA (Japan Aerospace Exploration Agency). The mission objectives are the global survey of the moon, and to develop technologies for the lunar orbit insertion and spacecraft attitude and orbit control. The global survey of the moon is made for better understanding the origin and evolution of the moon, measuring the gravity field, elemental/chemical composition, etc. It also includes the measurement of the lunar and solar-terrestrial environment, and research on the possibility of future utilization of the moon. The nominal SELENE observation period is planned for one year. 1) 2) 3)
Background: The SELENE project started in 1998 as a joint mission of the former ISAS (Institute of Space and Astronautical Science) and the former NASDA (National Space Development Agency) of Japan. The two organizations were merged into JAXA on Oct. 1, 2002.
SELENE consists of the Main Orbiter and two microsatellites (also referred to as subsatellites): Rstar (Relay Satellite) and Vstar (VLBI Radio Satellite). The Main Orbiter in turn employes two modules, UM (Upper Module) and the LM (Lower Module). Thermal control of SELENE has been achieved by using combination of passive and active control techniques. MLI (Multi layer insulation) and radiators are used as passive means. The active thermal control consists of thermal louvers and heaters.
Figure 1: Overview of the SELENE spacecraft configuration (image credit: JAXA)
Table 1: Main parameters of SELENE and the subsatellites
UPS (Unified Propulsion System): A dual-mode system using hydrazine for fuel and MON-3 for oxidizer. The design is of DRTS heritage. UPS features two gas tanks, two fuel tanks, one oxidizer tank, one 500 N bi-propellant engine, twelve 20 N monopropellant thrusters and eight 1 N monopropellant thrusters. A regulator controls the pressure of the propellant tanks as well as the thrust of the 500 N engine, referred to as OME (Orbit Maneuvering Engine), which is maintained at a constant level. 4)
Table 2: Main characteristics of the UPS
The UPS is controlled by the AOCS (Attitude and Orbit Control Subsystem). The 500 N thruster is the main thruster for LTO (Lunar Transfer Orbit) and LOI (lunar Orbit Injection) maneuvers. The 20 N thruster is mainly used for attitude control and the cluster of the 1 N thrusters is used as a backup for orbital maneuvers. Figure 2 shows the AOCS and the UPS of SELENE.
SELENE's AOCS has seven control modes, as presented Table 3, to satisfy various control requirements from different flight phases. The AOCS consists of attitude sensors of three different types, accelerometers, a fault-tolerant flight computer, wheels and their drivers, and thruster drivers (Figure 2). SELENE has two star trackers, two pairs of sun sensor heads, and an internally redundant IRU (Inertial Reference Unit) as attitude sensors. SELENE uses a ST (Star Tracker) and an IRU for attitude determination. Two attitude determination modes exist: the inertial attitude determination using only IRU and the ST-IRU integrated attitude determination with an extended Kalman filter. The SSH (Sun Sensor Head) is used for sun acquisition to measure the elongation between the sun direction and the direction of SSH view center. An accelerometer is used for the velocity increment cutoff of orbital maneuvers.
The AOCE (Attitude and Orbit Control Electronics) is a fault-tolerant flight computer system with three MPUs, on which the ACFS (Attitude and orbit Control Flight Software) calculates the attitude and orbit control commands and checks the behavior of the AOCS sensors and actuators. SELENE has four pairs of reaction wheels and drivers as actuators for precise attitude control. Furthermore, SELENE/Kaguya has internally redundant valve drivers, respectively corresponding to the 500 N thruster, 20 N thrusters and 1 N thrusters. 5) 6)
Table 3: Overview of SELENE/Kaguya control modes
Table 4: Main performance of KAGUYA's control mode
Figure 2: AOCS and UPS configuration of SELENE/Kaguya (image credit: JAXA)
Table 5: Main performance of Kaguya's control mode
Launch: A launch of SELENE/Kaguya took place on Sept. 14, 2007 on an H-IIA launch vehicle from TNSC (Tanegashima Space Center), Japan. About 46 minutes after liftoff, the separation of the spacecraft was confirmed.
RF communications: Science data from the Main Orbiter (Kaguya) in near-side lunar orbit are downlinked in X-band to the JAXA DSN (Deep Space Network) - consisting of the UDSC (Usuda Deep Space Center) or a backup station of USC. The downlink data rate is 10 Mbit/s (QPSK modulation). The TT&C data are transmitted in S-band at rates of 2 and 40 kbit/s. The modulation type is PCM (NRZ-L)/PSK-PM.
The 4-way tracking data of Main Orbiter on the far-side of the moon, are being downlinked via Rstar, using the UDSC DSN antenna. SOAC (SELENE Operation and data Analysis Center) is being constructed on the Sagamihara campus of JAXA/ISAS. All data of the SELENE mission are being interfaced at SSOC (Sagamihara Space Operation Center), and are being send to SOAC. Science data downlinked to UDSC or USC are transmitted to SOAC through SSOC, and stored in a raw level database.
Four antennas of JAXA ground network, NASA DSN, JAXA DSN of UDSC and USC, are redundantly assigned for telemetry-command operation of the Main Orbiter and the two subsatellites Rstar and Vstar. 7)
Figure 3: Two views of the SELENE launch configuration (image credit: JAXA)
Figure 4: Artist's view of the deployed SELENE spacecraft (image credit: JAXA)
LTO (Lunar Transfer Orbit): The H-IIA vehicle doesn't have an upper stage (like Apollo) - its active mission time is limited to 3000 seconds or 50 minutes. After much study and long analysis, the initial choice of a "direct transfer orbit" was dropped in favor of a robust phasing LTO for SELENE. The phasing LTO uses several phasing loops (elliptical orbits) around Earth prior to going into an orbit to the moon and reaching the lunar orbit insertion point. 8) 9) 10)
The phasing LTO can use parameterized rocket trajectories. Two phasing loops were selected by the project team for the LTO of SELENE (Figure 6). The subsatellites Rstar and Vstar are being used to measure the gravitational field of the moon. Rstar and Vstar will be separated from the Main Orbiter during the interval of lunar orbit injection (LOI) maneuvers. 11)
Figure 5: Overview of SELENE Flight Dynamics System and data distribution flow (image credit: JAXA)
Figure 6: Baseline concept of the Lunar Transfer Orbit (image credit: JAXA)
Figure 7: Outline of the Lunar Transfer Orbit (image credit: JAXA)
Figure 8: Lunar on-orbit configuration of SELENE (image credit: JAXA)
Figure 9: Artist's view od SELENE in lunar orbit (image credit: JAXA)
Mission status: (The Kaguya mission was launched on Sept. 14, 2007 — it impacted the Moon on June 10, 2009)
• January 2017: In a new study, Japanese researchers are reporting of observations from the Japanese spacecraft Kaguya of significant numbers of 1–10 keV O+ ions, seen only when the Moon was in the Earth's plasma sheet. Considering the penetration depth into metal of O+ ions with such energy, and the 16O-poor mass-independent fractionation of the Earth's upper atmosphere, the research team concludes that biogenic terrestrial oxygen has been transported to the Moon by the Earth wind (at least 2.6 x 104 ions cm-2 s-1) and implanted into the surface of the lunar regolith, at around tens of nanometers in depth. The team is suggesting the possibility that the Earth's atmosphere of billions of years ago may be preserved on the present-day lunar surface. 12)
- In 2008, the Moon and the Japanese lunar orbiter Kaguya both lay within the Earth's plasma sheet for tens of minutes to a few hours per month (Figure 10, Table 6). When the Moon moved into the central magnetosphere on 21 April 2008, Kaguya measured plasma sheet ions with an energy of several keV during the periods of 0:50–1:10 UT and 8:00–16:00 UT, and detected a weak signature of cold lobe ions in the remaining period (Figure 11). Moonward-moving magnetospheric ions were observed by the IEA (Ion Energy Analyzer), whereas the IMA (Ion Mass Analyzer) measured both the ions coming from the Moon (anti-moonward ions) and the magnetospheric ions by means of a wide energy FOV (Field of View).
- Figure10 illustrates the geometrical setting of solar wind, Earth, Moon and Kaguya and the direction of the mass spectrometer. The IEA is not equipped with a mass analyzer, and thus the moonward energy spectra include the signatures of all ions mixed together (Figure 11a). In the case of the anti-moonward ions, in contrast, the team was able to disentangle the H+ and the O+ contributions (Figure 11b,c). The temporal evolution of the moonward energy spectra (Figure 11a) is very similar to (Figure 11b), because ions in the plasma sheets are mainly H+ and are dense, hot and almost isotropic. The O+ anti-moonward energy spectra exhibit a series of low-energy signatures below ~1 keV, which appear only when Kaguya is in the dayside region. These signatures have been observed before and were mainly attributed to ions of lunar origin, produced by the photoionization of exospheric particles and/or ion emission from the lunar surface, and then accelerated by the electrical potential difference between the lunar surface and the spacecraft. This potential difference causes the cutoff energy to be about 1 keV in the energy distribution.
- The teams new finding in this study is the higher-energy O+ (1–10 keV) ions during almost the entire period of the plasma sheet encounters (Figure 11c), which were not detected when the Moon was outside the magnetosphere.
Figure 10: Geometrical setting of the instrumental apparatus. a) The solar wind, Earth, Moon and Kaguya, within the geocentric solar ecliptic coordinate system. The squares indicate the position of Kaguya during the measurements analyzed in this paper. The grey area is the position of the plasma sheet. The arrows schematize the direction and behavior of the solar wind around the Earth's magnetosphere. RE, Earth radius. b) Position of the two Kaguya ion sensors, IEA and IMA, with respect to the Moon (image credit: (Osaka University, JAXA, University of Nagoya)
Table 6: Densities of the magnetospheric ions measured by IEA, abundance ratios of the magnetospheric O+/H+ measured by IMA and net fluxes of the magnetospheric O+ to the lunar surface during the plasma sheet crossing of Kaguya in 2008
Legend to Table 6: In all the events, continuous (≥ 5 min), hot (widely distributed over 1 keV) and dense (≥ 1.0 x 10-2 cm-3) plasma sheet ion signatures were found in the central region of the Earth's magnetosphere (|Y| ≤ 15RE in Geocentric Solar Ecliptic coordinates). A dash indicates that no mass analysis data were available, because IMA was working in the other mode. The magnetospheric O+ fluxes were estimated by using H+ fluxes from the IEA data and O+/H+ ratios from the IMA data. February, August, October and December each had a couple of days on which observation was suspended in the magnetosphere for operational reasons such as the eclipse. Time is UT.
Figure 11: Kaguya ion observations in the Earth's magnetosphere on 21 April 2008. The Moon encountered plasma sheet ions during the periods of 0:50–1:10 UT and 8:00–16:00 UT, which are indicated by the thick black lines at the top of the Figure. a–c) Energy–time spectra of ions observed by the IMA and IEA instruments. The color indicates the differential energy flux. a) Moonward ions measured by IEA. b,c) Anti-moonward H+ and O+ ions measured by the IMA. The measured O+ consisted of lunar ions below ~1 keV (marked by white ellipses in Figure 11c) and magnetospheric ions in the higher energy range, particularly during the plasma sheet crossing. d) Densities of H+ and O+ ions. The H+ ions are derived from the IEA (black plus) and IMA (black dot) data. Low-energy (< 1 keV) O+ ions coming from the Moon are marked in red, and high-energy (> 2 keV) magnetospheric O+ in blue. e) Lunar latitude of the spacecraft that orbited near noon and midnight. The magnetospheric ions were measured in both the dayside and nightside (grey) regions, whereas the lunar O+ was only measured in the dayside region.
• May 26, 2016: The project developed an application scheme for conducting lunar calibration, one of the radiometric calibration methods for optical instruments onboard Earth-orbiting satellites and planetary explorers, with a newly developed hyperspectral lunar reflectance model based on SELENE/SP (Spectral Profiler) data. Because the model considers photometric properties (lunar surface reflectance and its dependences on incident, emission, and phase angles) with high spectral and spatial resolution (6–8 nm wavelength intervals and 0.5º grid meshes in lunar latitude and longitude), it enables us to simulate disk-resolved Moon radiance observed by not only multispectral but also hyperspectral sensors in space. Simulations of Moon observations conducted by ASTER with its three visible and near infrared bands, produced brightness profiles of simulated Moon images that show high correlation (more than 0.99) with the observed images in all bands, and the relative brightness of each pixel can be evaluated with 5% uncertainty. Consistency of dependence on phase angle and the libration effect between the SP model and another lunar reflectance model, ROLO, was also confirmed. The SP model will therefore be useful for evaluating the relative degradation of sensors in space. 13)
• August 2014: An international research team, led by Yuji Harada from the Planetary Science Institute, China University of Geosciences, has found that there is an extremely soft layer deep inside the Moon and that heat is effectively generated in the layer by the gravity of the Earth. These results were derived by comparing the deformation of the Moon as precisely measured by Kaguya/SELENE (Selenological and Engineering Explorer) and other probes (Change-1, LRO, and GRAIL) with theoretically calculated estimates. These findings suggest that the interior of the Moon has not yet cooled and hardened, and also that it is still being warmed by the effect of the Earth on the Moon. This research provides a chance to reconsider how both the Earth and the Moon have been evolving since their births through mutual influence until now. 14) 15)
When it comes to clarifying how a celestial body like a planet or a natural satellite is born and grows, it is necessary to know as precisely as possible its internal structure and thermal state. How can we know the internal structure of a celestial body far away from us? We can get clues about its internal structure and state by thoroughly investigating how its shape changes due to external forces. The shape of a celestial body being changed by the gravitational force of another body is called tide. For example, the ocean tide on the Earth is one tidal phenomenon caused by the gravitational force between the Moon and the Sun, and the Earth. Sea water is so deformable that its displacement can be easily observed. How much a celestial body can be deformed by tidal force, in this way, depends on its internal structure, and especially on the hardness of its interior. Conversely, it means that observing the degree of deformation enables us to learn about the interior, which is normally not directly visible to the naked eye.
Figure 12: This is the pattern diagram of the Moon shape changing by the Earth's gravitational force. It especially shows deformed shape by that the movement of the Moon to the Earth is out of complete circle. For clarity, it draws deformed larger than the actual. (image credit: NAOJ)
The Moon is no exception; one can learn about the interior of our natural satellite from its deformation caused by the tidal force of the Earth. The deformation has already been well known through several geodetic observations (also referred to as "selenodetic" observation as it is for the Moon). However, models of the internal structure of the Moon as derived from past research could not account for the deformation precisely observed by the above lunar exploration programs.
Therefore, the research team performed theoretical calculations to understand what type of internal structure of the Moon leads to the observed change of the lunar shape.
What the research team focused on is the structure deep inside the Moon. During the Apollo program, seismic observations were carried out on the Moon. One of the analysis results concerning the internal structure of the Moon based upon the seismic data indicates that the satellite (Moon) is considered to consist mainly of two parts: the "core", the inner portion made up of metal, and the "mantle", the outer portion made up of rock. The research team has found that the observed tidal deformation of the Moon can be well explained if it is assumed that there is an extremely soft layer in the deepest part of the lunar mantle. The previous studies indicated that there is the possibility that a part of the rock at the deepest part inside the lunar mantle may be molten. This research result supports the above possibility since partially molten rock becomes softer. This research has proven for the first time that the deepest part of the lunar mantle is soft, based upon the agreement between observation results and the theoretical calculations.
Figure 13: Artist's rendition of the internal structure of the Moon based on this science result (image credit: NAOJ)
Figure 14: An estimate of the Moon's interior viscosity structure replicate well the observational results in this research. The viscosity is one of the indicators of tenderness/hardness. For reference, the density structure and seismic velocity based on previous study are added (image credit: NAOJ)
Furthermore, the research team also clarified that heat is efficiently generated by the tides in the soft part, deepest in the mantle. In general, a part of the energy stored inside a celestial body by tidal deformation is changed to heat. The heat generation depends on the softness of the interior. Interestingly, the heat generated in the layer is expected to be nearly at the maximum when the softness of the layer is comparable to that which the team estimated from the above comparison of the calculations and the observations. This may not be a coincidence. Rather, the layer itself is considered to be maintained as the amount of the heat generated inside the soft layer is exquisitely well balanced with that of the heat escaping from the layer. Whereas previous research also suggests that some part of the energy inside the Moon due to the tidal deformation is changed to heat, the present research indicates that this type of energy conversion does not uniformly occur in the entire Moon, but only intensively in the soft layer. The research team believes that the soft layer is now warming the core of the Moon as the core seems to be wrapped by the layer, which is located in the deepest part of the mantle, and which efficiently generates heat. They also expect that a soft layer like this may efficiently have warmed the core in the past as well (Ref. 14).
• In the summer of 2011, the Kaguya science team has archived the Kaguya data and has made them available to the public; the team is involved in various studies in which the data are used. Although data analysis and science study are ongoing, the major scientific achievements to date are summarized in Table 7. 16)
Table 7: Science summary of the SELENE/Kaguya mission
• The SP (Spectral Profiler) device of the LISM instrument onboard SELENE/Kaguya revealed the global distribution of olivine on the lunar surface and its origin. This new finding provides the project important insight into the Moon's origin and evolution. This result was published in the British scientific journal "Nature Geosciences" on July 4, 2010. 17)
Note: Olivine is one of the dense silicate minerals. The mantle in the Earth is composed mainly of olivine.
• JAXA ended the SELENE/Kaguya mission on June 10, 2009 (UTC) when the main orbiter was maneuvered to drop onto the lunar surface near the location: 65.5º south latitude, 80.4º east longitude, near the GILL crater. The impact location was in the shaded area of the moon - providing a slight chance of an impact flash to be observed from Earth or a spacecraft. 18) 19)
Kaguya was the largest and most sophisticated lunar mission since the Apollo program. During the two years mission, it collected scientific data on elemental abundance, surface and subsurface structure, gravity fields, magnetic field, and lunar environment for lunar science. It also observed the solarterrestrial plasma environment from the lunar orbit. The high-quality motion pictures of the Earth and the moon were obtained by the HDTV cameras for publicity and educational purposes. - The major part of the scientific data are open to public in November 2009. The huge amount of the data will be used to study the origin and evolution of the moon, and to investigate the future plan for the lunar exploration and utilization. 20)
Figure 15: Summary of over-all KAGUYA mission operation (image credit: JAXA)
• The Kaguya spacecraft and its payload were operating nominally as of spring 2009 - in the extended mission period. 21)
• On October 31, 2008, the nominal operation period of KAGUYA in lunar orbit was successfully completed. The extended operation period will proceed until the summer 2009. During the extended operations period, KAGUYA changed its lunar altitude from 100 km to 50 km. This will be reduced again to less than 50 km, to gather further precious observation data. - From October 2009 onwards (i.e. ~ 2 years after KAGUYA launch), worldwide registered scientists can search and download Level-2 processed data via the Web browser. SELENE data will be available for users through the internet (Ref. 21).
• The relay satellite "Okina (Rstar)" made an impact on the lunar surface on February 12, 2009 (JST), and the four-way Doppler measurement mission was successfully completed. 22)
• SELENE/Kaguya is operating nominally as of 2008 providing observations of the lunar surface. This applies also to the operations of the two subsatellites, Vstar (Ouna) and Rstar (Okina). SELENE/Kaguya is considered the most sophisticated moon exploration mission in the post-Apollo era. 23) 24) 25) 26)
• The extended mission will start in November 2008. Considering the amount of fuel available at this stage, an extension of the mission for another half a year is possible. The operation scenario for the extended mission is under discussion, but one possible scenario is to lower the altitude of the main orbiter down to 50 km. This will provide a good opportunity to measure the lunar magnetic field much more accurately (Ref. 23).
• SELENE/Kaguya started nominal lunar operations on Dec. 21, 2007 - when the Kaguya orbiter completed a two-month initial phase to inspect the functioning of all the equipment before starting its main mission. Normal operations will continue for 10 months to collect data for lunar and other research. 27)
• On Oct. 30, 2007 (JST -Japan Standard Time), the first image shooting was carried out by the onboard high definition television (HDTV) camera - representing the world's first high definition image data acquisition of the lunar surface from an altitude about 100 km (Figure 16). On Oct. 31, 2007, Kaguya deployed the LMAG (Lunar Magnetometer) mast (12 m length), two pairs of the LRS (Lunar Radar Sounder) antennas (15 m in length), and the UPI (Upper Atmosphere and Plasma Imager) gimbals 28)
Figure 16: First image of the lunar north pole region taken by the HDTV camera on Oct. 30, 2007 (image credit: JAXA, NHK)
• On Oct. 18, 2007, SELENE /Kaguya was injected into a lunar orbit at an altitude of about 100 km. Then on Oct. 21, the spacecraft attitude control mode was enacted to observe the lunar surface at all times. Also, the initial checkout phase (or commissioning phase) was started at this time. After the checkout of bus system, the extension of four sounder antennas with 15 m length and the 12 m mast for magnetometer, and deployment of plasma imager were successfully carried out to start the checkout of the science instruments.
• On Oct. 12, 2007, the Kaguya spacecraft released one of its onboard microsatellites, namely the VRAD satellite (Vstar, the nickname is "Ouna").
• On Oct. 9, 2007, the Rstar (Relay satellite) microsatellite was released from Kaguya while the onboard HDTV camera observed the release; in addition, images of the moon's surface were captured.
• On Oct. 4, 2007, JAXA performed the first lunar orbit injection maneuver (LOI1) for the Kaguya (SELENE) spacecraft (insertion into a large elliptical orbit circulating the moon after passing the phasing orbit rounding the Earth with 2.5 times).
• On Sept. 29, the HDTV camera onboard Kaguya took imagery of the rising Earth from a distance of about 110,000 km from Earth in its cruise phase to the moon (Figure 17). The imagery was acquired at the JAXA Usuda Deep Space Center, and processed at NHK (Japan Broadcasting Corporation). 29)
Figure 17: An Earth-rise image - showing the west coast line of the South American Continent - taken by the HDTV camera on Sept. 29, 2007 (image credit: JAXA)
Figure 18: The projected mission schedule after launch of the SELENE spacecraft (image credit: JAXA, Ref. 25)
Sensor complement: [XRS, GRS, LISM (TC, MI, SP), LRS, LALT, UPI, LMAG, CPS, MAP-PACE (ESA-S1, -S2, IMA, IEA)]
SELENE carries a total of 14 instruments on the Main Orbiter and its subsatellites. These observations will provide a most comprehensive data set on the geophysical and geochemical parameters of the moon and will be used for studying the lunar origin and evolution, and future utilization of the moon. 30) 31) 32)
Figure 19: SELENE configuration and some instrument allocations (image credit: JAXA)
Table 8: Overview of the SELENE science instruments, grouped thematically, with some performance characteristics
XRS (X-Ray Spectrometer):
The scientific objectives of XRS are to provide the following observations:
1) Global mapping of major elements of lunar surface materials except for polar regions through remote XRF (X-Ray Fluorescence) spectrometry during day time observation
2) Understanding of the physical processes of lunar X-ray illumination in the night time that happens by impact of solar wind particles and comic rays as well as natural radioactivity in the uppermost layer of lunar surface
3) Regional variation of surface microscopic roughness as the results of particle size effect on XRF intensities.
The instrument is of XRS heritage flown on the Hayabusa/MUSES‐C spacecraft (launch May 9, 2003). The XRS is using an X-ray CCD detector array composed composed of three detectors: XRF-A, SOL-B, and SOL-C. The footprint of the XRS image is 20 km x 20 km. 33) 34) 35)
• XRF-A is the main detector with 16 X-ray CCDs (1 k x 1 k pixels). The objective is X-ray detection from the lunar surface. The total detection area of XRF-A is about 100 cm2 with a FOV (Field of View) of 12º. A beryllium foil of 5 µm in thickness is attached to avoid detection of visible light. The 80 cm2 radiators atop the XRF-A and SOL-BC are to keep the CCD chips sufficiently cool with passive radiation. For the control and onboard data handling, the 60 MHz and 32 bit fast RISC-type onboard computer (SH-OBC) is installed with the majority voting technique to improve radiation tolerance.
• SOL-B is a solar X-ray monitor using a PIN photo-diode detector instead of a CCD detector. SOL-B observes X-rays from the sun direction; it does not require the wide effective area as an X-ray CCD detector.
• SOL-C observes X-rays for the standard sample on SELENE. The elemental composition of the standard sample is determined to perform comparative X-ray fluorescence analysis.
Figure 20: Photos of the XRF-A (upper) and SOL-BC (lower) instruments (image credit: JAXA)
Table 9: Specification of the XRS instrument
XRS instrument initial checkout: All the functions of the XRS instrument showed good performance. However, some substantial degradation of the CCD detector performance in XRF-A must have occurred. The CCD detector is experiencing a larger numbers of improper events than expected. A possible explanation is that they occurred from deficiencies of damaged pixels due increased irradiation by high-energy charged particles. During the transfer phase (3 transfers through the terrestrial radiation belts) the XRS must have experienced more severe radiation conditions than designed for. 36) 37)
The XRS is now (2008) operated in the reduced mode, in which limited numbers of CCD chips are driven to read the data. This means the effective detection area becomes a quarter of original one, but the signal to backgrounds ratio becomes good enough for observation due to shorter integration time. Thus, the dark current level is much reduced below the event threshold level. Further optimization of the CCD operation is necessary to achieve full performance in the lunar orbit.
Nonetheless, X-ray data from the lunar surface and from the onboard standard sample were successfully obtained. The X-rays off the standard sample detected by SOL-C (calibrator) provide the X-ray spectrum with Mg-, Al-, and Si-Kα, and in some instances with Ca- and Fe-Kα elements. The X-ray flux is in fact in good agreement with the enhanced solar activity data of the GOES X-ray monitor. - The XRS elemental mapping observation is highly dependent on the solar activity, which has been at a solar minimum in 2007. Hence, the X-ray activity has still been at a very low level (under A0-Level) during most of the operational period so far. The quantitative elemental composition of XRS is still to be obtained.
GRS (Gamma-Ray Spectrometer):
The goal is the global mapping of major element distributions (study of the origin, evolution and structure of the moon), such as Mg, Al, Si, Fe, and natural radioactive elements like K, U, and Th, using a highly pure n-type Ge detector.
GRS consists of three subsystems: GRD (Gamma-Ray Detector), CDU (Cooler Driving Unit), and the GPE (Gamma-ray and Particle detector Electronics). The GRD is mounted on the lunar-facing side of the mission module. The GRS instrument observes the spectrum of lunar gamma rays from 0.1 to 12 MeV providing an energy resolution of 3 keV @ 1.33 MeV. The spatial resolution is 120 km.
A Stirling cryocooler (with vibration suppression) is being used to cool the detector to 80-90 K. The Ge detector has a volume of 252 cm3 and is hermetically encapsulated in a high vacuum-tight Al canister. To increase the sensitivity of GRS, it is essential to reduce the gamma-ray background. The major background components are cosmic ray particles entering the detectors, produced particles due to the primary and secondary cosmic ray interactions with materials of the spacecraft, and scattered gamma-rays produced in planetary surface and the detector itself. For the reduction of this background, the SELENE GRS employs BGO (Bismuth Germinate Crystals - Bi4Ge3O12) and plastic scintillators as an active shield. 38)
Figure 21: Schematic view of the GRS instrument (image credit: JAXA)
GRS instrument initial checkout: The spectrum of lunar gamma rays was accumulated for 1027 hours from Dec. 14, 2007 to Feb. 17, 2008. During this initial observation period, no solar proton events which might have caused large increases in the counting rates and distortions of the spectra, have been observed. 39)
LISM (Lunar Imager/SpectroMeter):
LISM consists of three subsystems: the Terrain Camera (TC) the Multiband Imager (MI), and the Spectral Profiler (SP). The subsystems share some components and electronics including the DPU (Data Processing Unit), DPCU (Data Processing and Control Unit), and PCDU (Power Control and Distribution Unit). 40)
• TC (Terrain Camera): TC is a two-line stereo pushbroom camera assembly (forward and aft looking units) with a spatial resolution of 10 m. The main objective of TC is stereo mapping of the moon with high spatial resolutions. TC consists of two slant telescopes, each of which has a linear CCD array detector, and takes images in pushbroom scanning mode. The swath of TC is 35 km from an orbital altitude of 100 km, so that images taken at serial paths are overlapped in the across-track direction. The DCT lossy compression method has been adopted to reduce the large source data volume. The data will be nominally compressed to 30% or less of the original volume. 41)
Table 10: Specification of the TC instrument
Note: The SP instrument will be operated during lunar daytime. The total amount of data of TC and MI is so large that TC and MI are not operated at the same time. Still, operations of TC and MI should be carried out in limited daytime. To identify where SP is observing, however, highly compressed TC images or one band images of MI are planned to be taken simultaneously with SP data.
Figure 22: Observation configuration of the TC instrument (image credit: JAXA)
The TC assembly provides: 1) global/local high-contrast mosaicked maps, and 2) DTMs of the moon's entirety with relative height resolution of a few tens of meters or better, and ultimately a DEM with absolute height information.
LISM-TC instrument checkout: The first checkout of TC took place on Nov. 3, 2007, sharing time with MI's checkout in three revolutions along longitudes of about 60ºE on the lunar night side and about 240ºE on the lunar day side as planned. The TC first image data demonstrated high resolution and high signal-to-noise ratio, as had been expected. Digital terrain models (DTMs) have been produced from the TC stereo-pair data. These data are considered fundamental for lunar science. 42)
• MI (Multiband Imager): MI observes in the spectral ranges of VIS/NIR (0.4 to 1.6 µm) in 9 bands with a spectral resolution of 20-50 nm and a spatial resolution of 20 m. MI consists of two telescopes, each of which has a 2D detector with bandpass filters. Data of several lines of the detectors, each of which correspond to band assignments, are read out. MI takes imagery by pushbroom scanning. The swath of MI is 20 km. A DPCM loss-less compression method is adopted to avoid irreparable loss of information about subtle differences in reflectance. 43) 44)
Table 11: Specification of the MI instrument
MI objectives: One of the important scientific goals of MI is to investigate small but scientifically very important areas such as crater central peaks and crater walls. Investigations of such small areas will help answer current questions such as the existence, chemical composition, and source of olivine at the central peaks of some craters. Topographic effects can be removed with MI which cause false reflectance values seen in the crater wall and crater central peak, by photometric correction with detailed topography. A digital terrain model will be derived from TC stereoscopic images, or MI band sets, that have 11.2º maximum parallax.
Another important objective of MI is to search for the most primitive lunar crustal materials such as magnesia anorthosites suggested to be located on the lunar far side from recent lunar meteorite studies.
LISM-MI instrument checkout: On Nov. 3, 2007, MI successfully provided the first lunar images using two orbits during SELENE's checkout period. To check the LISM-MI hardware functions, all possible observation parameters, such as exposure, compression table and nominal/SP support mode, were used and were confirmed to be normal. During the day of the first checkout, MI took more than 3500 images (in nine bands) of the lunar surface. 45)
The first nominal observation cycle of LISM-MI was started on 18 Jan. 18, 2008. The high lunar latitude region (60º - 90º) was observed in the nominal mode (loss-less compression and nine full bands). The second nominal cycle (in the latitude region of 30º-60º) was completed in March 2008. The third nominal observation cycle (in the latitude region of 0º-30º) was underway in April 2008. The imagery of all cycles (total of 6) will be combined for a moon surface portrait.
• SP (Spectral Profiler). The objective is to provide a mineralogical distribution on the moon's surface. Continuous spectral profiling in the range from 0.5 to 2.6 µm with a spectral resolution 6 to 8 nm and a spatial resolution of 500 m. The instrument consists of one optics element with three detectors for three ranges (VIS: 500 - 1000 nm, NIR1: 900 - 1700 nm, and NIR2: 1700 - 2600 nm). NIR2 is cooled to 220 K using a Peltier cryocooler. Periodic onboard calibration with halogen lamps is performed to obtain a high radiometric accuracy of the imagery. 46)
Table 12: Specification of the SP instrument
Figure 23: Schematic view of the SP instrument (image credit: JAXA)
LISM-SP instrument checkout: The initial functional check of the SP on November 3, 2007, successfully produced a series of spectra along a strip longer than 1,000 km on the far side of the moon.
Continuous visible and near infrared reflectance spectra are to be used to precisely determine the type and breakdown of minerals on the lunar surface. In addition, comprehensive information on the lunar surface material can be obtained by combining data from the Spectral Profiler with that taken with the Multi-band Imager (MI), which measures detailed spatial distribution of minerals, as well as that from the X-ray Spectrometer (XRS) and Gamma-ray Spectrometer (GRS), which measure spatial distribution of elements.
LRS (Lunar Radar Sounder):
The objective of LRS is to provide sounding of the subsurface structures of the moon to a depth of a few km (study of the tectonic evolution of the subsurface structure). LRS is using the FM/CW (Frequency Modulated/Continuous Wave) radar technique with a transmission power of 800 W. The transmission pulse frequency is swept linearly in the range from 4 to 6 MHz. The system consists of the frequency synthesizer, the sounder, power amplifier, the antenna system (two sets of dipole antennas with a tip-to-tip length of 30 m), the sounder receiver, the passive mode receiver, the digital signal processor, and the central processing unit, and the power supply unit (Figure 24). The pulse is transmitted through a set of two dipole antennas extending from opposite sides of the spacecraft. Returns from the ground are received by a second set of dipole antennas, identical but oriented orthogonally to the first set. 47) 48) 49)
In addition to the radar experiment, LRS provides an observation of natural radio and plasma waves in the frequency range from 10 Hz to 30 MHz. The radar echo signals and planetary radio waves are detected by the antennas and fed into the pre-amplifier units. The receiver system of LRS is also very sensitive to noise, not only to signals. The electromagnetic noise radiated from other components onboard the SELENE can be detected by the LRS antenna system. The potential difference between the SELENE body panels, which is produced by the unbalanced cable currents between components on the panels, can be noise for the LRS pre-amplifier units. The pre-amplifier input level of the sounder echo signal from a subsurface structure 6 km below the surface is estimated to be 0.3 µV, assuming transmitting power of 800 W, a spacecraft altitude of 100 km, and loss tangent value "tan delta" of 0.06.
Figure 24: Block diagram of the LRS system (image credit: JAXA)
Table 13: System parameters of the LRS instrument
LRS instrument checkout: Initial results on the lunar subsurface structure were obtained using the LRS sounder mode observation data collected on November 20 and 21, 2007. The received radar echo was as expected through computer simulation. The received strength of the echo signal is consistent. By performing the Fourier analysis on the echo waveform, the project team was able to obtain the sharp echo pulse from the surface and subsurface boundaries. Hence, the extraction of radar echoes reflected by subsurface structures was demonstrated to be satisfactory. 50)
LALT (Laser Altimeter):
LALT was developed at NEC Corporation, Tokyo, Japan. The objective of LALT is to measure the vertical distance between the Main Orbiter and the subsatellite point on the lunar surface to obtain the lunar topography. The instrument consists of two elements: LALT-TR (Transmitter) and LALT-E (Electronics). LALT-TR contains the laser transmit units, the optical system including two telescopes, the thermal control system, and a high voltage unit, whose size is 360 mm x 450 mm x 408 mm. LALT-E is comprised of a low voltage unit, an onboard computer for telemetry and command management, and an interface control unit between LALT and the Main Orbiter. The size of LALT-E is 241 mm x 301 mm x 88 mm. The total mass of LALT is 19.1 kg.
The structure of LALT is of a double-decker type (Figure 25). On the first floor of LALT, there are laser diode driver, high voltage unit, Q-switch driver and LALT control unit in the anti-clockwise direction. The second floor of LALT-TR without cover contains the following components: laser collimator, receiving telescope, and mirror are seen. The laser pulse detector and processing unit (yellow box) is attached to the end of receiving telescope. 51) 52) 53) 54) 55)
Table 14: Specification of the LALT instrument
Figure 25: Schematic view of the first (right) and second floor of LALT (left), image credit: NEC)
LALT instrument checkout: The first ranging experiment was carried out successfully on November 25, 2007. Nominal observations of LALT started on December 30, 2007. 56)
UPI (Upper atmosphere and Plasma Imager):
The near-Earth environment can be observed at high quality from lunar orbit due to the absence of a lunar atmosphere. Moreover, Earth may be observed from many different directions since the moon orbits Earth once a month. Hence, two telescopes are part of the payload to observe Earth in the visible and the extreme ultraviolet (EUV) light. The EUV telescope is called TEX (Telescope EUV Experiment) while the VIS telescope is named TVIS (Telescope Visible light experiment). The goal is the observation of the Earth's magnetosphere and aurora from the lunar orbit. 57)
• The UPI-TEX imager detects the O II (83.4 nm) and He II (30.4 nm) emission distributions scattered by ionized oxygen and helium, respectively. The targets of EUV imaging are the polar ionosphere, the polar wind, and the plasmasphere and the inner magnetosphere. The maximum spatial and time resolutions are 0.09 Re and 1 minute, respectively. -- UPI-TEX observes the resonance scattering emissions of oxygen ion (O II: 83.4 nm) and helium ion (He II: 30.4 nm) to take images of near-Earth plasmas.
• The UPI-TVIS imager detects the four emission lines (427.8, 557.7, 589.3, 630.0 nm) to simultaneously take auroral images around both of Earth's polar regions.
UPI has a gimbal system (UPI-G) to follow the Earth from the moon. UPI-G has two-axis control; one axis is parallel to the rotating axis and always moves during observational periods. The other is the perpendicular, and revolves by 360º every a month. The UPI pointing accuracy of the equatorial mounting is 3.24 arcseconds around the azimuthal axis and 2.68 arcseconds about the elevation axis. These correspond to the spatial resolution of 5 km on the Earth's surface. 58) 59)
Figure 26: UPI observation scheme of the Earth's magnetosphere from lunar orbit (image credit: JAXA)
The telescope in the visible range (TVIS) is a catadioptric system with an aperture of 136 mm in diameter, a focal length of 320 mm, and a FOV of 2.4º. It uses a CCD device to detect the photons. It has a filter turret composed of one shutter and five filters with bandpasses at 427.8 nm, 557.7 nm, 589.3 nm, 630.0 nm, and above 730 nm. The main science targets are the simultaneous observation of the aurora at both polar regions, and the atmospheric airglow in the equatorial region.
The TEX imager is a type of normal-incidence telescope. It uses a multilayer-coated mirror, a bandpass filter - parted into two regions for He+ and O+ emissions - and MCP (Microchannel Plate) detectors. TEX has an aperture of 12 cm diameter, a focal length of 168 mm, and a FOV of 10º. The main targets are the global distribution of the plasmaspheric He+ ions and the overall picture of O+ ions escape from the polar ionospheres.
Table 15: Specification of the UPI device
Figure 27: Schematic view of the UPI instrument (image credit: JAXA)
Figure 28: Schematic view of the TEX telescope components (image credit: JAXA)
UPI-TEX initial checkout: The project team was able to image near-Earth cold plasmas at O II (83.4 nm) and He II (30.4 nm) emissions with the UPI-TEX instrument. TEX has enough performance to detect the oxygen ion outflow, the transport route from the polar ionosphere into the magnetosphere, and the plasmasphere. 60)
LMAG (Lunar Magnetometer):
LMAG is a 3-axis fluxgate magnetometer, referred to as MGF-S (Magnetic Fluxgate Sensor), designed to measure the weak lunar magnetic field with a resolution of 0.5 nT. LMAG is mounted on the end of a 12 meter boom which extends from the top of the mission module. The sample rate of LMAG is 32 Hz. The LMAG data is quantized to 16 bit.
Table 16: Specification of the MGF-S device
Since there is no known magnetic field in the lunar orbit, LMAG is furnished with SAM-C (Sensor Alignment Monitor Coil) to generate precisely known magnetic fields. Two linearly independent fields can be generated by a bi-axis coil system of SAM-C. SAM-C is installed at the mounting of the mast, and it produces a magnetic field of about 2 nT at the position of the MGF-S with an electric current of 2 A. The spatial distributions of the SAM-C generated magnetic fields have been determined by ground experiments.
The alignment of MGF-S is monitored by measuring a known magnetic field. As there is no well-known ambient natural magnetic field available in the lunar environment, known magnetic fields are applied to MGF-S by the SAM-C system on board SELENE for in-orbit calibrations.
SAM-C has two set of coil systems, SAM-C (A) and SAM-C (B), to generate two distributions of linearly independent magnetic fields which are used to determine the position of the magnetometer, about 12 m away from the main body of the satellite, and three rotation angles. In total six unknowns can be determined simultaneously using the two independent SAM-C magnetic field vector distributions. Multiple measurements in a set of calibration reduce the error of determination of the position and rotation angles. SAM-C generates a 1 Hz triangular wave, which continues with the same amplitude for 10 seconds and then decays in 8 seconds.
LMAG initial checkout: In-flight calibrations of MGF-S alignment were made intensively during the check-out phase and several times afterwards. No clear temperature dependencies of the alignment of MGF-S are seen. The variations of the angles are well within 0.6º, which implies that the alignment of MGF-S has been stable in the orbit around the moon.
The MGF-S observation data are being used in the analysis to obtain magnetic anomaly maps of the moon and to detect an induction signature of the moon. 61)
CPS (Charged Particle Spectrometer):
The objective is to measure the energy spectrum of charged particles, including alpha particles, electrons and protons, over a wide range from lunar orbit. The instrument has two components, ARD (Alpha Ray Detector) and PS (Plasma Sensor), which pick up charged particles in the near-lunar environment. Alpha particles can arise from the decay of radioactive radon and polonium in the lunar surface. 62) 63) 64)
The objective of ARD is to detect alpha rays emitted by Rn and Po on the lunar surface for identification of gas emanation and obtaining information on the crustal movement during the last - 50 years.
The PS will observe solar and galactic cosmic rays around the moon. The objective is to protect human health in space travel from radiation particles and to obtain basic cosmic ray data around moon to be able to forecast the cosmic ray radiation distribution in the space.
The ARD and the PS instruments consist of Si semiconductor detectors with high energy resolution. An incident particle is identified by the method of ΔE/E using the information of energy deposited in the multilayer Si detectors, respectively.
The detector assembly consists of five silicon CCD sensors:
- HID (High-energy Isotope Detector)
- LPD-HE (Low-energy Particle Detector for Heavy Ions)
- LPD-p (Low-energy Particle Detector for protons)
- LPD-e (Low-energy Particle Detector for electrons)
- Alpha particle spectrometer covering the energy range of 4-6.5 MeV.
The HID and LPD units are silicon semiconductor telescopes consisting of layers of boron-doped silicon-lithium stuck detectors with diameters of 65 mm and thicknesses of 2, 3 or 6 mm. For the HID and LPD-HE, the top two layers are position sensitive detectors (PSDs) which are used to determine the trajectory and energy loss of the particle. Under the PSDs are layers of detectors (five layers for the HID, four for the LPD-HE) which get thicker towards the bottom. These are underlain by a silicon anti-coincidence detector of the same design. The HID covers isotopes in the energy range 18 - 470 MeV/n (Be to Xe) with a resolution of better than 0.35 amu at Fe and a geometric factor of 50 cm2 steradians (sr). The LPD-HE covers ions from He to FE, 3 MeV/n to 105 MeV/n, with a resolution of 0.4 amu at C and a geometric factor of 6 cm2 sr.
The LPD-p and LPD-e detectors have no PSDs. The LPD-p has four layers of detectors and covers the energy range 1 MeV/n to 50 MeV/n (protons to He) with a resolution of 200 keV and a geometric factor of 1. The LPD-e has one layer and is overlain with aluminum foil a few tens of microns thick to reject low energy ions. It covers electrons in the energy range 0.3 to 0.7 MeV/n with a resolution better than 20 keV and a geometric factor of 0.3.
The HID and three LPD detectors are housed in a box mounted on the edge of the nadir-facing side of the Main Orbiter along with three electronics boards holding a 32 channel pulse shaper, peak-hold systems and a data processing unit.
The objective of the alpha particle spectrometer is to search for alpha particles which are the result of radon gas emanation at the lunar surface. The instrument is mounted on the nadir facing side of the mission module, is 41 cm x 37 cm x 13 cm, has a mass of 3.65 kg and uses 3.4 W. The detector is an array of 48 silicon solid-state detector sensor chips. Each chip is 2.6 cm x 2.6 cm and has a thickness of 100 µm. A 450 µm thick anti-coincidence detector is below the chips. Collimators limit the field of view to 40º. The total sensitive area is 326 cm2. The energy resolution is 100 keV.
Figure 29: Photo of the PS device (image credit: JAXA)
Figure 30: Photo of the ARD assembly (image credit: JAXA)
MAP-PACE (Magnetic field Plasma experiment- Plasma energy Angle and Composition Experiment):
Low energy charged particles around the moon were vigorously observed by moon orbiting satellites and plasma instrumentation placed on the lunar surface in 1960s and 1970s. Many new discoveries concerning the lunar plasma environment were made during the period. Since that time, there has been almost no new information about the low energy charged particles around the moon. 65) 66) 67)
The overall objectives of MAP-PACE are to study:
• The solar wind interaction with the moon and the structure of the lunar wake and the behavior of plasma near the limb of the moon is a prime area of study. With almost no intrinsic magnetic field around the moon, there is actually no well-defined bow shock as can be found in the terrestrial magnetosphere.
• The moon - Earth's magnetosphere interaction
• Plasma observations of the Earth's magnetotail
• Electron reflectometer observations. If the moon has a remnant magnetic field, then the electrons moving with large angle around the ambient magnetic field will be mirror reflected back to the satellite. Measuring the pitch angle distribution of the reflected electrons, the remnant magnetic field on the lunar surface can be deduced.
MAP-PACE consists of 4 sensors: ESA (Electron Spectrum Analyzer)-S1, ESA-S2, IMA (Ion Mass Analyzer), and IEA (Ion Energy Analyzer). ESA-S1 and -S2 measure the three-dimensional distribution function of low energy electrons below 15 keV, while IMA and IEA measure the three-dimensional distribution function of low energy ions below 28 keV/q.
Specific scientific objectives of MAP-PACE are to:
4) Measure the ions sputtered from the lunar surface and the lunar atmosphere
5) Measure the magnetic anomaly on the lunar surface using two ESAs and a magnetometer (LMAG) onboard SELENE simultaneously as an electron reflectometer
6) Resolve the moon-solar wind interaction
7) Resolve the moon - Earth's magnetosphere interaction
8) Observe the Earth's magnetotail.
Since the start of continuous operation of MAP-PACE on 14 December 2007, MAP-PACE has been observing plasma around the moon. The electron sensors (ESA-S1 and S2) have been measuring solar wind electrons, electrons in the wake region of the moon and electrons in the Earth's magnetosphere. The IEA (Ion Energy Analyzer) has been measuring solar wind ions and ions in the Earth's magnetosphere. Though the operation period of IMA as a mass spectrometer is limited to several hours/day (during the commissioning period), IMA has already discovered the existence of alkali ions coming from the lunar surface or lunar atmosphere.
The MAP-PACE sensors revealed the low energy ion distribution in the dayside of the Moon for the first time. MAP also discovered new features of low energy ion intrusion into the lunar wake region. Mapping of the lunar magnetic anomalies from multiple altitudes was also realized. By analyzing all the data obtained by MAP during the mission life of KAGUYA, more information about the lunar plasma/magnetic field environment is expected to be obtained in the near future. 68)
Figure 31: Summary of low energy ions around the Moon (day side), image credit: JAXA
• ESA-S1 and ESA-S2 (Electron Spectrum Analyzer):
The ESA sensor basically employs a method of a top hat electrostatic analyzer placing angular scanning deflectors at the entrance and toroidal electrodes inside (Figure 33). The FOV is scanned between ±45º about the center of FOV which is 45º inclined from the axis of symmetry. A 3D electron distribution is observed with two identical ESA sensors that are placed in the +Z and -Z surface of the spacecraft.
Table 17: Specification of ESA instrument parameters
Figure 32: Illustration of the MAP-PACE sensors (image credit: JAXA)
Figure 33: ESA-S1 and ESA-S2 measurement concept (image credit: JAXA)
• IMA (Ion Mass Analyzer), and IEA (Ion Energy Analyzer):
The IMA device is comprised of the energy analyzer (similar to ESA) and the LEF (Linear Electric Field), TOF (Time Of Flight) ion mass analyzer.
The IEA device consists of only an energy analyzer that is the same as the energy analyzer of IMA (Figure 35). The upper and lower angular deflectors of the energy analyzer are provided with high voltage which are swept between 0 V and +5 kV. The inner toroidal electrode is also provided with high voltage swept between 0 V and -4 kV simultaneously with the angular scanning deflectors. Since the flux of the solar wind ions and the lunar-origin ions differs significantly, the sensitivity of the energy analyzer can be reduced electrically to about 1/10 in case of the solar wind ion observation. The ions transmitted through the energy analyzer are post-accelerated and enter into the LEF TOF mass analyzer part.
Table 18: Specification of IMA and IEA devices
Figure 34: Schematic illustration of the IMA device (image credit: JAXA)
Figure 35: Schematic view of the IEA device (image credit: JAXA)
During the Kaguya commissioning period, IMA discovered alkali ions generated in the lunar surface or lunar atmosphere. MAP-PACE ion sensors also discovered the existence of "reflected" solar wind ions -- this is contrary to general expectations based on the assumption that the solar wind ions colliding with the moon should be absorbed by the lunar surface (Ref. 67).
HDTV (High Definition Television) camera:
In addition to the nominal sensor complement, Kaguya is also furnished with a HDTV camera developed by NHK (Japan Broadcasting Corporation). The hardware consists of a telephoto and a wide-angle HDTV color camera using three CCD detectors of size: 2.2 Mpixel. 69)
Table 19: HDTV camera parameters
The camera mounted to the moon side of the explorer, it is capable of shooting imagery of Earth rise. A one minute video scene is compressed, stored, and transmitted to Earth in a period of 20 minutes.
Figure 36: Block diagram of the HDTV camera (image credit: JAXA)
Figure 37: Photo of the HDTV camera (image credit: JAXA)
Subsatellites: Rstar (Okina), Vstar (Ouna)
Both Rstar (also referred to as RSAT) and Vstar are spin-stabilized (10 rpm) microsatellites with octagonal main bodies (Figure 39). An S- and X-band coaxial vertical dipole antenna (S/X-ANT) on each subsatellite links signals toward the tracking and VLBI stations on the ground with toroidal beams of 32º width. Hence, the communications link of S/X-ANT requires an attitude stability to keep the angle between the normal of the dipole antenna axis and the direction of the ground station to be < 16º. The S-band omnidirectional patch antenna on Rstar is being used to communicate with the main orbiter, Kaguya (conical beams of 140º). Two antenna pairs for transmission and receiving are mounted on both upper (R-ANT1 and T-ANT1) and lower decks (R-ANT2 and T-ANT2) in axial symmetry. 70) 71) 72)
The subsatellites Rstar and Vstar contain the conventional subsystems like EPS (Electric Power Subsystem), TCS (Thermal Control Subsystem), and integration hardware and structure subsystems including the release mechanism depicted in Figure 40.
Each microsatellite has a size of 1 m x 1 m x 0.65 m, a mass of 45 kg, and electric power of 70 W provided by surface-mounted solar panels (highly efficient silicon solar cells). In addition, there is a NiMH battery of 13 Ah capacity. Neither of the subsatellites has thrusters to control orbits and attitudes, which yields precise measurements of orbital perturbations and enlightens the mass of the satellite system. The initial attitude conditions are determined by the performance of the release mechanism; the nutation generated by the separation is removed by a nutation damper. In addition, flare structures are used to maintain the attitude of each subsatellite against solar radiation pressure. Both Rstar and Vstar will be released with the spin-axis angle perpendicular to the plane of moon's path.
Figure 38: Photo of the RSAT microsatellite (image credit: JAXA)
Figure 39: Two views of Rstar (image credit: JAXA)
Figure 40: Schematic block diagram of the Rstar and Vstar bus elements/equipments (image credit: JAXA)
Legend to Figure 40: Asterisked (*) components are only implemented on Rstar. The umbilical cables were disconnected at the separation from Kaguya.
Lunar gravity field mapping:
The mission of the Rstar and Vstar subsatellites in their lunar orbits is dedicated to lunar gravity observations. The configuration selected employs four-way Doppler measurements between the Main Orbiter and the two subsatellites using RSAT (Relay Satellite Transponder) on Rstar and on the Main Orbiter. In parallel, differential VLBI observations will be performed for multi-frequency carrier waves emitted from differential VRAD (VLBI Radio Sources) on Rstar and Vstar. 73) 74) 75) 76)
The orbit of the Main Orbiter will be directly determined by the four-way Doppler measurements relayed as: ground - Rstar - Main Orbiter - Rstar - ground. The orbit of Rstar will be simultaneously observed by two-way RARR (Range and Range Rate).
Figure 41: Schematic diagrams of RSAT (left) and VRAD (right) experiments (image credit: JAXA)
Four-way Doppler measurement sequence: The RSAT-1 transponder (on Rstar) receives ranging signals in S-band, which are transmitted from the 64 m antenna at UDSC (Usuda Deep Space Center). Then, RSAT-1 returns the ranging signals to UDSC to conduct two-way RARR measurements. RSAT-1 also extracts carrier waves from the ranging signals and relays them toward the Main Orbiter. Signals, focused by the high gain antenna (HGA) on the Main Orbiter are acquired by a transponder in RSAT-2, and then returned to Rstar. RSAT-1 receives the signals from the Main Orbiter, converts the frequency into X-band, and relays them to UDSC. 77)
Differential VLBI observation sequence: The differential VRAD (VLBI Radio Sources) on Rstar and Vstar will accomplish multi-frequency differential VLBI observation. VRAD is composed of the radio sources on Rstar (VRAD-1), and those on Vstar (VRAD-2). Vstar will be separated from Main Orbiter and injected into the initial elliptical lunar orbit of 800-100 km in altitude. The radio sources of VRAD-1 and VRAD-2 will deliver carrier waves for differential VLBI observations of the ground stations. Each sub-satellite transmits three waves in S-band and one wave in X-band carrier signals to conduct multi-frequency VLBI methods. The distribution of S and X-band frequencies is chosen to calibrate the delay by the terrestrial ionosphere and to solve uncertainties of the phase delay which exceeds one wave length. The VLBI ground stations in this scenario are: Mizusawa, Iriki, Ishigaki-jima, and Ogasawara in Japan, Shanghai and Urumqi in China, Hobart in Australia, and Wettzell in Germany. 78)
Note: Rstar, the data relay microsatellite, is also referred to as Okina. Okina is in an elliptical lunar orbit with an apolune of ~2400 km and a perilune ~100 km.
Vstar, the gravity-field measurement microsatellite, is also referred to as Ouna. Ouna is in a near-circular lunar orbit with a mean altitude of ~100 km and an inclination of 90º.
Figure 42: Doppler measurements in SELENE: a) 4-way Doppler for Main satellite via Rstar, b) 2-way Doppler for Rstar and Vstar (image credit: JAXA)
Table 20: Periods of on-orbit events of Rstar and Vstar (Ref. 71)
Table 21: Initial elliptical lunar orbit of Rstar and Vstar (Ref. 71)
The communication subsystem as well as the other subsystems and sensors onboard Rstar and Vstar were functioning nominally after the initial checkout and initial observation periods. Consequently, it is expected that selenodetic measurements supported by the subsatellites and Kaguya can provide the gravity field data with enough accuracy for the scientific evaluations.
Differential VLBI is being used to accurately measure the trajectories of the satellites, both with the Japanese network VERA (VLBI Exploration of Radio Astrometry) and an international VLBI network. New technologies, such as the multi-frequency VLBI method, the same beam VLBI method, and a new method of measuring the phase characteristics of an antenna, have been developed during the course of the design and development phase of Kaguya. They have provided significant improvements to the measurement accuracy and the success of the VRAD experiment.
VLBI observations are being carried out by the Japanese domestic VERA network. In addition, 2-way and 4-way Doppler measurements will be conducted by the Usuda Station of JAXA. Since the accuracy of VLBI positioning depends on baseline length, observations with long baselines are desirable. Currently the Shanghai, Urumqi (China), Hobart (Australia) and Wettzell (Germany) stations are also participating in the international observations to improve the accuracy.
VRAD (differential VLBI RADio) sources are installed onboard the two sub satellites, Okina and Ouna. They are being used for differential VLBI observations of the trajectories of the subsatellites with the Japanese network VERA and the international VLBI network. The MFV (Multi-Frequency VLBI) method is being used to measure the angular distances between the two radio sources on Okina and Ouna using three frequencies in S-band, 2,212, 2,218 and 2,287 MHz, and one in X-band at 8,456MHz.
Through cross correlation between the four sets of two carrier waves received at two ground stations, the MFV method produces 4 fringe phases for each integration time. Four differenced fringe phases are obtained by differencing the corresponding fringe phases for the two radio sources. The final observables are the phase delays in the carrier wave at X-band, which are obtained by resolving the cycle ambiguities step-by-step from the lowest frequency in S-band to the one in X-band.
In summary, the researchers succeeded in making VLBI observations of Okina/Ouna with VERA and the international network, and confirmed that the receiving, the tracking, and the recording system work well, and that the signals from Okina/Ouna are normal. The project team succeeded in obtaining phase delays with an accuracy of several pico-seconds at S-band.
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The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: "Observation of the Earth and Its Environment: Survey of Missions and Sensors" (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (firstname.lastname@example.org).