Overview

“Kaguya” (SELENE) is a lunar exploration mission led by the Japan Aerospace Exploration Agency (JAXA), aimed at elucidating the origin and evolution of the Moon. It was launched on September 14, 2007, and concluded operations on June 10, 2009. This mission was designed to conduct detailed observations of the lunar surface, internal structure, magnetic and plasma environments, and gravity field. “Kaguya” was placed into lunar orbit and was equipped with various observation instruments.

“Kaguya” consists of a main satellite orbiting in a polar circular orbit at an altitude of approximately 100 km and two sub-satellites (“Okina” relay satellite and “Ouna” VRAD satellite) in higher elliptical orbits. The main satellite measures about 2.1m x 2.1m x 4.8m and weighs approximately 3 tons. At the end of the mission, it was deliberately impacted on the near side of the Moon on June 11, 2009.

Observation Instruments

X-Ray Fluorescence Spectrometer (XRS)

XRS is an instrument that observes fluorescent X-rays emitted from the lunar surface. This allows for the analysis of the elemental composition of the lunar surface. XRS is capable of detecting elements such as magnesium, aluminum, and silicon.

Gamma-Ray Spectrometer (GRS)

GRS is an instrument that observes gamma rays emitted from the lunar surface. GRS is employed a germanium semiconductor crystal cooled to below -180 degrees centigrade by a Stirling cryocooler. It can identify gamma rays of various energy levels originating from the lunar surface with high precision and measure the lunar abundance of more than ten elements, including radioactive elements such as uranium, thorium, and potassium.
By mapping the measurement data globally, we aim to shed light on the origin and evolution of the Moon. It will also be possible to investigate whether water exists in the lunar polar regions.

Lunar Imager,SpectroMeter (LISM)

LISM is an instrument that observe images and spectrum to understand structure and materials of planetary surface. It consists of three sensors: Terrain Camera (TC), Multi-band Imager (MI), and Spectral Profiler (SP).

Multiband Imager (MI)

MI is a camera that captures images of the lunar surface in multiple wavelength bands, ranging from visible light to near-infrared. MI are push-broom type imagers and continuously observe the lunar surface. It has two nadir-looking telescopes with two-dimensional detectors and bandpass filters to observe in nine different wavelengths.
This facilitates detailed analysis of the lunar mineral composition and geological structures.

Spectral Profiler (SP)

SP is a spectrometer observing the lunar surface below the spacecraft. Two diffraction gratings are used to disperse light in the visible to near-infrared range, reflected from the surface, into 296 wavelengths. This allows for detailed analysis of the lunar mineral composition.

Terrain Camera (TC)

TC is a camera that captures high-resolution images of the lunar surface. TC are push-broom type imagers and continuously observe the lunar surface. TC can also perform stereoscopic imaging to create three-dimensional topographic maps of the Moon. This enables to map the lunar terrain in detail.

Laser Altimeter (LALT)

LALT is a laser altimeter used to measure the altitude of the lunar surface. LALT determines the distance between the main satellite and the lunar surface by emitting a laser beam toward the Moon and measuring the time it takes for the reflected light to return. Since the main satellite orbits the Moon in a polar orbit, it acquires distance data covering the entire lunar surface, enabling the creation of a high-precision topographic map of the whole Moon.

Lunar Radar Sounder (LRS)

LRS is a radar instrument designed for sounding the surface and subsurface structures of the Moon. The LRS system transmits RF pulses with frequency of 5 MHz which is able to penetrate deep into the Moon’s subsurface region. When the radio waves meet with the discontinuity of subsurface material, it makes a subsurface echo signal. Observation of subsurface signal makes it possible to see subsurface structure of the Moon. Features such as the tilt of subsurface geological structures provide important clues for investigating the Moon’s origins, including its thermal history.

Lunar Magnetometer (LMAG)

LMAG is an instrument that observes the lunar magnetic field. The sensor is mounted at the tip of an ultra-lightweight mast extending 12 meters from the satellite body; designed to avoid interference from magnetic fields generated by the satellite’s own equipment, it is capable of detecting magnetic fields as weak as one-hundred-thousandth of the Earth’s magnetic field.
This helps elucidate the internal structure and origin of the lunar magnetic field.

Plasma energy Angle and Composition Experiment (PACE)

PACE is an instrument designed to observe the plasma environment around the Moon. It consists of two electron analyzers (ESA-S1 and ESA-S2) for measuring electrons, one ion analyzer (IEA-S) for measuring solar wind ions, and one ion mass analyzer (IMA-S) for measuring ions in the lunar vicinity. It is capable of measuring parameters such as the energy, density, velocity, temperature, and mass of electrons and ions.
These measurements allow for the elucidation of the factors determining the density and temperature of the plasma surrounding the Moon, while also enabling the investigation of the distribution of source materials—specifically alkali elements like sodium and potassium—on the lunar surface.

Charged Particle Spectrometer (CPS)

CPS is an instrument that observes high-energy particles around the Moon. The CPS consists of two sub-components, one is the Particle Spectroscopy (PS) and the other is Alpha Ray Detector (ARD).

Alpha Ray Detector (ARD)

ARD will detect alpha rays emitted by Rn and Po on the lunar surface for identification of gas emanation. By examining their distribution, it is possible to obtain information on the crustal movement during the past 50 years.

Particle Spectroscopy (PS)

PS will observe solar and galactic cosmic rays around moon. That is due to protecting human health in the space from radiation particles and to obtain basic cosmic rays data around moon for forecast of cosmic ray radiations in the space.

Radio Science (RS)

RS is an instrument used to precisely measure the lunar gravity field. This helps elucidate the internal structure and mass distribution of the Moon. RS measures the gravity field by utilizing the Doppler shift of radio waves.

Upper Atmosphere and Plasma Imager (UPI)

UPI is an instrument designed to observe Earth’s upper atmosphere and plasma from lunar orbit. It consists of two telescopes: the Extreme Ultraviolet Telescope (TEX), which observes the plasmasphere, and the Visible Light Telescope (TVIS), which observes the global distribution of auroras and airglow.

Extreme Ultraviolet Telescope (TEX)

TEX detects the resonance scattering emissions of oxygen ion and helium ion and observes Earth’s plasmasphere. The telescope employs a high-efficiency mirror and micro-channel plates with a resistive anode producing 128x128pixel images. The corresponding spatial resolution is 500km.
From TEX observations we will know the spatial plasma distribution around the Earth every 10 minutes. Especially, imaging of the entire globe by oxygen ion emission will promote the space plasma physics.

Visible Light Telescope (TVIS)

TVIS is equipped with a fast catadioptric optics and a high-sensitivity CCD to image swift aurora and dark airglow. TVIS has a field-of-view equivalent to the Earth disk seen from the Moon.Spatial resolution is about 30km on the Earth’s surface. Observation wavelengths can be changed by selecting filters.
Simultaneous imaging of the auroral ovals in both the Northern and Southern Hemispheres using TVIS enables a detailed, direct comparison of auroras at geomagnetically conjugate points. By airglow imaging global propagation of large-scale ionospheric disturbances will be clearly seen.

High-Definition Television Camera (HDTV)

HDTV transmits high-definition images of the Earth and lunar surface from lunar orbit. This hardware has a telephoto and a wide-angle HDTV color cameras used three 2.2M-pixel CCDs. This provides educational and public outreach opportunities by sharing these images alongside other scientific results.

Relay Satellite (RSAT) / VLBI Radio Source (VRAD)

RSAT is the radio relay function on the sub-satellite Rstar (Okina). While the main satellite is on the far side of the Moon, uplink radio wave from Usuda is relayed to the Main Orbiter via the relay satellite, which is returned to Usuda via relay satellite again. Then the Doppler frequency is measured at Usuda.
VRAD is an instrument mounted on the sub-satellite Vster (Ouna) that uses VLBI (Very Long Baseline Interferometry) to measure the position of the Moon with high precision. The orbit of the sub-satellite is precisely determined by tracking the radio source mounted on it using differential VLBI.
This enables Doppler measurements of the lunar far-side, making it possible to measure the distribution of gravity fields of the lunar far-side.

Achievements

The “Kaguya” mission has provided a wealth of important data on the origin and evolution of the Moon. Significant achievements include detailed topographic maps and mineral composition mapping of the lunar surface, elucidation of the lunar subsurface structure, and precise measurements of the lunar magnetic field and gravity field. These data have greatly contributed to understanding the formation processes and internal structure of the Moon.