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Space Topics: Kaguya (SELENE)

Kaguya's Science Instruments

First Kaguya Terrain Camera image of the Moon
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First Kaguya Terrain Camera image of the Moon
The Terrain Camera is the highest-resolution instrument on Japan's Kaguya lunar orbiter, capable of achieving 10-meter resolution on the surface of the Moon. The Terrain Camera actually consists of two cameras, one pointed forward and one pointed backward along Kaguya's orbit. Data from images of the same place gathered by the two cameras can be combined to create high-resolution, 3D views of local lunar topography. The image here was taken on the lunar farside of a region just 30 kilometers from the south pole. The enlarged version has a resolution of 20 meters per pixel, half the resolution of which the Terrain Camera is capable. Credit: JAXA / SELENE

The main Kaguya satellite is equipped with 11 science instruments, plus a high-definition television camera.  In addition, Kaguya deployed into lunar orbit two mini-satellites as part of a lunar gravity experiment.

Optical remote sensing instruments

Optical remote sensing instruments read electromagnetic radiation to determine the appearance, mineral composition, and elemental distribution of the surface of the Moon.  Kaguya's optical remote sensing instruments sample a broad range of the electromagnetic spectrum, from X-rays to infrared:

The X-ray Spectrometer (XRS) studies X-ray fluorescence from major elements in lunar surface rocks.  Incoming solar x-rays excite atoms in the surface materials to higher energy states.  When the atoms return to ground states, they emit x-rays at different wavelengths.  The wavelength of the emitted x-rays provides a "fingerprint" of which elements are present.  XRS will map the distribution of sodium, magnesium, aluminum, silicon, potassium, calcium, titanium, iron, thorium, and uranium, among other elements.  It can achieve resolutions better than 10 kilometers on the surface.

The Gamma Ray Spectrometer (GRS) studies gamma rays emitted from the Moon, which arise when galactic cosmic rays knock neutrons from atoms in the rock; the neutrons interact with other atoms, producing gamma rays.  Gamma rays are also produced by the decay of radioisotopes that naturally occur in the lunar crust.  Again, the specific wavelengths of the emitted gamma rays are diagnostic of which elements are present.  GRS will map the distribution of similar elements to XRS (though at a lower resolution of 50 kilometers), including magnesium, aluminum, silicon, potassium, calcium, titanium, iron, thorium, and uranium; in addition, it can detect lighter elements such as oxygen and hydrogen.  Hydrogen is particularly important in the search for water trapped within the lunar crust, a potential resource for future human missions to the Moon.

The Upper-atmosphere and Plasma Imager (UPI) is turned not on the Moon but on Earth.  Measuring radiation in ultraviolet and visible wavlengths, it will map Earth's aurora and nightside oxygen airglow, producing a global view of Earth's plasma weather every 10 minutes and showing the motion of large-scale ionospheric disturbances.

There are three instruments that measure reflected sunlight in the visible and near-infrared portions of the electromagnetic spectrum.  The highest-resolution of these is the Terrain Camera (TC), a broadband visible imager that takes black-and-white images of the lunar surface by looking in two directions simultaneously, forward and backward along Kaguya's orbit.  The images have an unprecedentedly high resolution of 10 meters per pixel.  The forward and backward images together make a stereo or 3D data set that will eventually cover the entire Moon.  The Multiband Imager (MI) is an imaging spectrometer with five visible and four near-infrared bands.  It will create a global reconnaissance map of lunar surface units at a resolution of 20 meters per pixel in the visible wavelengths and 62 meters per pixel in the near-infrared wavelengths.  This is a fivefold improvement over the spatial resolution of Clementine's ultraviolet and visible camera, which only had six visible and near-infrared bands.  The Spectral Profiler (SP) is a spectrometer that samples larger areas of the surface below the spacecraft (each sample covers an area about half a kilometer square) in 296 different wavelengths from visible through the near-infrared.  Together the Terrain Camera, Spectral Profiler, and Multiband Imager will provide a global data set permitting high-resolution geologic mapping of the Moon.

Earthset over the lunar south pole
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Earthset over the lunar south pole
The five images in this animation are cropped from frames of a high-definition movie captured by Kaguya on November 7, 2007 as it traveled over the south pole to the lunar farside. The spacecraft's motion makes Earth appear to set below the lunar limb. The individual frames were taken roughly 20 seconds apart. Credit: JAXA / NHK / animation by Emily Lakdawalla

In addition, there is a High Definition Television HDTV instrument provided to the mission by the Japan Broadcasting Corporation (NHK) to shoot high-definition video as Kaguya circles the Moon, primarily for public outreach purposes.  There are actually two cameras, one with a wide-angle view and one with a telephoto lens.  The HD movies are continuously transmitted to Earth within 20 minutes of their capture.  HD images provide a unique viewpoint on the lunar limb and will frequently capture beautiful movies of Earthrise and Earthset.

Active remote sensing instruments

Kaguya has two instruments that actively broadcast signals to the Moon and measure the reflected energy:

The Lunar Radar Sounder (LRS) transmits low-frequency (5-MHz) radio pulses toward the Moon, which can penetrate several kilometers into the lunar surface.  These pulses are reflected from layers in the subsurface, and the reflected signals picked up with two 30-meter-long antennas on the spacecraft.  Over time, LRS will build up a map of the structure of the top few kilometers of the lunar surface.

The Laser Altimeter (LALT) fires a high-powered laser at the surface and times the travel time of the reflected pulse in order to capture a precise topographic profile along the path of the spacecraft.  Data from the laser altimeter will eventually allow scientists to calculate a global topographic map of the Moon like the one that was produced by the Mars Orbiter Laser Altimeter experiment for Mars.  This data set is necessary for comparison to Kaguya's gravity data to determine the subsurface structure of the Moon.

Space environment sensors

Several experiments measure the plasma environment and magnetic field encountered by Kaguya as it flies around the Moon:

The Lunar Magnetometer (LMAG) will map the lunar magnetic field with high precision.

The Charged Particle Spectrometer (CPS) has two components, an alpha ray detector (ARD) and plasma instrument (PS), 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.  Measurement of all charged particles will inform Earth scientists about the radiation hazards presented to future human travelers to the Moon.

The Plasma energy Angle and Composition Experiment (PACE) measures the distributions and energies of electrons and ions using four detectors.  Many of these ions are "sputtered" from the lunar surface by incoming solar radiation, producing a tenuous atmosphere around the Moon.

Kaguya's Mini-satellites

Kaguya deploys its mini-satellites
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Kaguya deploys its mini-satellites
Views from a small onboard camera show the successful deployment of the lunar orbiter Kaguya's two mini-satellites. Each mini-satellite is an octagonal prism one meter in diameter with solar cells covering its 0.65-meter-long sides; the satellites will be used in a radio interferometry experiment to map the Moon's gravity. The first satellite, Okina (also known as the Relay satellite or Rstar), was released on October 9, 2007 into an elliptical orbit 100 by 2,400 kilomters in size (first two frames). The second satellite, Ouna (also known as the VRAD satellite or Vstar), was released on October 12, 2007 into an elliptical orbit 100 by 800 kilometers in size (last two frames). Okina and Ouna, meaning "honorable elderly man" and "honorable elderly woman," refer to the old couple who adopted the moon princess Kaguya in the Tale of the Bamboo Cutter. Credit: JAXA

Kaguya's two mini-satellites, Okina (also referred to as RSAT, RStar, or the Relay satellite) and Ouna (also referred to as VRAD, VStar, or Differential VLBI Radio Source) will permit Kaguya to perform a unique and detailed survey of the lunar gravity field.  The three spacecraft are in separate orbits, with Kaguya closest to the Moon, then Ouna (VRAD), then Okina (RSAT).  The gravity field will be mapped through precise tracking of signals broadcast from and between the three satellites by two different methods.  One method employs only Kaguya and the highest-altitude satellite, Okina (RSAT).  In "4-way Doppler tracking," a signal is broadcast from the Japanese radio antenna at Usuda to Okina (RSAT).  Okina (RSAT) returns this signal to Usuda and also relays the signal to Kaguya.  Kaguya returns the signal to Okina (RSAT), which relays it back to Usuda.  By timing the four-times-relayed signal and measuring its Doppler shift, scientists can map out how tiny perturbations in the lunar gravity field are affecting the orbits of the two satellites.

In addition, the lunar gravity field will also be mapped with Very Long Baseline Interferometry experiments performed between a global network of radio dishes and the two mini-satellites.  The network includes radio antennas located in Wettze, Germany; Urumqui and Shanghai, China; Iriki, Ishigaki, Mizusawa, and Ogasawara, Japan; and Hobart, Australia.  The same data set will also permit scientists to see if the signals from the lower-altitude satellite, Ouna (Vstar) are attenuated by passage through a lunar ionosphere, which has never been detected.