Selene / Kaguya

Selene, also known as Kaguya, was a Japanese lunar orbiter mission launched by the Japan Aerospace Exploration Agency (JAXA) in 2007. It was one of the most ambitious lunar exploration missions of its time, aimed at obtaining comprehensive data on the Moon’s origin, geological structure, topography, and gravitational field. The mission was officially named SELENE (SELenological and ENgineering Explorer), while its popular name Kaguya was derived from Princess Kaguya, the heroine of the famous Japanese folktale The Tale of the Bamboo Cutter.

Background and Objectives

The Selene/Kaguya mission was designed to conduct global and detailed scientific observations of the Moon, following earlier missions such as NASA’s Clementine (1994) and Lunar Prospector (1998). It represented Japan’s first full-scale lunar exploration programme and was intended to:

• Investigate the composition and structure of the Moon’s surface and interior.
• Study the distribution of minerals and chemical elements to understand the Moon’s formation and evolution.
• Analyse the gravitational and magnetic fields to reveal subsurface structures.
• Observe the lunar environment, including plasma, radiation, and the effects of solar wind.
• Map the topography of the entire lunar surface with high precision.

By combining scientific and engineering objectives, Kaguya also tested advanced space technologies that could support future lunar missions, including potential human exploration.

Launch and Mission Profile

The Selene spacecraft was launched on 14 September 2007 aboard an H-IIA rocket from the Tanegashima Space Centre in Japan. After a series of orbital adjustments, it entered lunar orbit on 3 October 2007.
The mission consisted of a main orbiter and two smaller subsatellites:

1. Okina (Relay Satellite) – maintained communication with Earth when the main orbiter was behind the Moon.
2. Ouna (VRAD Satellite) – used for measuring the Moon’s gravitational field through radio-wave observations.

The main orbiter operated in a polar orbit approximately 100 kilometres above the lunar surface, enabling comprehensive coverage of the Moon’s terrain.

Spacecraft Design and Instruments

The Selene/Kaguya spacecraft had a mass of about 3,000 kilograms, making it one of the largest lunar orbiters ever built. It carried 15 scientific instruments designed to study the Moon’s geology, magnetism, gravity, and surface composition.
Key instruments included:

• Lunar Imager/Spectrometer (MI, SP, XRS): For mineral and elemental mapping.
• Terrain Camera (TC): Produced high-resolution stereoscopic images of the lunar surface.
• Lunar Radar Sounder (LRS): Probed the Moon’s subsurface layers to reveal geological structures.
• Gamma-Ray Spectrometer (GRS): Detected surface elements such as uranium, thorium, and potassium.
• Laser Altimeter (LALT): Measured topography with unprecedented accuracy.
• Charged Particle Detectors and Plasma Analysers: Monitored the lunar environment and interaction with solar wind.
• High-Definition Television (HDTV) Cameras: Captured stunning visual imagery of the Moon and Earth, including the iconic “Earthrise” video in high definition.

Major Discoveries and Scientific Results

The Selene/Kaguya mission produced a wealth of scientific data, contributing significantly to lunar science.
1. Topographical Mapping: Kaguya generated the most accurate 3D global map of the Moon at the time, revealing precise measurements of crater depths, mountain heights, and basin structures.
2. Gravitational Anomalies: Data from the Okina and Ouna subsatellites allowed scientists to create a detailed gravity map, identifying mass concentrations (mascons) beneath major lunar basins. These findings helped explain the Moon’s internal composition and tectonic history.
3. Geological Insights: Spectral analysis confirmed variations in surface composition between the lunar highlands (rich in anorthosite) and mare regions (basaltic plains). The data supported theories that the Moon underwent global magmatic differentiation early in its history.
4. Magnetic Field Studies: Kaguya detected small, localised magnetic fields on the lunar surface, remnants of an ancient global magnetic field that existed billions of years ago.
5. Subsurface Structure: Using radar soundings, the mission provided evidence of buried lava flows and layered rock formations, giving insight into volcanic activity and crustal evolution.
6. Environmental Observations: Instruments studying plasma and radiation conditions helped to understand how the solar wind interacts with the Moon’s surface, an important factor for planning future lunar habitats.
7. Visual Achievements: The onboard HDTV cameras transmitted some of the most visually striking images of the Moon and Earth, including full-Earth views and shadow transitions, which inspired renewed global interest in lunar exploration.

End of Mission

After completing its primary scientific objectives, Selene/Kaguya was deliberately impacted onto the lunar surface on 10 June 2009, near Gillis crater, to prevent orbital debris and conserve communications resources. By the end of its mission, Kaguya had transmitted approximately 10 terabytes of data, making it one of the most productive lunar missions in history.

Significance and Contributions

The Selene/Kaguya mission represented a major milestone for Japan’s space programme and contributed significantly to international lunar research. Its findings:

• Provided the foundation for later missions such as NASA’s Lunar Reconnaissance Orbiter (LRO) and India’s Chandrayaan-1.
• Enhanced understanding of lunar geology and internal structure, helping refine models of the Moon’s formation—especially theories related to the giant impact hypothesis.
• Advanced lunar cartography and 3D modelling, essential for navigation and landing site selection in future lunar missions.
• Demonstrated international scientific collaboration, as data were shared with agencies including NASA and ESA.

Legacy and Future Influence

Kaguya’s success revitalised Japan’s commitment to deep-space exploration, paving the way for subsequent missions such as SELENE-2, SLIM (Smart Lander for Investigating Moon), and Lunar Polar Exploration (LUPEX) in collaboration with India.
Scientifically, it established Japan as a leading contributor to planetary science and reaffirmed the importance of multi-instrument, high-resolution observation in understanding celestial bodies.