Last week, we talked about U.S. space policy speeches, particularly related to crewed lunar missions. This week we have an uncrewed lunar mission from Japan – SELENE, also called Kaguya.

Japan’s space program may have fewer resources than other countries’ space programs, but they still manage to complete innovative missions despite their tight budgets. One such mission was the SELenological and ENgineering Explorer, or SELENE. The largest object put into orbit around the Moon since the Apollo missions in the late-1960s and 1970s, SELENE’s goal was to survey the Moon’s surface from a very low orbit inclined 90 degrees to the lunar equator. This orbit allowed the close study of the Moon but required more propellant due to the lunar mass concentrations skewing its orbit.

SELENE launched on September 14, 2007, on an H-IIA rocket from the Tanegashima Space Center in Japan and entered orbit around the Moon on October 4. Once in its high capture orbit, SELENE deployed two small satellites. The first was a relay satellite to ensure the main satellite could maintain communication with Earth while on the far side of the Moon and the other to perform Very Long Baseline Interferometry in cooperation with radio telescopes on Earth.

After the insertion of those two satellites, SELENE lowered itself into its operational orbit of 100 kilometers above the Moon, where it stayed for the next fifteen months until it was lowered into a 50-kilometer circular orbit. The orbit’s lowest point, the perilune, was further lowered to 30 kilometers two months later. Finally, after SELENE had exhausted most of its propellant and had a failing reaction wheel, the spacecraft was deorbited into the lunar surface on June 10th, 2009.

What did it do during those months and what have scientists learned by analyzing the data since? Lots! SELENE had fourteen instruments onboard and also conducted an investigation in conjunction with its two smallsats.

SELENE’s primary objective was to measure the Moon’s gravity, topography, and mineral composition. It measured topography by combining pictures with data from a laser altimeter and tracking data from Earth. Gravity was also measured indirectly by tracking the spacecraft’s speed in orbit from Earth. Its orbital speed changed slightly depending on the local gravity. To measure gravity on the far side of the Moon, SELENE used its relay satellites. Measuring the gravity of the Moon helps understand what’s below the surface, which gives some clues to how it might have formed and changed.

Combining these three methods, SELENE discovered that the lunar far side has very different gravity concentrations than the near side. On the near side, there are more areas of stronger gravity, namely mountains and other mass concentrations or mascons. On the far side, there are no mascons but instead large numbers of basins and craters. The reason for this difference is thought to be a result of differential cooling, with the far side being cooler and thus more resistant to changes. SELENE’s measurements determined a possible age for when the geologic activity on the Moon ended: 2.5 billion years ago. A paper using SELENE data was published this year, 2022.

SELENE also mapped the entire Moon in 10-meter stereo resolution. This means it had two cameras with different perspectives – one on the front of the spacecraft and the other on the back – that combined to produce three-dimensional images.

SELENE’s mission overlapped with several other lunar orbiters, including China’s Chang’e 1 launched one month later and India’s Chandrayaan-1 launched ten months after that. Chang’e 1’s mission ended before SELENE, deliberately impacting the Moon on March 1, 2009. Chang’e 1 was followed by four other Chang’e lunar missions, including one more orbiter, two rovers, and a sample return.

NASA’s Lunar Reconnaissance Orbiter, which entered lunar orbit thirteen days after SELENE was deliberately crashed into the surface, is still in operation today. Chandrayaan-1 is inactive but still in orbit around the Moon.

One other historical event of note this week is the birth of famous amateur telescope maker John Dobson on September 14, 1915. His combination of existing technologies in the Newtonian reflector optical design and rocker box alt-az mount allowed amateurs to make large telescopes that were previously very costly to build. Commonly known as a Dob, it has its roots in sidewalk astronomy and continues to be popular with astronomers to this day.

More Information

KAGUYA (SELENE) mission page (JAXA)

SELENE Mission Instruments (JAXA)