Japan launches Hiten, the country's first lunar probe, the first robotic lunar probe since the Soviet Union's Luna 24 in 1976, and the first lunar probe launched by a country other than Soviet Union or the United States.

Hiten: Japan's Pioneering Lunar Probe and Deep Space Innovator

The Hiten spacecraft, known in Japan as ひてん (Japanese pronunciation: [çiteɴ]) and given the evocative English name Celestial Maiden, stands as a pivotal achievement in space exploration. Prior to its launch, this groundbreaking mission was designated MUSES-A, an acronym for "Mu Space Engineering Spacecraft A," signifying its role as the inaugural mission within Japan's ambitious MUSES Program. Developed by the esteemed Institute of Space and Astronautical Science (ISAS) of Japan, Hiten embarked on its historic journey on January 24, 1990.

Hiten marked several significant "firsts" for Japan and the global space community. It was definitively Japan's first lunar probe, heralding the nation's entry into deep space exploration. Furthermore, Hiten broke a thirteen-year hiatus in lunar missions, being the first robotic lunar probe launched since the Soviet Union's Luna 24 in 1976. This distinguished it as the first lunar probe launched by any country other than the traditional space superpowers, the Soviet Union or the United States, demonstrating Japan's growing technological prowess. The spacecraft's name, Hiten, carries cultural depth, derived from the flying heavenly beings, or Apsaras, often depicted in Buddhist art and mythology, symbolizing grace and celestial movement.

Hiten's Challenging Orbital Insertion and Swift Correction

Hiten's primary mission objective was to be injected into a highly elliptical Earth orbit, designed to achieve an impressive apogee – its furthest point from Earth – of 476,000 kilometers. This carefully calculated trajectory would allow the spacecraft to perform crucial swing-by maneuvers past the Moon, leveraging lunar gravity for subsequent operations. However, the initial orbital injection encountered an unexpected challenge: a slight but significant delta-v deficit of 50 meters per second (m/s). Delta-v, or change in velocity, is a critical parameter in orbital mechanics, representing the impulse needed to perform maneuvers. This deficit meant Hiten's initial apogee was considerably lower than planned, reaching only 290,000 kilometers.

Despite this initial setback, the highly skilled engineers at ISAS demonstrated remarkable agility and technical acumen. They swiftly identified the discrepancy and successfully executed precise orbital correction maneuvers using the spacecraft's onboard propulsion systems. This critical intervention restored Hiten to its intended trajectory, allowing the probe to continue its ambitious mission as planned, underscoring the robust design and operational flexibility of the spacecraft.

Deployment of the Sub-Satellite Hagoromo

During its very first lunar swing-by, a key moment in its mission, Hiten successfully deployed a diminutive sub-satellite named Hagoromo (はごろも). This name, meaning "feather mantle" or "robe of feathers," was chosen to complement Hiten ("Celestial Maiden"), drawing inspiration from the same Japanese folklore where a celestial maiden wears such a garment. Hagoromo's intended purpose was to enter lunar orbit, potentially acting as a demonstration of micro-satellite deployment in deep space or for supplementary data collection.

Regrettably, shortly after its release, the transmitter aboard Hagoromo experienced a critical failure, rendering it unable to communicate telemetry back to Earth. Despite this technical hitch, the mission team showcased impressive ground observation capabilities. They were able to visually confirm Hagoromo's successful orbital insertion around the Moon from Earth, a testament to sophisticated tracking technologies even without direct radio contact from the sub-satellite itself. This visual confirmation was a vital indicator of the successful deployment and orbital mechanics, proving the sub-satellite's trajectory even if its data could not be relayed.

Pioneering Aerobraking: A Landmark Achievement

One of Hiten's most significant and enduring legacies was its pioneering demonstration of the aerobraking technique. After completing its eighth lunar swing-by, Hiten executed this groundbreaking maneuver on March 19, 1991. The spacecraft deliberately dipped into Earth's upper atmosphere, flying at a remarkably low altitude of just 125.5 kilometers over the Pacific Ocean, while traveling at an incredible speed of 11.0 kilometers per second (approximately 24,600 miles per hour).

During this precise atmospheric pass, the subtle yet effective forces of atmospheric drag acted upon Hiten. This drag successfully lowered the spacecraft's velocity by a precisely measured 1.712 m/s and, critically, reduced its orbital apogee altitude by a substantial 8,665 kilometers. This event marked a monumental achievement: it was the first aerobraking maneuver ever performed by a deep space probe. Aerobraking is a highly valuable technique in spaceflight, allowing spacecraft to alter their orbits and reduce speed by using the drag of a planet's atmosphere, thereby conserving precious propellant that would otherwise be needed for propulsive maneuvers. This successful demonstration by Hiten paved the way for future missions, significantly influencing the design and efficiency of later deep space endeavors, particularly planetary orbiters.

Following its ninth lunar swing-by and a second successful aerobraking maneuver performed on March 30, 1991, the primary objectives of the Hiten probe's mission were officially concluded, leaving behind a remarkable legacy of innovation and scientific firsts.

Frequently Asked Questions about the Hiten Spacecraft

What was the primary purpose of the Hiten spacecraft?
Hiten's primary purpose was to be Japan's first lunar probe, demonstrating advanced navigation techniques such as lunar swing-bys, and pioneering orbital maneuvers like aerobraking for deep space missions. It also aimed to deploy a small sub-satellite, Hagoromo, into lunar orbit.
What was the significance of Hiten's aerobraking maneuver?
Hiten's aerobraking maneuver, performed on March 19, 1991, was historically significant as it marked the first time this technique was successfully demonstrated by a deep space probe. This proof-of-concept showed that a spacecraft could use a planet's atmosphere to reduce its orbital energy and conserve propellant, a method now routinely used by missions to Mars and other planets.
Was Hiten the first lunar probe ever launched?
No, Hiten was not the first lunar probe ever launched. However, it held significant distinctions: it was Japan's first lunar probe, the first robotic lunar probe launched since the Soviet Union's Luna 24 in 1976, and notably, the first lunar probe launched by a country other than the United States or the Soviet Union.
What happened to the Hagoromo sub-satellite?
Hagoromo was successfully released into lunar orbit by Hiten during its first lunar swing-by. Unfortunately, its transmitter failed shortly after deployment, meaning it could not send data back to Earth. However, its orbit around the Moon was visually confirmed by ground observations from Earth.
What does "Hiten" mean?
"Hiten" (ひてん) is a Japanese name referring to flying heavenly beings, or Apsaras, often depicted in Buddhist art and mythology. The name "Celestial Maiden" was given as its English equivalent, while the sub-satellite "Hagoromo" refers to the celestial maiden's feather mantle or robe.