Hiten Mission Caitlin Ahrens NASA Goddard Space Flight Center, Greenbelt, MD, USA Definition The Hiten mission (formerly the MUSES-A) was the first space engineering experimental satellite of the Institute of Space and Astronautical Science (ISAS) and was launched from Kagoshima Space Center (JKSC) in Japan in January 1990. After the successful insertion into a highly elliptical orbit around the Earth, the Hiten mission carried out several objectives to demonstrate space technologies for swing-by (or gravity assist) techniques and insertion of a sub-satellite into orbital space around the Moon. The results from these engineering missions have also led to follow-on missions, including aerobraking techniques and an excursion to the Lagrange points of the Earth-Moon system. Mission Objectives and Instrumentation The main mission objectives of the Hiten mission include several demonstrations of engineering technologies: (1) Trajectory-control, acquisition and verification of the double lunar swing-by technique for the purpose of determining precise orbital design and accuracy, and optimization of multiple lunar encounters; (2) insertion of a sub-satellite into an orbit around the Moon; (3) optical navigation experiments in proximity of the Moon using a CCD optical system equipped on a first-time spin- stabilized system; (4) tests of a fault-tolerant onboard computer and data processing for the telemetry system; (5) aerobraking experiments near the end of the mission life (planned 1 year) by descending the perigee altitude down to ~120 km; and (6) detection and measurement of the velocity and mass of micro- meteorite dust particles between the Earth-Moon system (the Munich Dust Counter MDC was developed by Technische Universit Michen in Germany; Iglseder et al. 1996). These specific engineering-specific experiments were selected due to their technological importance for lunar science and future technological prospects at the time. After its launch in January 1990 to the end of March 1991, Hiten carried out every primary mission mentioned successfully, including the first cis-lunar aerobraking experiments (Abe et al. 1991; Uesugi 1996). This MDC was the only onboard science instrument on Hiten that provided dust environment analyses between the Earth and the Moon until April 1993. Hiten also carried an optical navigation camera system (ONS). Due to its small chip size and low-resolution imagery, this instrument was solely for navigational and optical verification purposes. The ONS consisted of two CCD image detectors with a 384 x 490 pixel array, then the images were converted into a 4-bit digital signal that was relayed to Earth. The resolution of the ONS was ~1 arcminute (Nishimura and Kawaguchi 1993). Hiten was a cylindrical shape with a weigh of 197 kg (fully fueled), with a 1.4 m in diameter and 0.8 m high (Uesugi 1996). The 12 kg, polyhedral-shaped Hagoromo lunar orbiter was mounted on top of Hiten. Solar cells on the main Hiten spacecraft supplied power of 110 W, with a small onboard backup battery. The Hiten propulsion and communications systems consisted of: eight 23-N and four 3-N hydrazine thrusters, two spin-type Sun aspect sensors, three accelerometers, a star scanner, a steerable horizon crossing indicator, and control electronics with an onboard processor. The spin stabilization of the spacecraft was at 10 – 20.5 rpm (Uesugi et al. 1991). Hiten also released a small component, Hagoromo, into lunar orbit. The onboard transmitter ultimately failed, but the orbit insertion burn was verified optically. Hagoromo S-band transmitter had malfunctioned since February 1990, nearly 27 minutes before the closest approach to the Moon. The apolune and perilune altitudes of Hagoromo were ~20,000 km and 7,4000 km respectively, with an orbital period of ~2 days. Multiple Lunar Swing-by Hiten was launched on a Mu-3SII-5 rocket from Kagoshima Space Center in Japan into a highly elliptical Earth orbit in January 1990. Originally, the injection velocity was ~50 m/s less than the nominal value (Uesugi et al. 1991; Uesugi 1996), but trajectory correction maneuvers were performed for a nominal orbit. Such an injection error caused the first apogee distance to decrease about 180,000 km lower than the planned trajectory of 476,000 km. About 10 hours after launch, the first trajectory correction maneuver was conducted to accomplish the first lunar swing-by as planned. Most of the maneuvers were executed before Hiten’s perigee passages. On March 19th, 1990, Hiten encountered the Moon with its closest approach at a distance of 16,472 km. The first swing-by increased the apogee altitude from 440,000 km to 720,000 km. Hiten flew around the Earth 3.5 times on this orbit, with two TCMs were carried out (Uesugi et al. 1991; Farquhar 1991). The next apogee altitude was then lowered to 560,000 km as a result of the second lunar swing-by. Since then, Hiten successfully continued to conduct multiple lunar swing-by operations. The subsequent perigee and apogee altitudes were at 2,500 km and 425,000 km respectively by the eighth lunar swing-by. The first aerobrake demonstration was performed after a TCM was executed just prior to the 16th apogee and decreasing the altitude on March 19th. For the first time, a man-made object traveling near escape velocity demonstrated aerobraking in a planetary atmosphere. Hiten was at an altitude of 125.5 km at a speed of 11 km/s, then atmospheric drag effects decreased the spacecraft velocity by 1.71 m/s (Abe et al. 1991). Another TCM was done on March 23rd, 1991, where the perigee altitude was further lowered to 120 km for a second aerobrake demonstration, completed on March 30th. Follow-on Missions Hiten’s primary objectives were accomplished by the end of March 1991. The spacecraft had ~12 kg of fuel for further use, in which follow-on missions were planned and performed successfully. The Lagrange points, L4 and L5, in the earth-moon system, micro-meteorite particles conglomerate around these stable libration points. Hiten flying around these L4 and L5 points tried to find this “cloud” by the onboard dust counter (Iglseder 1996). Due to limited fuel, this follow-on mission utilized the gravity assist of the Moon and also perturbation force of the Sun (Kawaguchi et al. 1995). Two lunar swing-by maneuvers were necessary for this to stretch the orbit over several months, reaching the L4 point by October. With the lunar gravity assists and the perturbation of the Sun, Hiten could approach the Moon with low relative velocity (Menon et al. 1993). The last of the fuel also supported the decision to purposefully direct the landing (crash) of the spacecraft to the nearside of the Moon and land at a specified time while visible from the Usuda Deep Space Center (UDSC) (Nishimura and Kawaguchi 1993). After two TCMs in February and March 1993, UDSC observed cease of signals from Hiten on April 11th, 1993. The impact point was at latitude 34.0°S and longitude 55.3°E, near Stevenius and Furnerius craters, very close to the planned target location (Burnham 1993; Kato et al. 2008). References Abe, T., Kawaguchi, J., Saito, S., Ichikawa, T., & Uesugi, K. (1991, June). The World's First Cis-Lunar Aerobrake Experiment: Preliminary Report of the Results. In Proceedings of the 47th Annual Meeting of The Institute of Navigation (1991) (pp. 29-39). Burnham, D. L. (1993). Hiten mission ends. 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Advances in Space Research, 18(11), 69- 72. Uesugi, K., Matsuo, H., Kawaguchi, J., & Hayashi, T. (1991). Japanese first double lunar swingby mission “Hiten”. Acta Astronautica, 25(7), 347-355. .