OMOTENASHI Spacecraft

Demonstrate the technology for low-cost and very small spacecraft to explore the lunar surface with a landing probe.

3 modules: orbiting module, retro motor module, and surface probe.
Demonstrate the technology for low-cost and very small spacecraft to explore the lunar surface. This technology could open up new possibilities for future missions to inexpensively investigate the surface of the moon.

The CubeSat will also take measurements of the radiation environment near the moon as well as on the lunar surface.

Super-Small Solid Rocket Motor to deaccelerate 2500 m/s just before surface impact. Surface probe will have a semi-hard landing at about 50 m/s.

Tumbling; early signal, no signal now.
Initial detection for 80 minutes then nothing further. RS provided data and plots to project to help assess spacecraft condition and what may have happened.

The first contact pass from NASA DSN Madrid station began shortly after the separation. However, telemetry from OMOTENASHI could not be locked for a while. In order to check the status of the spacecraft, we sent a “high power transmission mode” command, and the telemetry locked. Because the attitude control system showed “Sun pointing” mode, the +Y panel with solar cells should face the sun. However, the status of Sun-pointing mode was “Converging”, which meant that the attitude control system was in the process of trying to point towards the sun. The solar angle, which is the angle between +Y panel normal and the direction of the sun, was approximately 145 degrees. That meant the solar cell panels faced almost in the opposite direction of the sun. In fact, the generation power of the solar cells was zero and the battery voltage showed it was almost depleted. Moreover, the spacecraft was rotating around Y axis at about -80 deg/s. It was beyond the control limit of the attitude control system and the Sun-pointing control was not completed. Finally, the battery was depleted and communication with the spacecraft lost. Considering the attitude when the communication lost, the spacecraft power was assumed to be reactivated from March or April in 2023. However, the spacecraft would fly-by the moon on 21 November. Therefore, we gave up the lunar landing experiment and decided to restart the operation from March 2023.

We estimated the cause of the anomaly from the telemetry data (Fig. 5) and the time history of the receiving signal recorded by Madrid station (Fig. 6). By a Fault Tree Analysis (FTA), we think the failure scenario as follows, 1. “Rate dump” control using gas jet propulsion was activated automatically because the disturbance of the separation from OSA was over the angular momentum limit of the attitude control system. The Rate-dump control seemed to be converged normally and move to “Normal control” in which RWs are used instead of the gas jets. Therefore, gas jet propulsion units were automatically switched off. 2. However, before the completion of the Rate-dump control, one or two of thruster valves were not completely closed and the leakage of the gas continued. It cased the disturbance torque to the spacecraft and the spacecraft started rotation around its Y axis. 3. The thrusters are connected to a plenum tank, which is connected to a propellant tank with two valves. It cannot be considered that three valves had open failures at the same time. Therefore, if the propellant in the plenum was gaseous state, the leakage would stop soon because the volume of the plenum was small. Unfortunately, however, liquid propellant seemed to exist in the plenum, because gas leaked from the propellant tank through the valves in three years (from the filling of the propellant in September 2019 to the launch in November 2022) became liquid in the plenum tank. Even the leak rate of the valves was very small, the long period has caused a lot of propellant leak and the propellant liquefied inside the plenum. 4. Because the rotation of the spacecraft, that is, its angular momentum became big beyond the threshold, the attitude control system stopped the drive of the reaction wheels. Therefore, Sun-point control was not completed. To restart Sun-point control, we tried to reduce the angular momentum by initiating Rate-dump control manually. But there was not enough time to reduce the momentum, and the battery was depleted.

From March 2023, we started the search and rescue operation. Since the tracking time of the spacecraft at the first contact pass was very short, the trajectory estimation of OMOTENASHI had a large error. The uncertainty was due to the estimation error of the separation velocity from OSA. To narrow the search area, we tried to reduce the estimation error of the separation velocity as much as possible. All CubeSats were deployed from OSA while spinning with about 6 deg/s. The spin axis direction of OSA and the attached positions of the deployers to OSA were informed from NASA. However, the rotational phase of OSA at the instance of the separation of OMOTENASHI was unknown. We estimated it from other CubeSats’ trajectories, though only three of ten CubeSats made precise orbit determination. The antenna beam width of JAXA ground stations are about 0.1 degrees. It was very small compared with OMOTENASHI’s position error. Therefore, we searched one direction for two minutes, which were required to send commands for the search and rescue. And we changed to search another direction, and so on. The spacecraft is going far from Earth. We estimated that the communication with the spacecraft was possible till the end of September 2023. So, we gave up the operation then.

Though we gave up “semi-hard landing on the moon”, some newly developed or procured instruments were checked at the first contact pass. Power system except solar cells, COM, OBC, and some functions of the attitude control system were confirmed. Moreover, we measured the radiation environment outside the Earth’s magnetosphere for the first time in Japan. Fig. 7 shows the measured counts of both Ch.1 (Proton + GCR) and Ch.2 (GCR) sensors. The number obtained by subtracting the Ch.2 counts from the Ch.1 counts corresponds to protons. A detailed analysis to correct the shielding effect of the spacecraft body and calibrate the sensor outputs based on ground tests is underway [23]. 

One of 13 SLS 6U Interplanetary or Lunar CubeSats

• ADCS - Blue Canyon XACT

• ArgoMoon
• BioSentinel
• CuSP
• EQUULEUS
• LunaH-Map
• Lunar IceCube
• LunIR
• Miles
• NEA-Scout
• OMOTENASHI