The Present status of the Japanese Penetrator Mission: LUNAR-A

2006年07月25日

Hiroaki Shiraishi1,Satoshi Tanaka1,Akio Fujimura1, and Hajime Hayakawa1
1Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency (ISAS/JAXA)
siraisi@planeta.sci.isas.jaxa.jp

Abstract. LUNAR-A mission is a Japanese penetrator mission to the moon. The development of the mission has been suspended since 2004 mainly because of a recall and replacement of thruster bulbs and sequence skip of the program after the penetration experiment performed as the qualification test in November, 2003. After the review boards held in 2004, we have decided to concentrate on the accomplishment of the penetrator technology for three years since fiscal year 2005. For the evaluation of the progress, three major phases were considered each of which accompanies the penetration experiment with QT level. The first phase is the accomplishment of newly development technologies against the possible malfunctions, the second is the confirmation of the complete function by using full integrated penetrator, and the third (final) step is authentication of accomplishment of the technologies using a full specification penetrator. We have already passed the “phase-one” experiment successfully and the next one will be performed in the near future (May, 2006). By the accomplishment of this technology, we expect to provide not only for the penetrator for the LUNAR-A mission, but to be widely applicable for the future missions.

The Japanese lunar penetrator mission, LUNAR-A, whose image view is shown in Fig 1, will be launched using the M-V launch vehicle which can send a spacecraft of about 540kg into a lunar transfer orbit. Although the launch date was changed many times in the past for various reasons, the essential mission concept has been kept.



FIGURE 1. Image view of the LUNAR-A penetrator mission.

The penetrator is a missile-shaped instrument carrier, which is about 14cm in diameter, 75cm in length, and about 14kg in weight without attitude control system. It contains a two-components seismometer and heat flow probes together with other supporting instruments such as a tilt meter and an accelerometer.

The main purpose of the LUNAR-A mission is to investigate the internal structure of the moon (e.g. Mizutani et al., 2003, Tanaka et al., 1999). We expect to obtain some important clues about the size of metallic core by the seismic observation and about the bulk chemical composition by the heat flow observation respectively. Besides these science goals, this mission will demonstrate the usefulness of the penetrator technology for future planetary missions. In this respect, the LUNAR-A mission should be thought of as the first step toward the more long-range goal of planetary geophysics.

Although, the LUNAR-A mission has been suspended since 2004 mainly because of a recall and replacement of thruster bulbs of the spacecraft. The penetrator qualification test (QT) performed in November, 2003 resulted in a sequence skip of the program was also one of the reasons for the suspension of the mission development at that time. Since then, the Lunar-A project had been reviewed in all aspects, including technical problem, management strategy, and so on. From the recommendation by the review boards in the processing of the mission program, we made a decision to concentrate on the development and accomplishment of the penetrator technologies prior to the development of the spacecraft.

In the technical problems of the penetrator, two major points must be considered as follows;

1) Assurance of robustness on communication link between the penetrator and spacecraft, including the data acquisition during deployment phase.

2) Addition of CPU reset circuit for possible malfunction which occurred in the last experiment.

In order to clear these problems, some modification and re-design of electronics circuits and a development of a new sensor were required. We estimated about 3 years to accomplish these and the project started from fiscal year 2005.

For the evaluation of the progress at some crucial points, three major phases were considered, each of which accompanies the penetration experiment with QT level. For the first phase, newly development technology against the possible malfunctions must be accomplished, the second one must involved the confirmation of the complete function by using full integrated penetrator, and the third (final) step places the accomplishment of the development in a total sense.

In order to avoid the possible malfunction during and right after the penetration event, we developed a reset sensor and electronics circuits of the CPU triggered by the deceleration force at the penetration event. This “phase-1” experiment had already been performed in November, 2005 and we could get successful result both for the sensor and for the electronics. We also could get some successful results of shock durability for the electronics parts in substitution for the products which can not be obtained. The phase-2 experiment will be performed in the near future in May, 2006. In this experiment, we expect successful communication and complete function after the penetration event. In order to confirm this, we have been manufacturing a fully integrated penetrator, but some of the specification could not be satisfied. Especially the performance of the communication link, whose robustness was required by the review board mentioned above, could not be realized in this phase. For the phase-3 experiment, so ca
lled “the final step”, the flight model quality will be tested in fiscal year 2007.

On the other hand, high-precision verification tests for the sensors and instruments after the impact experiment have been examined so far. For example, the seismic sensors were calibrated by observing the micro tremor ground motion and compared with the data obtained by a reference seismometer (Yamada et al., 2005). To execute this experiment, the penetrator was placed at a seismic observatory at where the ground motion is almost equivalent to that of the largest deep moonquakes. From our result, we could get good consistency with the data obtained by the reference seismometers. Other sensors and instruments such as the rotational mechanism for adjustment of the angle of the seismometers were also confirmed their complete function and satisfied with the specifications after the penetration experiment. Therefore, we have confirmed that the most of its technology has been established.