SLIM: Small explorer for technology demonstration of lunar pinpoint landing

Seisuke FUKUDA1), Shin-ichiro SAKAI1), Eiichi SATO1), Shujiro SAWAI1), and SLIM WG

1)Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, Sagamihara, Japan

SLIM (Small Lander for Investigating Moon) is a small spacecraft launched by the Epsilon rocket for demonstration of lunar pinpoint landing technology. SLIM aims at achieving accurate navigation to a specific landing point on the moon. Various researches concerning the pinpoint landing technology, which are for example image-based navigation, landing shock absorbers, navigation control algorithms, landing radar, and the advanced propulsion system, are intensively performed by the WG members containing university people. Also SLIM is an extremely light-weight spacecraft, so a lot of innovative technology for reducing the bus weight is applied. In this paper, mission sequence, spacecraft design, and demonstrated technology of SLIM are overviewed.

SLIM: 小型探査機による高精度月面着陸技術実証

福田盛介 1)，坂井真一郎 1)，佐藤英一 1) ，澤井秀次郎 1)，SLIM WG

1)宇宙航空研究開発機構 宇宙科学研究所 (ISAS/JAXA)

SLIM はイプシロンロケットで打ち上げる小型探査機により、月面への高精度着陸技術の実証を行うミッションである。SLIM 提案に際し、画像航法や着陸誘導、着陸レーダ、高性能推進系、着陸衝撃吸収機構など、ピンポイント着陸技術に係る様々な技術が、多くの大学関係者を含むワーキンググループメンバーにより精力的に検討されてきた。また、SLIM では、極限的な探査機軽量化のための多くの試みがなされており、その成果は後続の探査ミッションに極めて有用である。本稿では、SLIM のミッションシーケンスと探査機設計、及び技術実証項目を概説する。

1. Introduction

SLIM (Small Lander for Investigating Moon) is a small spacecraft for demonstration of lunar pinpoint landing technology. Now SLIM survives to the last stage in a selection process for the next mission launched by the Epsilon rocket. SLIM aims at achieving accurate navigation (~ 100 m) to a landing point on the moon; the Marius Hills Hole (MHH) discovered by KAGUYA/SELENE [1] is provisionally considered to be the landing point. The pinpoint landing can bring a paradigm shift in the field of celestial body landing from ‘landing where easy to land’ to ‘landing where desire to land’. The paradigm shift requires a number of novel technologies, and it is reasonable to demonstrate with the small lander at first. At the present, various researches concerning the pinpoint landing technology, which are for example image-based navigation, landing shock absorbers, navigation control algorithms, landing radar, and an advanced propulsion system, are intensively performed by the SLIM WG members containing a lot of university people.

When we consider a lunar lander launched by the Epsilon rocket, weight resources are very severe. So SLIM is designed as an extremely light-weight spacecraft whose dry mass is less than 130 kg including the propulsion tank. Therefore a lot of innovative technology for reducing the bus weight is applied. The outcome will be surely useful for future exploration projects including Mars missions.

2. Mission sequence and spacecraft

The orbital sequence of SLIM is depicted in Fig. 1. SLIM will be launched by the Epsilon rocket with the fourth kick stage into an elliptical orbit of 250 19000 km. SLIM will be released from the rocket in a spin-stabilized attitude. Via a few rotation in a phasing orbit, SLIM will be injected into the Lunar Transfer Orbit. SLIM goes through the Van Allen radiation belt at each rotation, so the rotation times decide requirements of radiation tolerance in the spacecraft design. The Lunar Orbit Insertion (LOI) will be fired when real-time communication between SLIM and ground stations is available. As the baseline ground stations, JAXA’s Usuda Deep Space Center and Uchinoura

Space Center are taken into account. The LOI maneuvers of several times will be carried out in a divisional manner, so that the shape of lunar circling orbits will be shrunk. From the final circling orbit of 100 15 km, the powered descent will be started with continuous burning of the 500 N main thruster. When SLIM arrives above the landing point, the altitude will be about 3.5 km from the lunar surface. Thereafter SLIM performs the final vertical descent.

A candidate landing point of SLIM is an area within 100 m around the hole discovered by KAGUYA/SELENE Terrain Camera observation in the Marius Hills region [1]. Fig. 2(a) is an image of the MHH acquired by Lunar Reconnaissance Orbiter (LRO) of NASA. The diameter and depth of the MHH exceed 50 m, quite unlike normal craters on the Moon. The MHH is a possible skylight of a lava tube, and it exists on a belt-like terrain (Fig. 2(b)). Since the width of the terrain is as narrow as several hundred meters, aiming for the MHH is considered to be appropriate for the demonstration of the precise landing.

When exploration missions with the Epsilon rocket are considered, it is crucial to reduce the weight of spacecrafts to rock bottom. The present mass budget of SLIM compared with other scientific satellites is shown in Table 1. More than three-quarters of SLIM’s launch mass is propellant. Consequently the dry mass has to be suppressed to microsat-class; as a matter of fact, it can be seen in Table 1 that SLIM’s dry mass except the propulsion subsystem is about as light as REIMEI/INDEX.

The spacecraft configuration of SLIM is sketched out in Fig. 3. The PAF (Payload Attachment Fitting) to the rocket is located on the top of the body. SLIM will be attached to the launch vehicle at upside down attitude. Two component boxes are installed on a deck panel around the propulsion tank. Solar cell sheets are wrapped outside the

Fig. 1 Orbital sequence of SLIM

Fig. 2 Marius Hills Hole: (a) LRO image and (b) belt-like terrain around MHH

(a) (b)©NASA

spacecraft. A bi-propellant Orbital Maneuvering Engine (OME) is at the bottom side. Its nominal thrust is 500 N. Eight 20N-class small bi-propellant thrusters are located under the deck panel to conduct attitude control and small orbit change. Four tilted rectangle antennas at the bottom side are the equipment of the landing radar. In this baseline configuration, SLIM has four legs with shock absorbers of porous aluminum.

The drastic reduction of SLIM’s dry mass is investigated under the concept of innovative functional integration or consolidation. Various challenges will be tried in each subsystem as follows:

Integrated single tank shell storing both of fuel (hydrazine) and oxidizer (MON-3), which has charge of a load paths from the PAF as main structure of the spacecraft Light-weight computer into which DHU (Data Handling Unit), AOCP (Attitude & Orbit Control Processor),

communication I/F, and an image processing board are integrated Digital-controlled power unit combined with HCE (Heater Control Electronics) Thermal conjunction of the tank and batteries (SUS-laminated Li-ion), or the tank and component boxes

through oscillating heat pipes Space solar sheets sewn outside the MLI (Multi Layer Insulation)

SLIM AKATSUKI/Planet-C

HISAKI/SPRINT-A

REIMEI/INDEX

Communication 3.5 26.6 8.2 2.4

DHAOCS

14.5 6.5 9.4 4.1

37.6 58.7 8.4

EPSThermal

15.6 51.9 70.3 16.5

16.2 6.5

Structure 29.3 41.8 50.5 12.0

Instrumentation 15.5 29.9 33.0 13.8

Balance weight 17.4 4.2

Ignition 0.9

Mission(inc. margin)

2.0 34.6 111.8 11.4

Propulsion 46.5 59.3

Propellant 418.8 196.7

Total (dry) 126.8 322.8 348.4 72.9

Total (wet) 545.6 519.5

Table 1 Mass budget of SLIM compared with other scientific satellites

PAF

tank

Deck panel

Component box

Space Solar Sheet

OME(500N)

Landing radar antennas

Fig. 3 Spacecraft configuration of SLIM

Fig. 5 Landing radar

3. Technology demonstration for pinpoint landing SLIM is purely engineering demonstrator, and engineering community including professionals of many university,

industry, and JAXA makes major contributions to each technology for pinpoint landing.

Image-based navigation (Meiji Univ., Univ. of Electro-Communications, and ISAS) At first, crater extraction from lunar surface images by onboard cameras are executed. The PCA-based algorithm

can estimate the position and size of craters [2]. Subsequently, as shown in Fig. 4, the Evolutionary Triangle Similarity Matching (ETSM) method searches the current spacecraft location by matching extracted craters with a crater database prepared before launch [3]. Here our thorough study has revealed that it is critical for matching success to fail to extract craters registered in the crater database, whereas it is not critical to extract craters unregistered in the database. Also hardware size and speed are carefully evaluated when the algorithms of crater extraction and matching are implemented on a space-grade FPGA.

Hazard detection and estimation (ISAS and Sokendai)

To realize the smart landing, autonomous hazard recognition and avoidance is one of essential technologies. Image-based recognition algorithms, for example with shadow information, are being considered. A method of detecting sub-pixel size hazards across multiple pixels is also studied [4], which is useful for searching safe landing areas in rather higher regions of the vertical descent phase.

Guidance control (Yokohama National Univ. and ISAS)

Guidance algorithm has two sub phases. At the first phase, altitude profile is a major target of the guidance. In the contrary, at the second phase, the lateral position and velocity become the major concern to achieve pinpoint landing. At the second phase, the modified LOS (Line Of Sight) guidance is introduced. The polynomial guidance laws [5], which sometimes include some coasting, are also studied.

Landing gear and shock absorber (Tokyo Metropolitan Univ., Shizuoka Univ., Kyushu Univ., and ISAS)

Porous aluminum is to be used as shock absorber at the landing. The sintered aluminum fibers are set at the tip of four legs, and they will absorb both of the vertical and lateral motion without any link mechanism. Drop test using a 1/1 scale model has revealed effectiveness of the porous aluminum as a shock absorbing material [6].

High performance propulsion (ISAS, JEDI, and ARD)

Ceramic thruster, which is employed in AKATSUKI/Planet-C, are modified by improved injector design pursuing heat-resistant characteristics of the ceramics.

Landing radar (ISAS and Melco)

The C-band pulse radar provides not only altitude information but also relative velocity against the surface (Fig. 5). Since field experiments using a helicopter have already done for several times, manufacturing the Engineering/Flight Model is ready. Some studies for simulating the radar performance are proceeded [7].

Fig. 4 Evolutionary Triangle Similarity Matching (ETSM) [3]

4. Conclusion

The mission sequence, spacecraft design, and demonstrated technology of SLIM are overviewed. After the pinpoint landing technology and the light-weight bus system are acquired through the development and operation of SLIM, it will be sure that a variety of exploration missions can be successively planed. References [1] J. Haruyama et al., “Possible lunar lava tube skylight observed by SELENE cameras,” Geophysical Research Letters, 36,

L21206, 2009. [2] I. Nomura, T. Takino, S. Nagata, J. Irie, H. Kamata et al., “Study on the evaluation of the crater detection method using

principal component analysis,” this symposium, A-11, 2014. [3] T. Harada, R. Usami, K. Takadama et al., “Computational time reduction of evolutionary spacecraft location estimation

toward smart lander for investigating moon,” International Symposium on Artificial Intelligence, Robotics and Automation in Space (i-SAIRAS), 10C-04, Turin, Italy, 2012.

[4] S. Kanazawa, S. Fukuda, S. Sawai, and Y. Morita, “Autonomous precision landing system with avoidance system using single camera for small launders,” 51st AIAA Aerospace Sciences Meeting, AIAA2013-1151, Grapevine, TX, 2013.

[5] M. Kawasaki, “Study on polynomial guidance law for the Smart Lander for Investigating Moon,” 28th International Symposium on Space Technology and Science (ISTS), 2011-d-70s, Ginowan, Japan, 2011.

[6] K. Kitazono, T. Sekino, E. Sato, and S. Sawai, “Impact energy absorption mechanism of sintered aluminum fiber at landing of SLIM explorer,” 58th Space Sciences and Technology Conference, 2H08, Nagasaki, 2014 (in Japanese).

[7] S. Fukuda, T. Sakai, and T. Mizuno, “Landing radar simulation with digital terrain models,” Trans. JSASS Space Tech. Japan, vol.10, no.ists28, pp.Pd_61-Pd_66, 2012.