Analysis of Autonomous Rendezvous Docking and Sample Transfer Technology for a Space Probe in the Jupiter Trojan Region

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1 Analysis of Autonomous Rendezvous Docking and Sample Transfer Technology for a Space Probe in the Jupiter Trojan Region Yuki TAKAO 1), Shigeo KAWASAKI 2), Thoshihiro CHUJO 1), Shota KIKUCHI 1), Kazuaki IKEMOTO 1), Satoshi KITAO 3), Hideki KATO 2), Osamu MORI 2) 1) The University of Tokyo, Tokyo, Japan 2) Japan Aerospace Exploration Agency, Sagamihara, Japan 3) Aoyama Gakuin University, Sagamihara, Japan In the exploration mission of Jupiter Trojans by a solar power sail which is planned at present in JAXA, sampling from the asteroid and sample return by use of the lander spacecraft are considered. Communication delay in the Jupiter Trojan region is however so large that the remote manipulation from the ground station is almost impossible. This study analyzes the technology of rendezvous docking with autonomous navigation and guidance, sample transfer technology from the lander to the motherspacecraft, and evaluates the feasibility of the system. 1 Introduction Japan Aerospace Exploration Agency (JAXA) is currently planning to explore the Jupiter Trojan asteroids by use of a solar power sail. Solar power sail is an extended concept of solar sails, whose sail membrane is equipped with thin-film solar cells. Owing to the large area of the sail, a solar power sail can generate large amount of electric power even in the distant region from the sun. JAXA launched the small solar power sail demonstrator IKAROS in 2010, and succeeded to demonstrate the technology of both solar sail and solar power sail[1]. Figure shows the concept of solar power sail, and the appearance of IKAROS. Utilizing a solar power sail makes it possible to drive ion engines in the outer regions. This achieves the hybrid propulsion system of ion engines and solar sailing, which can be widely applied in the future deep space missions. JAXA has proposed to explore the Jupiter Trojan region with the next solar power sail for the first time in the world[2]. In this mission, the sail is fully equipped with thin-film solar cells, and the large V is acquired by ion engines in combination with the Earth and Jupiter gravity assist. Figure 2 shows the concept of the next solar power sail mission. One of purposes of the mission is to collect samples from the surface of the asteroid, and takes them back to Earth (sample return). Since it is difficult for the solar power sail, which is spin stabilized with the large sail, to land on the asteroid, the small lander is used for the sampling (Fig. 3). The lander is separated from the solar power sail mother spacecraft (MSC), lands on the asteroid, and collects samples from the asteroid. Then, it returns to and docks with the MSC. This rendezvous and docking sequence must be performed autonomously because of the large communication delay in the Jupiter Trojan region. The Japanese ETS-VII mission successfully demonstrated the autonomous rendezvous docking in 1998 for the first time in the world[3]. ETS-VII consisted of two Fig.1 Concept of solar power sail, and appearance of the small solar power sail demonstrator IKAROS. satellites, the chaser and the target. They utilized the global positioning system (GPS), a rendezvous laser radar (RVR), and a camera-type proximity sensor (PXS) for their navigation. However, the GPS cannot be used in the Jupiter Trojan region in case of the solar power sail mission. In addition, the target spacecraft in the next solar power sail mission (i. e. solar power sail MSC) is basically non-cooperative because it is spin-stabilized with the large amount of moment of inertia, while the target of ETS-VII could perform the attitude control for the rendezvous docking. Also, the lander must transfer the collected samples to the re-entry capsule which belongs to the MSC. This paper presents the newly developed rendezvous docking and sample transfer system. This method can be widely applied to the future asteroid exploration missions. 1

2 Altitude (Not to scale) HP: 250km H3: 50km 1km Time (Not to scale) Fig.4 Mission sequence related with sample return. Fig.2 Concept of Jupiter Trojan asteroid exploration mission by the next solar power sail. Size of the sail is about 50m 50m. Fig.5 the takeoff. When the lander reaches the relative distance of 1km from the MSC, the lander switches the navigation sensor to the optical navigation camera (ONC). When the lander reaches the relative distance of about 2m, it docks with the MSC by use of a method called berthing. Finally, the lander transfers the samples to the re-entry capsule. Fig.3 Design of the lander for the Jupiter Trojan exploration mission. 2 Sample Return Scenario Figure 4 shows the mission sequence related with sample return. The solar power sail (MSC) stays at the home position (HP), whose altitude is 250km from the asteroid, in the ordinary operation. When the lander-separation operation starts, the MSC descents to 1km altitude to separate the lander. After the separation, the lander starts its descent, sampling, and in-situ analysis. The MSC simultaneously ascends to 50km altitude (docking altitude) and keeps that position until the RVD is completed. After finishing the in-situ analysis of the samples, the lander starts ascending toward the MSC, and performs the RVD. Then, the samples inside the lander are transferred to the re-entry capsule of the MSC. Finally, the lander is again separated from the MSC to reduce the system weight in the return path to Earth. In the following sections, detailed ways of the rendezvous docking and sample transfer are described. 3 Sequence of the rendezvous docking. 3.1 Rendezvous RF sensor Radio frequency (RF) sensor utilizes the characteristics of an active integrated phased array antenna (AIPAA) to determine the relative relations between the two antennas[4, 5]. This sensor is to make it possible to estimate the position and attitude without conventional ones such as GPS. Phased array antenna is an aggregate of small antennas: phases of their electromagnetic radiations can be electronically controlled by phase shifters to control the beam direction with high accuracy. The AIPAA is the one with amplifiers added to its elements and phase shifters. Figure 6 shows the appearance of the AIPAA. The RF sensor in this mission not only allows the two way communication by the AIPAA, but has ranging function, Doppler shift measurement function, and retrodirective function. The retrodirective function can measure the direction from which the pilot signal comes, and return the signal back to that direction. The principle of the function is that the direction of the pilot signal is calculated from the difference of phase of the signal received by each adjacent antenna element. This makes it possi- Rendezvous Docking Figure 5 shows the sequence of the rendezvous proposed in this study. The lander and the MSC perform navigation by use of RF sensors, which are described later, after 2

3 10~15cm Fig.7 Fig.6 Sequence of the berthing method. Appearance of the RF sensor ble for the lander and the MSC to know the direction in which they exist each other. RF sensors can be used from the takeoff to the relative distance of 1km because their accuracy deteriorates in the closer region Optical navigation From the relative distance of 1km through 100m, the ONC captures the whole image of the sail of the MSC. The relative position of the lander to the MSC is calculated from the size of the sail photographed by the ONC. The attitude of the lander is calculated by use of inertial reference unit (IRU) through the whole sequence. When the lander reaches the relative distance of 100m, the ONC captures LED markers attached on the bottom side of the main body of the MSC because the ONC cannot capture the whole image of the sail at the distance. The relative position is calculated from the distribution of the markers in the image. When the lander reaches the relative distance of about 2m, it stops ascending and performs docking by use of the berthing method. 3.2 Fig.8 Extension boom[7]. The tip of the boom is connected with the MSC by the electromagnetic force between the magnetic substance on the boom and the electromagnet on the connection part. This is to make the second separation of the lander easier compared to the use of latching structures. In addition, the connection part is designed to be rotation free to avoid the torsion of the boom, which may be caused by the relative spin between the lander and the MSC after the connection. The lander must be fixed to the MSC, which is a spinning object, to secure the sample transfer path. To achieve this, the lander has protrusions on its side while the MSC has the triangle-shaped grooves on its holding space (Fig. 9). First, the lander roughly adjusts its rotation to the MSC by use of the ONC. Next, the lander revolves its boom. The protrusions of the lander is then guided along the grooves of the MSC, and the relative rotation is finally fixed. Docking Berthing method Berthing originally means to bring a vessel to a berth, for example by use of something like ropes. In the proposed method, the lander uses an extension boom (Fig. 8) instead of ropes. The extension boom is a telescopic rolled-up boom which is made of tri-axial woven fabric CFRP[6, 7]. On the other hand, the MSC has a holding space for the lander, the top of which is tapered. As shown in Fig. 7, the lander extends the extension boom as the first step of the berthing method. Next, the lander approaches toward the MSC to insert the boom into the holding space. Due to the tapered shape of the holding space, the tip of the boom is guided to the connection part of the MSC regardless of some errors of guidance, navigation and control. After the connection of the boom with the MSC, the lander revolves the boom to complete the docking Simulation The feasibility of the proposed berthing method is verified by the numerical simulation. The lander first stays at the initial position; 2m beneath the MSC with certain errors. Then, the lander starts ascending toward the MSC by one V with errors. The collision of the tip of the boom and the wall of the holding space is modeled by the spring-damper system. The reaction force and the 3

4 Table.2 Requirements for the berthing obtained by the simulation. initial state error V taper angle position: 15 cm attitude: 1 deg requirement: 8 cm/s permissible error: 1 cm/s 30 Grooves Protrusions Fig.9 Structures to fix the relative rotation between the lander and the MSC. (a) Initial Table.1 Conditions for the berthing simulation. extension boom length diameter Young s modulus buckling point mass 1.5 m 40 mm 10 GPa 15 MPa 95 kg lander diameter 650 mm height frictional force are given as follows. 630 mm F r = kx cẋ (1) F f = µf r (2) where x is the displacement (depression) of the wall, k is the spring constant, c is the damping coefficient, µ is the coefficient of friction.the collision characteristics such as k, c, and µ are measured by the experiment. The conditions for judging the berthing to be successful are as follows. The tip of the boom reaches the connection part and is captured by the electromagnet. The extension boom does not buckle through the sequence. Materials of the boom such as elasticity, buckling point, are measured by the strength test. These conditions for the simulation are summarized in Table. 1. The berthing motion is simulated multiple times with various initial states and errors of position, velocity and attitude of the lander. As a result, requirements for the successful docking are obtained as shown in Table 2. Figure 10 shows one example of the successful trials. (b) Final Fig.10 Verification of the berthing method by the multi-body simulation. The red line Describes the path the tip of the boom followed. 4 Sample Transfer The lander transfers samples to the MSC after the rendezvous docking. At the sampling, samples are contained in a container called sample catcher via induction pathway. When the rendezvous docking is completed, the lander pushes up this sample catcher toward the re-entry capsule by use of an extension mast (Fig. 11, 12). The feasibility of the sample transfer method is verified by experiments. First, we confirmed that the required amount of samples are contained in the sample catcher. In this experiment, collected samples are blown through the sampler hone, callow cell and induction path way. Then, mass of samples contained in the sample catcher is measured. Next, an experiment is conducted which simulates the sample catcher transfer (Fig. 13). As a result, the sample catcher is confirmed to be successfully transferred to the capsule regardless of friction. 4

5 Capsule Callow cell Induction pathway References MSC [1] Tsuda, Y., Mori, O., Funase, R., Sawada, H., Yamamoto, T., Saiki, T., Endo, J., Kawaguchi, J.: Flight status of IKAROS deep space solar sail demonstrator, Acta Astronautica, Vol. 69 (2011), pp [2] Mori, O., et al.: Jovian Trojan Asteroid Exploration by Solar Power Sail-craft, Trans. JSASS Aerospace Tech. Japan, Vol. 14, ists30 (2016), pp. Pk_1-Pk_7. [3] Kawano, I., Mokuno, M., Kasai, T., and Suzuki, T.: Result of Autonomous Rendezvous DockingExperiment of Engineering Test Satellite-VII, Journal of Spacecraft and Rockets, Vol. 38 (2001), pp [4] Wu, C. T. M., Choi, J., Kawasaki, S., Itoh, T.: A Novel Miniaturized Polarization Orthogonalizing Active Retrodirective Antenna Array for Satellite Use, IMS2013, TH3C-2, Seattle, [5] Ju, H., Kawasaki, S., Kawahara, Y., and Asami, T.: Compact Mixer Type Retrodirective Hybrid Itegrated Circuit, KJMW 2014, TH5A2, Suwon, Korea, [6] Higuchi, K., Watanabe, K., Watanabe, A., Tsunoda, H., and Yamakawa H.: Design and Evaluation of an Ultra-light Extendible Mast as an Inflatable Structure, Proceeding of the 47th AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics, and Materials Conference, AIAA Paper, Vol. 1809, [7] Sakamoto, H., Furuya, H., Satou, Y., Okuizumi, N., Takai, M., and Natori, M. C.: Wrapping Fold and Deployment Characteristics of Boom-Membrane Integrated Space Structures, 2nd AIAA Spacecraft Structures Conference, AIAA SciTech Forum, Florida, Sample catcher Ablator Lander Extension mast Sampler Samples Fig.11 Schematic chart of the sample transfer. Fig.12 Extension mast used for the sample transfer. Fig.13 fer. An experiment of the sample catcher trans- 5 Conclusion An autonomous rendezvous docking and sample transfer strategy for the Jupiter Trojan exploration mission is proposed. In this mission, the small lander utilizes newly developed RF sensors and berthing method. This rendezvous docking technique allows a completely autonomous operation. Also, the sample transfer method using inflatable extension mast is an unprecedented method. This paper presented the operation scenario, and confirmed the effectiveness of the system. The use of small lander is considered to be crucial in future deep space exploration mission since it is unrealistic that a mother spacecraft lands on and explores a large, heavy celestial bodies. This study gives an effective insight to such missions. 5