Direct Exploration of using Solar Power Sail

By Osamu MORI1), Hideki KATO1), Jun MATSUMOTO1), Takanao SAIKI1), Yuichi TSUDA1), Naoko OGAWA1), Yuya MIMASU1), Junichiro KAWAGUCHI1), Koji TANAKA1), Hiroyuki TOYOTA1), Nobukatsu OKUIZUMI1) , Fuyuto TERUI1), Atsushi TOMIKI1), Shigeo KAWASAKI1), Hitoshi KUNINAKA1), Kazutaka NISHIYAMA1), Ryudo TSUKIZAKI1), Satoshi HOSODA1), Nobutaka BANDO1), Kazuhiko YAMADA1), Tatsuaki OKADA1), Takahiro IWATA1), Hajime YANO1), Yoko KEBUKAWA2), Jun AOKI3), Masatsugu OTSUKI1), Ryosuke NAKAMURA4), Chisato OKAMOTO5), Shuji MATSUURA6), Daisuke YONETOKU7), Ayako MATSUOKA1), Reiko NOMURA1), Ralf BODEN8), Toshihiro CHUJO8), Yusuke OKI8), Yuki TAKAO8), Kazuaki IKEMOTO8), Kazuhiro KOYAMA8), Hiroyuki KINOSHITA9), Satoshi KITAO10), Takuma NAKAMURA10), Hirokazu ISHIDA8), Keisuke UMEDA8), Shuya KASHIOKA11), Miyuki KADOKURA11), Masaya KURAKAWA10) and Motoki WATANABE10)

1)The Institute of Space and Astronautical Science, JAXA, Sagamihara, Japan 2)Department of Chemistry, Chemical Engineering and Life Science, Yokohama National University, Yokohama, Japan 3)Project Research Center for Fundamental Science, Osaka University, Osaka, Japan 4)Information Technology Research Institute, AIST, Tsukuba, Japan 5)Department of Planetology, Kobe University, Kobe, Japan 6)Department of Phisics, Kwansei Gakuin University, Sanda, Japan 7)Faculty of Mathematics and Physics, Kanazawa University, Kanazawa, Japan 8)Department of Aeronautics and Astronautics, The University of Tokyo, Tokyo, Japan 9)Department of Aeronautics and Astronautics, Tokai University, Hiratsuka, Japan 10)Department of Mechanical and Engineering, Aoyama Gakuin University, Sagamihara, Japan 11)School of Physical Sciences, The Graduate University for Advanced Studies, Sagamihara, Japan

The solar power sail can generate sufficient electric power to drive the high specific ion engine in the outer planetary region by thin-film solar cells attached to the entire surface of the spin-type . This paper proposes the direct exploration of the outer planetary region using solar power sail-craft. The target is an unexplored D/P-type Jupiter Trojan asteroid. A lander is separated from the solar power sail-craft to collect surface and underground samples of the Trojan asteroid and perform in-situ analysis in Plan-A/B. In addition, the lander delivers samples to the solar power sail-craft for sample return to in Plan-B. Scientific observations during the interplanetary cruise are also implemented.

Key Words: Solar Power Sail, Jupiter Trojan Asteroid, Lander, In-Situ Analysis, Sample Return

1. Introduction studied in Europe and the USA achieve only flyby and rendezvous.7) Direct exploration mission of small celestial bodies, inspired by The use of an ion engine should be adopted in order to decrease Hayabusa1), are being actively carried out. Hayabusa-2,2) fuel mass. The higher the specific impulse of the ion engine, the OSIRIS-REx,3) and ARM can be mentioned as specific examples. lager the required electric power becomes. Besides the use of Due to constraints of resources and orbital mechanics, the current solar cells, a nuclear power source could be considered. However, target objects are mainly near-Earth and the adopting nuclear power is inefficient for small and medium-sized satellites, and . However, in the near future it can spacecraft from the perspective of weight. Thus the solar power be expected that the targets will shift to higher primordial bodies, sail can be a solution to this problem, since it can generate located farther away from the . sufficient power from a large area thin-film solar cell to drive In the primordial celestial exploration, it is required to high specific impulse ion engines in the outer planetary region. scientifically investigate the internal structure of small celestial IKAROS,8) which was launched in 2010, demonstrated thin-film bodies by in-situ analysis of pristine underground samples which solar power generation as well as photon propulsion successfully. have not been exposed to space weathering. The exploration of Based on the above, we propose a mission to explore the higher primordial celestial objects has been enhanced by landing primordial Trojan asteroids directly using the solar power missions such as MINERVA,4) MASCOT5) and Philae.6) sail-craft.9) At the Trojan asteroid, a lander is separated from the In the navigation of the outer planetary region, ensuring electric solar power sail-craft to collect surface and underground samples, power becomes increasingly difficult and ΔV requirements and perform in-situ analysis in Plan-A/B. In addition, the lander become large. It is not possible to perform landing or round trip delivers samples to the solar power sail-craft for sample return in missions to asteroids beyond the main belt with the combination Plan-B. of solar panels and chemical propulsion system, even with a large This paper shows a direct exploration mission of the outer launch vehicle. Trojan asteroids exploration missions which are planetary region by the solar power sail-craft in detail and presents an initial design for this spacecraft.

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9) Surface and underground sampling, in-situ analysis 2. Direct Exploration Mission of Outer Planetary Region 10) Rendezvous using an RF sensor, berthing and sample by Solar Power Sail-craft transfer from lander to solar sail spacecraft (in Plan-B) 11) High speed re-entry capsule (in Plan-B): Re-entry speed = 2.1. Mission outline 13~15km/s, Vinf = 10km/s A spin-type large solar sail with an area of 2500m2 (10~15 The main features of this mission are the following. times larger than that of IKAROS) can be an ultra-light power 1) World’s First Photon / Electric Hybrid Sail Propulsion generation system (1kW/kg) and generate high electric power 2) World’s Highest Performance Ion Engines in the outer planetary region ([email protected]) by attaching 3) World’s First Background Emission Mapping thin-film solar cells on the entire surface of the sail membrane, 4) World’s First Access to a Trojan Asteroid as shown in Fig. 1(a). This electric-generating capacity is 10 5) World’s First Sample Analysis of a Trojan Asteroid times larger than that of the solar panels of JUNO10) 6) World’s First Round Trip to the Outer (in ([email protected]), shown in Fig 1(b). Even if using thin-film Plan-B) solar cell panels with a rigid-frame, the power generation 7) World’s Highest Re-entry Speed Capsule (in Plan-B) system cannot achieve significant light weight and large sizes. A high-performance ion engine with a specific impulse of 7000 seconds (2~3 times larger than that of Hayabusa) is driven by this high electric power, and is capable of achieving a large ΔV in the outer planetary region. This is much more than the ΔV by chemical propulsion of , but much less fuel mass is required because of the high specific impulse. (a) Solar power sail-craft (b) JUNO The mission sequence is shown in Fig. 2. The spacecraft is Fig. 1. Spacecraft in the outer planetary region supposed to make the world’s first trip to a Jupiter Trojan asteroid around the L4 point of the Jupitar-Sun system using Trojan asteroid both Earth and Jupiter gravity assists. After arriving at the Mainbelt (~ 3AU) (~5.2 AU) Trojan asteroid, a lander is separated from the solar power sail-craft to collect surface and underground samples and Earth Sun perform in-situ analysis in Plan-A/B. In addition, the lander (1AU) delivers samples to the solar power sail-craft for sample return Jupiter (~5.2 AU) to Earth in Plan-B. In this mission, by probing the Trojan Launch asteroids directly, it is possible to examine the planetary Earth Swing-by Jupiter Swing-by Plan-A/B movement model of gas giants as the latest hypothesis of solar Arrival at Trojan asteroid system formation theory empirically. Landing, Sampling and In-situ analysis (Lander) Departure from Trojan asteroid This mission also aims to provide several new innovative Jupiter Swing-by Plan-B first-class astronomical science observations during the deep Return to Earth space cruising phase as shown in Fig. 3. Infrared telescope Fig. 2. Mission sequence (EXZIT), gamma-ray burst polarimeter (GAP2), dust detector (ALDN2) and magnetometer (MGF) utilize features of cruising phase. They can greatly contribute to the progression of planetary science, and space physics. The solar power sail-craft will carry out demonstrations of the following new technologies that will be required for future solar system exploration. 1) A large membrane space structure including spin deployment strategy: Area = 2500m2, Heat fused film 2) High power generation using Thin-film solar cells: Power = [email protected], CIGS, the bending prevention by sputtering 3) Sail control using a reflectivity control device: Spin rate control, spin axis direction control 4) Hybrid propulsion using both solar sail and ion engines: Hybrid navigation using photon propulsion and electric propulsion 5) Ultra-high specific impulse ion engines: Specific impulse = 7000 seconds, service life = 4000 hours 6) Reaction control system capable of operation at very low temperatures: Ignition temperature = -40 degree C 7) Long-distance determination: USO, ΔVLBI technology Fig. 3. Cruising Phase Science 8) Autonomous operation and landing of a lander

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2.2. Mission purpose based astronomy as a new scientific field.

The purpose of this mission is to demonstrate the direct Mars Asteroid Jupiter , exploration in the outer planetary region using a solar power zone zone EKBO Flyby ●U, ●U ●R, U ●U, R ●J, U, ● ● ● ●U ●U ●U: sail to lead the future of solar system exploration. R, J E, R U, E, C U U New ▲J: ■J Horizons (1) Navigation technology: Navigation technology using a Procyon Orbiter/ ●U, ●U ● ● ● ●J, U, R ●U ●U ■U solar power sail-craft is demonstrated in the medium scale R, J, ■E/J: Rendez ■E/J: R, U, E U, R, E, E E, C, JUICE ▲Under operation vous Bepi ▲J: I plan, in order to transport the payload needed for landing I Colombo Akatsuki ■C, J ■U ●Achievements () ■Under development operations, to the outer planetary region, and in the round trip. Under investigation Landing ●U, ●R ●U ●E/G ●J, U ●U ●E ■U (2) Exploration technology: The following exploration R, C ■U ■E, J (CG ■E/G ■J () ■ I, J comet) (Trojan techniques which are necessary for achieving mission success asteroid) ● ■ ● ● ■ ■ Sample U, U, E, U J: J J Frontier at 2016 are demonstrated together: rendezvous with a Trojan asteroid, return R R ■U Hayabusa (Trojan (Encela ▲ ＝Yellow ■U, J: asteroid) dus) (comet Hayabusa2 C, I, J nucleus) surface and underground sampling, and in-situ analysis by the ■U, E Frontier at 2035 Manned ●U ■U: ＝Orange lander; sample return (in Plan-B). (3) Scientific observations: round trip ■C ARM Solar power sail

Scientific observations during the deep space cruising phase U = USA; R = Russia (& USSR); J = Japan; E = ESA; C = China; I = India; G = Germany and at the Trojan asteroid are implemented. 2.3. Mission significance Fig. 4. Frontier of solar system exploration (1) Navigation technology: Ensuring electric power is difficult and ΔV becomes large in the outer planetary region navigation. 2.4. Mission positioning It is not possible to perform round trip missions including The relationship between Hayabusa, IKAROS, Hayabusa-2 landing to asteroids beyond the main belt, even with a large and the solar power sail-craft is shown in Table 1. In regard to rocket, with a combination of solar panels and chemical navigation technology, the solar power sail-craft is composed propulsion systems. Therefore, it is assumed that a system of a high-Isp ion engine and a large solar sail which have both which drives an electric propulsion engine with nuclear power been successfully demonstrated by Hayabusa and IKAROS will eventually play an important role for large scale respectively. In regard to exploration technology, Hayabusa interplanetary transport in future. On the other hand, electric landed directly on an asteroid to collect a surface sample. In propulsion which utilizes the power generated by large solar addition to this, Hayabusa-2 will collect underground samples cells without any frames will be adopted for small and using a small carry-on impactor. On the other hand, the lander medium-sized spacecraft because of the inefficiency and high which is separated from the solar power sail-craft, lands on the weight of nuclear systems for spacecraft of this size. The solar asteroid to collect surface and underground samples and power sail enables landing and round trip missions to celestial carries out in-situ analysis. Regarding scientific observations, bodies within the distance of Saturn. As a consequence, Japan the S-type asteroid Itokawa and C-type asteroid Ryugu can secure superiority in sample return exploration, regarding (1999JU3) are the targets of Hayabusa and Hayabusa-2, frontier solar exploration missions (Fig. 4). respectively. For the solar power sail-craft, a D/P-type Trojan (2) Exploration technology: In the exploration of primordial asteroid is selected as the target. In addition, the solar power celestial bodies, it is important to enhance landing missions by sail-craft performs cruse science observations, like IKAROS. collecting pristine underground samples which have not been influenced by space weathering. When landing on celestial Table 1. Mission positioning bodies larger than 10km in diameter, the fuel consumption Spacecraft Navigation Landing Sampling Science increases. Therefore, it is necessary to investigate such Hayabusa Ion engine Mother Surface Itokawa [S-type] Spacecraft asteroids by a separate lander. In addition, it is also required to IKAROS Solar sail - - Cruising observation obtain in-situ analysis because of the long duration of a round Hayabusa-2 Ion engine Mother Surface Ryugu (1999 JU3) trip mission. Considering the above, in this plan, the Spacecraft [C-type] technology demonstration is carried out in conjunction, to Solar Power High-Isp Ion engine Lander Surface and Trojan asteroid Sail-craft Large solar sail underground [D/P-type] realize the following mission sequence: The solar power sail Cruising observation performs a rendezvous with a Trojan asteroid and lets a lander land on the surface. The lander then collects surface and 3. Initial Design of Solar Power Sail-craft Mission underground samples, and carries out in-situ analysis. Sample return is also performed in Plan-B. 3.1. Trajectory design (3) Scientific observation: It can be considered that the targets If the diameter of the asteroid is more than 20km, there is a for small celestial body exploration will shift to D/P-type high probability of it being D/P type. From the point of view asteroids, as higher primordial and farther small celestial of landing, the gravity should be as small as possible. bodies. In particular, unexplored D/P-type asteroids are Therefore, Trojan asteroids with diameters between 20~30km predominant in the population of Jupiter Trojan asteroids. In are selected as target candidates. The prerequisite of the this mission, direct exploration which includes both landing trajectory design is as follows; on a Trojan asteroid and a round trip mission, can be achieved. -Launch: 2022 Furthermore, several astronomical science observations by -Initial mass at Earth departure/launch: 1300kg EXZIT, GAP2, ALDN2 and MGF are expected to provide -Initial mass at Trojan departure: 1100kg first-class scientific results in the early stages of this mission. -IES specific impulse: 7000 seconds It is also expected to provide a breakthrough of deep space

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-IES thrust: 26.1mN per unit at 100% slot ring -IES use power: 1600W per unit at 100% slot ring -IES available power: max 2600W at 5.2AU, sun angle 0deg -IES number of operating units: max 2 units during 1.4-year-EDVEGA and max 3 units during 2-year-EDVEGA -IES operation rate: max 70% Fig. 6. Rosetta and Philae -Sun angle: max 45deg -Stay period at Trojan asteroid: min 1 year The overview of each sub-system is as follows. A trajectory example for Plan-B is shown in Fig. 5. The -Ion engine system (IES): 6 ion thrusters are mounted. In launcher configuration is H2A204; the target object is order to spin up / down, three thrusters each are inclined 2001DY103; the outbound transfer period is about 13 years. towards the spin up / down direction. Four IES power The required fuel mass is decreased and inclination is changed processing units (IPPU) are equipped account for redundancy. by a Jupiter swing-by. In Plan A, the flight time to the same -Communication system (COM): Two X-band transponders asteroid can be shortened to about 11 years by using are equipped. Low gain antenna (LGA-A, LGA-B) are used in additional ΔV, because the sample return is not performed. the Earth neighborhood. The medium gain antenna (MGA) is S/C mainly used during cruising phase. The high gain antenna 1 Earth (HGA) is mainly used during the rendezvous phase. HGA and 0.5 MGA can be pointed to Earth by a despun platform. The 0 Y[AU] communication rate of the HGA is aimed to provide a data -0.5

-1 transmission rate of 16Kbps by using the DSN in the Jupiter

-2 -1.5 -1 -0.5 0 0.5 1 zone. X[AU] -Thermal control system (TCS): The -Z plane is set as the 2-year-EDVEGA 1 main radiating face and the IPPU is mounted on the -Z plane. Jupiter S/C Asteroid Asteroid 0.8 In order to cope with the significantly changing thermal

0.6 environment and to reduce the heater power, thermal louvers S/C and the thermal switches are utilized. 0.4 Earth SJ-Fix[-] S/C -Electric power system (EPS): A Sub solar cell (SUBCELL) 0.2 mounted on the +Z plane is used prior to deployment of the Asteroid 0 power sail. Since the voltage of the cells is significantly 0 0.2 0.4 0.6 0.8 1 1.2 SJ-Fix[-] changed by the distance from the sun, it is equipped with a Earth to Jupiter Jupiter to Asteroid

0.2 function to switch the series number. S/C Asteroid Earth 0 -Reaction control system (RCS): A chemical propulsion

S/C -0.2 system driven at ultra-low temperatures with much less heater

-0.4 power is adopted. The piping system is divided into two lines Asteroid SJ-Fix[-] -0.6 for redundancy. S/C -0.8 -Attitude control system (ACS): Star tracker (STT), Spin type Jupiter Asteroid -1 sun sensor (SSAS), inertial reference unit (IRU) and optical 0 0.5 1 1.5 SJ-Fix[-] navigation camera (ONC) are equipped. The IES, reflectivity Jupiter to Earth Asteroid to Jupiter control device (RCD) and RCS are used to control attitude. Phase Start End IES dV[m/s] -Data handling unit (DHU): Each device is connected through 2-year-EDVEGA 25/9/2022 24/7/2024 1442 a space wire router (SpW). Mission system equipment is Earth to Jupiter 24/7/2024 22/12/2026 - connected to the mission processing unit (DE). Jupiter to Asteroid 22/12/2026 1/1/2036 1471 -Mission system (MS): EXZIT, GAP2, ALDN2, MGF, Rendezvous 1/1/2036 1/1/2037 - Asteroid to Jupiter 1/1/2037 12/9/2050 3686 imaging spectrometer (IS), lander and re-entry capsule are Jupiter to Earth 12/9/2050 29/7/2053 - carried.

The structural design and equipment layout of the solar Fig. 5. Trajectory for Plan-B power sail-craft is shown in Fig. 7. The entire structure is 3.2. System design composed of octagonal side panels and upper / lower panels. The initial design of the solar power sail-craft has been A cylinder structure is equipped inside of the octagon. A path conducted. Wet mass, which includes a 100kg lander, is for transferring the sample from lander to re-entry capsule is 1285kg. This means that the trajectory plan is feasible, as the provided in the cylinder. Re-entry capsule, HGA, MGA, and estimated spacecraft mass is within the considered mass of SSAS-H are located on the +Z plane. Lander, EZXIT, GAP2, 1300kg. Thus a solar power sail-craft, with a mass of about IS, ONC-T/W, STT-A/B and ion engine thrusters are located 1.3 tons will be able to transport a 100kg lander to the Trojan on the -Z plane. CIGS, ALDN2, MGF and RCD are attached asteroid. On the other hand, Rosetta with its mass of 3 tons on the sail. transported the Philae lander of the same 100kg mass to the comet 67P/Churyumov–Gerasimenko, which is located closer than the Trojan asteroids (Fig. 6). This difference indicates the superiority of the solar power sail.

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Upper Panel Re-entry MGF Capsule Φ 1999 mm

CIGS Xe Tank Side Panel Sail Storage Hydrazine Oxidizer 1100 mm Structure Tank Tank ALDN2 Lander

RCD Lower Panel

Φ 1860 mm ZSC

XSC Sail 48.7m YSC

Distance between Fig. 7. Structural design and equipment layout X Side panels SC 1600 mm 3.3. Mission Analysis at Trojan Asteroid

Path Since the solar power sail-craft has a large sail and spins, it is very risky for it to land on the asteroid itself. A lander is Center Cylinder therefore separated and performs the landing, sampling, and Φ 750 mm in-situ analysis. It delivers a sample to the solar power sail-craft by rendezvous and docking for sample return in Plan-B. The operational policy after the arrival at the Trojan asteroid is as shown in Fig. 8. 1) The solar power sail-craft is placed in a hovering home position (HP) above the asteroid, identical to Hayabusa. Since Y RCS SC the size of the target asteroid is large, the HP altitude is Thruster x8 250~1000km to conserve fuel needed for maintaining this HP. XSC SUBCELL 2) The solar power sail-craft performs global mapping and XLGA-A scientific observations from the HP. These are used for the SSAS-H landing site selection.

XHGA 3) The solar power sail-craft performs a rehearsal of the lander deployment by descending to a lower altitude and ascending again. This makes it possible to investigate the asteroid surface around the selected landing point in detail. Re-entry Capsule XMGA 4) The solar power sail-craft descends again, and separates the lander at an altitude of 1km. 5) The solar power sail-craft ascends to an altitude of 50km and waits for lander. 6) The landing method is not a free fall but a soft landing utilizing an RCS. Antenna for 7) The lander collects the surface and underground samples. Lander GAP2 8) The collected sample is conveyed to the mass spectrometer LGA-B ONC-T/W and in-situ analysis is performed. IS 9) The lander takes off to rendezvous with the solar power STT-A/B sail-craft. 10) The lander docks with the solar power sail-craft and delivers the sample. EXZIT 11) The lander is separated again and decommissioned. Meanwhile, the solar power sail-craft returns to its original Thermal HP. Lander Louver x 5 The structural design and equipment layout of the lander is Ion Engine shown in Fig. 9. It is composed of an octagonal shape with Thruster X SC legs at the bottom. Unlike the Philae, the lander is equipped x6

YSC with an RCS consisting of 12 cold gas thrusters. Since the sun distance of Trojan asteroids is about 5.2AU, significant power

5 generation by solar cells cannot be expected. Therefore, the In rendezvous phase, lander navigation is supported by the lander is exclusively driven by a battery. Science equipment is solar power sail-craft. In particular, this is a new point to assigned a mass of 20kg, including sampling devices. The wet measure the relative direction between the solar power mass is satisfying the mass of 100kg assigned to the lander. sail-craft and the lander by using the lander’s phased array The lander has two sampling devices to collect the surface antenna as an RF sensor. and underground samples. For the former, a sampler horn, Because the solar power sail-craft is spinning and the delay similarly to that of Hayabusa is used. For the latter, a time is large, the docking should be conducted autonomously pneumatic drill excavates a 1m regolith layer using as shown in Fig. 11. At first, the lander extends the extension high-pressure gas as shown in Fig. 10. This device has a boom. Next, the lander approaches toward the solar power telescopic structure. At first, extension and excavation are sail-craft to insert the boom into the holding space. Due to the performed at the same time. Next, additional high-pressure tapered shape of the holding space, the tip of the boom is gas is released and underground samples are collected. If there guided to the connection part regardless of some errors of is a rock, a projectile is shot to generate the fragments of the guidance, navigation and control. The tip of the boom is samples. connected by electromagnetic force. This is to make the HP (250~1000km) second separation of the lander easier. Plan-B Figure 12 shows the sample transfer sequence. The samples are first transferred to the container called sample catcher via Plan-A the induction pathway at the sampling. The extension mast pushes the sample catcher up to the re-entry capsule after the docking is completed. 50km Sail-craft 1km

Fig. 8. Operational policy at the Trojan asteroid 650mm

400mm 630mm

Fig. 11. Sequence of the berthing method

830-930mm

Sail-craft

Fig. 12. Schematic chart of the sample transfer

Science Power 4. Conclusion RCS DHU AOCS Communication Sampler horn Pneumatic drill In this paper, a direct exploration mission of the outer planetary region by a solar power sail-craft was proposed. In Fig. 9. Structural design and equipment layout of the lander regard to navigation technology, the transportation of a lander

and a first trip to a Jupiter Trojan asteroid is demonstrated. In regard to exploration technology, landing, surface and underground sampling, and in-situ analysis by a lander is 1m demonstrated. Scientific observations during the Regolith Layer of the Asteroid interplanetary cruise as well as at a D/P-type Trojan asteroid (2) Sampling by High-Pressure Gas are also implemented. Sample Flow Projector (3) Shooting Projectile (1) Extension & Excavation With this mission, the solar power sail can lead future solar if necessary by High-Pressure Gas system exploration, as well as provide a breakthrough of space Fig. 10. Underground Sampling astronomy as a new scientific field.

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