Trans. JSASS Aerospace Tech. Japan Vol. 10, No. ists28, pp. Tk_37-Tk_40, 2012 Topics

The Magnetospheric Orbiter and the Explorer in EJSM ( Jupiter System Mission)

1) 2) 2) 2) 3) By Sho SASAKI , Masaki FUJIMOTO , Hajime YANO , Takeshi TAKASHIMA , Yasumasa KASABA , 4) 4) 2) 2) 2) Yukihiro TAKAHASHI , Jun KIMURA , Yuichi TSUDA , Ryu FUNASE , Osamu MORI 2) 2) Stefano CAMPAGNOLA , Yasuhiro KAWAKATSU

1)National Astronomical Observatory of Japan, Oshu, Japan 2)The Institute of Space and Astronautical Science, JAXA, Sagamihara, Japan 3)Tohoku University, Sendai, Japan 4)Hokkaido University, Sapporo, Japan (Received June 27th, 2011)

EJSM (Europa Jupiter System Mission) is an international Jovian system mission with three . Coordinated observation of Jovian is one of the important targets of the mission in addition to icy satellites, atmosphere, and the interior of Jupiter. JAXA will take a role on the magnetosphere spinner JMO (Jupiter Magnetospheric Orbiter), whereas ESA will launch JGO (Jupiter Orbiter) and NASA will be responsible for JEO (Jupiter ). One possibility is to combine JMO with a proposed mission of JAXA for Jupiter and one of Trojan . Since Trojan asteroids could be representing raw solid materials of Jupiter or at least outer bodies, involvement of Trojan observation should enlarge the scope and enhance the quality of EJSM.

Key Words:Jupiter, Jovian Magnetosphere, Trojan Asteroids, Europa, Ganymede

1. Introduction: EJSM to investigate the 3D spatial structure of magnetosphere, the multi-spacecraft mission is desiable1). From the beginning of Jupiter is the largest in the solar system. It is a EJSM (former Laplace), JAXA and Japanese members rapidly rotating gaseous body whose main composition is committed with the mission planning with ESA and European hydrogen and . Jupiter has various satellites, four of scientists. The initial plan was that JAXA will take a role on which were discovered by 400 years ago. Jupiter has the magnetosphere spinner JMO (Jupiter Magnetosphere 1) the strongest planetary magnetosphere in the solar system . Orbiter) and JMO would be launched and transported together And recent discoveries of exoplanets, especially hot , with ESA’s orbiter. The original plan was similar to the suggest that Jupiter should represent a body not only in the framework of the BepiColombo mission where ESA 2) solar system but in the . is responsible for the launcher and MPO (Mercury Planetary The Jupiter System, with Jupiter and its satellites, can be Orbiter) and JAXA provides MMO (Mercury Magnetosphere considered as a small . The Jupiter system Orbiter). EJSM (formerly "Laplace") was selected as one of was observed by several flyby missions such as future ESA mission candidates of L-class Cosmic Vision and 11, and 2, Cassini, and (2015-2025) 1). Then NASA with the Europa orbiter investigated by Galileo orbiter and its atmospheric probe. participated in the mission plan. ESA will take charge of However, data amount from Galileo spacecraft was very JGO () and NASA will be limited due to the malfunction of high-gain antenna. We responsible for JEO (). However, due now have more information on the Saturnian System (being to the increase in the resource and mass of JGO, JAXA is provided by Cassini) than that on the Jupiter system. requested to launch JMO by a vehicle of its own. Then, we by NASA was launched in 2011 and will start observation of seek a possibility to combine JMO with a proposed solar Jupiter in 2016. Since the main target of JUNO is structure power sail mission of JAXA for Jupiter and one of Trojan and composition of Jupiter and JUNO takes polar , asteroids. information of Jovian satellites would not increase. JUNO JAXA already started a study of a solar sail for deep space has a magnetometer and some plasma instruments but explorations in early 2000s. The ISAS (Institute of Space and preferable multi-satellite observation cannot be fulfilled. Astronautical Science) of JAXA once evaluated a mission EJSM (Europa Jupiter System Mission) is a planned Jovian proposal for a new engineering verification spacecraft, which system mission with three spacecraft aiming at coordinated is called the solar power sail, a hybrid propulsion system of observations of the Jovian satellites especially Europa and the solar sail and ion engines. It would demonstrate 1) magnetosphere, atmosphere and interior of Jupiter . In order technologies necessary to explore the outer planet region in

Copyright© 2012 by the Japan Society for Aeronautical and Space Sciences and ISTS. All rights reserved.

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the solar system. Together with a solar sail (photon to Jupiter’s rotation by the auroral current system, generation propulsion), it should have very efficient ion engines where of Jupiter’s intense radiation belts4). electric power is produced by very thin solar panels within the The Jovian magnetosphere is the most intense particle sail. JAXA has already experienced ion engine in the accelerator in the planetary system. Sulfur ions that came successful mission 4). out of at the energy of less than 1 eV are somehow An engineering mission IKAROS (Interplanetary Kite-craft energized up to > 10 MeV within the magnetosphere. It is Accelerated by Radiation Of the ) 5) was launched in 2010 obvious that turbulence driven by the fast rotation of the together with Climate Orbiter AKATSUKI 6). planet and magnetic reconnection in the magnetodisk are IKAROS was operated successfully where the folded sail playing crucial roles. (20m x 20m) was developed from the spin-stabilized satellite, Ganymede has an intrinsic magnetic field and occupies a and then it succeeded in utilizing the solar radiation pressure small magnetosphere in the huge Jovian magnetosphere7). for the navigation and in the tests of thin-film solar power Interaction of Jovian particles and Ganymedean panels. magnetosphere is another intriguing target of plasma science. From the experience of IKAROS, we consider that a It is desirable that JMO shall monitor the Jovian mission with a large (100m-scale) solar power sail can transfer magnetosphere while JGO is observing Ganymedean large payload mass to Jovian system. The original plan will environment. During the initial stage of orbital maneuver, take about 4-6 years to go to Jupiter, and the extend mission JMO by itself will flyby with Ganymede and observe its will reach to the Jovian L4 Trojan asteroids after using environment. As for JEO, JMO can monitor the Jovian gravitational swing-by with Jupiter 3). At present we are magnetosphere while JEO is observing Europa. However, studying a mission to Jupiter and one (or two) of Trojan because of strong radiation environment around Europa’s asteroids, which are primitive bodies with information of the orbits, JMO approach to Europa would be limited. early solar system as well as raw solid materials of Jovian system. JMO will be released and inserted to the around Jupiter using a chemical thruster, before the main 3. Orbits and Instruments of JMO spacecraft flies by Jupiter to direct Trojan asteroid using gravity assist. Apoapsis of JMO around Jupiter will be During the initial periods of Jupiter observation phase of decreased by chemical thrusters and gravity assists by EJSM, the Jovian magnetosphere is investigated by the three satellites. The main spacecraft will cruise for about 5 years spacecraft. JMO will take the largest periapsis (> 100RJ: RJ to one of Trojan asteroids. being the radius of Jupiter). If possible, we set the periapsis

as small as Io’s orbit (5.9RJ) to observe the role of particles from Io on the magnetosphere. After JEO and JGO enter 2. Jovian Magnetosphere orbits around Europa and Ganymede, respectively, JMO will take equatorial orbits with periapsis at Europa or The Jovian magnetosphere is driven by the fast rotation of Ganymede’s orbit and apoapsis around 100RJ. JMO will Jupiter and populated by ions coming mainly from its determine the outer boundary condition of when JEO and satellites, especially Io. Io has a lot of active volcanoes that JGO observe the satellite-magnetosphere interaction. In the supply SO2 particles and ions are distributed to form a torus final stage where JEO and JGO would have stopped along Io’s orbit. The jovian magnetosphere is the entrance to operation, using satellite flybys, we can increase the orbital the astro-plasma world. Explosive phenomena require inclination of JMO to observe off-equatorial characteristic of ideal-MHD to break down. How this can be done in an magnetosphere and polar regions of Jupiter. Numerical astro-plasma situation where the basic scale length is much simulations by Stefano Campagnola show that using multiple larger than the non-MHD characteristic scales (ion and gravity assists by the inclination of JMO can be electron scales) is not clear at all. This question cannot be enhanced up to 35deg in a half year and 55deg in two years8). answered without in-situ observation around the magnetically High inclination enables JMO to observe polar region of energetic planet Jupiter. Jupiber and off-equatorial (large volume in space) region of Important astrophysical plasma processes that can be Jovian magnetosphere. investigated in Jovian magnetosphere are (i) dynamics of a JMO will have energetic particle instruments package magnetodisk, with different mechanisms of angular including low-energy plasma spectrometers (< a few keV), momentum exchange and dissipation of rotational energy, (ii) medium energy particle detectors (1 – 80keV), energetic electro-dynamical coupling between a central body and its particle detectors (50keV – 1MeV), very high energetic satellites, and (iii) global and continuous acceleration of particle detectors (>1MeV) and DC electric field / magnetic particles. Electrodynamic interactions of the Jovian system field and plasma wave instrument package including a include generation of plasma at the Io torus, magnetosphere / magnetometer and plasma/wave instrument. JMO will also satellites interactions, dynamics of a giant plasma disc coupled have remote sensing instruments such as an ENA imager, an

Tk_38 S. SASAKI et al.: Jupiter Magnetospheric Orbiter and Trojan Explorer in EJSM (Europa Jupiter System Mission)

EUV spectrometer, and a camera for the observation of execute cooperative observation with JGO and JEO when they Jupiter, Io torus, and magnetosphere. JMO will have a dust are orbiting around Ganymede and Europa. There is detector to investigate interaction between motion of fine possibility that cruise time from the launch to Jupiter is as dust particles and strong magnetic field. Both JGO and short as 4 years. Then we can conduct nearly full JEO will also have plasma instrument packages with a cooperative observation from the beginning. Collaborating magnetometer 3). with plasma instruments on board JEO and JGO, JMO will investigate the fast and huge rotating magnetosphere to clarify the energy procurement from Jovian rotation to the magnetosphere, to clarify the interaction between the solar wind the magnetosphere. JMO will clarify the characteristics of the strongest accelerator in the solar system. JMO will investigate the role of Io as a source of heavy ions in the magnetosphere.

5. Trojan Asteroids

Trojan asteroids are located at the L4 and L5 Lagrange points in the Jupiter-Sun system. At present, there are two models for the origin of Trojan asteroids9. One is that they are leftovers of the that formed the Jovian systems. Another is so-called that Trojan Fig. 1. Cooperative observation among JMO, JEO and JGO. During asteroids are captured when Jupiter and entered 1:2 9) the stages around Jupiter, JEO will target Io and Europa and JGO will resonance orbits . They should have come from outer solar target Ganymede and Callisto. JGO will enter orbits around Ganymede system, probably some of objects. The key before JEO will enter orbits around Europa. Even if JMO is launched in scientific targets involves characterization of the chemical and 2022, JMO can execute cooperative observation with JGO and JEO. mineralogical composition of asteroid surface, characterization of the bulk physical properties and interior structure of a Trojan (especially using density evaluation), and 4. Launch Timing for International Collaboration. observation of the present surface geology and its relevance to past evolution operated by surface processes including The Japanese Solar Power Sail mission with JMO will take impact-related processes. 10) about 6 years to Jupiter and 5 years more to a Trojan asteroid Most of Jovian L4 Trojan asteroids are D-type asteroids , around L4. Since synodic encounter period between the poorly understood taxonomic types from neither ground and Jupiter is about 400 days, we have the launch observation nor meteoritic analyses due to the window nearly every year. Currently JGO and JEO will be lack of possible analog chondrites discovered on the earth. launched in 2020 if JGO is finally selected as the first L-class D-type asteroids could be remnants of short-period cometary mission of Cosmic Vision *. Since the cruising time of nucleus. If detailed observation revealed the connection JMO is nearly the same as JEO and JGO, JMO can observe between Trojan asteroids and cometary nucleus, this would Jovian magnetosphere cooperatively from the beginning if we support Kuiper belt origin model. can launch JMO in 2020 (Figure 1). The stages of orbiting Based on the heritage of HAYABUSA and HAYABUSA-2 around Jupiter are more than 2 years for JGO and JEO before missions, we propose basic instrument packages consisting of entering circumsatellite orbits.. (1) a multiband UV-Visible high-resolution imager (for geology and basic composition), (2) an infrared spectrometer with wavelength up to 3 micron (for water/ice detection), (3) a * Additional note to the proof: thermal IR sensor (for surface properties such as regolith/rock Since JEO was not placed at the first priority among planetary ratio), and (4) a laser altimeter (for geology, shape, and flagship missions of NASA in the latest Decadal Survey of geodesy). We may have more instruments according to Planetary Sciences, it became difficult to launch JEO by mid various mission scenarios. 2020’s. Now JGO group proposes the single launch of JGO in Some additional instruments are proposed for the cruising 2022. The revised mission called JUICE(The JUpiter ICy science, since it will take more than 10 years between the Explorer) includes two Europa flybys and high inclination phases. launch and the arrival at a Trojan asteroid. Those involve a If JMO is launched by 2024, JMO can collaborate with JGO. dust detector, an instrument for measuring cosmic background, Therefore, even if JMO is launched in 2022, we can still and a gamma-ray burst detector.

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6. Conclusion References

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