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Names Jupiter Trojan Asteroid Explorer
Mission type Technology demonstration,
possible sample return
Operator JAXA
Mission duration ≈12 years
>30 years for optional sample-return
Spacecraft properties
Spacecraft type Solar sail
Manufacturer ISAS and DLR
Launch mass 1,400 kg[1]
Landing mass ≈100 kg (optional lander)
Payload mass Spacecraft: 30 kg
Lander: 20 kg[1]
Dimensions Sail/solar panel:
40×40 m (1,600 m2)[2]
Lander: 65 × 40 cm[1]
Power Max: 5 kW at Jupiter[2]
Start of mission
Launch date Proposed: 2026 [2]
Rocket H-IIA or H3[1]
Jupiter Trojan lander
Landing date 2039 [2]
Main telescope
Wavelengths Infrared
Band X band
Capacity 16 Kbps [3]
Large-Class Missions

OKEANOS (Oversize Kite-craft for Exploration and Astronautics in the Outer Solar System) is a proposed mission concept to Jupiter's Trojan asteroids using a hybrid solar sail for propulsion; the sail is covered with thin solar panels to power an ion engine. In-situ analysis of the collected samples would be performed by either direct contact or using a lander carrying a high-resolution mass spectrometer. A lander and a sample-return to Earth are options under study.[4]

OKEANOS is a finalist for Japan's ISAS' 2nd Large-class mission to be launched in 2026,[2][5][6] and possibly return Trojan asteroid samples to Earth in the 2050s.[6][7]


The OKEANOS mission is a concept first proposed in 2010 to fly together with the Jupiter Magnetospheric Orbiter (JMO) as part of the cancelled Europa Jupiter System Mission - Laplace.[8]

In its latest formulation, the OKEANOS mission and LiteBIRD are the two finalists of Japan's Large Mission Class by the Ministry of Education, Culture, Sports, Science & Technology. LiteBIRD is a proposed cosmic microwave background astronomy telescope.[9]

Analyzing the composition of the Jupiter Trojans may help scientists understand how the Solar System was formed. It would also help determine which of the competing hypotheses is right:[10] remnant planetesimals during the formation of Jupiter, or fossils of building blocks of Jupiter, or captured trans-Neptunian objects by planetary migration. There are several options for this mission, and the most ambitious one proposes to include a lander that would perform some in situ analyses, and even send samples to Earth for extensive investigations.[11] If selected in December 2018 for development, the spacecraft would launch in 2026,[2] and may offer some synergy with Lucy spacecraft that will flyby multiple Jupiter Trojans in 2027.[12]


The spacecraft is projected to have a mass of about 1,285 kg (2,833 lb) if it includes a lander[3] and in any instance it would be equipped with solar electric ion engines.[5] The 1,600 m2 sail would have a dual purpose of solar sail propulsion and solar panel for power generation. If a lander is included, it must have a mass no larger than 100 kg and it would collect and analyze asteroid's samples. A more complex suggested concept would have the lander take off again, rendezvous with the mothership and transfer the samples for their transport to Earth.

Solar sail and solar panels[edit]

The unique sail is a hybrid that provides both photon propulsion and electric power, that JAXA calls Solar Power Sail.[3][13] The sail is made of a 10 μm-thick polyimide film measuring 40 × 40 meters (1,60000 m2), [2] and it is also covered with 30,000 solar panels 25 μm thick capable of generating up to 5 kW at Jupiter, which is 5.2 Astronomical Units from the Sun.[6][7][10] The main spacecraft would be located at the center of the sail and it would be equipped with a solar-electric ion engine for maneuvering and propulsion, especially for a possible sample-return trip to Earth.[4][6][7]

The spacecraft uses solar sail technology initially developed for the successful IKAROS (Interplanetary Kite-craft Accelerated by Radiation of the Sun) that launched in 2010, whose solar sail was 14 m × 14 m in size.[6][13] As with the IKAROS, the solar angle of the sail will be changed by dynamically controlling the reflectivity of liquid crystal displays (LCD) on the outer edge of the sail so that the sunlight pressure would produce torque.[14]

Ion engine[edit]

The ion engine intended for the mission is called μ10 HIsp and its specific impulse is 10,000 sec, power of 2.5 kW, and a maximum thrust magnitude of 27 mN for each of the four engines.[15][16] The electric engine system is an improved version of the engine from the Hayabusa mission, and it would be used for maneuvering and especially for an optional sample-return trip to Earth.[16][13] A preliminary study indicates the need for 191 kg of xenon propellant if it is decided to bring a sample back to Earth.[16]


Mass ≤ 100 kg (220 lb)
Dimensions Cylindrical, 65 cm diameter
40 cm height
Power Non-rechargeable battery
(≤ 20 kg)
Sampling Pneumatic
Depth: ≤1 m

The mission concept is still in development and several scenarios, targets, and architecture are being assessed. The most ambitious scenario contemplates in situ analysis and a sample-return using a lander. This lander concept is a collaboration among the German Aerospace Center (DLR) and Japan's JAXA, started in 2014.[3] The spacecraft would deploy a 100 kg lander[4][1] on the surface of a 20–30 km Trojan asteroid to analyze its subsurface volatile constituents, such as water ice, using a 1-meter pneumatic drill powered by pressurized nitrogen gas. Some subsurface samples would be transferred to the on board mass spectrometer for volatile analysis.[4] The lander's scientific payload mass, including the sampling system, would not exceed 20 kg. The lander would be powered by batteries and perform an autonomous descent, landing, sampling and analysis.[3] Some samples would be heated up to 1000 °C for pyrolysis for isotopic analysis. The conceptual payload for the lander would include a panoramic camera (visible and infrared), infrared microscope, Raman spectrometer, a magnetometer, and a thermal radiometer.[18] The lander would operate for about 20 hours using battery power.[1]

If a sample-return is decided, the lander would then take off, rendezvous and deliver the surface and subsurface samples to the mothership hovering above (at 50 km) for subsequent delivery to Earth within a reentry capsule.[5][3] The lander would be discarded after the sample transfer.

Conceptual scientific payload[edit]

On the lander
On the spacecraft
Attached to the sail

GAP-2 and EXZIT are instruments for astronomical observations, and are not intended to be used for studying Trojan asteroids. The two will conduct opportunistic surveys that will take advantage of the mission's trajectory. For GAP-2, the maximum 5.2 AU distance from Earth makes it possible to locate the position of Gamma-ray bursts with high precision by pairing it with terrestrial observatories. For EXZIT, as zodiacal light gets significantly weak beyond the asteroid belt, it enables the telescope to observe the cosmic infrared background for uncovering the comic dawn. MGF-2 is a successor of the MGF on board the Arase satellite, and ALADDIN-2, GAP-2 are successors of the respective instruments onboard IKAROS.

See also[edit]


  1. ^ a b c d e f g h SCIENCE AND EXPLORATION IN THE SOLAR POWER SAIL OKEANOS MISSION TO A JUPITER TROJAN ASTEROID. (PDF). T. Okada, T. Iwata, J. Matsumoto, T. Chujo, Y. Kebukawa, J. Aoki, Y. Kawai, S. Yokota, Y. Saito, K. Terada, M. Toyoda, M. Ito, H. Yabuta, H. Yurimoto, C. Okamoto, S. Matsuura, K. Tsumura, D. Yonetoku, T. Mihara, A. Matsuoka, R. Nomura, H. Yano, T. Hirai, R. Nakamura, S. Ulamec, R. Jaumann, J.-P. Bibring, N. Grand, C. Szopa, E. Palomba, J. Helbert, A. Herique, M. Grott, H. U. Auster, G. Klingelhoefer, T. Saiki, H. Kato, O. Mori, J. Kawaguchi. 49th Lunar and Planetary Science Conference 2018 (LPI Contrib. No. 2083).
  2. ^ a b c d e f g h i INVESTIGATION OF THE SOLAR SYSTEM DISK STRUCTURE DURING THE CRUISING PHASE OF THE SOLAR POWER SAIL MISSION. (PDF). T. Iwata, T. Okada, S. Matsuura, K. Tsumura, H. Yano, T. Hirai, A. Matsuoka, R. Nomura, D. Yonetoku, T. Mihara, Y. Kebukawa, M. ito, M. Yoshikawa, J. Matsu-moto, T. Chujo, and O. Mori. 49th Lunar and Planetary Science Conference 2018 (LPI Contrib. No. 2083).
  3. ^ a b c d e f Direct Exploration of Jupiter Trojan Asteroid using Solar Power Sail (PDF). Osamu Mori, Hideki Kato, et al. 2017.
  4. ^ a b c d Sampling Scenario for the Trojan Asteroid Exploration Mission (PDF). Jun Matsumoto, Jun Aoki, Yuske Oki, Hajime Yano. 2015.
  5. ^ a b c Trajectory Design for Jovian Trojan Asteroid Exploration via Solar Power Sail (PDF). Takanao Saiki, Osam Mori. The Institute of Space and Astronautical Science (ISAS), JAXA. 2017.
  6. ^ a b c d e JAXA Sail to Jupiter's Trojan Asteroids. Paul Gilster, Centauri Dreams. 15 March 2017.
  7. ^ a b c Huge sail will power JAXA mission to Trojan asteroids and back. Shusuke Murai, The Japan Times. 21 July 2016.
  8. ^ Sasaki, Shio, et. al. (2010). "Jupiter Magnetospheric Orbiter and Trojan Asteroid Explorer" (PDF). COSPAR. Retrieved August 26, 2015.
  9. ^ Roadmap 2017 — Fundamental Concepts for Promoting Large Scientific Research Projects (PDF). 28 July 2017.
  10. ^ a b The Solar Power Sail Mission to Jupiter Trojans (PDF). The 10th IAA International Conference on Low-Cost Planetary Missions. 19 June 2013.
  11. ^ Science exploration and instrumentation of the OKEANOS mission to a Jupiter Trojan asteroid using the solar power sail. Tatsuaki Okada, Yoko Kebukawa, Jun Aoki, etal. Planetary and Space Science. Volume 161, 15 October 2018, Pages 99-106. doi:10.1016/j.pss.2018.06.020.
  12. ^ ISAS Small Body Exploration Strategy. Lunar and Planetary Laboratory, The University of Arizona-JAXA Workshop (2017).
  13. ^ a b c IKAROS and Solar Power Sail-Craft Missions for Outer Planetary Region Exploration (PDF). J. Kawaguchi (JAXA). 15 June 2015.
  14. ^ Liquid Crystal Device with Reflective Microstructure for Attitude Control. Toshihiro Chujo, Hirokazu Ishida, Osamu Mori, and Junichiro Kawaguchi. Aerospace Research Central. doi:10.2514/1.A34165.
  15. ^ Lineup of Microwave Discharge Ion Engines. JAXA.
  16. ^ a b c Mission Analysis of Sample Return from Jovian Trojan Asteroid by Solar Power Sail (PDF). Jun Matsumoto, Ryu Funase, et al. Trans. JSASS Aerospace Tech. Japan Vol. 12, No. ists29, pp. Pk_43-Pk_50, 2014.
  17. ^ Science experiments on a Jupiter Trojan asteroid on the solar powered sail mission (PDF). O. Mori, T. Okada1, et al. 47th Lunar and Planetary Science Conference (2016).
  18. ^ Trojan asteroid probe (PDF) (in Japanese). JAXA.
  19. ^ EXZIT Telescope. JAXA.
  20. ^ Jupiter Trojan’s shallow subsurface: direct observations by radar on board OKEANOS mission. Alain Herique, Pierre Beck, Patrick Michel, Wlodek Kofman, Atsushi Kumamoto, Tatsuaki Okada, Dirk Plettemeier. EPSC Abstracts Vol. 12, EPSC2018-526, 2018. European Planetary Science Congress 2018.