OKEANOS

OKEANOS, short for Oversize Kite-craft for Exploration and Astronautics in the Outer Solar System, was a concept for a mission to the Trojan asteroids that share Jupiter’s orbit. It would have used a hybrid solar sail, covered with thin solar panels, to generate power for an ion engine. Samples collected from the asteroids could have been analyzed in place, either through direct contact or with a lander equipped with a high-resolution mass spectrometer. Returning samples to Earth was also considered.

The mission was first proposed in 2010 as a companion to the Jupiter Magnetospheric Orbiter, part of the canceled Europa Jupiter System Mission – Laplace. Later, OKEANOS became a finalist for Japan’s Institute of Space and Astronautical Science (ISAS) 2nd Large Mission Class, competing against LiteBIRD, a cosmic microwave background telescope, which ultimately won. Had it been selected in April 2019 for development, OKEANOS would have launched in 2026 and might have returned Trojan samples to Earth in the 2050s, potentially coordinating with NASA’s Lucy spacecraft, which will fly by several Trojans in 2027.

Studying the composition of Jupiter’s Trojans could clarify how the Solar System formed, helping to decide between competing theories: that they are leftover planetesimals from Jupiter’s formation, fossils of the planet’s building blocks, or trans-Neptunian objects captured during planetary migration. The latest proposal included a lander for in situ analysis, with the most ambitious option involving sample retrieval and return to Earth.

The spacecraft would have weighed about 1,285 kilograms, including a possible lander of no more than 100 kilograms. It would have used solar-electric ion engines and a 1,600-square-meter sail made of 10-micrometer-thick polyimide film, covered with 30,000 solar panels each 25 micrometers thick. At Jupiter’s distance of 5.2 astronomical units from the Sun, the sail could generate up to 5 kilowatts of power. The sail design built on JAXA’s successful IKAROS mission, which launched in 2010 with a 14-by-14-meter sail. Like IKAROS, the sail’s orientation would have been adjusted by controlling the reflectivity of liquid crystal displays on its outer edges, using sunlight pressure to create torque.

The ion engine, called μ10 HIsp, was an improved version of the one used on the Hayabusa mission. Each of the four engines would have a specific impulse of 10,000 seconds, a power of 2.5 kilowatts, and a maximum thrust of 27 millinewtons. For a sample-return trip, the spacecraft would have needed 191 kilograms of xenon propellant.

The lander, a collaboration between the German Aerospace Center (DLR) and JAXA starting in 2014, would have weighed up to 100 kilograms and been deployed on a Trojan asteroid 20 to 30 kilometers in diameter. It would have used a 1-meter pneumatic drill powered by pressurized nitrogen gas to collect subsurface samples, including volatile materials like water ice. Some samples would be transferred to an onboard mass spectrometer for analysis. The lander’s scientific payload, including the sampling system, would not exceed 20 kilograms. It would operate on batteries for about 20 hours, performing autonomous descent, landing, sampling, and analysis. Samples could be heated to 1,000 °C for pyrolysis and isotopic analysis. The payload would include a panoramic camera (visible and infrared), an infrared microscope, a Raman spectrometer, a magnetometer, and a thermal radiometer.

For a sample-return mission, the lander would take off after sampling, rendezvous with the mothership hovering 50 kilometers above, and deliver surface and subsurface samples to a reentry capsule for return to Earth. The lander would then be discarded.