OKEANOS is a proposed mission concept to Trojan asteroids, which share Jupiter's orbit, 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 sample-return to Earth is an option under study. OKEANOS was a finalist for Japan's ISAS 2nd Large-class mission to be launched in 2026, and possibly return Trojan asteroid samples to Earth in the 2050s. The winning mission was LiteBIRD.
Overview
The OKEANOS mission is a concept first proposed in 2010 to fly together with the Jupiter Magnetospheric Orbiter as part of the cancelled Europa Jupiter System Mission - Laplace. In its latest formulation, the OKEANOS mission and LiteBIRD were the two finalists of Japan's Large Mission Class by the Ministry of Education, Culture, Sports, Science & Technology. LiteBIRD was selected, and it is a cosmic microwave background astronomy telescope. 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: remnant planetesimals during the formation of Jupiter, or fossils of building blocks of Jupiter, or captured trans-Neptunian objects by planetary migration. The latest proposal included a lander to perform in situ analyses. There are several options for this mission, and the most ambitious one proposed to retrieve and send samples to Earth for extensive investigations. Had it been selected in April 2019 for development, the spacecraft would have launched in 2026, and may had offered some synergy with Lucy spacecraft that will flyby multiple Jupiter Trojans in 2027.
Spacecraft
The spacecraft is projected to have a mass of about if it includes a lander and in any instance it would be equipped with solar electric ion engines. 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
The unique sail is a hybrid that provides both photon propulsion and electric power, that JAXA calls Solar Power Sail. The sail is made of a 10 μm-thick polyimide film measuring 40 × 40 meters, 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. 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. The spacecraft would use solar sail technology initially developed for the successful IKAROS that launched in 2010, whose solar sail was 14 m × 14 m in size. As with the IKAROS, the solar angle of the sail would be changed by dynamically controlling the reflectivity of liquid crystal displays on the outer edge of the sail so that the sunlight pressure would produce torque to change its orientation.
Ion engine
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. 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. A preliminary study indicates the need for 191 kg of xenon propellant if it is decided to bring a sample back to Earth.
Lander
The mission concept considers several scenarios, targets, and architectures. The most ambitious scenario contemplates in situ analysis and a sample-return using a lander. This lander concept was a collaboration among the German Aerospace Center and Japan's JAXA, started in 2014. The spacecraft would deploy a 100 kg lander 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. 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. 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, infrared microscope, Raman spectrometer, a magnetometer, and a thermal radiometer. The lander would operate for about 20 hours using battery power. If a sample-return was to be performed, the lander would then take off, rendezvous and deliver the surface and subsurface samples to the mothership hovering above for subsequent delivery to Earth within a reentry capsule. The lander would be discarded after the sample transfer.
GAP-2 and EXZIT are instruments for astronomical observations, and are not intended to be used for studying Trojan asteroids. The two would conduct opportunistic surveys that would 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 instrument on board the Arase satellite, and ALADDIN-2, GAP-2 are successors of the respective instruments onboard IKAROS.
