OKEANOS (Oversize Kite-craft for Exploration and Astronautics in the Outer Solar system) was a proposed mission concept to Trojan asteroids, which share Jupiter's orbit, using a hybrid solar sail for propulsion; the sail was planned to be covered with thin solar panels to power an ion engine. In situ analysis of the collected samples would have been performed by either direct contact or using a lander carrying a high-resolution mass spectrometer. A sample-return to Earth was an option under study.

OKEANOS was a finalist for Japan's Institute of Space and Astronautical Science (ISAS)' 2nd Large Mission Class to be launched in 2026, and possibly return Trojan asteroid samples to Earth in the 2050s. The winning mission was LiteBIRD.

The OKEANOS mission was 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.

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 and Technology. LiteBIRD, a cosmic microwave background astronomy telescope, was selected.

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 were 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.

The spacecraft was projected to have a mass of about 1,285 kg (2,833 lb) including a possible lander and would have been equipped with solar electric ion engines. The 1600 m² sail would have had a dual purpose of solar sail propulsion and solar panel for power generation. If a lander had been included, its mass would have been no greater than 100 kg. The lander would have collected and analyzed samples from the asteroid. A more complex suggested concept would have had the lander take off again, rendezvous with the mothership and transfer the samples for their transport to Earth.

The unique proposed sail was a hybrid that would have provided both photon propulsion and electric power. JAXA referred to the system as a Solar Power Sail. The sail would have been made of a 10 μm-thick polyimide film measuring 40 × 40 meters (1600 m²), covered with 30,000 solar panels 25 μm thick, capable of generating up to 5 kW at the distance of Jupiter, 5.2 Astronomical Units from the Sun. The main spacecraft would have been located at the center of the sail, equipped with a solar-electric ion engine for maneuvering and propulsion, especially for a possible sample-return trip to Earth.

The spacecraft would have used 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. As with the IKAROS, the solar angle of the sail would have been 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 to change its orientation.