A Solar Sail gathers sunlight as propulsion by means of a large membrane while a Solar "Power" Sail gets electricity from thin film solar cells on the membrane in addition to acceleration by solar radiation. What's more, if the ion-propulsion engines with high specific impulse are driven by such solar cells, it can become a "hybrid" engine that is combined with photon acceleration to realize fuel-effective and flexible missions.
JAXA is studying two missions to evaluate the performance of the solar power sails. The project name for the first mission is IKAROS (Interplanetary Kite-craft Accelerated by Radiation Of the Sun). This craft will be launched with the Venus Climate Orbiter "AKATSUKI", using an H-IIA launch vehicle. This will be the world's first solar powered sail craft employing both photon propulsion and thin film solar power generation during its interplanetary cruise.

Vast, thin, and strong solar sail

A solar sail can move forward without consuming propellant as long as it can generate enough energy from sunlight. This idea was born some 100 years ago, but it had lots of technical hurdles such as the appropriate material and deployment method for the sail. Recently, we have finally seen some prospect of using this technology practically. The sail of the IKAROS is a huge square some 20 meters in a diagonal line, as thin as 0.0075 mm, and made from polyimide resin. On the membrane of the sail are not only thin film solar cells but also an attitude control device and scientific observation sensors. This thin and light solar sail membrane will be deployed using the centrifugal force of spinning the main body of the IKAROS before its tension is maintained. The deployment is in two stages. The first stage is carried out quasi-statically by the onboard deployment mechanism on the side of the main body. The second stage is the dynamic deployment. As this deployment method does not require a strut such as a boom, it can contribute to making it lighter, thus can be apply for a larger membrane.