Not long ago we looked at IKAROS, an interesting solar sail concept out of JAXA, the Japanese space agency. Osamu Mori, project leader for the sail mission, offers up further background in an interview available at the JAXA site. IKAROS is notable because rather than relying solely on photons for propulsion, it would use solar cells covering part of the sail to generate electricity. In addition, the sail will operate with a unique attitude-control system. Here’s what Osamu Mori says about the latter:
The solar-powered attitude-control system uses a technology that controls the reflectivity of the sail. It works just like frosted glass: normally, the entire area of the sail will reflect sunlight, but by “frosting” part of the film, we can reduce the reflectivity of that area. When the reflectivity is reduced, that part of the sail generates less solar power. So by changing the reflectivity of the left and right sides of the sail, we can control its attitude.
Interesting stuff, and it fits into a broader context when you think about it in terms of a Jupiter mission. IKAROS is actually meant to be a technology demonstrator for evaluating solar sail performance in interplanetary flight. It will carry an ion engine along with the solar sail because at the distance of Jupiter, solar cells will provide only four percent of the efficiency they would offer near Earth. Japan’s intention is to go to Jupiter using solar cells, so both the ion engine and sail reflectivity adjustments can be seen as ways of stretching a known technology to see what is functional at these distances.
Not that there is any intention of going for Jupiter with the first mission. JAXA has been running vibration and thermal vacuum tests to shake out the systems of a small sail, no more than 14 meters to the side, made of polyimide resin some 7.5 micrometers thick (by comparison, a human hair is about 100 micrometers thick). Rather than Jupiter, the demonstrator will be launched along with the Venus Climate Orbiter AKATSUKI, deploying its sail a month after launch. Passing by Venus, IKAROS will navigate around the Sun as its systems are tested.
And as anyone working with sail concepts knows, deployment is a huge issue. Mori says that IKAROS will launch wrapped and folded around the body of the spacecraft. Centrifugal force generated by spinning the spacecraft will unfurl the sail, which will continue to be spin-stabilized, eliminating the need for a support structure. The method is illustrated in the image below:
Image: Deployment procedure for IKAROS sail. Credit: JAXA.
Spin-stabilization is beneficial not just given the complexity of deploying a supporting truss but also in terms of keeping the sail’s weight as low as possible. Says Mori:
IKAROS’s sail is small for a solar sail, but I think sails with a diameter of 50 to 100 meters will be developed in the near future. Unfurling such a thin film by spinning is still very difficult, though. We have gone through a long process of trial and error to figure out how we should fold the film so that it spreads smoothly. We conducted many experiments on the ground, and also launched the film aboard a sounding rocket. We even sent it high up in the sky in a big balloon, to spread the film in a near-vacuum environment. We experienced many failures, but we kept searching for the most reliable deployment method, and that led us to the model we’ve now built. I believe it will be successful.
This is an ambitious demonstrator, and the Jupiter mission that could develop out of it would be even more interesting as Japan develops its Jupiter and Trojan Asteroids Exploration Program. Can solar cells generate enough power to drive the ion engine, and will controlling the sail’s reflectivity prove useful for navigation? We’ll know more soon, as the launch date for AKATSUKI and IKAROS has now been set for May 18. More in this JAXA news page for IKAROS.