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JAXA Explores the Planets of the Solar System Space Yacht: using sunlight to cruise through space Osamu Mori Project Leader, Solar Power Sail Demonstrator IKAROS

World's first solar power sail demonstration

Q. What is the purpose of the Solar Power Sail Demonstrator IKAROS?

1/40 scale model of IKAROS
1/40 scale model of IKAROS

Attitude control system for IKAROS
Attitude control system for IKAROS
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Full view of IKAROS. The sail film wraps around the body of the probe.
Full view of IKAROS. The sail film wraps around the body of the probe.

The purpose of IKAROS is to demonstrate the technology of the Solar Power Sail. Simply put, the solar sail is a "space yacht." A yacht moves forward on water, pushed by wind captured in its sails. A solar sail is propelled by sunlight instead of wind, so it's a dream spaceship - it doesn't require an engine or fuel. Part of IKAROS's sail is covered by a solar cell made of an ultra-thin film, which generates electricity from sunlight.
IKAROS will be the world's first solar-powered sail. The mission has four parts:

1.
To deploy a large-scale thin sail in space. IKAROS has a square sail that is approximately 14 meters long on each side. The solar cell is made of a film that's only 7.5 micrometers thick. Human hair is 100 micrometers thick, so you can imagine how thin this film is.
2.
To generate electricity through the ultra-thin solar cell.
3.
To see if the sail-powered spacecraft can accelerate with the help of sunlight, as we have theorized.
4.
To develop solar-sail navigation technology. We are going to adjust the direction of reflected sunlight, so that IKAROS will move along a targeted orbit, and not just travel randomly depending on where the sun happens to be. In order to do that, IKAROS carries a gas-powered jet engine to change the direction of the sail. It also has a system that will allow it to change direction using only solar power, without using gas. We are going to test both of them.

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.
IKAROS will be launched together with the Venus Climate Orbiter AKATSUKI (PLANET-C), and for much of the journey they will travel to Venus side by side. IKAROS will deploy its sail about a month after launch, and our first step will be to make sure the thin-film solar cell is generating electricity. For about six months after that, we will be demonstrating the solar sail's acceleration and orbit-control functions. AKATSUKI will decelerate to enter the Venus orbit, but IKAROS will pass by Venus and navigate around the Sun. Where it heads will depend on how well the solar sail's orbit control function performs.


Q. Will there be any observation devices aboard IKAROS?

The purpose of IKAROS is to demonstrate the solar power sail, but since it is going to space anyway, we will take the opportunity to put two observation devices on it. One is a dust counter, which will evaluate dust distribution in the region of inner planets. The other is a device to observe polarization and direction of gamma ray bursts, which are thought to be explosion phenomena triggered by the birth of a star. I hope that IKAROS will make scientific discoveries as well as helping us demonstrate its technology.

Ultra-thin film created using Japanese technology

Q. What are the origins of solar-sail technology?

The shape of IKAROS's sail.
The shape of IKAROS's sail.
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The idea for a solar sail first appeared about 100 years ago. Since then, there has been a lot of research done on this in western countries as well as Japan, but so far no one has made a breakthrough. One of the reasons for this is that the technology didn't exist to reliably produce a lightweight thin film for the sail, which is very important. This film has to be made from a material that's not just lightweight but can withstand extreme radiation and heat in space. The material that meets these conditions is polyimide resin, which is used as a foam insulation for satellites. Once such a high-quality material became available, the development of a solar sail came much closer to reality. Today, Japan has the largest market share in the world for polyimide resin. We are currently leading the race to develop applications for this technology, and it would mean a great deal to us to be the first in the world to build a working solar sail.
Polyimide resin allows us to create a much lighter sail. As well as being extremely strong, it doesn't need glue, because it can be joined using heat sealing. Polyimide resin is originally yellow, but one side of IKAROS's sail is silver. This is because aluminum is vapor deposited on one side of the film, in order to reflect sunlight more efficiently. In addition, the film is reinforced in such a way as to prevent it from splitting all the way if it's ripped. If the solar sail is torn, its performance will decline slightly, but it can still continue its space travels.

Unique Japanese spin deployment system

Q. What's the current development status of IKAROS. Was there anything particularly difficult in the development process?

Deployment procedure for IKAROS's sail.
Deployment procedure for IKAROS's sail.
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U.S. LightSail-1 (Courtesy of the Planetary Society)
U.S. LightSail-1 (Courtesy of the Planetary Society)

Currently, we are testing. We're running vibration tests, thermal vacuum tests, etc. with actual spacecraft, in order to confirm that all the devices work properly. Such integrate test will continue right up to launch.
The most difficult thing was to develop the technology that would deploy the sail. The sail film doesn't have a supporting frame, and for storage at launch it will be folded and wrapped around the main body of the spacecraft. To unfurl the sail, the spacecraft will spin, generating centrifugal force. Once the sail is deployed, the main body will continue to spin, to maintain that centrifugal force and keep the sail open. This eliminates the need for a supporting frame for the sail film, so the spacecraft can be very light. Using the centrifugal-force method, a bigger sail is easier to unfurl. 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.

Q. What kind of solar sails have other countries built in the past?

The solar sail spacecraft Cosmos 1 was launched by the Planetary Society in the United States in 2001 and 2005, but both missions ended in failure due to rocket trouble. The Planetary Society is scheduled to launch LightSail-1 into Earth orbit at the end of 2010.

  
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