Solar Sailing Comes of Age With IKAROS and Lightsail I

This is an article about lightsails, an idea that has been around for a long time, and is now approaching reality.  Both the Japan Aerospace Exploration Agency and America’s Planetary Society have plans underway to launch and test prototype lightsails.  The two projects are not in competition with each other, but are working together in a complementary effort to initiate and explore this new lightsail technology, which ultimately promises to be one of the most revolutionary concepts in space exploration.

For space kids who grew up reading science fiction, this idea requires no explanation.  For those unfortunate readers who did not have this experience, we offer this quick summary:

The warm, gentle sunlight that we feel here on Earth is really only a tiny fraction of the sun’s full output.  Even the hottest places on Earth- say, Death Valley or the Sahara Desert- are only receiving a small percentage of the solar radiation that hits the atmosphere above them.  Luckily for us, we are protected from most of it by that thick blanket of air.  Outside of that protection, the wind from the sun is a blasting torrent, a constant tsunami of radiation and particles.

And of course, sunlight exerts a certain amount of pressure.  The pressure is very weak down here on Earth, but if you get off the Earth and move into the full blast of the solar wind, everything changes.  Suddenly you’re in the full tsunami, and the pressure exerted by it is much greater.

Consider what you’ve got here.  It’s a stream of propulsive force which, in human terms, is inexhaustible- and unlike the intermittent thrust of rockets, this is constant propulsion, which allows you to build up enormous speed over time.  In the world of space exploration, this is the Holy Grail.  It is the thing that can finally free us from that necessary evil of space flight: fuel.  The sad fact is that when you’re using conventional rockets, the fuel is the biggest part of the weight.

Now, we’ll always need a big push to get out of the atmosphere and attain orbital velocity, and chemical rockets are still the only way to get that (though other ideas have been discussed- more on this in future articles).  However, once you get into orbit and you’ve got all that sunlight,  why not use it?  Throw out a kite and ride!

That’s the definition of a lightsail: a kite that uses the solar wind to move a spacecraft.  In the old days, this was pure sci-fi, because we didn’t have any materials that were strong and light enough to do the job, but recent advances in materials science have provided lightweight plastics that are bringing the goal within reach.   Not only that, but we now have a couple of possible embellishments that build on the basic concept and use the power of light in different ways.

As mentioned above, there are actual prototypes being readied for launch this year.  The Japan Aerospace Exploration Agency, a rising power in the field of space exploration, is planning to send up a craft called IKAROS in May.  In an effort to cut costs, the craft will be launched aboard the same rocket with Japan’s Venus Climate Orbiter, the partner to the European Space Agency’s Venus Express which we discussed a few weeks ago.

IKAROS takes the idea of a lightsail a step or two further.  Here, the plastic membrane is not only used for propulsion, but also contains three other systems: a thin-film electrical power generation system, a set of steering devices and a dust-counter.  They do all this on a layer of polyimide that is only .0075 mm thick.  When fully deployed, IKAROS will be a square with a diagonal length of 20 m.  Its mission will be in two stages.  In the first stage, the sail will be deployed and used to generate electricity.  This will be the first time a lightsail has been used for this purpose, and if the mission ended right there, it would have already started a revolution in the field of space electronics.

But hopefully, IKAROS will keep on going.  The second phase of its mission is to actually use solar power to navigate the craft.   The destination of IKAROS is uncertain, but it will be steered toward Venus.  As mentioned above, it will be sent into orbit on the same rocket with the Venus Climate Orbiter, and hopefully both craft will eventually arrive at that planet.

IKAROS is the first of two proposed Japanese missions.  The second one will take place in the late 2010′s, and will consist of a hybrid craft which combines all of the technology of IKAROS with an ion propulsion system.

The ion drive uses electricity generated by solar power to excite xenon fuel.  The excited fuel is focused into a jet by passing it between two powerful magnetic fields, and leaves the engine at high velocity.  The advantage of an ion engine is that the xenon fuel is capable of delivering a large amount of thrust in proportion to its weight, which means that a spacecraft can carry enough fuel to keep going for years.

When you combine this system with the lightsail idea, you get a hybrid craft that can use both systems to maximum advantage.   For instance, such a craft might use its lightsail while it’s near the sun, riding the solar wind and storing up electricity.  Later, when it gets farther out where the sunlight is weaker, it might fold up its sail and use the stored electricity to run the ion drive.

Here in the US, the Planetary Society is making its second attempt at testing a lightsail.  Their first one, Cosmos I, was tragically lost when its launch rocket crashed, but now the Society has embarked on an ambitious project to deploy three sails over the next few years.   While the Japanese project is focused on broad technologies that will be used for multiple projects in the future, the Planetary Society is focusing more on practical and specific jobs, such as monitoring the sun for solar storms and providing stable Earth observation platforms.

Their first sail, Lightsail I, will be launched this year and will demonstrate the deployment of the sail and its use for propulsion.  The second sail will do the same, but will move to a much higher Earth orbit.  The third sail in the Planetary Society’s program will leave Earth orbit and navigate to the Earth-Sun libration point, L1.  This will be an ideal location for weather-sensing satellites and other devices, which in the future will hopefully be propelled into their positions by lightsails.

The Planetary Society’s ambitions are set on greater goals someday.  Louis Friedman, the Society’s executive director, recently posted an article on their website about the glowing possibilities offered by this line of research.  One possibility that he brought up is the idea of using a lightsail with an Earth-based laser for propulsion instead of sunlight.  With something like that, you could send a beam of coherent light at another star and ride it all the way there.  When you arrived, you could set up another laser and point it back at Earth, then ride the beam back home.  Whereas the basic solar sail idea only allows travel within the solar system, the laser idea could give us access to the stars.

It’s a long way off, but someday it could happen.  In the meantime, we need to do the basic groundwork, and that’s what’s about to happen.  Both the JAXA craft and Lightsail 1 will go up this year, and the results of those projects will show the way to the future.

In 1964, Arthur C. Clarke wrote a short story called “The Sunjammer,” which was about a race between solar-propelled spacecraft.  It was published in a popular boys’ magazine, and was read by a whole generation of kids in love with space.  Some of those kids are probably working on these projects today- and some of them are also reading and writing about them.

Stick with us, and you won’t miss a thing.

Sources:
Satellites and Spacecraft: Small Solar Power Sail Demonstrator “IKAROS” at JAXA website:  jaxa.jp/projects/sat/ikaros/index_e.html

IKAROS Project page at JAXA website: jspec.jaxa.jp/e/activity/ikaros.html

Let’s Set Sail for the Solar System by a Solar Yacht!  at JSPEC website: jspec.jaxa.jp/ikaros_cam/e/03.html

Lightsail- the Future of Solar Sailing at the Planetary Society website:  planetary.org/programs/projects/solar_sailing/

Friedman, Louis D.: Lightsail: A New Way and a New Chance to Fly on Light at the Planetary Society website:  planetary.org/programs/projects/solar_sailing/tpr_lightsail.html