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.

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Japan will dominate solar-sail technology if this one works, but then maybe that’s a good thing? JAXA has an up-and-down track record for getting its bigger ideas funded – hopefully this one will find consistent funding.

Hi Folks

Assume we have a total ship and sail mass of about 300,000 metric tons. The ship will initially start its journey with an acceleration of 0.1 G or about 0.981 meters/[sec EXP 2]. Further assume that the beam power is adjusted so that the beam power received by the ship is always 9 x 10 EXP 16 Watts and the beam force is always 3 x 10 EXP 8 Newtons on the ship’s sail. As a result, the power of the beam emitted at the source grows by a factor of z for a differential emitted beam element that eventually impinges on the sail of the space craft traveling a velocity such that the power transferred to the sail by the differential emitted beam element is always 9 x 10 EXP 16 watts. In other words, the beam power is increased to match the condition of constant beam power received by the sail where the constant power is always 9 x 10 EXP 16 watts.

As our outward bound craft reaches:

v = + 0.9 C, the portion of the beam impinging on the craft was emitted at a power of (Z)[9 x (10 EXP 16)] Watts where Z + 1 = (Lambda o)/(Lambda s) = {[1 + (0.9c/c)]/[1 – (0.9c/c)]} EXP (1/2). For z = 3.3589, the source beam power is 3.02 x (10 EXP 17) watts. The relativistic gamma factor for the ship is {1/[1 – [(v/c) EXP 2]]} EXP (1/2) = {1/[1 – [(0.9c/c) EXP 2]]} EXP (1/2) = 2.294.

v = + 0.99 C, the portion of the beam impinging on the craft was emitted at a power of (Z)[9 x (10 EXP 16)] Watts where Z + 1 = (Lambda o)/(Lambda s) = {[1 + (0.99c/c)]/[1 – (0.99c/c)]} EXP (1/2). For z = 13.1067, the source beam power is 1.1796 x (10 EXP 18) watts. The relativistic gamma factor for the ship is {1/[1 – [(v/c) EXP 2]]} EXP (1/2) = {1/[1 – [(0.99c/c) EXP 2]]} EXP (1/2) = 7.0888

One can consider the case of ever higher gamma factors such as might be obtainable for a cosmically distant future, although still within the star forming and burning era wherein a distribution of beaming stations has been deposited over hundreds of billions and even trillions of light years as follows:

v = + {c – [(10 EXP – 30)c]} , the portion of the beam impinging on the craft was emitted at a power of (Z)[9 x (10 EXP 16)] Watts where Z + 1 = (Lambda o)/(Lambda s) = {[1 + {{c – [(10 EXP – 30)c]}/c}]/[1 – {{c – [(10 EXP – 30)c]}/c}]} EXP (1/2). For the resulting z = 1,414,210,000,000,000, the source beam power is 1.27279 x (10 EXP 32) watts. The relativistic gamma factor for the ship is {1/[1 – [(v/c) EXP 2]]} EXP (1/2) = {1/[1 – [{{c – [(10 EXP – 30)c]}/c}EXP 2]]} EXP (1/2) = 707,100,000,000,000.

The beamed power can be increased for greater acceleration and shorter acceleration paths.

Naturally, some of the caveats are: 1) The ability to mitigate the effects of space time expansion based Doppler losses; 2) The ability to cloak the ship from extremely energetic cosmic rays; 3) The ability to negate drag energy or reprocess it so that its effects are neutralized; 4) The ability to produce sail materials that can meet the previous 2 conditions and yet still interact with the driving beams.

The beauty of solar sailing and beam sailing is that no fuel is required to be carried on board.

Just as we have gradually increased the speed at which we can do transportation on Earth, I see attaining ever higher gamma factors as an open ended problem to which ever more cleaver, innovative, and revolutionary technologies will be brought to bear. My guess is that the best known manners for which ever higher gamma factors can be obtained is by riding a beam of light.

We should also take note of the Planetary Society’s projects with solar sailing. I just got a call today from one of their representatives and have agreed to donate a small amount of money each month to help fund their efforts.

I wish Japan great sucess with their solar sailing projects.

I’m excited for JAXA and Japan – they’re on the verge of being the first to successfully use a solar sail. At the same time, I’m sad it isn’t the US, who’ve had an amazing track record of space firsts. I remember when the Planetary Society tried to get their solar sail up. From what I read, they’re going to try again this year.

It’s high time someone got moving on this technology. I suspect a lot of science agencies are watching JAXA. Hopefully, JAXA will not only succeed but also open up a whole new frontier in spaceflight.

Hi Folks;

Oops! My big mistake. The driving pressure of the beam for constant source output decreases by gamma squared. So take the square root of all of the above calculated values for gamma and keep the associated redshift and the results will be in the right ball park.

Once again, sorry for the miscalculations. This was a very large error on my part for which I should have noticed. It pays not to blog into the early morning hours.

Regards;

Jim