Small Lightsail Will Propel Cubesat

If you read science fiction, you are probably familiar with the idea of a light or solar sail. A very large and lightweight sail catches solar “wind” that accelerates a payload connected to the sail. Some schemes replace the sun with a laser. Like most things, sails have pros and cons. They don’t require you to carry fuel, but they are also maddeningly slow to accelerate and require huge sails since there isn’t much pressure produced by a star at a distance. So far not many real spacecraft have used the technique, IKAROS was the first back in 2010. However, this month should see the launch of a crowdfunded cubesat that will use a solar sail to move to a higher orbit.

The 5 kg satellite built by Georgia Tech students is about the size of a loaf of bread. Once in orbit, it will deploy solar panels and a square solar sail nearly 20 feet long on each side. Despite the nearly 350 square feet of area, the sail is less than 5 microns thick. You can see more details about the mission in the video below.

It is tempting to think of solar sails as a true analog to wind sails, but the analogy only goes so far. Tacking with a sailboat depends on the interaction between the wind pressure above and water pressure below. In space, there’s no other pressure to work with. However, according to the Solar Sails Wiki, you can change the angle of attack on the sail to cause your orbit around the sun to get bigger or smaller, which is a bit like tacking.

Since light pressure can constantly accelerate a sailing vehicle, you can achieve interstellar speeds with some patience. Then again, some people think alien light sails are already whizzing through our solar system.

19 thoughts on “Small Lightsail Will Propel Cubesat

      1. Well yes, but the rewards essentially give you an absurdly large solar sail made of plasma, and the device is about the size of a pickle jar. The power consumption is well within the capabilities of modern spacecraft. I’ve been following this quiet technology for a very long time, and I’m dumfounded as to why nobody has bothered to try it out in space.

        1. I’m having trouble with the “size of a pickle jar” reference, as I’ve seen various sizes of pickle jars.
          Could you give me a better comparative reference, in bananas, perhaps, or Olympic swimming pools?
          TIA!

        2. It gives you an absurdly large *magnetic* sail, not a solar sail. It pushes against the solar wind, not the light from the Sun.

          It’s not actually right to compare the size of the two, though, because again, they’re pushing against totally different things. The actual pressure the M2P2 system would feel would actually be *far less* than a solar sail (the solar wind is absurdly sparse, measured in particles per cubic centimeter).

          “I’ve been following this quiet technology for a very long time, and I’m dumfounded as to why nobody has bothered to try it out in space.”

          It’s a scale problem. You can’t test it on the ground because you need far too large a volume to test it in, and *simulating* it is a total nightmare because the device itself is small, but the plasma it generates is huge, so the simulations take forever.

          It’s an MHD problem, so really, the reason why it’s taking so long is the same reason that fusion reactors are taking a long time – magnetohydrodynamics is just a nightmare.

          1. Not that I’ve tried personally, but I’ve also heard that simulating this stuff is computationally intense. All the more reason to do physical tests in space rather than simulations. There’s enough indication that this thing works and works well that we ought to fire one up in space and have a look at what happens.

          2. I don’t know why you think there’s “more than enough indication that this thing works.” Specifically for the M2P2 case, the magnetosphere inflating is not guaranteed, since initially the solar wind will act to collapse it. The overall *idea* is certainly possible but actually finding the right design space to have it work at all isn’t trivial.

            Moreover you can’t use space as a skunkworks for this anyway – you need to understand how the plasma bubble is interacting with the solar wind in order to understand what thrust you can generate, and you can’t do that in space because you can’t instrument it.

            It’s even *more* important for a low-thrust propulsion system like this because it’s a constant burn. For engines it’s different, because you can always burn, measure what you got, and then burn again to correct, etc. But with a constant burn you don’t have that ability because the changes take a long time to develop.

    1. Por que no los dos?

      I’ve heard of hybrid craft designs that use both methods of propulsion to allow you to tack against the solar output like a a terrestrial sailcraft.

    1. I liked the Goldenwing Cycle books by Alfred Coppel. The first was the best in the series though, before the story got too tangled up with monsters from subspace or whatever.

      I loved the idea of the giant sailing star ships delivering and trading goods between systems like in the olden days and the description of the ship in orbit appearing like a great jeweled moth in the sky when viewed from the ground really captured me.

      1. Not sure if you’ve misinterpreted my comment. I literally mean ‘shinier’ as in ‘shine’ i.e. bright in the sky.
        I’m certainly not going to see the car with my naked eyes, but something so close, and with such a large surface area could be very shiny.

  1. “A very large and lightweight sail catches solar “wind” that accelerates a payload connected to the sail.”

    I know wind is in “quotes” there, but it’s still misleading: it’s catching light, not the solar wind, which is the particle flux from the Sun.

    “It is tempting to think of solar sails as a true analog to wind sails, but the analogy only goes so far. Tacking with a sailboat depends on the interaction between the wind pressure above and water pressure below. In space, there’s no other pressure to work with.”

    1) Solar sails really, really are a total analog to wind sails. It’s space that isn’t a good analogy to the ocean.

    2) There’s totally another pressure to work with – gravity. Which is how “tacking” a solar sail works. Ask a KSP player to navigate around the Solar System using only delta-V burns that are at least *partly* radially outward from the Sun (even if only marginally). It’s not a big deal. You want to go inward, you thrust against your orbit, and you fall towards the Sun.

    It’s just orbital mechanics, not anything specific to solar sails. If you’re at escape velocity, there isn’t anything you can do with a solar sail to stop that, for instance.

      1. That’s right – in fact, that’s the entire point of the sail, to make it as lightweight as possible while still making it as reflective as possible. Lightweight means that it has very little matter to interact with, so the vast majority of the particles making up the solar wind just zip right through it.

        You can actually create a sail to actually ride the solar wind: that’s a magnetic sail (the M2P2 design mentioned above is one of those), which creates a magnetic bubble that the charged solar wind pushes against. That would require power (and “propellant” in some cases to generate a plasma) and in many cases is a total MHD nightmare of a simulation, which isn’t a great thing for precision space travel. A proper solar sail is a much simpler problem.

  2. Lightsail 2 was not built by students at Georgia tech, but at Cal Poly San Luis Obispo. Developed and played for by the Planetary Society. At launch it is housed inside Prox-1 which was built at Georgia tech.

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