What are solar sails?
Solar sails are a creative and unique way to harness the sun’s energy for transportation, requiring no other fuel. Solar sails act just as the name implies: They are a physical sail, pushed by the sun’s energy. The sun constantly spews out a high-energy stream of photons. (Photons create the images “caught” by camera film, digital camera CCDs, and the rods and cones in our eyes. “Photograph” and “photon”… It’s not a coincidence. In fact, a solar sail is sometimes called a “photon sail”. But for this article, the literal – and perhaps more appropriate – name loses out to the conceptual one: Photon sails are out, and we’ll stick with “solar sails”.
The picture below is a simple example of how a solar sail works. (It’s also proof I’ve never taken an art class beyond third grade. Trust me: Keep reading, and the pictures get a LOT better.)
The sun shines, spraying photons in all directions. The solar sail is just a large panel, highly reflective and very lightweight. The force of the photons pushes the against the solar sail. The sail is attached to a spaceship (the green pickle-like thing above), and the force of the photons moves it away from the sun. The direction of movement can be changed by changing the angle of the solar sail. There’s not much to it, apart from mind-melting high-level mathematics needed to steer the thing, designing an ultra-light yet strong frame, and knowing how to manage the gigantic sail itself. A fairly simple concept, but executing it requires heavy brainpower.
In real life, of course, a solar sail is more than just a digitized mess like my poorly drawn example. For more detail, I got information from NASA’s Jet Propulsion Laboratory.
Neil Murphy is the Supervisor of the Space and Astrophysical Plasmas Group at NASA’s Jet Propulsion Laboratory in Pasadena, California, at the California Institute of Technology. Don’t let the long title fool you. The Space and Astrophysical Plasmas Group has many projects, but a few easily defined areas of interest. It explores our solar system and what it’s made of. It uses technology to gather information about planets and suns, and that information is used for furthering human knowledge and making our lives better on Earth. You may know the JPL from their Cassini-Huygens spacecraft mission, which was the first spacecraft to explore Saturn’s beautiful ring and moon system from orbit.
Neil does exciting work, at the edge of technology and off the edge of our planet. In fact, that’s part of the JPL’s creed, as exploring the outer ranges of space is a primary reason the organization exists.
Andy: Weight and density of a solar sail are extremely important. You mentioned sail material using mylar, aluminum or nanotubes. Does the sail itself need to be a physical sail? My understanding is that photons have some detectable affect on things like magnetic fields. Is it possible to design a gigantic non-physical solar sail to react to photons, or is this more within the realm of science fiction?
Neil Murphy: We use aluminized plastics and nanotubes. You really do have to have a physical sail. Magnetic fields interact, but not in the same way. It’s not a matter of can we do it, but a matter of how do we design it. This is where the difficulty comes in: Solar sail craft are a different beast. There’s no technological barrier in us doing this. We just have to do the job. The lighter you make the spacecraft, the better off you are.
Andy: The solar sail is extremely light, thin and flexible. What is the solar sail physical design like?
Neil Murphy: It’s a thin membrane supported by inflatable booms. The boom – a fibrous membrane – uses temperature, UV or other techniques to control rigidity. Use air to inflate it, but you maintain geometry in other ways. There are supporting guy wires that help keep it rigid. When we started working three or four years ago, one of the major concerns was how to handle this micron-thick film. [A micron is a millionth of a meter. A grain of salt measures about 60 microns.] Building a thin film so large and being able to handle it without ripping it to shreds, we now know how to do that. It’s so large and flexible, it had been very difficult to work with.
Andy: What are the restrictions preventing this technology from being developed and used today?
Neil Murphy: I don’t believe there are any. I think we’re ready to build and fly a flight mission. The first mission [probably mission ST9] needs to demonstrate the technology works. Because of the complexity with flying and controlling, we’ll use that information to design the next missions.
What does a solar sail look like?
The JPL was kind enough to provide pictures of proof-of-concept solar sails. Solar sails, needing to catch as many photons as possible, are by necessity very large. Can you spot the humans in each picture?
From Neil, the image below is from an ATK sail successfully deployed at ambient pressure and vacuum at NASA’s Glenn Research Center. The sail is 20 meters on a side.
Below is the sail by L’Garde, Inc, proposed for NASA’s ST9 mission. Picture provided by Tim Van Sant, ST9 Solar Sail Team Lead, NASA Goddard Space Flight Center.
Andy: What does the sail material actually look like?
Neil Murphy: It’s a little crinkly. Early on, the fact it was wrinkled was a concern as a loss of efficiency. But it’s not a very large loss. Multiple deployments show this is not a problem, and it even distributes stress more effectively.
Andy: What is the range of a solar sail flying in our solar system?
Neil Murphy: The effective range of a solar sail is roughly 2-3 AU.
Andy: An update request: The Solar Sails page for the JPL hasn’t been updated in a while. For specifics, I’ll reference this mission roadmap. Can you update us on the status of the projects?
Neil Murphy: The program evolved quite a bit, and the website got left behind, but the program has been very active across NASA.
What upcoming NASA missions will utilize solar sails? The missions below are listed in upcoming chronological order.
Mission name: ST9 (Space Tech 9)
Tentative launch date: 2010-2011
Estimated sail size: 40 meters on a side
Description: Neil says, “Ground demos have been very very successful,” and ST9 will put ground theory to the test. It will launch test a 40 meter sail into orbit. This will confirm that the solar sail design and implementation is not only feasible, but durable and functional. If this mission is successful, engineers believe that scaling the sail size up (from 40 to 100-150 meters on a side) is doable. At these sizes, you only have to extend your original calculations and data. You don’t have to invent a new way of doing things. This mission is led by Goddard, in collaboration with JPL and Langley.
Mission name: Heliostorm
Tentative launch date: 2016-2020
Estimated sail size: 150 meters on a side
Description: Heliostorm would alert scientists of solar storms that cause problems with Earth-based communication systems. Currently, we have about three hours warning before communications problems occur. Thanks to a solar sail, mission Heliostorm would place a spacecraft closer to the sun than our current monitoring equipment. Because of this, we could double our warning time before communications problems.
Mission name: SPI (Solar Polar Imager)
Tentative launch date: 2020-2035
Estimated sail size: 150 meters on a side
Description: The SPI mission is one that requires continual thrust, and therefore is perfect for a solar sail. The SPI mission will allow a spacecraft to orbit the sun at a high angle – giving a good view of the sun’s polar regions. Maintaining position would be easy for a solar sail, but impossible for conventional systems that need to burn rockets to maintain position. Neil comments, “There’s a huge amount of stuff we don’t know about how the sun works, and many questions could be answered by getting a polar view of the sun.”
Mission name: Interstellar Probe
Tentative launch date: 2031
Estimated sail size: Several hundred meters on a side
Description: Neil says, “We’re making our first steps in understanding how solar systems interact with our solar system.” The Interstellar Probe will dive close to the sun (for maximum thrust), then fly out an incredible distance, 200+ AU away from our sun. Deep in interstellar space, it will gather data. Compared to the energy requirements of traditional rocket engines, solar sails are the best bet for this application.
Andy: So solar sails will orbit the Earth, our sun, and travel in deep space. Are there any down-to-earth applications of solar sail technology, or are foreseeable applications limited to off-planet? Would Earthers see any uses of it in the next 10-20-30 years?
Neil Murphy: I’m not really sure. There’s a lot of clever people out there. It’s very important in a development point of view that you don’t think of this when designing. With that said, solar sails are not just for pure science missions, but for application missions. Mission Heliostorm helps provide early warning to Earth about solar storms. What we currently do to get advance warning, is to have a spacecraft at the L1 point, looking for flares and gamma emissions, and indicators that something’s going on that could interrupt Earth’s communications systems. When the emissions pass this L1 point, you have only a few hours warning. This is becoming a real concern – Spacecraft and communications are interrupted. A solar spacecraft could sit closer to the sun. We think we could get twice the warning time of a space weather event before it hits the earth.
Also, a little while ago NOAA put out a call for commercial activities that would use solar sails. I got involved with a small group of people that believed they could build and fly a commercial solar sail spacecraft.
Andy: How do you determine solar sail performance? Do you assume a standard sail size? If so, what is that size? If not, what is the formula to determine how much force a certain size sail will generate? When a solar sail accelerates and moves, does it move fast enough to watch, or is all movement slow?
Neil Murphy: A practical sail is about 100m on a side. When we model spacecraft trajectories, it’s a difficult task. And that assumes you take a spacecraft moving ballistically. Solar sails don’t do that. With a continuous thrust, modeling the flight is a much more complex issue. The effect of acceleration is millimeters per second squared. You would not see a rapid change just watching it. The reason solar sails work is that it’s a continually gradual thrust, unlike a rocket where it’s a brief large thrust. The best way to fly a mission is to use a rocket to get out of earth orbit. Get the most bang for your buck from the rocket and let the sail take over.
Andy: Regarding this page about laser-assisted solar sailing. Do we have lasers powerful enough today for intrasolar or extrasolar solar sailing?
Neil Murphy: I think the simple answer is no, but we probably know how to build one. The trick is getting the laser and power in orbit. We have a long way to go before we talk about laser assisted sails. I think it would be most efficient to do from orbit, but you’d need a large power supply, and that’s not in our near-term plans. People have talked about laser and microwave assisted sails, but it’s not what we call a near-term technology.
Andy: Regarding this page about spin-stabilized solar sails, the sail was 80cm in diameter. I assume this was just a proof of concept for a larger implementation, as your website states full size sails could be the size of a football field. If so, have further tests been made with larger sails? Is a spin-stabilized sail still a workable model, or have better ideas been developed?
Neil Murphy: That sail was just to prove a concept for the larger one. The current technologies are NOT spin stabilized. The tradeoff is basically between having a potentially lighter structure, versus the ease of controlling the spacecraft attitude. The whole goal in solar sailing is dependant on controlling the orientation of a spacecraft, and it’s a little harder to do that with a spinning sail. Conceptually we knew what we wanted to do. But there were serious technological challenges to get one of these into flight reliably.
Andy: The solar sail is literally a gigantic mirror. Could you see this feature being utilized in other ways at the same time, perhaps as a solar energy conversion system for a solar-sailing ship?
Neil Murphy: This is a good question and an interesting point. We’ve talked about this doing several things. If you made this mirror parabolic, you’d get a huge amount of energy. The problem is, Newton’s laws get in the way. Any momentum would be lost almost in proportion to the energy you get out of it. With that said, there are a couple other things that have been looked at, like 1) an antenna for communications, to help focus radio transmissions from earth, 2) a dust detector – spacecraft are hit by dust continually when in space, and the number of dust impacts would be increased and we’ve thoughts of using solar sails to measure it. 3) Etching out parts of the solar sail surface, and using that as antenna to pick up radio waves. We’d use this as a phased array radio receiver. 4) Make a frenel lens using an interference effect. You focus the light by making gaps in the sail. Once people realize these technologies are available, they’ll also come up with intriguing applications.
Andy: Thanks again for your time and all the information. You have a fascinating job!
Neil Murphy: I have too many things to do, but they’re all very interesting! Technical and scientific literacy is very important. People like you, a bridge between us and the public, are needed to communicate what we’re doing.
For more information on Solar Sails, check out the following resources:
Solar Sailing from the Planetary Society (also see the page about Cosmos 1, an ill-fated solar sail mission that never had a chance to spread its photon-collecting wings)
Solar Sail Technology Development at NASA’s Jet Propulsion Laboratory
Solar Sail media (including video) at L’Garde
Encyclopedia of Astrobiology, Astronomy & Spaceflight
Solar Sails Fundamental Physics
The book Solar Sailing, by Colin McInnes