Figure 1. The solar sail ‘Sunjammer’ successfully deployed as part of the demonstration mission. Retrieved from https://www.nasa.gov/mission_pages/tdm/solarsail/index.html
Interplanetary travel has always been theorized by many creative scientists and engineers, as well as in art and media. Sci-Fi movies have long portrayed future spacecrafts as ones that utilize unique sources of energy, such as nuclear fusion. But as we progress further and further from the Apollo missions and the Saturn V rocket, we are now seeing one of those futuristic models beginning to realize–the Solar Sail.
As the name suggests, the idea involves using a large planar sail that gets “pushed” by sunlight. For a while, scientists believed that it would be possible to harness the power of the Sun by riding its solar wind. Solar wind is composed of stray protons or electrons released from the Sun’s magnetic field in the form of plasma. This idea has been realized, but with a slight difference. Derived from Maxwell’s electromagnetic theory of light, the idea of light radiation hitting a reflective surface leads to the existence of solar pressure. When multiple photons from the rays of sunlight bounce off of a planar reflective surface, the object may be propelled forward by this pressure. By relying on a seemingly unlimited source of energy, the efficiency far surpasses that of any engine or thrust mechanism.
Figure 2. Diagram of photon collision with solar sail to produce thrust. Adapted from “Solar Sail Propulsion” by Les Johnson, NASA.gov.
There have already been multiple successful test missions involving different prototypes of the solar sail. On November 19, 2010, NASA launched the NanoSail-D solar sail into the Earth’s orbit. The payload was a small satellite dubbed “FASTSAT”, one of many new minisatellites that NASA has developed for better cost and size efficiency. This was the first time NASA had launched a solar sail into low-Earth orbit. The NanoSail-D solar sail was an easily collapsible sail built from 100-square-feet of polymer material. After being deployed from the satellite on January 20, 2011, NanoSail-D successfully orbited the Earth for 240 days. The mission’s completion demonstrated the viability of the solar sail idea, which was all theoretical up until that point.
The U.S. wasn’t the only country pursuing solar sail endeavors. Japan had also accomplished a similar feat, using the spacecraft IKAROS (Interplanetary Kite-craft Accelerated by Radiation Of the Sun). JAXA, or the Japanese Aerospace Exploration Agency had developed a solar sail made of polyimide to provide the thrust for IKAROS. The payload for this craft was a climate orbiter called “AKATSUKI (Planet-C)”, designed to scout out our sister planet Venus. Probing the planet’s thick atmosphere helped to provide more information on Venus’s climate history as well as Earth’s due to their similarity. The primary goal of the mission, however, was to verify the viability of using a solar sail to propel and help navigate the trajectory of a spacecraft carrying equipment. Launched on May 21, 2010, the 20-square-meter solar sail successfully carried its payload from Earth to Venus in the span of about 6 months.
Although these missions have helped answer the questions surrounding the viability of traveling by solar sail, there is a lot more that needs to be done before we can begin looking towards long distance sailing. The solar sail’s thrust, efficiency, and cost of production and handling are all core parts that can be improved going forward. All of these improvements can be made with a change in material selection for the sail.
Solar sails require a film material that is extremely thin, highly reflective, and heat resistant. Dynamic changes in temperature on the film can lead to mechanical deformation due to thermal expansion. The most popular choice for solar sails presently has been commercial-off-the-shelf (COTS) polymeric materials. These are cheap, easy-to-acquire materials that excel in efficiency due to their low weights. NASA’s NanoSail-D used CP1 Polyimide for its solar sail membrane, while the publicly funded LightSail 2 from The Planetary Society used Mylar® (polyethylene terephthalate).
The downsides to using these polymeric materials are more prevalent during long duration missions, such as those that are planned for solar sail usage in the future. One of the main issues is that polymeric materials experience changes in material properties (from brittle to rubber-like), with this transition temperature (glass transition temperature Tg) in between the range of -105ºC to 150ºC. These thermal effects may end up permanently distorting the membrane. Polymeric materials are also vulnerable to multiple types of radiation in space. UV radiation can cause photochemical effects on the membrane, meaning that the material’s chemical properties will change due to the unnatural state of excited electrons. Ionizing radiation can cause the electrons to even be removed from the atoms, thus leaving them ionized. These physical and chemical changes in the membrane can lead to material loss, accelerated degradation, and consequently lowered thrust and efficiency.
Figure 3. Material changes of Mylar® due to thermal effects over time. Adapted from “Mylar® polyester film Product Information”.
There are multiple ways to mitigate these issues. Thermally conductive coating is one way to reduce drastic changes in material temperature. By absorbing most of the heat from the environment, the inside film membrane can be insulated from thermal effects. Reflective coatings can help protect the membrane from radiation effects. The past solar sail demonstration missions have used these techniques to improve the membrane’s durability, but at the cost of thrust and efficiency. The added thickness and weight from adding support to the structure end up hindering the performance of the solar sail. On top of that, the process of assembling the membrane by combining the COTS polymeric materials with the rigid support structures causes the handling and packaging of the parts to be impractical for larger sails.
In order to implement these fixes realistically, solar sail membranes will need to be composed of ultra-thin materials customized during assembly for the specific mission at hand. The process of putting together support structures with the COTS polymeric materials will no longer be viable for longer missions going forward. The membrane will need to be easily integrated with its supportive coating and reinforcements without hindering its weight, thickness, or ability to be handled and packaged.
Figure 4. NEA Scout’s solar sail in construction. Adapted from “NASA Tests Solar Sail For Exploration Mission 1’s NEA Scout” by Jim Sharkey, SpaceFlight Insider.
The next NASA mission utilizing a solar sail is Artemis-1, which will launch around November 2021. The mission will feature 13 CubeSats, which are miniature cost-efficient satellites that are easily compatible with other payloads aboard a single spacecraft. One of the CubeSats, dubbed NEA Scout (Near-Earth Asteroid Scout), will fly by a near-Earth asteroid and return with collected data, all while propelled by solar sail. This will require an 85 m2 sail built for long-distance capabilities, larger than any of the past. The sail is composed of a polymer membrane coated in aluminum, similar to previous solar sails. However, this time the sail is structured as one large membrane piece attached to a single spindle for physical support, which is much less delicate than the previous designs. With a real long-distance mission for a solar sail, the viability of the polymer material and structure used for a large solar sail will finally be tested. If the NEA Scout successfully completes its flight to and from the asteroid, the future of solar sail designs for even longer travel could be just on the horizon. Solar sailing could finally be a practical means of space travel that will provide the efficient thrust we need in order to reach deeper parts of the universe.
References
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2. Newton, K. (2011, January 21). NASA’s First Solar Sail NanoSail-D Deploys in Low-Earth Orbit. NASA. https://www.nasa.gov/mission_pages/smallsats/11-010.html
3. Anderson, J. L. (2011, November 29). NASA’s Nanosail-D ‘Sails’ Home — Mission Complete. NASA. https://www.nasa.gov/mission_pages/smallsats/11-148.html
4. Mahoney, E. (2020, January 14). NEA Scout. NASA. https://www.nasa.gov/content/nea-scout
5. Johnson, L., Whorton, M., Heaton, A., Pinson, R., Laue, G., & Adams, C. (2011). NanoSail-D: A solar sail demonstration mission. Acta Astronautica, 68(5-6), 571-575. https://doi.org/10.1016/j.actaastro.2010.02.008
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7. JAXA. (n.d.). IKAROS Mission Overview. JAXA. https://global.jaxa.jp/countdown/f17/overview/ikaros_e.html
8. Mylar® polyester film Product Information. (2003, June). DuPont Teijin Films. https://usa.dupontteijinfilms.com/wp-content/uploads/2017/01/Mylar_Physical_Properties.pdf
9. Bryant, R. G., Seaman, S. T., Wilkie, W. K., Miyauchi, M., & Working, D. C. (2014). Selection and Manufacturing of Membrane Materials for Solar Sails. In M. Macdonald (Eds.), Advances in Solar Sailing. Springer Praxis Books. https://doi.org/10.1007/978-3-642-34907-2_33
10. Johnson, L. (2018, July 26). Solar Sail Propulsion [PowerPoint slides]. https://ntrs.nasa.gov/api/citations/20180005252/downloads/20180005252.pdf
11. Sharkey, J. (2018, July 5). NASA TESTS SOLAR SAIL FOR EXPLORATION MISSION 1’S NEA SCOUT. SpaceFlight Insider. https://www.spaceflightinsider.com/missions/solar-system/nasa-tests-solar-sail-for-exploration-mission-1s-nea-scout/
