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Projects: LightSail - The Future of Solar SailingLightSail: A New Way and a New Chance to Fly on Lightby Louis D. Friedman Let us create vessels and sails adjusted to the heavenly ether, and there will be plenty of people unafraid of the empty wastes.
A sail powered by sunlight alone will not be able to reach the stars. Such a trip will require large solar-powered lasers that will beam concentrated light over interstellar distances. But solar sailing—flying on sunlight—will allow us to get around the solar system without fuel and to hover at important places in space, countering the effect of the Sun’s gravity. It will enable us to monitor the Sun and protect Earth, and then to open up the solar system and the way to other solar systems after that. These practical applications will happen much sooner than interstellar travel. In 2005, with the support of our sponsoring partner, Cosmos Studios, The Planetary Society built the first solar sail spacecraft and attempted the first solar sail flight, only to be thwarted by the Earth-bound rocket that was supposed to deliver us to space. The Volna rocket carrying our spacecraft was lost in the Arctic sea. We have not lost our vision or our determination to make it happen. Technology has advanced, and a new class of “nanosat” spacecraft makes solar sailing even more practical, with smaller sails giving higher accelerations and greater performance for flight (see box on page XX). These ultra-light spacecraft point the way to the future. A spacecraft designed for interstellar flight will have to be “atosat” in size, weighing perhaps only a few grams. For now, we will take the step of reducing mass by a factor of 25 while flying farther and faster. We have dubbed our project LightSail. The concept of a nanosat sail was pioneered by NASA, but it was not designed for solar sail flight. NASA’s Nanosail application was an atmospheric drag device to help de-orbit satellites. NASA’s solar sail development program was terminated several years ago. The Planetary Society is applying the nanosail concept to develop a new class of LightSail spacecraft that will take us step by step first into the solar system and then, someday, to the stars. Thanks to a major new donation and the continued support of our members, we are now embarking on the first LightSail. Our first mission, LightSail-1, will demonstrate solar sail flight. LightSail-2 will use the solar sail to increase the orbit energy and move us away from Earth, and LightSail-3 will take us on a mission for which a solar sail spacecraft is uniquely suited: creating a solar weather monitor to provide early warning of solar storms that can affect Earth. LightSail-1LightSail-1 will be made up of three cubesat spacecraft. One cubesat will form the central electronics and control module, and two additional cubesats will house the solar sail module. Cubesats were developed about a decade ago, primarily for student spacecraft projects at universities, notably Stanford University. Since then, they have been used much more widely by industry, the military, and NASA for Earth orbit missions.
We will adapt the cubesats to make a customized spacecraft for the LightSail missions. The central electronics and control module will house a TV camera to image the sail, a radio system to send and receive data, a computer to control the spacecraft and the sail, an accelerometer to measure the force of sunlight, and a Global Positioning System (GPS) to provide navigation information about the effect of the light force on the spacecraft’s position and velocity. Taken together, these instruments make possible a controlled solar sail flight. The sail module, housed in the other two cubesats, will deploy a sail approximately 5.5 meters on a side made of aluminized Mylar 4.6 micrometers (microns) thick. The spacecraft’s mass will be less than 4.5 kilograms, and thus the mass-to-area ratio will be 140 grams per square meter. This will provide a characteristic acceleration (the acceleration from sunlight at the distance of Earth) of 6.5 micro-g (6.5 times one millionth the force of gravity), or 5.7 x 10-5 meters per second squared. This is better than Cosmos 1, which had a mass-to-area ratio of 160 grams per square meter. The Planetary Society sail will fly in an orbit greater than 800 kilometers (500 miles) above Earth atmosphere, where the only nongravitational force is that of sunlight pressure. The actual orbit will be determined by the launch vehicle on which we piggyback (fly as a secondary payload). Fortunately, our spacecraft is light and small, and there should be little trouble finding a ride into space. In fact, we have a Space Act Agreement with NASA Ames Research Center to cooperate on nanosat development, and this should help us find even more opportunities for flight. In any case, we will fly high enough to be out of the atmosphere even during times of solar maximum, when the atmosphere expands due to heating from solar wind activity. The mission will fly long enough to enable us to detect the increase in orbit energy caused by the effects of sunlight reflecting off the sails. We will do this by integrating the spacecraft’s acceleration measurements, observing the position and velocity changes with the GPS, and tracking the spacecraft from the ground. The onboard television will verify the sail’s deployment and dynamic behavior, and we will also arrange for Earth-based imaging to take pictures of the sail in flight. LightSail-1 will be built over the course of 18 months and be ready for flight by the end of 2010. The launch vehicle possibilities include both U.S. rockets—the Minotaur, an Atlas V, and the Falcon—and Russian rockets—the Cosmos 3M and the Soyuz-Fregat. All of these have recent successful experience launching multiple satellites. LightSailing Into the FutureLightSail-2 and LightSail-3 will advance solar sailing, with trips increasing from days of light to months, and add spacecraft payload to permit more scientific information to be obtained. The goal of LightSail-3 is to fly instruments to monitor the solar output of protons and electrons before they hit the Earth’s ionosphere. Large solar magnetic storms create coronal mass ejections that can disrupt communications and power grids when the particles reach the ionosphere, as well as damage spacecraft in Earth orbit. Currently, spacecraft to monitor the Sun and provide advance warning can be placed only at L1, a Lagrangian gravitational stable point between the Earth and Sun located approximately 1.5 million kilometers (900 million miles) from Earth. A solar sail will allow monitoring much closer to the Sun, making for better observations and longer warning times to help us protect our planet’s electrical grid. LightSail-3 will have its own propulsion unit, enabling the solar sail to be placed on a deep space trajectory and to provide more flexibility in choosing the launch vehicle for the fight. Together, We’re Making It HappenSail forth—steer for the deep water only,
Reckless O soul, exploring, I with thee, and thou with me, For we are bound where mariner has not yet dared to go, And we will risk the ship, ourselves, and all. —Walt Whitman Can The Planetary Society do this? Is this a job for a public membership organization that accepts no government funding and that instead relies on private contributions from people who share its vision of exploring new worlds? Having been in the space business for 40 years, I cannot deny that the program is audacious. The funding I had for our study of solar sailing in the 1970s was more than the budget for either our Cosmos 1 mission or this new LightSail-1 mission. NASA’s cost estimate to do a mission to meet our first flight goals was nearly $100 million (in a proposal they developed three years ago). Nonetheless, we accept the challenge, and we will meet it. We don’t aspire to be an aerospace industry contractor or miniature space agency. Instead, we seek to seed and prod governments and industry with creativity and innovation to further space exploration. Taking the first step to the stars by demonstrating an innovative new technology fits that role perfectly. Using small, excellent teams focused on a single purpose of flight is now a proven way to open up space to innovators and private entrepreneurs. It’s called new space, and we are adopting the approach. Our team brings together experienced professionals and youth, including students. Our project manager, Jim Cantrell, has worked with almost all the major aerospace companies as an engineer, manager, and consultant for both military and civilian space projects. He first worked with The Planetary Society on Mars balloon development (and even was assigned to the French space agency to help it adopt the technology advances we made on that project). He is now the CEO of Strategic Space Development. Although this spacecraft will be different from Cosmos 1 and built in the United States, we deeply respect our Russian colleagues who built that spacecraft, and we value their experience. After all, Cosmos 1 is the only flight-ready solar sail spacecraft ever built. The Russians will continue to work with us on both the sail module and the spacecraft, and we keep open the option for flying on a Russian launch vehicle (although not the Volna). When we step up to LightSail-2 and LightSail-3, we will likely need their expertise and experience. Creating something new requires a fortuitous combination of circumstances. The greatest solar sail design is useless if we can’t find an affordable launch to space. As I am fond of saying, there is no such thing as a free launch. Cosmos 1 was enabled not just by a neat spacecraft design and a clever team but also by funding from an Internet venture start-up and continued support by a visionary—Carl Sagan’s widow and collaborator, Ann Druyan, and her Cosmos Studios company. Planetary Society members are the other key component. Your support has been sensational and unique, making it possible to continue this quest for flight on light. LightSail is made possible by one anonymous member who provided us with a $1 million grant and challenged us to commit to flight. We accept. That said, we have rigorously put together a detailed budget and plan and have engaged an outstanding team of space veterans to review our project and question our realism and technical approach. Included on this review team are Glenn Cunningham, former Mars Global Surveyor project manager; Bud Schurmeier, former Voyager and Galileo project manager; Donna Shirley, former head of JPL’s Mars Program; and Viktor Kerzhanovich, a veteran of nearly every Soviet planetary mission as well as several U.S. planetary mission developments. International Interest in Solar SailingThis era of solar sailing resembles the early days of ballooning and of aircraft, with technology expanding rapidly and little certainty about what kinds of approach will work and what will fail. It’s an exciting time to push the technology that will someday take us to the stars. So throw off the bowlines. Sail away from the safe harbor. Catch the trade winds in your sails. Explore. Dream. Discover.
—Mark Twain When we started our solar-sailing effort in 2000, both NASA and ESA (the European Space Agency) had solar sail development programs. ESA was scheduling an Earth orbit flight before 2005, and NASA was building a much larger sail for a technology development mission. In addition, two other private organizations (one in the United States and one in Europe) also were trying to develop missions. Within a couple of years, all those projects ceased. Both space agencies had their technology development budgets slashed, and the cutbacks that followed affected much more than the solar sail program. Around the same time, the Japanese Space Exploration Agency (JAXA) started a different kind of program—development of a hybrid propulsion system for an eventual Jupiter orbiter. The hybrid included both a solar sail and an ion drive electric propulsion engine. In this innovative design, solar power for the electric engine comes from solar cells embedded in the sail. The Japanese performed a successful suborbit sail deployment test in 2004. Now they have built a technology development spacecraft with their sail and solar cells, and they will fly it next year as a piggyback on their Venus Climate Orbiter (currently known as Planet-C) mission. They will deploy the sail as the spacecraft departs from Earth, and if all goes well, they will try to fly it as a solar sailer. They have invited The Planetary Society to collaborate with them to disseminate public information about the mission and to share technical results. We will do that. Back in 1993, the Russian Space Agency conducted a deployment test of what could be called a solar sail in Earth orbit. It was at low altitude, observed from the Mir space station, and was meant not for solar sailing but for the development of large reflective mirrors for a possible application to light up permanently dark regions of the Siberian north at night. Seven tests of solar sail deployment have been conducted, and only one attempt (ours) at flight. The attempts are listed in the following table. The Elegance of Sailing ’Twas all so pretty a sail it seemed Two unique characteristics lured me into solar sailing. The first is that solar sails are beautiful. Spaceships tend to look like functional engineering boxes. Oops—that is exactly what they are. Solar sails look romantic. The second unique characteristic is that they require no fuel; hence, they alone (from what we know) can make the trip to the stars. Even for them, it will be difficult. Although I have long argued other benefits—round-trip interplanetary ferries, robotic sample return missions, and missions to Lagrangian points (gravitational equilibrium points) such as L1, for example—they are not quite as interesting because other spacecraft also can accomplish these missions. My colleague and friend Slava Linkin added a conjecture that solar sails are antigravity machines: they permit exactly countering the Sun’s gravity anywhere in interplanetary space. That makes hovering at nonequilibrium points possible—for example, at two or three times the distance toward the Sun of L1, where we could get two or three times the warning time of an impending solar storm. This is special—no other spacecraft can do this—and I am now much more hopeful of seeing a near-term application of solar sailing for this purpose. It’s special, and it’s also rather elegant: we balance the force of the Sun’s gravity with the force of the Sun’s electromagnetic radiation (light). That jibes perfectly with our physical intuition—gravity may be the most pervasive force in the universe, but it is also the weakest. The strongest force is electromagnetic radiation. If we harness this fundamental property of physics to achieve a practical and important space application such as a solar weather station, then it will feel like just the right cosmic thing to do. We have lingered for too long on the shores of the cosmic ocean; it’s time to set sail for the stars. Louis D. Friedman is executive director of The Planetary Society.
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