SURVEY OF THE ELECTRIC SOLAR WIND SAIL: THE FASTEST MAN-MADE DEVICE EVER BUILT Nick R. Purtle 1 University of Oklahoma, Norman, OK, 73069 In recent years, scientists have discovered that there are planets outside of our solar system that may or may not have the capabilities to support life. Because of these discoveries, astronomers and engineers from around the world have banded together to design and build spacecraft that could voyage beyond the boundary of influence within our solar system and continue into other star systems for scientific advancement. The question for the past few decades has been can we reach the edge of the solar system? But, in recent years, that question has shifted to how fast can we reach the edge of the solar system? AU = Astronomical Unit NOMENCLATURE E-sail or ESAIL = Electric Solar Wind Sail, or Electric Sail EU = European Union FMI = Finnish Meteorological Institute HEO = High Earth Orbit IHP = Interstellar Heliopause Probe mph = miles per hour NASA = National Aeronautics and Space Administration O I. INTRODUCTION N September 5, 1977, NASA launched two twin space craft, Voyager 1 (Figure 1), the fastest and most distant human-made object in the universe, and Voyager 2, for the purpose of performing flybys of both Jupiter and Saturn. Since completing their mission in 1980, the Voyagers mission was modified to include finding and studying the termination shock, heliosheath, and heliopause located at the edge of the solar system. For all intents 1 Student, College of Aerospace and Mechanical Engineering, University of Oklahoma
and purposes, the heliosphere includes everything that is influenced by the sun s solar winds; that includes all of the planets, the terminal shock, and the heliopause). A better depiction of the heliosphere and its components can be seen in Figure 3. From its launch in 1977, the Voyager mission has lasted 33.27 years and is expected to reach the heliopause in 2015 a total mission time of 38 years [8, 10]. Recent extrasolar discoveries have made us anxious to reach other solar systems at faster rates. It is because of this anxiety that faster means of space travel are being designed and created. Figure 1: Concept Drawing of Voyager 1 and Voyager 2 The European Union is funding a three-year project which began in 2007 at the Finnish Meteorological Institute to build the fastest man-made device in the universe [1]. FMI is in the process of creating an electric solar wind sail, or E-sail, intended to be capable of reaching Pluto in just five years time. The ESAIL has no internal propulsion system to propel the craft through space; instead, it is propelled by solar wind dynamic pressure. II. HOW THE ESAIL WORKS The ESAIL consists of 50 to 100 long, thin metallic tethers (wires) that retract from the body of the craft in a circular array around the space craft. Each tether is approximately 12 miles in length and carries a potential up to +20 kv provided by the on-board electron gun that is powered by solar panels. Each tether has an effective width of 65 feet which creates a sail, catching solar wind from the sun at speeds up to 1.8 million mph, and will propel the craft to 220,000 mph (Voyager 1 is leaving the solar system at 39,000 mph) [2]. There are also two propulsive arms on the space craft that create the initial spin of the craft. This spin has a period of approximately 20 minutes and
keeps the tethers outstretched around the space craft. Each of these parts can be seen in Figure 1. The orange arrows represent the solar wind acting on the sail and the red arrows represent the spin of the space craft provided by the propulsive arms. Figure 2: Diagram of ESAIL After launch, the tethers are unfurled into an array and, once outside of Earth s atmosphere, the propulsive arms (each with a small rocket attached to the tip) fire to create the spin mentioned above and the electron gun is turned on to create the electron field between each wire for catching solar wind. This solar wind will propel the space craft to 67,000 mph in the first year [3]. At this rate, the E-sail should reach the edge of the solar system, 120 AU (17.6 billion miles) away, in 15 years [4, 5]. III. WHAT TO WATCH FOR Because of the many components of this space craft, the craft must be monitored at all times. As mentioned above the craft must spin around approximately once every twenty minutes. If the vehicle were to stop spinning, the tethers would begin to furl and twist thus losing acceleration and speed. After launch, the spinup fuel tanks can be jettisoned to save fuel in case the craft were to stop spinning. Another way to generate spin is to make spinplane turning maneuvers simultaneously with synchronized reel in/reel out procedures during the deployment while the electron gun is turned on [4]. In this case, the spin of the craft can be generated mostly by the solar wind to minimize the amount of spinup fuel used. Due to slight inhomogeneities in the solar wind stream and/or mechanical or electrical differences between the wires, a single tether can rotate at a different angular velocity than the others. This could then continue until that
tether collided with a neighboring tether which could then lead to a chain reaction causing a complete mix-up of tethers. To avoid such a problem, the space craft is allowed to reel in or out the tether in question to increase or decrease its angular speed. IV. POSSIBLE MISSION PROFILES OF THE ESAIL [4] One main mission for the E-sail would be for flyby purposes, much like the Voyagers initial mission profile. In this mission, navigational accuracy is crucial in order to reach a specified target such as the outer planets or even objects beyond the solar system. Another mission would be to reach the heliopause of our solar system, which is the boundary where the Sun s solar wind is stopped by the interstellar medium. The ESAIL will be able to reach this point in 15 years time as opposed to 25 to 30 years like in chemical rockets. This mission is much like the first except there would be no need for navigational accuracy because all the ESAIL would need to do is set a trajectory away from the Sun. Such a mission could be cheap due to the lightweight of the components in the spacecraft. If the E-sail were to be used to reach the heliopause it would be considered an Interstellar Heliopause Probe, IHP. Figure 3: Depiction of Heliosphere and Interstellar Medium Next, the E-sail can be used as an off-lagrange point solar wind monitor by placing the craft at the L1 Lagrange point. In such a mission, the ESAIL s purpose would be to predict space weather by monitoring the solar wind as it reaches Earth s magnetosphere.
The scientific and monitoring application listed above are not the only missions available. There are also commercial uses for the E-sail such as an orbital fuel factory. An orbital fuel factory will provide rocket fuel to high Earth orbit satellites. The fuel factory is based on asteroid water and saves money because it minimizes the amount of fuel shuttled to HEO from Earth. Such a mission for the E-sail would include carrying the water mined from an asteroid to HEO. V. CONCLUSIONS The ESAIL is a revolutionary means of space propulsion in a world of rapid technological advancement especially in the astronautical industry. It is expected to be easily constructed and deployed at a fraction of the cost of chemical and ion propulsion rockets. With only a few challenges met so far (producing and reeling of the tethers), the E-sail is on a path to be the fastest man-made space craft in the universe. While challenges have yet to be encountered, none are expected to end the project in its entirety.
References 1. Dillow, C. (2010, December 9). EU-Backed 'Electric Sail' Could Be the Fastest Man-Made Device Ever Built. Retrieved December 11, 2010, from Popular Science: http://www.popsci.com/technology/article/2010-12/eu-backed-electric-sail-could-be-fastestman-made-device-ever-built 2. Gilster, P. (2007, April 2). Electric Sail Rides the Solar Wind. Retrieved December 11, 2010, from Centauri Dreams: http://www.centauri-dreams.org/?p=1144 3. Janhunen, P. (2006, December 5). The electric solar wind sail. Retrieved December 11, 2010, from Finnish Meteorological Institute - Space Research Activities: http://www.space.fmi.fi/~pjanhune/esail/index.html 4. Janhunen, P. (2008). The Electric Sail - A New Propulsion Method Which May Enable Fast Missions to the Outer Solar System. Journal of the British Interplanetary Society. 5. Journey Through the Galaxy. (2006, September 14). The Edge of the Solar System. Retrieved December 11, 2010, from Journey Through the Galaxy: http://burro.astr.cwru.edu/stu/solarsystem_edge.html 6. Mengali, G. (2007). Missions to the outer solar system and beyond. Fifth IAA Symposium on Realistic Near-Term Advanced Scientific Space Missions. Aosta, Italy: International Academy of Astronautics. 7. Space Today Online. (2005). Voyagers are leaving the Solar System. Retrieved December 11, 2010, from Space Today Online: http://www.spacetoday.org/solsys/voyagers20years.html 8. Steigerwald, B. (2005, May 24). Voyager Enters Solar System's Final Frontier. Retrieved December 11, 2010, from NASA: http://www.nasa.gov/vision/universe/solarsystem/voyager_agu.html 9. Wikipedia. (2010, December 1). Heliospere. Retrieved December 11, 2010, from Wikipedia: http://en.wikipedia.org/wiki/heliopause#heliopause 10. Wikipedia. (2010, December 7). Voyager I. Retrieved December 11, 2010, from Wikipedia: http://en.wikipedia.org/wiki/voyager_1#current_status