Until the 1970s, Kepler’s vision and Compton’s physics were good science but for space travel they were primarily an intellectual curiosity. With the anticipated return of Halley’s Comet in 1986, NASA commissioned a study of the feasibility of using a solar sail to rendezvous with the comet. The project never got off the ground, but it did get many space scientists and engineers thinking about solar sailing as something real, and the pace of sail technology development accelerated. The first big step was taken by Russia with the launch of their Znamya mirror in 1993. Znamya was a large, lightweight mirror flown in space to test the idea of using reflected sunlight to illuminate large areas on the ground at night. The mirror was made from very lightweight reflective materials and looked, for all practical purposes, like a solar sail.
In the late 1990s, the Europeans entered the picture with the ground-based development of a one hundred foot sail manufactured by the German company DLR (Deutschen Zentrums für Luft- und Raumfahrt). Though the sail never left the laboratory, it inspired NASA to develop a similar capability during the early 2000s that culminated in the testing of two different solar sails in the world’s largest vacuum chamber, which is located at the NASA Glenn Research Center’s Plumbrook Station. The two solar sails were one hundred feet in diameter, made from materials thinner than a human hair, and autonomously deployed under space vacuum conditions to test their space worthiness. Figure 5 shows the sail developed for NASA by L’Garde, Inc. just after a deployment test in the vacuum chamber.
Figure 5. NASA and L’Garde, Inc. tested a 100-foot diameter prototype solar sail in the mid-2000’s. Shown in the picture are the fully deployed solar sail and with four of the sail engineers standing in the foreground to show scale. (Image courtesy of NASA.)
Japan took the next major step in solar sailing by actually flying a sail in space and using it as a primary propulsion system. The IKAROS (Interplanetary Kite-craft Accelerated by Radiation Of the Sun) was launched in May 2010 on a trajectory that will take it on a voyage near Venus. Though smaller than the NASA and DLR ground demonstration sails, the sixty-five foot diameter sail showed the world that solar sails can be used in space for propulsion. Figure 6 shows the IKAROS in space after deployment.
Figure 6. The Japanese Aerospace Exploration Agency launched the IKAROS solar sail on a mission to Venus in 2010. Shown in the figure is an actual picture of the IKAROS sail after deployment taken by a small robotic camera ejected from the spacecraft during flight. (Image courtesy of the Japan Aerospace Exploration Agency.)
In 2010, NASA launched the NanoSail-D into low Earth orbit. NanoSail-D, (where D stands for drag) is not a functioning solar sail since it is not using the force of sunlight in a controlled manner for propulsion. The ten-square-foot NanoSail-D might instead be a space demonstration of more conventional windsailing. As NanoSail-D skimmed through the Earth’s uppermost atmosphere, the wind created by its passing caused the spacecraft to slow and eventually re-enter. The wind caused drag, giving NanoSail-D its name.
Other groups are planning small sail missions that will actually use sunlight pressure for propulsion. Chief among them is the Planetary Society’s LightSail-1. Similar in weight to NanoSail-D, LightSail-1 will have a sail three times larger and be capable of pointing toward the Sun in order to use the sunlight for propulsion. CU Aerospace and The University of Surrey have similar sails in development.
Following the successes of IKAROS and NanoSail-D, there has been renewed interest in solar sailing, and several countries are considering the development of even more ambitious sails for use in missions throughout the solar system. We have a long way to go, however, before we will have a sail that can be used to send a spacecraft beyond the edge of the solar system into the abyss between the stars.
Some may be wondering how a solar sail, which derives its thrust from sunlight, can possibly take a spacecraft from one solar system to the next. After all, sunlight gets rather dim and is almost nonexistent when we get beyond the orbit of Pluto—let alone when we are in true interstellar space. Without sunlight, there is no force acting on the sail, hence no acceleration. So, how can it be done? There are two answers: 1) solar sails with very close solar approaches and 2) laser-augmented solar sails.
As discussed above, the thrust on a solar sail increases as its distance from the Sun decreases. Some pioneering work by Drs. Gregory Matloff and Roman Kezerashvili shows that an approximately one mile diameter solar sail spacecraft weighing no more than seven hundred pounds passing very, very close to the Sun, within about nine million miles, could achieve a solar-system exit velocity of two hundred and fifty miles per second. A craft traveling this fast would pass the Earth in four days, Jupiter in twenty one days and reach the Alpha Centauri system in just over three thousand years. By comparison, the fastest rocket we’ve ever sent into space won’t cover the distance to the Alpha Centauri system for another seventy-four thousand years! By increasing the sail size, and keeping the payload mass the same, we can see an engineering path to building a sail that could cover this immense distance in about a thousand years. For you and me, there isn’t much difference between a thousand years and seventy four thousand years. But in the lifetime of civilizations, the difference between these numbers is significant. We have recorded history going back a thousand years and there is no reason to assume that we won’t have similar records going forward; however seventy-four thousand years goes back well beyond the origins of human civilization.
You might have noticed another problem with the relatively near-term solar sail—it weighs only seven hundred pounds. Unfortunately, to carry a larger mass—millions of tons are required to carry and sustain humans on such a voyage—would require a solar sail of immense proportions (think the size of continents) made of incredible materials (“unobtainium” comes to mind). While such sails don’t violate any known laws of physics, we currently are almost clueless regarding how to engineer them.
One approach to creating these massive sails is to build them in space, so that they don’t have to experience the stresses of riding a rocket to get them there. This would solve two problems at the same time. First of all, the rocket launch will be the most stressful of the mechanical environments which the sail must be designed to survive. Rockets are not known for slow and graceful acceleration or for being a smooth ride. Quite the opposite is true; consequently, building a gossamer sail strong enough to ride on a rocket will be difficult. Second, the manufacturing of extremely large, lightweight and fragile solar sails in Earth’s gravity will be nearly impossible. The forces experienced by just being here on the surface may be sufficient to cause tears in the sail. Overcoming the stresses experienced as the sail is folded and packaged, as well as surviving the effects of Earth’s gravitational acceleration, will likely be both complex and expensive. When compared to the Earth, the space environment is much kinder to solar sails.
Building sails in space will not be so easy either. Manufacturing anything in space implicitly assumes there is some sort of facility or location where the construction will take place. This place itself must be built and launched. Then there’s the raw materials part. Sails, though conceptually simple, are anything but simple when we consider their subsystems and components: lightweight, highly reflective membranes; lightweight structures; moving parts for attitude control; electronics for deployment, attitude control, and navigation; plus many others. All of these, at least here on Earth, come through an extensive supply chain all the way from the extraction of the raw materials from which they are made to the final fabrication in a factory somewhere in the world. It’s only after the system integrator orders all the right parts that the engineers and technicians can even begin putting it together. All of this would have to be re-created in space to enable in-space manufacturing of a very large solar sail.