“What a delightful irony it will be if the real age of sail has yet to dawn – not only on the oceans of Earth, but also in the far wider seas of space”
This is how Arthur C. Clarke envisioned the future of interstellar exploration in the great anthology of essays, poems, and essays titled Project Solar Sail, compiled by the World Space Foundation. The collection is replete with science fiction stories about solar sails voyaging through space driven by none other than the fuel-less pressure of the sun’s light.
This method of interstellar travel was first proposed by physicist Robert Forward in 1962 and popularized by Arthur C. Clarke in his 1964 novel “Sunjammer” about a solar sail race to the moon. Since then, the potential wonders of such an idea have fired the imagination of scientists and researchers. Sunjammer inspired the name of NASA’s first such project, a 1200 squaremeter light sail, the plans of which were unfortunately cancelled in late 2014. In 2011 though, NASA had already launched Nano-Sail D, deployed from a small satellite to unfurl a 10 square meter reflective sail. This was the first solar sail spacecraft to be launched in low earth orbit. However, the vehicle was just aimed to be a mere demonstration and it burned up during re-entry having spent up to 240 days in orbit. The project was rather intended to show that sails can indeed be used to increase the atmospheric drag in what could be used in decommissioning satellites through a de-orbit manouevre. It was not much of a solar sailing attempt.
However, the attempts to transfer such a fantastical imagination into a daring reality of real science far from dwindled. Indeed, the Planetary Society headed by Bill Nye the Science Guy have been working hard to bring this ambitious concept into being. The Planetary Society is an organization that seeks to empower the world’s citizens to participate in space initiatives and advance space exploration efforts. The Planetary Society’s first project, Cosmos1, was backed by the private contributions of members worldwide. The concept was to deploy eight triangular sails for a total surface area of 600 square metres. Each sail was to unfurl an extended area of fifty feet. Motors were to be attached to the sails for controllable solar sailing and attitude control and inflatable blades were to be used for varying the pitch angle of the blades. In 2005, Cosmos 1 was finally launched from a submarine aboard a converted ballistic missile, the Volna rocket. Unfortunately, however, the rocket misfired and Cosmos 1 was destroyed.
Unwaveringly, the Planetary Society announced new plans in 2009 to launch LightSail-1. Based on Nano-Sail D, LightSail-1 was designed to have a 40 square metre light-weight solar sail. The sail points towards the sun in order to be propelled by the pressure of the photon particles. The spacecraft is essentially a triple CubeSat configuration where the first unit houses the electronics compartment while the other two units constitute the solar sail module. It was originally planned to be positioned at an orbital altitude of 800 metres, high enough to be under the sole influence of Earth’s gravity and the pressure of solar light but the actual flight saw a lower orbit.
Parts of the cost of such a project were backed by citizens worldwide. Bill Nye the Science Guy started a crowdfunding goal on KickStarter which garnered thousands of supporters worldwide. The campaign reached a final stretch goal of 1.24 million dollars!
LightSail-1 hitchhiked a ride on the Atlas V rocket in May 2015 for a test flight to verify sail deployment. However, it did not soar high enough on this mission and was released into a low orbital altitude of 720 kilometres, where atmospheric drag brought down the spacecraft 7 days later to re-enter the atmosphere. But, that was the point of the mission as it was merely a test flight to verify spacecraft deployment and function, which it successfully did. This paved the way for the 2016 concept. LightSail B is planned to be flown at a higher orbital altitude of 800 km in 2016 or 2017 in order to demonstrate controlled sailing. It will be deployed by the Prox-1 satellite and both will hitch a ride aboard SpaceX’s new Falcon Heavy rocket. The mission aims to accomplish a complete demonstration of solar sailing at an altitude high enough to allow the thrust of sunlight to be tested without the force of atmospheric drag impinging.
Solar sail technology depends upon the fact that photons carry momentum since they are quantized particles of light and thus a lightweight reflective solar sail would be able to utilize the momentum of photon pressure. The photons bounce off backwards thus propelling the spacecraft forwards. Over a prolonged period of time, such accelerative force incrementally generates immense velocities. Solar sails must, accordingly, have a huge surface area in order to intercept as much photons as possible since each square kilometre would receive a total force of 10 newtons. The sail can be tilted in order to control the direction of the thrust. Since the acceleration is inversely proportional to mass as per Newton’s second law, low mass structures are essential. A few micron thick material would have a mass of 1 gram for each square metre. Accordingly, a solar sail with an area of 10,000 square metres would have a mass of 10 kilograms. The photon pressure from the sun to Earth is 4.6 * 10 ^ -6 pascals (N/m^2). However, since the photons are being reflected off the surface rather than being absorbed, the pressure imparted would be twice this amount as per Newton’s laws, 9.2 * 10 ^ -6 Pa. Thus, the force acting on the sail would be 9.2 * 10 ^ -2 N. Following Newton’s second law, the acceleration would be 0.92 cm/sec^2.
Over time, the solar sail can utilize the accumulated acceleration to reach tremendous speeds. This ultimately results in a continuous thrust propulsion system. Such a technology might even pave the way further for more sophisticated solar sails, ones that are propelled by a focused beam of laser power – a propulsion system that would be able to reach the nearest star system (Alpha Centauri) in a few decades.
I wish Arthur C. Clarke were still alive to witness this. As he would put it, the age of sail is indeed yet to dawn. It is remarkable how a once-science fiction concept, told of only in the fanciful musings of fictional novels and stories, is now being transferred into the pantheon of possible scientific endeavors to finally unfurl its sails into tangible existence, thanks to the efforts of organisations like the Planetary Society and space advocacy groups alike.
Featured image credit: Planetary Society.