Sailing to the Stars
Harnessing Light for Interstellar Travel
Since the dawn of civilization, humanity has gazed into the vast abyss of darkness above and dreamed of a way to visit the countless points of light which pepper our night sky. Ancient philosophers imagined sailing from one celestial body to another, akin to the way that colonial explorers sailed the endless oceans of Earth. By the 17th century, aeronauts collecting atmospheric data from balloons began to realize that outer space was devoid of air, and thus, absent of the winds which pushed on the sails of vessels charting the mysterious new lands across Earth’s expansive seas. It seemed that exploring space would require investment in new technologies, the likes of which our species hadn’t yet dreamed of. By the 20th century, humanity had become the master these technologies, economizing our own orbit, sending probes to all corners of our Solar System, and even placing footprints on the surface of our nearest celestial neighbor: the Moon. Yet, despite all this advancement, the stars still are seemingly unreachable.
The reason for this is simple physics — a rocket requires propellent to achieve Earth orbit, but also requires additional propellent in order to launch from orbit to its destination. To carry this additional propellent, a rocket must then have even more fuel just to get this fuel into orbit. The effect is summed up in the Tsiolkovsky rocket equation which states, in a nutshell, that travelling farther (or faster) through space will require exponentially more fuel. This hasn’t proven to be a huge issue for chemical rockets launching relatively small payloads throughout the local Solar System, but for any prospect of interstellar travel, chemical rockets won’t do the trick. The Tsiolkovsky effect can be stunted by increasing a rocket’s exhaust velocity by utilizing theoretical forms of high-impulse propulsion, but the fact of the matter remains the same; conducting interstellar travel within human timescales using fuel-based propulsion is arduous.
There are, however, several possible ways to overcome the Tsiolkovsky rocket equation for interstellar travel. One such method is to collect the fuel you need enroute to your destination, such as in a Bussard Ramjet design. Another solution could involve launching fuel modules to an already accelerating spacecraft, employing large linear accelerators, which are precisely timed to arrive at the spacecraft while matching its velocity for rendezvous and pick-up. However, perhaps the most elegant way of overcoming the Tsiolkovsky effect is a method that requires no on-board fuel at all. It was Johannes Kepler who first noted that a comet’s tail curls away from the Sun, suggesting some form of light-based pressure emanating from its fiery surface. In a letter to Galileo Galilei in 1610, he wrote, “Provide ships or sails adapted to the heavenly breezes, and there will be some who will brave even that void.” It would be over two centuries before this effect was scientifically verified, and nearly another before Tsiolkovsky himself would suggest the use of the phenomenon to propel spacecrafts between the stars. The concept of the solar sail was born.
The underlying principles of a solar sail are quite simple — a payload is attached to a large, thin, reflective sail which uses radiation pressure from the Sun in order to acquire thrust. The intensity of this thrust is determined by the mass of the payload being transported, the surface area of the sail, and the intensity of the solar radiation (dependent on distance from the Sun). Compared to traditional forms of propulsion, the accelerations involved are tiny, making the solar sail a low-thrust form of propulsion. However, accumulated over large timescales, the effects can be noteworthy. On interplanetary scales, a 10-person spacecraft of 7.5 tons destined for Mars could open a 1-kilometer wide solar sail at Earth’s orbit, begin a transfer to Mars’ orbit, reorient the sail partway through the mission to slow down upon approach, and arrive at the Red Planet in a little over a year using no propellant. If the need for a rendezvous is suspended, such a spacecraft could make a flyby of Mars in under 6 months. Though impressive, the velocities and accelerations involved still pale in comparison to the enormous distance to our nearest stellar neighbor, Proxima Centauri, which resides a distance of nearly 540,000 times further than Mars at its closest approach.
To conceptualize this, let’s imagine a 100 person mission to Proxima Centauri utilizing a traditional, 10-kilometer diameter solar sail. SpaceX’s Starship, which is planned to have a passenger capacity of 100, will have a dry mass of about 100 tons (excluding propulsion components). Unfurling its sail at Earth orbit and aligning it for maximum velocity increase, the modified Starship begins its journey. After just 3 months, the spacecraft whizzes past Mars after accruing over 18 km/s of velocity. Though it would seem the spacecraft is destined for the stars, radiation pressure from the Sun drops off quickly beyond this distance, and the trajectory of the spacecraft becomes governed by the gravity of the Sun. More than a decade after its mission began, the Starship finally leaves the Sun’s sphere of influence and enters interstellar space. Drained of its velocity from the gravity of the Solar System, the craft saunters away at a measly 32 km/s, a 40-millennia year journey to Proxima still ahead. A more effective strategy could be to first use the sail to slow the orbit of the Starship to make a close encounter with the Sun, utilizing the extremely powerful solar radiation at this distance to gain an extra boost. Such a maneuver would take a few months to execute, but when complete would propel our modified Starship out of the Solar System at upwards of 100 km/s, an astonishing 6 times faster than the New Horizons probe. Even at this velocity, the craft would still take more than 11,000 years to reach the target star.
The fact that solar pressure decreases exponentially with distance from the Sun is what prevents the solar sail from being a reliable means of interstellar travel. If instead the intensity of the light source could remain constant, the solar sail could prove to be a more effective means of traversing the stars. Such a concept was a pipe dream until the advent of the laser in the 1960s, which proved that light can be stimulated and amplified to maintain a tight beam which hardly spreads out over vast distances. One could imagine a powerful laser engineered to focus its beam on an interstellar vessel’s sail, allowing it to experience a nearly constant acceleration as it cruised out of the Solar System. Even a laser, however, must abide by the known laws of physics. In an effect known as beam divergence, a laser too will lose its intensity over long distances due to diffraction. This diffraction is dependent on the wavelength (color) of light being used, and the size of the emitting aperture. By decreasing the light’s wavelength and increasing the aperture size, beam divergence is minimized.
Alternatively, longer wavelengths of light could be employed for the opportunity of a lower density sail. Light cannot pass through a hole which is smaller than its own wavelength. This is why it is safe to put your face up to a microwave oven; the wavelength of light that a microwave uses to cook your food (~12 cm) is larger than the size of the holes in the microwave’s window (~1 mm). Instead of a continuous, solid sail built to reflect visible light, a mesh of thin wires thatched together could be used to reflect microwave light emitted from a microwave laser — a maser — greatly decreasing the amount of material required for the fabrication of the sail. The concept, known as “Starwisp”, was proposed by Robert L. Forward in 1985, and would use just 1% of the material that a traditional sail requires. This would allow the manufacturing of a sail 100 times larger than a traditional sail with the same mass of material. This comes at the cost of the beam being more difficult to keep focused on the sail over great distances due to its higher degree of diffraction.
The issue with exercising any form of artificial beam propulsion is that it leaves no means for the spacecraft to slow down upon approach to the target star (unless an equally powerful emitter is placed at the destination as well). With no way to decelerate, interstellar beam-propelled craft would be limited to unmanned probes on flyby missions. The Breakthrough Starshot program, founded by Yuri Milner, Stephen Hawking, and Mark Zuckerberg in 2016, envisions a fleet of ~1,000 tiny probes wielding miniaturized solar sails propelled by a powerful array of ground-based lasers. Destined for Proxima Centauri, each “Starchip” spacecraft would be armed with a tiny 2-megapixel camera, a 150 milligram battery, and 4 sub-gram processors for reporting back to Earth on the findings of the alien planetary system. One by one, the tiny spacecraft would be blasted into deep space with 100 gigawatts of combined power, attaining 20% of the speed of light within just 10 minutes. Some are vaporized at launch, misaligned by a fraction of a degree or manufactured with the slightest asymmetric defect. More still are annihilated by cosmic dust on the dangerous two decade trek across the abyss of space. Several hundred, however, safely reach the planetary system, overcoming the numerous hazards of their voyages to become the first artificial objects to cross the expanse of interstellar space.
In 6 decades of space travel, we have advanced chemical rocketry from a juvenile experiment to a finely exacted science. Using this new technology, we have ventured beyond the confines of our protective atmosphere, scattering small pieces of ourselves throughout the Solar System in an attempt to explore and unveil the secrets of the universe. Soon we will aspire to further harness this technology to build bases on the Moon, permanently colonize our rusty red neighbor, and send explorers to the mysterious realm of the gas giants. But our use of this technology is rapidly approaching a limit. One day humanity will be at a crossroads, poised to explore the stars that have inspired our species for millennia, yet unsure of exactly how to get there. At this point, it may be necessary to look towards the past for inspiration on how to trek into the final frontier. In the same way that ancient explorers extended their sails into the wind to chart the new lands of the Earth, so may we extend our sails into a wind of light to propel us into the great unknown.
