First Steps Across the Stars: Interstellar Travel Using Solar Sails

Nitin Kanchinadam Thomas Jefferson High School for Science and Technology

This article was the 1st place winner in the 11th-12th grade division of the Teknos 2020 Summer Writing Competition.

Humanity has been around for millions of years, yet is still only a newborn child when compared to the age of the universe we live in. However, in the approximately 200,000 years we have been on Earth, we have always had an ambition to explore the world around us. On October 4, 1957, the Soviet Union initiated the exploration of the sky above us with the launch of Sputnik 1. Since then, we have sent missions to every planet in the solar system. The next step in our exploration of the universe is to venture into the star systems around us, starting with the closest one. Despite the nearest star system, Alpha Centauri, being “only” four light years away, the distance between it and our own solar system is orders of magnitude higher than the greatest distance any man-made spacecraft has traversed. However, using a relatively new method of travel, scientists may be able to propel us to Alpha Centauri and beyond within our lifetimes.

From Sputnik 1 to the recent SpaceX Crew Dragon mission, all of our space expeditions have used rocket engines, in which the spacecraft carries its own propellant in addition to the payload. While these rocket engines may have been viable for travel within our solar system, they are not capable of reaching relativistic speeds of 0.2-0.3c (c being the speed of light) due to their mass. These speeds are required for interstellar travel within a human lifetime [1]. The fastest spacecraft we have made is the Parker Solar Probe, which, after several gravity assists, reached a top speed of 700,000 kilometers per hour [6]. Unfortunately, this is still only a mere 0.00065c. A theoretical engine using antimatter, which is matter that is composed of the antiparticles of the corresponding particles of “ordinary” matter, is the only possible rocket engine that could reach speeds of 0.2-0.3c. Unfortunately, obtaining the amount of antimatter to even build such an engine is infeasible [1]. To be able to travel to the stars, we need a spacecraft that does not need to carry its propellant, enabling it to be flung beyond the outer reaches of our solar system.

German astronomer Johannes Kepler first theorized the solution in 1608, which was to adapt the sails that ships use to catch “the heavenly breezes” rather than the normal winds [9]. These heavenly breezes turned out to be the sunlight all around us, and the solar sail was born. Solar sails use the momentum of photons to propel a spacecraft forward. When photons come into contact with the reflective material of the sail, they impart some of their momentum into the sail, pushing it forward before they are reflected away from the spacecraft. Solar sails are made of Mylar, a lightweight material, that is just 4.5 microns (1/10000th of a centimeter) thick and covered with a metallic coating to reflect light [9]. The main advantage of solar sails over rocket engines is that solar sails allow for continuous acceleration as long as there are photons pushing on the sail, enabling solar sails to propel spacecraft to speeds unattainable by rocket engines [9]. However, by itself, the sun is unable to accelerate the spacecraft to sufficient speeds due to the intensity of the sun’s rays on the spacecraft decreasing according to the inverse-square law as it ventures further from Earth. Rather, artificial alternatives must be used to accelerate the spacecraft to its top speed.

To accelerate the spacecraft to the speed required for interstellar travel, an Earth-based phased array of lasers (a light beamer) can be used. The output of the light beamer would need to be around 100 gigawatts to reach sufficiently fast speeds of 0.2-0.3c [2]. Lasers with petawatt-level outputs have already been created, so creating a 100 gigawatt light beamer should be easily attainable, but the pulse energy of the light beamer would have to be orders of magnitude greater than the petawatt-level laser for continuous acceleration [7]. Increasing the pulse width of the laser and decreasing the period between pulses can increase the average energy output of the light beamer. A suitable light beamer can take as little as 9 minutes to fully accelerate the spacecraft [7]. Using a phased array of lasers would make it possible for us to accelerate a spacecraft to 0.2c, allowing it to cross the interstellar space between us and Alpha Centauri in only 20 years.

When traveling at relativistic speeds of 0.2-0.3c, the entirety of the Alpha Centauri system can be traversed in just a few hours [5]. Therefore, the deceleration of an interstellar solar sail spacecraft is just as important as the acceleration to ensure that 20 years of waiting does not amount to nothing. One method of decelerating a spacecraft would be a combination of magnetic sails, which use a static magnetic field to deflect interstellar matter, and electric sails, which use the pressure of the solar wind to decelerate the spacecraft [8]. Magnetic sails perform better at higher velocities while electric sails perform better at lower velocities, so a combination of the two types of sails could reduce deceleration time and the total travel time by several years. In addition, the solar sail, which was previously used for acceleration, could be used in conjunction with the magnetic and electric sails for even faster deceleration of the spacecraft as it approaches the target system [8]. As the spacecraft approaches the target system, the increased gravitational influence and photon pressures of the system’s star(s) act in opposition to the spacecraft’s current velocity by applying a force to the spacecraft in the direction of Earth, therefore causing further deceleration [5]. With a combination of these methods of deceleration, we can place our spacecraft in orbit 40 trillion kilometers from Earth.

Unsurprisingly, given the amount of research being put into solar sails, light beamers, and other methods to begin to give humanity access to the night sky, technology billionaire Yuri Milner founded Breakthrough Initiatives and the Breakthrough Starshot project on August 24, 2016. Breakthrough Starshot’s primary goal is to get solar sail spacecraft to the Alpha Centauri system, specifically to Proxima b, the first exoplanet to be discovered in the system [3]. Pete Worden, a former NASA research director, directs Breakthrough Starshot and Stephen Hawking even endorsed the project, calling it the “next great leap into the cosmos” before his death in 2018 [4]. The project aims to outfit each solar sail nanocraft with a StarChip, which has a mass of less than a gram, but is still able to carry a camera, photon thrusters, a power supply, and navigation and communication equipment, allowing it to take pictures of targets and send them back to Earth [2]. Breakthrough Starshot plans to first launch a “mothership” containing thousands of nanocrafts since there is a high probability that some will collide with interstellar dust on the route and fail to finish the mission [2]. Then, they will use the light beamer on each one for 9 minutes to accelerate them to the speeds necessary for interstellar travel [2]. Twenty years into the mission, when the remaining nanocraft arrive at the Alpha Centauri system, they will begin to transmit pictures and other scientific data of Proxima b back to Earth, which will arrive 4 years later [2]. Before, humanity could only shoot for the stars, but with the Breakthrough Starshot initiative, we can finally reach them.

Only 60 years ago, we launched our first satellite into space. Now, we are starting to explore other star systems. As our ambition continues to drive us to explore the farthest reaches of our world and our universe, we will develop new technologies to rise to the challenge and discover new worlds in the process. We can adapt these technologies to better our lives in other ways such as using solar sails for travel within our own solar system, enabling us to travel between Earth and Mars in a single hour, and using the light beamer as an extremely powerful telescope [3]. Humanity may still be a crawling baby when compared to the vast universe, but there comes a time when every baby must take its first steps.

References

[1] Andrews, D. G. (2004). Interstellar propulsion opportunities using near-term technologies. Acta Astronautica, 55(3-9), 443-451. https://doi.org/10.1016/j.actaastro.2004.05.038

[2] Breakthrough Initiatives. (2016). Concept. Breakthrough Initiatives. Retrieved August 7, 2020, from https://breakthroughinitiatives.org/concept/3

[3] Breakthrough Initiatives. (2016). Target. Breakthrough Initiatives. Retrieved August 7, 2020, from https://breakthroughinitiatives.org/target/3

[4] Greene, K. (2019, June 26). Inside Starshot, the audacious plan to shoot tiny ships to Alpha Centauri. MIT Technology Review. Retrieved August 7, 2020, from https://www.technologyreview.com/2019/06/26/134468/starshot-alpha-centauri-laser/#:~:text=He%20outlined%20his%20plan%20to,Centauri%20in%20just%2020%20years.

[5] Heller, R., & Hippke, M. (2017). Deceleration of High-Velocity Interstellar Photon Sails into Bound Orbits At α Centauri. The Astrophysical Journal Letters. https://arxiv.org/pdf/1701.08803.pdf

[6] National Aeronautics and Space Administration. (2019, July 31). Parker Solar Probe. NASA. Retrieved August 7, 2020, from https://solarsystem.nasa.gov/missions/parker-solar-probe/in-depth/#:~:text=At%20closest%20approach%2C%20Parker%20Solar,%2C%20D.C.%2C%20in%20one%20second.

[7] Parkin, K. L. G. (2018). The Breakthrough Starshot system model [Abstract from ScienceDirect]. Acta Astronautica, 152, 370-384. https://doi.org/10.1016/j.actaastro.2018.08.035

[8] Perakis, N., & Hein, A. M. (2016). Combining magnetic and electric sails for interstellar deceleration. Acta Astronautica, 128, 13-20. https://doi.org/10.1016/j.actaastro.2016.07.005
[9] The Planetary Society. (2020). What is Solar Sailing? The Planetary Society. Retrieved August 7, 2020, from https://www.planetary.org/articles/what-is-solar-sailing#:~:text=When%20was%20the%20solar%20sail,Cosmos%201%20solar%20sail%20spacecraft.