How to Sail on Starlight

The Challenges and Opportunities of Solar Sailing Technology

Solar sailing technology has been around for decades, but recent experiments have shown its potential as a viable means of space propulsion. The Light Sail project, launched by NASA's Deep Space Network, demonstrated that solar sails can be used to manage the orbital energy of a spacecraft. By adjusting the orientation of the sail to take advantage of the changing position of the Earth and Sun, scientists were able to increase the altitude of the Light Sail's orbit.

However, this constant adjustment of orientation also presents several challenges. As the Earth blocks the Sun for about half of its orbit, the solar sail is only able to harness thrust for about one quarter of its orbital period. This means that reliable control moment gyros are essential for maintaining the sail's position and ensuring a stable orbit. The size of the equipment needed to achieve this becomes increasingly demanding as the sail grows in area.

Despite these challenges, the technology does exist. In fact, the Light Sail was able to raise its Apogee by 2 kilometers in just its first week of solar sailing. However, the lowest point of its orbit or perigee has been decreasing at a faster rate, indicating that the spacecraft's orbital energy is actually decreasing. This is largely due to the small traces of atmosphere found at 720 kilometers above Earth's surface, which cause significant drag on the spacecraft.

The implications of this are significant. The Light Sail is expected to deorbit in about a year, which could have been avoided if it had been launched into a higher orbit where it would experience more solar pressure and less atmospheric drag. Any useful solar sail would need to be launched above 800-1,000 kilometers above Earth's surface to achieve this balance.

Despite the limitations of its current design, the Light Sail project has proven that solar sailing could be a useful technology for future space missions. However, there are many other potential applications for this technology that scientists and engineers are eager to explore. One of the most promising areas is the development of an early warning system for solar flares.

A spacecraft positioned directly between the Earth and the Sun would be able to detect changes in the Sun's energy output before they reached us. This would provide a vital warning time, allowing scientists and policymakers to prepare for potential solar storms and protect our satellite systems. However, achieving this requires significant technological advancements, particularly in terms of propulsion and materials.

Solar sails offer a promising solution to these challenges. By harnessing the power of the Sun's radiation pressure, a spacecraft can achieve more thrust at closer distances than would be possible with traditional propulsion systems. This means that it may be possible to sail even closer to the Sun than previously thought, allowing for Sun diving maneuvers and exploration of the outer solar system.

The potential applications of solar sailing technology are vast and varied. From early warning systems for solar flares to in-orbit manufacturing and asteroid deflection, scientists and engineers are eager to explore new possibilities. To learn more about Kepler's laws of orbital mechanics and how they apply to solar sailing, there is a course quiz available on the Brilliant website.

Furthermore, daily challenges presented by Brilliant provide thought-provoking problems that can help learners develop their understanding of complex concepts like orbital mechanics. By applying these principles to real-world problems, scientists and engineers can unlock the full potential of solar sailing technology and make new discoveries about our universe.

If you're interested in learning more about solar sailing and other cutting-edge technologies, Brilliant offers a range of courses and resources on its website. For now, however, it's clear that solar sailing has made significant progress in recent years and holds much promise for future space missions.

"WEBVTTKind: captionsLanguage: enthis episode of real engineering is brought to you by brilliant a problem solving website that teaches you to think like an engineer on November 3rd 1973 the Mariner 10 probe set sail for Venus where it gave us our first close-up look of the planet all the while lending its gravity to boost the probes velocity to its next destination mercury this mission gave humankind many of its first experiences with space travel it was the first time a gravity assist had ever been performed using a planet's gravity it was the first time two planets had been visited up close in a single mission and it was the first time solar pressure had been used to control a spacecrafts attitude but that wasn't by design this mission came incredibly close to being a complete failure as a result of debris floating around the spacecraft interfering with the start tracking navigation camera each time it happened Mariner 10 needed to perform a corrective role to reacquire a lock-on Canova's its reference star to make matters worse this occurred more frequently as the brightness of the particles increased as Mariner 10 approached the Sun Mariner 10 managed to complete its gravitational slingshot by Venus and was now limping on to mercury but was quickly running out of nitrogen the propellant needed to perform these maneuvers to solve this problem the engineers came up with a genius plan by tilting the solar panels they could use solar pressure the force exerted by photons to control its roll and stabilize its position a huge moment that proved the viability of solar sails for future use in spacecraft and just this year light sail 2 provided a valuable technology demonstration of solar sails ability to provide control at very low cost for small satellites Before we jump into what light sail 2 achieved let's first see how it works although photons don't have mass they do have momentum which can be transferred into an object with a reflective surface this is a bit counterintuitive for anyone that has learned like me that momentum equals mass by velocity so we need a new equation to describe momentum of a photon and here it is the momentum of a single photon can be found by dividing Planck's constant by the wavelength of the photon since Planck's constant is well a constant the momentum of a photon is entirely determined by the wavelength with shorter waves having higher momentum finding the momentum of a wavelength of 500 nanometers we can find a momentum of just 1.3 by 10 to the minus 27 kilogram meters per second that's obviously tiny but when dealing with the vacuum of space that can add up when every single collision of a photon with the solar sails provides an increase in velocity of the spacecraft thanks to our friendly neighborhood laws of physics or the conservation of momentum to be exact which states that the change in momentum of a photon must involve an equal and opposite change in momentum for the object it interacted with we can maximize this transfer of momentum by ensuring the sale is as close to a perfect mirror as possible wasting as little energy as possible to the heating of the sails but this isn't the only factor that varies the force a light source can in Paris on a solar sail a less friendly neighborhood law of physics the inverse square law states that the intensity of a point source of energy say the Sun will decrease with the square of the distance away from it this is a result of the energy spreading over a greater distance as it radiates outwards this basically just means less photons will hit the sail the further away it is from the Sun so if a solar sail was double the distance from the Sun it would only receive a quarter of the photons triple the distance and the sail would receive 1/9 this makes the solar sail increasingly ineffective as we traveled to the outer edges of the solar system and beyond the other primary factor that affects the force imparted on the sail is the angle at which the photons strike the sail you can think of this like a game of pool where the angle the Q ball strikes another ball will determine the direction it travels in or you can dispense with the useless analogies and think of it as a photon bouncing off mirror the direction of thrust will always be perpendicular to the mirror and vary with the cosine of the angle of incidence when the sail is perpendicular to the incoming photons the incidence angle will be zero and the cosine of zero is one and thus it will be at a maximum of course the effective area of the sail also decreases as it angles away from the Sun the effective area can be found by multiplying the total area by the cosine of the incidence angle and thus the force really varies with the cosine squared of the incidence angle putting all this theory to work we can see how the light sail to achieved its goals light sail to was a project by the Planetary Society a company formed by Carl Sagan Bruce Murray and Louis Friedman back in 1980 with a goal of raising funds for exciting missions through crowdfunding it is currently run by my buddy Bill Nye as you can see here he totally knew who I was and didn't think I was just a fun nervously asking him for a photo light sail to is a tiny craft weighing just five kilograms an eleven point three centimeters in width and 34 centimeters long but when deployed the sails had an area of 32 metres squared this compact size allowed it to ride share on a falcon heavy mission in June 2019 launching into orbit around Earth at 720 kilometers when light sail too is moving away from the Sun it faces it sails towards the Sun to maximize the acceleration then as it moves back towards the Sun it turns on its side so the solar sail isn't slowed down but as the earth blocks the Sun for about half of its orbit we are only really getting thrust for about one quarter of its orbital period this constant adjustment of orientation will make a solar sail in planetary orbit heavily dependent on reliable control moment gyros and as the sail grows an area the more demanding this requirement will be both on the size of the equipment needed and the energy required but the technology does exist by performing this maneuver every period light sail to managed to raise its Apogee by 2 in its first week of solar sailing however the lowest point of its orbit or its perigee has been decreasing at a faster rate meaning the orbital energy of light sail - is actually decreasing this is happening primarily because the spacecraft still comes into contact with Earth's atmosphere even at 720 kilometers since the solar sail by design has a large surface area with very little mass the small traces of atmosphere found at 720 kilometers are enough to significantly slow down the spacecraft's velocity this drop in velocity causes the spacecraft to dip further into the atmosphere where the drag increases slowing the spacecraft down even more because of this light sail - is expected to deorbit in about a year this could have been mostly avoided if the spacecraft was placed into a higher orbit where it would experience more solar pressure and less atmospheric drag the point at which these two pressures equals is around 800 to 1,000 kilometers above Earth's surface so any useful solar sail would need to be launched higher than this nevertheless the increasing apogee of light sail - has proven that solar sailing could be a useful technology for future space missions but I'm not sure why we needed to prove that since Mariner 10 did that 40 years ago is in the least provided additional data and design verification for future solar sails so beyond this not so ideal application how could we potentially use solar sails to develop new technologies perhaps one of the most important could be an early warning system for solar flares to do this we would need a spacecraft to keep a position directly between the Earth and the Sun anyone familiar with Kepler's laws of orbital mechanics will know this is difficult as any object orbiting closer to the Sun will have a higher velocity and a shorter orbital period so it would be racing ahead of Earth to maintain this position a spacecraft would need to constantly push back into position and obviously propellant would pretty quickly run out so solar sails are the perfect solution for this and they will be able to achieve more thrust the closer they get to the Sun we already have several satellites for this purpose Parkton Lagrange point one which is the point at which the gravity exerted by Earth and the gravity exerted by the Sun is the same however a solar sail with a 67 by 67 meter area would allow a satellite to double its distance from Earth and thus provide double the warning time at about two hours this would provide earth vital time to prepare for a solar storm and protect our satellite in orbit and our grid system larger sails would allow the satellite to park even closer to the Sun but we begin to run into engineering challenges as we approach the Sun with unpacking the larger sail and protecting it from the sun's heat if we had suitable materials that could be both extremely thin reflective light and heat resistant we could sail even closer to the Sun to perform Sun diving maneuvers to capture the sun's energy to increase our velocity and explore the outer solar system there are countless other applications for this technology that you can learn about with the reference material in the description but you may have difficulty understanding them without a decent understanding of orbital mechanics I started to read up on this subject myself in anticipation for the new Kepler space program game and brilliance course on gravitational physics will teach you everything you need to know about Kepler's laws of orbital mechanics and much more or you could complete one of brilliants daily challenges each day brilliant presents you with interesting scientific and mathematical problems to test your brain each daily challenge provides you with the context and framework that you need to tackle it so that you can learn the concepts by applying them if you like the problem and want to learn more there is a course quiz that explores the same concept in greater detail if you are confused and need more guidance there's a community of thousands of learners discussing the problems and writing solutions daily challenges are thought-provoking challenges that will lead you from curiosity to mastery one day at a time if I've inspired you and you want to educate yourself then go to brilliant org for it slash real engineering and sign up for free and the first five people that go to that link will get 20% off the annual premium subscription so you can get full access to all their courses as well as the entire daily challenges archive as always thanks for watching and thank you to all my patreon supporters if you'd like to see more from me the links to my Instagram Twitter subreddit and discord server are belowthis episode of real engineering is brought to you by brilliant a problem solving website that teaches you to think like an engineer on November 3rd 1973 the Mariner 10 probe set sail for Venus where it gave us our first close-up look of the planet all the while lending its gravity to boost the probes velocity to its next destination mercury this mission gave humankind many of its first experiences with space travel it was the first time a gravity assist had ever been performed using a planet's gravity it was the first time two planets had been visited up close in a single mission and it was the first time solar pressure had been used to control a spacecrafts attitude but that wasn't by design this mission came incredibly close to being a complete failure as a result of debris floating around the spacecraft interfering with the start tracking navigation camera each time it happened Mariner 10 needed to perform a corrective role to reacquire a lock-on Canova's its reference star to make matters worse this occurred more frequently as the brightness of the particles increased as Mariner 10 approached the Sun Mariner 10 managed to complete its gravitational slingshot by Venus and was now limping on to mercury but was quickly running out of nitrogen the propellant needed to perform these maneuvers to solve this problem the engineers came up with a genius plan by tilting the solar panels they could use solar pressure the force exerted by photons to control its roll and stabilize its position a huge moment that proved the viability of solar sails for future use in spacecraft and just this year light sail 2 provided a valuable technology demonstration of solar sails ability to provide control at very low cost for small satellites Before we jump into what light sail 2 achieved let's first see how it works although photons don't have mass they do have momentum which can be transferred into an object with a reflective surface this is a bit counterintuitive for anyone that has learned like me that momentum equals mass by velocity so we need a new equation to describe momentum of a photon and here it is the momentum of a single photon can be found by dividing Planck's constant by the wavelength of the photon since Planck's constant is well a constant the momentum of a photon is entirely determined by the wavelength with shorter waves having higher momentum finding the momentum of a wavelength of 500 nanometers we can find a momentum of just 1.3 by 10 to the minus 27 kilogram meters per second that's obviously tiny but when dealing with the vacuum of space that can add up when every single collision of a photon with the solar sails provides an increase in velocity of the spacecraft thanks to our friendly neighborhood laws of physics or the conservation of momentum to be exact which states that the change in momentum of a photon must involve an equal and opposite change in momentum for the object it interacted with we can maximize this transfer of momentum by ensuring the sale is as close to a perfect mirror as possible wasting as little energy as possible to the heating of the sails but this isn't the only factor that varies the force a light source can in Paris on a solar sail a less friendly neighborhood law of physics the inverse square law states that the intensity of a point source of energy say the Sun will decrease with the square of the distance away from it this is a result of the energy spreading over a greater distance as it radiates outwards this basically just means less photons will hit the sail the further away it is from the Sun so if a solar sail was double the distance from the Sun it would only receive a quarter of the photons triple the distance and the sail would receive 1/9 this makes the solar sail increasingly ineffective as we traveled to the outer edges of the solar system and beyond the other primary factor that affects the force imparted on the sail is the angle at which the photons strike the sail you can think of this like a game of pool where the angle the Q ball strikes another ball will determine the direction it travels in or you can dispense with the useless analogies and think of it as a photon bouncing off mirror the direction of thrust will always be perpendicular to the mirror and vary with the cosine of the angle of incidence when the sail is perpendicular to the incoming photons the incidence angle will be zero and the cosine of zero is one and thus it will be at a maximum of course the effective area of the sail also decreases as it angles away from the Sun the effective area can be found by multiplying the total area by the cosine of the incidence angle and thus the force really varies with the cosine squared of the incidence angle putting all this theory to work we can see how the light sail to achieved its goals light sail to was a project by the Planetary Society a company formed by Carl Sagan Bruce Murray and Louis Friedman back in 1980 with a goal of raising funds for exciting missions through crowdfunding it is currently run by my buddy Bill Nye as you can see here he totally knew who I was and didn't think I was just a fun nervously asking him for a photo light sail to is a tiny craft weighing just five kilograms an eleven point three centimeters in width and 34 centimeters long but when deployed the sails had an area of 32 metres squared this compact size allowed it to ride share on a falcon heavy mission in June 2019 launching into orbit around Earth at 720 kilometers when light sail too is moving away from the Sun it faces it sails towards the Sun to maximize the acceleration then as it moves back towards the Sun it turns on its side so the solar sail isn't slowed down but as the earth blocks the Sun for about half of its orbit we are only really getting thrust for about one quarter of its orbital period this constant adjustment of orientation will make a solar sail in planetary orbit heavily dependent on reliable control moment gyros and as the sail grows an area the more demanding this requirement will be both on the size of the equipment needed and the energy required but the technology does exist by performing this maneuver every period light sail to managed to raise its Apogee by 2 in its first week of solar sailing however the lowest point of its orbit or its perigee has been decreasing at a faster rate meaning the orbital energy of light sail - is actually decreasing this is happening primarily because the spacecraft still comes into contact with Earth's atmosphere even at 720 kilometers since the solar sail by design has a large surface area with very little mass the small traces of atmosphere found at 720 kilometers are enough to significantly slow down the spacecraft's velocity this drop in velocity causes the spacecraft to dip further into the atmosphere where the drag increases slowing the spacecraft down even more because of this light sail - is expected to deorbit in about a year this could have been mostly avoided if the spacecraft was placed into a higher orbit where it would experience more solar pressure and less atmospheric drag the point at which these two pressures equals is around 800 to 1,000 kilometers above Earth's surface so any useful solar sail would need to be launched higher than this nevertheless the increasing apogee of light sail - has proven that solar sailing could be a useful technology for future space missions but I'm not sure why we needed to prove that since Mariner 10 did that 40 years ago is in the least provided additional data and design verification for future solar sails so beyond this not so ideal application how could we potentially use solar sails to develop new technologies perhaps one of the most important could be an early warning system for solar flares to do this we would need a spacecraft to keep a position directly between the Earth and the Sun anyone familiar with Kepler's laws of orbital mechanics will know this is difficult as any object orbiting closer to the Sun will have a higher velocity and a shorter orbital period so it would be racing ahead of Earth to maintain this position a spacecraft would need to constantly push back into position and obviously propellant would pretty quickly run out so solar sails are the perfect solution for this and they will be able to achieve more thrust the closer they get to the Sun we already have several satellites for this purpose Parkton Lagrange point one which is the point at which the gravity exerted by Earth and the gravity exerted by the Sun is the same however a solar sail with a 67 by 67 meter area would allow a satellite to double its distance from Earth and thus provide double the warning time at about two hours this would provide earth vital time to prepare for a solar storm and protect our satellite in orbit and our grid system larger sails would allow the satellite to park even closer to the Sun but we begin to run into engineering challenges as we approach the Sun with unpacking the larger sail and protecting it from the sun's heat if we had suitable materials that could be both extremely thin reflective light and heat resistant we could sail even closer to the Sun to perform Sun diving maneuvers to capture the sun's energy to increase our velocity and explore the outer solar system there are countless other applications for this technology that you can learn about with the reference material in the description but you may have difficulty understanding them without a decent understanding of orbital mechanics I started to read up on this subject myself in anticipation for the new Kepler space program game and brilliance course on gravitational physics will teach you everything you need to know about Kepler's laws of orbital mechanics and much more or you could complete one of brilliants daily challenges each day brilliant presents you with interesting scientific and mathematical problems to test your brain each daily challenge provides you with the context and framework that you need to tackle it so that you can learn the concepts by applying them if you like the problem and want to learn more there is a course quiz that explores the same concept in greater detail if you are confused and need more guidance there's a community of thousands of learners discussing the problems and writing solutions daily challenges are thought-provoking challenges that will lead you from curiosity to mastery one day at a time if I've inspired you and you want to educate yourself then go to brilliant org for it slash real engineering and sign up for free and the first five people that go to that link will get 20% off the annual premium subscription so you can get full access to all their courses as well as the entire daily challenges archive as always thanks for watching and thank you to all my patreon supporters if you'd like to see more from me the links to my Instagram Twitter subreddit and discord server are below\n"