In 1976, astronomer Carl Sagan appeared on Johnny Carson’s The Tonight Show to talk about solar sailing, a technology he said could let spacecraft travel using particles and radiation from the Sun as propellant.
Forty years later, Sagan’s vision is becoming a reality. On June 24, The Planetary Society’s LightSail 2 spacecraft is scheduled to hitch a ride aboard a SpaceX Falcon Heavy rocket launching from the Kennedy Space Center in Florida. When packed, the spacecraft weighs just over 11 pounds and is roughly the size of a loaf of bread. Once deployed, the solar sail stretches as wide as a regulation boxing ring.
LightSail 2 will be deployed seven days after the initial launch. In the week that follows, the spacecraft will swing open its immense solar panels, unfurl its sails, turn toward the Sun, and cruise like a dinghy around Earth. The goal is to increase the spacecraft’s orbit to around 450 miles over the course of a month, before coasting at that distance for a year.
If the mission is successful, LightSail 2 will be a milestone in spaceflight: the first spacecraft to raise its orbit around the planet using just the power of sunlight. The goal is to demonstrate the potential of solar sailing for other small spacecraft in a bid to make spaceflight more accessible for teams with tight budgets.
To learn more about the LightSail 2, we spoke with Bill Nye the Science Guy and Planetary Society CEO, about the mission’s goals and what he thinks the future holds for spaceflight.
This interview has been edited and condensed for clarity.
Digital Trends: The Planetary Society plans to send into orbit a small spacecraft that can sail on sunlight. For people who aren’t familiar with the concept, what is solar sailing and how does it work?
Bill Nye: It might surprise many of us to learn that photons, particles of light, have no so-called “rest mass,” but they are nevertheless pure energy and they carry momentum. In the vacuum of space, a very low mass spacecraft with a very large shiny area, a sail, gets a continuous push from sunlight. Unlike a rocket engine, a solar sail spacecraft never runs out of fuel. And although the momentum, the “solar pressure” of sunlight is quite small, the pressure is continuous, non-stop, for months or years in space. Solar sailing is in its infancy, but it may become a game-changer. We’ll soon be able to send our solar sail spacecraft to all sorts of destinations in our Solar System and perhaps to another star system one day.
Carl Sagan discussed his vision for a solar sail more than four decades ago, but the concept dates back centuries. Where did the idea of solar sailing originate?
The concept of solar sailing dates back to the 1600s. In 1607, a bright comet appeared in the night sky, which both frightened and fascinated people. Johannes Kepler, after whom Kepler’s Laws of Planetary Motion are named, thought deeply about what he saw that year. He deduced that the dazzling tail, of what we now call Comet Halley, was probably caused by the Sun’s warmth, which was somehow evaporating or liberating material from the comet’s surface. The notion of sunbeams interacting with a celestial object inspired Kepler to imagine a spaceship sailing with the starlight just as a ship sails with the wind. In 1608, he wrote in a letter to his friend Galileo Galilei that said, “Provide ships or sails adapted to the heavenly breezes, and there will be some who will brave even that void.”
Spaceflight has seen significant advances in the forty years since Sagan discussed his vision for a solar sail. How has the technology behind solar sailing changed in that time? What advancements have made spacecraft like the LightSail possible?
The physics are the same as in Sagan’s day. Sunlight carries momentum, and it can propel a suitably designed spacecraft. The instruments on board LightSail 2 are far more compact than they could have been 40 years ago. The cameras especially are higher resolution, more versatile, and require less power than any available back then. The “stitch-welded” cobalt steel booms would have been tough to manufacture, too. The main thing is that we’re really flying this thing in Earth’s orbit. We’ll be able to increase our orbital altitude and energy, because we can tack and maneuver LightSail 2 quickly. One test is worth a thousand expert opinions; we’ll learn what it really takes to fly a highly maneuverable solar sail.
What is the primary mission objective for LightSail 2? After that objective, what does the Planetary Society team hope to demonstrate?
The mission objective of LightSail 2 is to get the spacecraft to raise its orbit around the Earth using sunlight. This mission will also demonstrate the application of solar sailing for small spacecraft called CubeSats. CubeSats are small spacecraft that make spaceflight more affordable for academics, government organizations, private institutions, and new space-faring countries. There is a global effort to lower the cost of space exploration and we believe this innovative method of propulsion could dramatically open up space to all kinds of interested people. Some would say it will “democratize” space exploration.
Because there’s no fuel, there’s no limit to the propulsive force available, albeit once you’re on orbit around Earth. There is also no cost in handling dangerous rocket fuels like hydrazine. It’s a chance for more space exploring organizations to fly farther and deeper into space at lower cost.
The Planetary Society launched a similar spacecraft in 2015. How does LightSail 2 differ from its predecessor? What new technologies is LightSail 2 equipped with?
LightSail 1 demonstrated that we could deploy the sails and communicate with the spacecraft in orbit. We took some pictures as well. Playing the hand we were dealt by the rocket, LightSail 1 remained at a low enough orbital altitude that it was always subject to atmospheric drag. LightSail 2 is going to higher orbit where there are fewer air molecules to fly through and much less drag.
With LightSail 2 we plan to really sail. The spacecraft will tack edge-on toward the Sun, then twist to present its sails so they face directly at the Sun’s rays. We plan two 90-degree rotations with each orbit. With its solar panels powering an electric motor, LightSail 2’s momentum wheel can control its orientation in space. Since LightSail 1 didn’t fly high enough to solar sail, it didn’t carry a momentum wheel. We also expect LightSail 2 to last almost a year in Earth’s orbit. The spacecraft may even be visible in the night sky for a year to observers within 42 degrees of the equator, which is as far north as New York, and as far south as Auckland, New Zealand.
Another improvement to LightSail 2’s hardware is the incorporation of small, corner reflectors that will enable ground-based lasers to precisely measure the spacecraft’s orbit. It’s a ranging system using light rather than radar. Since our primary mission objective is measuring a change in LightSail 2’s orbit due to solar sailing, adding the reflectors will tell us a great deal about the spacecraft’s performance. This is courtesy of the International Laser Ranging Service.
Let’s talk about future solar sailing applications. How might solar sailing be used in the future to help spacecraft travel throughout the solar system? Can you give some concrete examples?
There are a few missions for which a solar sail spacecraft is uniquely suited. Anytime we want a spacecraft to stay aligned or “keep station” with Earth in its orbit, a solar sail can provide the force needed to compensate for an orbit that would be closer to the Sun than we are. Spacecraft that monitor solar activity on the Sun or asteroids that cross Earth’s orbit are the classic examples. A spacecraft monitoring solar coronal mass ejections could provide 9 hours warning of an event that could ruin a huge fraction of our communication systems here on Earth. A spacecraft with an infrared telescope pointed away from the Sun could find asteroids that are very difficult to track any other way.
And of course, NASA’s Near Earth Asteroid Scout mission to an asteroid scheduled to launch in 2020 or so will use a sail largely derived from LightSail 2’s design.
When people really start thinking big, they envision a solar sail driven by lasers on Earth to fly all the way to another star system. An idea that is almost literally “far out.”
What are some of the challenges that need to be addressed before solar sailing is used more broadly?
Someday we want materials that withstand the higher heat and radiation environments near the Sun so that we could accelerate a spacecraft to very high speeds quite quickly. The main challenge is to the imaginations of space explorers. Asteroids, Jupiter, Saturn, Venus, and Mars are all destinations within reach of custom-built solar sail spacecraft.
LigthSail 2 may be visible from Earth. Given that missions like StarLink have caused a stir among astronomers, who say these bright objects interfere with stargazing, I’m wondering if a future full of solar sails will have a similar impact on the night sky.
Rather than over 700 twinkling satellites in low Earth orbit, LightSail 2 is one craft on a predictable path. Astronomers can avoid it easily. LightSail 2 will be visible to billions of Earthlings for about a year. We hope it inspires space explorers everywhere.
Given that renewed interest in spaceflight dubbed the Space Race 2.0, can you make some predictions about where the current space race is headed in the next decade? What are some milestones we should look out for?
Lower cost, low-emission rocket fuels. Densified methane fuel made from carbon dioxide drawn from the atmosphere and renewably produced electricity. The enormous Space Launch System rocket flying at last, which will take human explorers and planetary payloads farther and deeper into space. Evidence of life on Mars and close-up looks at Europa. These will change the world for Earth’s citizens everywhere.
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