Successful Solar Sailing

The Planetary Society’s LightSail 2 spacecraft launched into orbit on June 25, 2019 and deployed its 32-square meter sails almost a month later on July 23. In the time since sail deployment, it raised its orbit some seven kilometers. Here we see a great shot of the LightSail 2 over Australia.

LightSail 2 image courtesy The Planetary Society

In 1619, astronomer Johannes Kepler noticed that comet tails always point away from the sun and realized that some solar force must produce that effect. Two centuries later, in 1862, James Clerk Maxwell suggested that light has momentum and it was finally demonstrated in 1900. The goal of the LightSail 2 mission was to show that momentum from light could propel a spacecraft. The process of solar sailing is not unlike sailing on the water, only in space, gravity acts like water currents while light acts like wind. As with a sail at sea, the sails must be turned in flight to take the best advantage of the “wind” while the ship moves on the “current.” LightSail 2 accomplishes this via internal reaction wheels. The gif below shows how the sail is turned to take the best advantage of sunlight and raise its orbit.

Josh Spradling / The Planetary Society

The following graph shows the elevation of LightSail2 over the course of the mission. The apogee is the highest point in orbit, the perigee is the lowest point. At LightSail2’s elevation, there are two forces working on the sail besides gravity. The first is light pressure from the sun. The other is atmospheric drag from the Earth. Even though LightSail 2 is in a relatively high orbit, it’s still in the most tenuous parts of the upper atmosphere. The upshot is that the apogee increases because of light pressure, but the perigee decreases because of atmospheric drag. That noted, LightSail 2 has accomplished the mission it set out to do, demonstrate that light can propel a spacecraft.

Image from the LightSail Mission Control Page supported by the Planetary Society

You can keep up to date with the LightSail mission and visit their mission control page by going to http://www.planetary.org/explore/projects/lightsail-solar-sailing/#the-lightsail-2-mission.

It’s worth noting this is a technology in its infancy. That said, with the lessons learned from this mission, there’s promise that solar sails could be used for small unmanned probes in the not-too-distant future, or perhaps they could be used to deflect dangerous asteroids, especially if caught early enough that they only need a small nudge.

In the long run, with much more development and technical innovation, I would like to think solar sails could be used to propel humans across the solar system as I imagine in my novel The Solar Sea. In a little under a week, on Friday October 11, Lynn Moorer will interview me about the novel on her radio show from 12:30-1:00 pm mountain daylight time. If you’re in Las Cruces, you can tune in to 101.5 FM on your radio dial. If you aren’t in Las Cruces, or just don’t listen to shows on the radio, you can stream the show at  https://www.lccommunityradio.org/stream.html. It looks like KTAL Community Radio is building up their archive of past shows, so I’m hoping I can share these interviews with you soon. If you want to read the book before the interview, you can find all the places it’s available by visiting http://www.davidleesummers.com/solar_sea.html.

Solar Sails in Science Fiction

From_the_Earth_to_the_Moon_Jules_Verne

As promising—and romantic—a technology as solar sails are, it’s perhaps not surprising that they have found their way into fiction numerous times. Perhaps the first mention of the idea of using light to propel a spacecraft is in Jules Verne’s 1865 novel From the Earth to the Moon. In Verne’s novel, a giant cannon is used to send a spacecraft to the moon. However, Verne writes that such a projectile has limited velocity. “Is it not evident, then, I ask you, that there will some day appear velocities far greater than these, of which light or electricity will probably be the mechanical agent?” asks Verne. The thing is, Jules Verne was right on top of the scientific achievements of his day. He knew that James Clerk Maxwell had recently discovered that light exerts a pressure on objects.

Lady Who Sailed

Compelling as this idea is, it seems that no one pursued it further until an engineer named Carl Wiley wrote an article for Astounding Science Fiction in 1951 about how solar sails could be built in orbit and used for space travel. The article was called “Clipper Ships in Space” and was written under the pseudonym Russell Saunders. This article influenced more than one science fiction writer. The first was Cordwainer Smith who published the story “The Lady Who Sailed The Soul” in Galaxy Magazine in 1960. The story was mostly about the romance of two characters, but it also does a fairly good job of describing a solar sail spaceship.

The next appearance of a solar sail in science fiction was in Pierre Boulle’s novel Planet of the Apes in 1963. To quote from the novel: “In those times, interplanetary travel was commonplace, though interstellar ventures were still an exception. Rocket ships would take the tourists to fabulous locations on Sirius or the finance people to the stock markets of Arcturus and Aldebaran. But Jinn and Phyllis, a wealthy and free couple, were known through the Cosmos to be young originals, with a bit of craziness, and they would cruise through the Universe just for the fun of it—with their sailcraft.” Boulle then goes on to describe the craft: “Their ship was a kind of sphere with a shell—the sail—made of amazingly thin material, and it would move through space, just pushed by the pressure of light beams.”

Boys Life Sunjammer

A year later, Arthur C. Clarke published the story “The Sunjammer” in Boy’s Life Magazine that told the story of seven solar sails racing from the Earth to the Moon. This story in particular captured many scientists’ imaginations and caused them to seriously ask the question of whether or not these kinds of craft could be developed. In fact, NASA’s first solar sail mission to deep space has been dubbed “Sunjammer” in Clarke’s honor. You can visit the project’s homepage at: sunjammermission.com

Light sails have continued to appear in science fiction since then. Notable appearances include the episode of Star Trek: Deep Space Nine called “Explorers” and Star Wars Episode II: Attack of the Clones. No doubt solar sails will continue to appear in science fiction. My own novel, The Solar Sea, about a solar sail spacecraft that’s used to explore the outer planets was published in 2009. You can read more about it at thesolarsea.com

Science and science fiction are closely intertwined with solar sails. In the middle of the nineteenth century, James Clerk Maxwell discovered that light exerted a pressure on objects. Soon after, Jules Verne posited that such a force could be used to move a spacecraft. In the 1950s an engineer published an article about light sails in a science fiction magazine and a few years later, science fiction writers were publishing stories about such craft. Now, scientists and engineers are working to turn the idea of solar sails into reality while science fiction writers continue to dream of sailing to distant planets and star systems.

The Solar Sea

Is there something about solar sails you’d like to know and I didn’t cover? Let me know in comments and I’ll see if I can find out for you and post next week. I’ve already been asked how to steer a solar sail. I’ll do my best to explain!

My novel The Solar Sea is available at:

Solar Sail Projects

Light pressure has been demonstrated as a force that can, indeed, move a spacecraft. Back in 1974, when Mariner 10 was running low on fuel, NASA utilized light pressure against its solar panels to adjust its trajectory. Although this was a limited demonstration, light sail technology holds sufficient promise that scientists around the world are trying to turn this science fictional idea into reality. Public and private teams in the United States, Japan, and Europe are all actively working on solar sail projects.

NanoSail-D

Although it’s not as heavily publicized as some projects, NASA has been on the forefront of light sail development. On November 19, 2010, they launched a solar sail “nanosatellite” called NanoSail-D. It was a 100-square-foot solar sail that was deployed from NASA’s FASTSAT Satellite on January 20, 2011. Four booms with sails deployed from a craft about the size of a lunchbox. The image above is an artist’s conception of the craft, but to learn more about NanoSail-D and to see photos of the sail during it’s 240-day voyage, visit: http://www.nasa.gov/mission_pages/smallsats/nanosaild.html

In the longer term, NASA is developing a somewhat larger prototype solar sail. This larger sail would have an area approximately 13,000 square feet with a payload of 110 pounds. This makes it about seven times larger than the largest solar sail ever deployed. While the NanoSail-D project was essentially a simple sheet of reflective material designed to test the concept of solar sailing, the larger solar sail would include four small steering sails to test maneuverability of the craft. Currently, this larger prototype sail is scheduled to launch on a Falcon 9 in 2015. You can read more about this mission at: http://www.nasa.gov/mission_pages/tdm/solarsail/solarsail_overview.html

LightSailEarth

Perhaps the most prominent private group working to build a solar sail is the Planetary Society. They’re a non-profit organization in the United States dedicated to the support of space exploration. They attempted to launch a solar sail called Cosmos 1 back in 2005. Unfortunately, it was lost when the launch vehicle it was aboard—a Volna rocket launched from a Russian submarine—exploded on the way to orbit. The original craft looked like an octagonal mirror. Eight triangular sails surrounded a central hub. Each sail was mounted to the hub by a rotating mast. By changing the angle of the sails, the craft’s course could be adjusted. The updated project is called LightSail-1 and appears to be a much simpler design as shown in the photo at the top of the paragraph. For more information, visit: http://www.planetary.org/explore/projects/lightsail-solar-sailing/

Interest in solar sail technology isn’t limited to the United States. In 2011, the Japan Aeronautic Exploration Agency or JAXA successfully deployed a small solar sail called IKAROS (for Interplanetary Kite-craft Accelerated by Radiation Of the Sun). The test successfully demonstrated both sail deployment and the ability of a craft to fly by photon pressure. Later this decade, they hope to launch a probe to Jupiter that will be propelled by solar sails. An artist’s conception is shown below. You can learn more about the IKAROS mission, and see a cool gif of the sail in flight at: http://www.jspec.jaxa.jp/e/activity/ikaros.html

Japan Jupiter Mission

The European Space Agency has also been working on a prototype solar sail, but there is little information at this point about when they might try a flight. They appear to have a near-term project using solar sail nanosatellites to help remove debris from Earth orbit.

As we move toward the middle of the 21st century’s second decade, we find ourselves searching for “green” technologies that utilize the resources we have available as wisely as possible. Solar sails appear to be a promising, green technology that would allow humans to explore space wisely. Indeed, it is the only propulsion technology within our reach that could conceivably propel a spacecraft to near the speed of light.


The Solar Sea

In the next installment, I’ll look at solar sails in science fiction. In the meantime, you can learn more about my novel, The Solar Sea, which is set aboard a solar sail spacecraft at TheSolarSea.com. The novel is available at:

Solar Sailing

In 2008, when my novel The Solar Sea was released, I gave a presentation about solar sails at several science fiction conventions. I summarized that information in an article called “Sailing the Winds of Space” which appeared in Strange Weird and Wonderful Magazine online. The issue no longer appears to be available and I thought it was worth updating some of the information. I’ll be presenting the article over the next three weeks here at the Web Journal. I hope you enjoy this look at solar sailing!


Pottery depicting masted ships. Photo by Einsamer Schütze.  License: CC BY-SA

Pottery depicting masted ships.
Photo by Einsamer Schütze. License: CC BY-SA

Sailing is one of the oldest transportation technologies. The simple use of wind to propel a craft across the water goes back to at least 3500 BC when the first representation of a ship under sail appeared on an Egyptian vase. Scientists around the world have explored ways to adapt this ancient technology for use in space flight. Sail technology is a way to make space flight more cost effective. Not only that, it’s possible that ancient sailing technology could propel a craft to near the speed of light. Sails for spacecraft would utilize light instead of wind. Such a sail is known as a light sail—or a solar sail if the primary light source is the Sun. This simple but powerful technology has also been an inspiration to numerous science fiction writers over the last fifty years.

Photons—individual particles of light—have momentum. When something with momentum strikes another object, it imparts some of its momentum to that object. Think of what happens when you play pool and the cue ball strikes another ball. The cue ball bounces off and the other ball moves in some direction. If you shine light at a mirror, the light bounces off, but it also imparts some momentum to the mirror. The reason you don’t see mirrors moving every time you shine light on them is that here on Earth, air pressure and gravity overwhelm light pressure. Light pressure from the Sun at Earth only produces about two pounds of force for every square kilometer. However, in space, where gravity is significantly less than it is on the ground and air pressure is no longer a factor, even a gentle force such as light pressure becomes significant.

Photo by No-w-ay in collaboration with H. Caps.  CC BY-SA

Photo by No-w-ay in collaboration with H. Caps. License: CC BY-SA

Even though gravity and air pressure are no longer factors in space, solar sails must be built out of very lightweight material. Think about the billiard balls, but imagine replacing the cue ball with a marble. If you shoot a marble at a billiard ball, the marble will still impart momentum to the billiard ball, but it will take a lot more effort for the marble to move the billiard ball. By the same token, solar sails must be built out of the lightest possible material so that photon pressure will have the greatest effect when propelling the spacecraft. The materials currently being investigated for solar sails are somewhere between 40 and 100 times thinner than a piece of writing paper.

Fuel is one of the greatest costs in contemporary space flight and spacecraft must be designed to start their journey with all of the fuel they will need for the duration of the voyage. This is a significant engineering challenge. However, in the solar system, the sun produces an abundant, steady stream of photons that could be harnessed by a spacecraft. As such, light sails become a very attractive means of space propulsion.

Venus transit with telescope

Because light pressure is a very gentle force, solar sails would accelerate very slowly. However, as long as there is a supply of light, there is nothing that will stop the acceleration of a solar sail. Theoretically, a solar sail will continue to accelerate until it reaches the same speed as the particles striking it—the speed of light. According to Newton’s first law, an object in motion will remain in motion unless some force acts upon it. In space, where there is no friction, once a solar sail reaches the speed of light, it could continue at the speed of light. As such, light sails could theoretically be used for interstellar travel.

A common misconception is that solar sails are propelled by the solar wind. The solar wind is a stream of charged particles emitted from the sun. This stream of charged particles will transfer some momentum to any object it strikes, just as photons will. However, photon pressure from the sun is about 5000 times greater than the force from the solar wind.


In the next installment, I’ll discuss some actual solar sail projects.

The Solar Sea

My novel The Solar Sea, which imagines a voyage aboard a solar sail spacecraft, is available at: