Louis D. FriedmanJun 15, 2007

Making Light Work

Professional Pilot Magazine asked me to contribute a prediction about the future of flight for the next century. Naturally, I wrote about solar sailing. Writing the article allowed me to step back a bit from our efforts to make this first solar sail flight happen and focus my thoughts on future applications for solar sailing -- uses within our solar system and beyond.

The promise of light sails enabling new missions of application and exploration is what motivates The Planetary Society and Cosmos Studios to reach for the stars.

Theory evolves toward practice—high-speed travel in the frictionless medium of space.

from Professional Pilot Magazine, June 2007

Anyone in the prediction business knows that long-range forecasts are almost always wrong, but even if everything I predict here proves false, I won’t be penalized much.

Space travel is the form of flight I know best. Today we have robotic spacecraft exploring the solar system from end to end, but human spaceflight seems to be lagging behind. I’d like to examine the future of humans in space, and what we can expect in the way of travel to other solar systems.

By the end of this century, we should have made some progress on both fronts. Humans should have visited Mars and perhaps other places in the solar system. I hope this is achieved in half that time --  by 2050. By century’s end, some version of Buzz Aldrin cycler orbits -- trajectories that permit round-trip missions without large propulsive maneuvers -- should be ferrying humans regularly back and forth to Mars.

Interstellar travel, however, is unlikely to be practical even by then -- space is big! Still, even today we can recognize the technology that will eventually take us to the stars, and can foresee the steps leading to such a journey. By the end of the century, we may be ready to start construction on a massive, powerful laser which can beam light over interstellar distances. If such a laser is built, then in the following 85 years, say, we will be able to launch a "light sailor" spacecraft kilometers across. Such a spacecraft might be able to visit another star system and return data to the same generation of humans that launched it. By developing the world’s first solar sail spacecraft, the Planetary Society is working now to make it happen.

Sailing by light is the only technology we currently know that might one day take us to the stars, and this is what motivates the high interest in solar sailing. It is certainly practical for interplanetary missions, especially round-trip missions in the inner solar system to Mercury, Venus, Mars and asteroids -- and it has some enormously important near-term advantages for setting up solar weather and other monitoring stations in nearby interplanetary space -- but I am motivated most by the long-range view that this technology might one day make interstellar flight possible.

Flight by solar sailing is enabled by the force that results from the momentum transfer of light photons reflecting off a highly reflective and large but very lightweight sail attached to a spacecraft. There are several possible uses of solar sailing in the near and more distant future.

Solar weather monitoring

A solar storm occurs when charged particles streaming from the Sun interact with Earth’s magnetic field and cause perturbations in our ionosphere. These well observed perturbations affect radio communications on Earth and between Earth and space. Solar storms have been known to cause communication blackouts, which are not just inconvenient but may also prove dangerous at critical times. They can also cause disruptions in the power grids.

The flux of charged particles is caused by storms within the Sun, which we can see as mass ejections from its corona. When the particles reach Earth, we can observe their interactions with our ionosphere. Occasionally an interplanetary spacecraft positioned between Earth and Sun, outside the Earth’s radiation belts, detects the solar storm as it erupts from the Sun -- but this happens only when the spacecraft is lined up between the Earth and Sun at just the moment when the stream of particles from the solar storm has reached the spacecraft’s distance from the Sun. For the interplanetary spacecraft to pass the right place at the right time is a matter of pure luck.

Sailing by light is the only technology we currently know that might one day take us to the stars, and this is what motivates the high interest in solar sailing.

However, there is one place where an ordinary spacecraft can "hover" on the line between the Earth and Sun and provide alerts of incoming solar storms. That stable point is known in physics as a "libration point," and is where the gravity fields of the Sun and Earth balance, allowing the spacecraft to stay in position. This point is nearly 1.5 million km from Earth, and NASA has been operating a spacecraft there -- the Advanced Composition Explorer (ACE) -- for nearly 10 years, making scientific observations. ACE is now reaching the end of its lifetime, and civilian and military authorities in the US and elsewhere are interested in replacing it with a spacecraft that can monitor solar weather more accurately. One option is a simple replacement satellite, but a longer-lived, more capable satellite with advanced station- keeping capabilities would be preferable. In fact, a solar sailing spacecraft would be ideal for this purpose, harnessing the constant force of sunlight for its station-keeping maneuvers.

But solar sailing permits something even better -- monitoring at points much closer to the Sun than the libration point, without relying on the special gravitational balance. If we can monitor the solar flux 2 or 3 times closer to the Sun, we will get 2 or 3 times greater warning of any potential disruption to the ionosphere. This can provide significant economic advantages to operators of communications and power grids. A solar sail could hover at points closer to the Sun than the gravitational equilibria.

Planetary Society Chairman Dan Geraci has suggested broadening the concept of weather monitoring to other observation applications. He proposes setting up a fleet of "sentinel" station-keeping satellites for solar weather monitoring, interplanetary and interstellar particle observation, detection and tracking of near-Earth objects, and Earth observation. All these missions could be accomplished with a standardized solar sail spacecraft stationed at various points in interplanetary space.

Interplanetary flight

It is intuitively obvious that using the power of sunlight would help missions whose trajectories took them toward the Sun, such as those targeted at Mercury or Venus. The latter is relatively easy to approach with conventional propulsion, but reaching Mercury demands higher energy and is therefore far more difficult. This is where a solar sail, which will receive its energy directly from the nearby Sun, would have an enormous advantage.

With Special Thanks to Cosmos Studios!

As the major sponsor and partner of Cosmos 1, we thank Cosmos Studios and its founder and CEO, Ann Druyan, for years of financial and inspirational support for this cutting edge project that will pave the way to future travel to the stars.

Your unwavering commitment, and that of Planetary Society Members around the world, will take our dream of a "first flight with light" to reality.

Together, we will make our way to the stars.

Solar sailing offers a practical means of carrying heavy payloads to Mars, and would also be effective for missions to the asteroid belt and to comets -- missions that require high energy but are not overly distant from the Sun. It was with these types of mission in mind that we began developing the solar sail concept at the Jet Propulsion Laboratory (JPL) in the mid-1970s. Our goal was to build a solar sailing spacecraft to rendezvous with Halley’s Comet, matching its position and velocity.

Solar sails could also be used to propel a spacecraft rapidly to the outer solar system and beyond. As we discovered at JPL when studying the Halley’s Comet mission, this can be done by diving in close to the Sun, where the sunlight power is greatest, and building up speed at the closest point of approach (perihelion) for the outbound mission.

Velocity increases at perihelion are the most efficient for increasing orbital energy. For example, if you wanted to get to Pluto fast, you could use your solar sail to spiral as close to the Sun as possible without damaging the sail material. This would increase the spacecraft’s speed to the velocity needed to escape the solar system altogether as you aimed the vehicle toward Pluto. Once you were past the asteroid belt, the distance from the Sun would render the sail useless, so it could be jettisoned.

With sails of a reasonable size (ie, hundreds of meters in diameter) and a relatively lightweight spacecraft (under 500 kg), a 3 to 4-year trip to Pluto would be possible. Such a quick trip would mean that it would be very difficult to slow the spacecraft down near Pluto, or to enter orbit around it. In fact, such a maneuver would require an enormous propulsion system -- maybe impractically so.

Round-trip missions are a more likely interplanetary application of solar sails. Today, using conventional propulsion, attempts to collect and return samples from Mars, Venus, Mercury and the main asteroid belt, require highly demanding missions. Such an approach would be impractical in the future, when large payloads will need to be transported to and from the planets. Unlike a conventional spacecraft, a solar sail does not need to carry with it the fuel for a return trip from its target. This makes a round trip inherently simpler, and may even make possible some day a sample return from Jupiter’s moons.

We can imagine a solar sail spacecraft carrying a deployable lander with an ice driller to Europa -- the site of the only accessible extraterrestrial ocean known today. The lander would be deployed from the main spacecraft and drill for a few weeks, collecting ice from the surface and, hopefully, reaching the underground ocean to collect water samples. It would then take off again and rendezvous with the orbiting solar sail spacecraft.

Initially the spacecraft would use rocket propulsion to leave Jupiter orbit and head back to Earth. Eventually, as it came closer to the Sun, the pressure of sunlight would be great enough to control its trajectory and bring the (now) solar sailing spacecraft back to Earth. I doubt that a case can be made that fish are swimming in Europa’s subterranean ocean, but the journey would still be one heck of a fishing trip. I’d like to believe that a mission such as this might be less than a century away, but I may be over-reaching.

A Mars sample return is far more likely. That mission has been sought by scientists for 2 decades now, and surely we’ll have several Mars sample returns under our belt by century’s end. After a number of these, we can expect increasingly complex Mars landers and the establishment of a base. Whether occupied by humans or not, a Mars base will require large masses of cargo -- and here again a solar sailing spacecraft can have a vital role. It would be the ideal transport system for a ferry service bringing cargo to Mars and returning material to Earth for study.

Interstellar flight

It is possible to imagine the solar sail flights I describe above taking place within the next 100 years. As I’ve said, getting to the stars is a daunting challenge and will take longer, but there is no known technology apart than solar sail flight that can do it. The sails required for such a journey would be enormous --  on the order of several kilometers across -- and would have to be ultra-thin -- 0.1 micron or less --  to reach the necessary speed for interstellar flight. Naturally, as a spacecraft moves away from the Sun, the sunlight quickly becomes irrelevant, and the vehicle would be propelled forward by laser beam. (Unlike diffuse sunlight, lasers concentrate their energy in a very narrow beam which can be focused over huge interstellar distances.)

As to what could power such a gigantic laser -- on the order of 100 gigawatts -- the answer is sunlight (which means that, in a sense, we could still call it solar sailing). The huge laser that would be used to propel the sails on an interstellar flight would be located in space in ever-present sunlight -- probably in the vicinity of the orbit of Venus, where solar radiation is double what it is on Earth.

I expect the laser would be transported there, and its power station delivered and assembled, with the aid of solar sail transports from Earth.

In all probability, there will be an operational laser in space this century, but not one big enough to power light sails on an interstellar flight. There are other difficult technological challenges as well. Sail construction would require extremely thin films, perhaps made with plastics that evaporate, leaving only aluminum molecules to reflect the sunlight. To keep the weight down, the spacecraft’s electronics might perhaps be sprayed onto the sail rather than attached to it in a separate compartment. Overall, the vehicle would need to weigh only a few kilograms, and the sail’s total area would be at least 1 square kilometer. With these technological challenges in mind, I venture to predict that practical interstellar flight is at least 2 centuries in the future.

First Flight with Light

What is exciting is that we know the way forward. We don’t have to invent some new physics (like matter/ antimatter engines) and we don’t have to conjure up new technologies from science fiction (such as interstellar ramjets scooping up and using interstellar hydrogen molecules). Rather, it’s all a matter of engineering -- make the light sail materials thinner, the spacecraft lighter and the lasers more powerful. If this is correct, and if there are other civilizations out there, it’s reasonable to think that they will have done all this already -- in which case we may learn about this new technology from them, and faster than we think.

The Time is Now.

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