Going Interstellar (collection)

SOLAR AND BEAMED ENERGY SAILS Les Johnson

We can’t feel it, but the light from the Sun is pushing on us. It’s a small push, less than an ounce per square football field. Whenever we are in sunlight, or any light, we are being pushed. This solar pressure is much smaller than the other forces we experience in our everyday lives. The force of the wind from the room air conditioner vent is far stronger than the force we experience in full sunlight. It is so small that very sensitive instruments are required to measure it. And it can only be measured in a vacuum because the various forces around us will otherwise swamp the effect. But solar pressure is real, it is constant, and it can be used to propel a spacecraft to incredible speeds.

About four hundred years ago, Johannes Kepler observed that the tail of a comet appeared to be created by some sort of cosmic breeze and postulated that this breeze could be used to move ships in space in a manner similar to which the sailing ships of his day were propelled by wind. While Kepler was wrong about the nature of these cosmic winds, he was correct in his observation that something coming from the Sun, which we now know is sunlight itself, can be used to move a spacecraft.

An earthly sail moves a ship by transferring the momentum of the wind to the ship by reflecting it from a sail. The force exerted on the sail pushes the ship, causing it to move. In physics, momentum is defined as the product of mass times velocity. Lots and lots of air molecules, each having mass and some velocity, reflect from a sail and transfer their momentum to it. The ship then begins to move, its momentum coming from the wind.

In 1923, the physicist Arthur Compton observed that photons (particles of light) have momentum even though they have no rest mass. In other words, these massless particles that we call light have momentum even though they would have no mass if we could catch one and slow it down to weigh it. This is yet another weird property of light—but one that will be very useful for taking us to the stars.

Imagine a large, very thin, lightweight and very highly reflective sail deployed in space for the sole purpose of reflecting sunlight. We’ve just imagined a solar sail and they are far from imaginary. Solar sails reflect sunlight, transferring the tiny momentum of each reflected photon to the sail, causing the sail to move. The force is tiny. At the Earth’s distance from the Sun (ninety-three million miles), the force from sunlight is about five pounds per square mile. In other words, we’d have to have a sail area of one square mile to feel five pounds of force. For comparison, just one of the Space Shuttle’s main engines produces about five hundred thousand pounds of thrust. The primary difference is that the shuttle’s engines can only produce this thrust for a very short period of time before running out of fuel while a solar sail can produce thrust as long as it remains in sunlight. And since the distances involved in space travel are so large, the sail will remain in sunlight for a very long time no matter its destination.

In this case, the space shuttle engine is the hare and the solar sail is the tortoise. Chemical rockets will never take us to the stars, but solar sails might. It is important to note that while solar sails may one day take us to Alpha Centauri, they will never get us off the surface of the Earth. To lift from the surface of the Earth, we need a propulsion system that can produce more thrust than the rocket weighs. Chemical rockets are capable of producing these high thrust levels; solar sails cannot.

Before we start building our solar sail-propelled starship, we need to discuss a few more critical issues that will affect our design. First of all, the sail will still be subject to Newton’s Second Law, which states, “a body of mass (m) subject to a force (F) undergoes an acceleration a that has the same direction as the force and a magnitude that is directly proportional to the force and inversely proportional to the mass.” In other words, to get a mass to accelerate, we need to apply a force. In order to get the accelerations needed to achieve very high speeds, such as those required for interstellar travel, we need a large force or a small mass, or in this case, we need both.

Newton’s Second Law requires our solar sail design to be very large so the sail can capture as much sunlight as possible in order maximize solar photon thrust. It also requires us to use very lightweight materials so that we can make our ship as low mass as possible. The sail must also be highly reflective so that we can capture as much momentum from each photon as possible.

Is there anything we can do to increase the force acting on the sail from the sunlight? Even though we have the benefit of time, five pounds of thrust per square mile is ridiculously small. We would require a sail almost one hundred thousand square miles in area to equal the thrust produced by one space shuttle engine. Such a sail would have roughly the same surface area as Alabama and Mississippi combined! Surely we can do something to increase our thrust so that we can make a smaller sail.

It turns out that another interesting fact about sunlight allows us to do just this. We can dramatically increase the force acting on the solar sail by flying closer to the Sun thanks to a property of sunlight called The Inverse Square Law. According to this law, if we move an object twice the distance from the light source, it will receive only one quarter of the illumination. Two times the distance (2) means one-fourth (¼) the illumination—two squared is four. If we move out to four times the distance from the Sun, the illumination drops to one sixteenth of the previous amount—four squared is sixteen. Less illumination translates directly into less force. Fortunately, we can use this geometric property to our benefit by moving closer to the Sun. If we reduce the distance to ½ its previous value, we get four (4) times the force. If we reduce it to one fourth, then we get sixteen times the force. And if we get sixteen times the force per square mile, then we can reduce the overall surface area of the sail by the same factor. And when we are talking about sails the size of US states, a factor of sixteen is significant.

This all sounds great, but are solar sails real? Have they been built and tested in space? Has anyone actually used one for sending a spacecraft anywhere? Yes, yes, and yes!

Until the 1970s, Kepler’s vision and Compton’s physics were good science but for space travel they were primarily an intellectual curiosity. With the anticipated return of Halley’s Comet in 1986, NASA commissioned a study of the feasibility of using a solar sail to rendezvous with the comet. The project never got off the ground, but it did get many space scientists and engineers thinking about solar sailing as something real, and the pace of sail technology development accelerated. The first big step was taken by Russia with the launch of their Znamya mirror in 1993. Znamya was a large, lightweight mirror flown in space to test the idea of using reflected sunlight to illuminate large areas on the ground at night. The mirror was made from very lightweight reflective materials and looked, for all practical purposes, like a solar sail.

In the late 1990s, the Europeans entered the picture with the ground-based development of a one hundred foot sail manufactured by the German company DLR (Deutschen Zentrums für Luft- und Raumfahrt). Though the sail never left the laboratory, it inspired NASA to develop a similar capability during the early 2000s that culminated in the testing of two different solar sails in the world’s largest vacuum chamber, which is located at the NASA Glenn Research Center’s Plumbrook Station. The two solar sails were one hundred feet in diameter, made from materials thinner than a human hair, and autonomously deployed under space vacuum conditions to test their space worthiness. Figure 5 shows the sail developed for NASA by L’Garde, Inc. just after a deployment test in the vacuum chamber.

Figure 5. NASA and L’Garde, Inc. tested a 100-foot diameter prototype solar sail in the mid-2000’s. Shown in the picture are the fully deployed solar sail and with four of the sail engineers standing in the foreground to show scale. (Image courtesy of NASA.)

Japan took the next major step in solar sailing by actually flying a sail in space and using it as a primary propulsion system. The IKAROS (Interplanetary Kite-craft Accelerated by Radiation Of the Sun) was launched in May 2010 on a trajectory that will take it on a voyage near Venus. Though smaller than the NASA and DLR ground demonstration sails, the sixty-five foot diameter sail showed the world that solar sails can be used in space for propulsion. Figure 6 shows the IKAROS in space after deployment.

Figure 6. The Japanese Aerospace Exploration Agency launched the IKAROS solar sail on a mission to Venus in 2010. Shown in the figure is an actual picture of the IKAROS sail after deployment taken by a small robotic camera ejected from the spacecraft during flight. (Image courtesy of the Japan Aerospace Exploration Agency.)

In 2010, NASA launched the NanoSail-D into low Earth orbit. NanoSail-D, (where D stands for drag) is not a functioning solar sail since it is not using the force of sunlight in a controlled manner for propulsion. The ten-square-foot NanoSail-D might instead be a space demonstration of more conventional windsailing. As NanoSail-D skimmed through the Earth’s uppermost atmosphere, the wind created by its passing caused the spacecraft to slow and eventually re-enter. The wind caused drag, giving NanoSail-D its name.

Other groups are planning small sail missions that will actually use sunlight pressure for propulsion. Chief among them is the Planetary Society’s LightSail-1. Similar in weight to NanoSail-D, LightSail-1 will have a sail three times larger and be capable of pointing toward the Sun in order to use the sunlight for propulsion. CU Aerospace and The University of Surrey have similar sails in development.

Following the successes of IKAROS and NanoSail-D, there has been renewed interest in solar sailing, and several countries are considering the development of even more ambitious sails for use in missions throughout the solar system. We have a long way to go, however, before we will have a sail that can be used to send a spacecraft beyond the edge of the solar system into the abyss between the stars.

Some may be wondering how a solar sail, which derives its thrust from sunlight, can possibly take a spacecraft from one solar system to the next. After all, sunlight gets rather dim and is almost nonexistent when we get beyond the orbit of Pluto—let alone when we are in true interstellar space. Without sunlight, there is no force acting on the sail, hence no acceleration. So, how can it be done? There are two answers: 1) solar sails with very close solar approaches and 2) laser-augmented solar sails.

As discussed above, the thrust on a solar sail increases as its distance from the Sun decreases. Some pioneering work by Drs. Gregory Matloff and Roman Kezerashvili shows that an approximately one mile diameter solar sail spacecraft weighing no more than seven hundred pounds passing very, very close to the Sun, within about nine million miles, could achieve a solar-system exit velocity of two hundred and fifty miles per second. A craft traveling this fast would pass the Earth in four days, Jupiter in twenty one days and reach the Alpha Centauri system in just over three thousand years. By comparison, the fastest rocket we’ve ever sent into space won’t cover the distance to the Alpha Centauri system for another seventy-four thousand years! By increasing the sail size, and keeping the payload mass the same, we can see an engineering path to building a sail that could cover this immense distance in about a thousand years. For you and me, there isn’t much difference between a thousand years and seventy four thousand years. But in the lifetime of civilizations, the difference between these numbers is significant. We have recorded history going back a thousand years and there is no reason to assume that we won’t have similar records going forward; however seventy-four thousand years goes back well beyond the origins of human civilization.

You might have noticed another problem with the relatively near-term solar sail—it weighs only seven hundred pounds. Unfortunately, to carry a larger mass—millions of tons are required to carry and sustain humans on such a voyage—would require a solar sail of immense proportions (think the size of continents) made of incredible materials (“unobtainium” comes to mind). While such sails don’t violate any known laws of physics, we currently are almost clueless regarding how to engineer them.

One approach to creating these massive sails is to build them in space, so that they don’t have to experience the stresses of riding a rocket to get them there. This would solve two problems at the same time. First of all, the rocket launch will be the most stressful of the mechanical environments which the sail must be designed to survive. Rockets are not known for slow and graceful acceleration or for being a smooth ride. Quite the opposite is true; consequently, building a gossamer sail strong enough to ride on a rocket will be difficult. Second, the manufacturing of extremely large, lightweight and fragile solar sails in Earth’s gravity will be nearly impossible. The forces experienced by just being here on the surface may be sufficient to cause tears in the sail. Overcoming the stresses experienced as the sail is folded and packaged, as well as surviving the effects of Earth’s gravitational acceleration, will likely be both complex and expensive. When compared to the Earth, the space environment is much kinder to solar sails.

Building sails in space will not be so easy either. Manufacturing anything in space implicitly assumes there is some sort of facility or location where the construction will take place. This place itself must be built and launched. Then there’s the raw materials part. Sails, though conceptually simple, are anything but simple when we consider their subsystems and components: lightweight, highly reflective membranes; lightweight structures; moving parts for attitude control; electronics for deployment, attitude control, and navigation; plus many others. All of these, at least here on Earth, come through an extensive supply chain all the way from the extraction of the raw materials from which they are made to the final fabrication in a factory somewhere in the world. It’s only after the system integrator orders all the right parts that the engineers and technicians can even begin putting it together. All of this would have to be re-created in space to enable in-space manufacturing of a very large solar sail.

There is another approach that takes advantage of the Earth’s well-established manufacturing infrastructure and the unique environment of space to solve the manufacturing and launch problems: build the sail on Earth, but make it more robust—thicker—than the mission requires and make the extra thickness out of materials that won’t easily tear when in the Earth’s gravity and that will not damage easily during launch. But, design the more robust sail so that the heaviest part will evaporate when exposed to a selected portion of the Sun’s ultraviolet light—which only happens when we are above the Earth’s atmosphere. Voila! The thick and heavy sail that was easier to make and launch quickly becomes the wispy, lightweight sail needed for rapid propulsion through interstellar space.

This might just work.

The single largest constraint on an interstellar spacecraft propelled by a solar sail is the “solar” part. If the ship must get all of its thrust from the Sun, then it is constrained to do so before it passes the orbit of Jupiter (in just a couple of weeks) because the Sun gets very dim at this point and the additional thrust the ship would obtain from the ever-more-distant Sun is minimal. It is very difficult to get enough energy from the Sun for a voyage to another star—especially in a few days or weeks. How then can we build a sail and continue to use light pressure to accelerate even after the sail is beyond the reach of sunlight?

Lasers may solve this problem. A laser provides a tightly focused beam of light across large distances and might be capable of providing enough light to continue pushing our sail during its journey through interstellar space. An interesting approach to using laser energy for interstellar solar sailing was described by the late physicist, engineer and author extraordinaire, Dr. Robert Forward. As early as 1962 Forward was publishing technical papers describing how a future sail might be pushed through deep interstellar space by a powerful laser orbiting the Sun.

On the scales that we typically use lasers, say in the few tens of feet or less, the beam appears to be tightly focused without significant divergence, or beam spread. But over millions of miles, even the best laser beam will diverge and become more diffuse. In order to keep a relatively small beam focused on our interstellar sail, we will need to build a six hundred mile diameter focusing lens at about the orbit of Jupiter through which we will shine our laser.

Using a spacecraft of similar weight to the one described above for the Sun-only solar sail, and using a sail of about the same size, Forward calculated that a sixty-five Gigawatt laser could accelerate our sail to a velocity of one-tenth the speed of light. This would enable our spacecraft to reach Alpha Centauri in only a little more than forty years after launch. A substantial improvement over three thousand years!

Unfortunately, we don’t know how to build continuously operating sixty-five GW lasers, nor do we know how to build six hundred mile diameter lenses orbiting the Sun near Jupiter. Our physics is once again ahead of our engineering—but we won’t let that stop us!

Forward went on to show that a sail craft of much more interesting (from the point of view of future human interstellar exploration) sizes, say six hundred miles in diameter and weighing almost two million pounds, could have the same forty-year trip time if a seven-Terawatt laser were used (Figure 7). I should point out that the annual total power output for the human race is approximately 1 TW. Again, there is no physical reason this cannot be done. The challenge, as physicists are often fond of saying, is in the engineering. BUT IT IS POSSIBLE.

Figure 7. Robert Forward’s interstellar light sail concept shown as it appeared in his Advanced Space Propulsion Study for the Air Force Astronautics Laboratory in 1986.

Figure 8. A possible roadmap for developing solar sail propulsion from that we can build today to that which will be required to take us to the stars. (Image courtesy of NASA.)

Forward further proved that we could slow down and rendezvous with a target star’s planets by having a detachable inner sail that uses laser light reflected from the outer ring (of the sail) to slow it down. This same approach could be used to send spacecraft to virtually any nearby star system with commensurately longer trip times—though they will be measured in decades rather than millennia.

So how do we get from where we are today, flying solar sails that are only a fraction of the size required for true interstellar travel, to those that will give us the stars? First of all, we start flying them for more near-term exploration of our own solar system. As the technology matures, we build increasingly large and lighter-weight sails, eventually crossing the threshold to use beamed energy to augment their sunlight provided thrust. Figure 8 shows one strategy for getting from here to “there.”

Solar and laser-driven sails can give us the stars. But, as with virtually every other propulsion system that might enable the greatest voyages in human history, their sizes will be immense and pose engineering challenges we cannot yet imagine. But what could be more fitting for the future explorers than journeying to the stars using the only other thing in the universe that already makes such trips with regularity—light! Just remember when you are next out stargazing that the light you see from the distant starts has been traveling for years, decades, or even millennia before it touched you—yes, before it gently pushed you—into thinking about taking such a journey yourself.