Going Interstellar (38 page)

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.”