Going Interstellar (collection)

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.