Introduction: Sailing the Void
In the early days of civilization, human beings discovered the use of sails. Woven sheets or rough textile caught the wind, and the momentum of that moving air was transferred to the floating ship, which then moved without a water current, and even against one. It was a wondrous advance. Until this innovation, ships could move upstream only by the unending application of human muscle. Of course, the wind might not always blow, or if it did, it might not blow in the right direction. Nevertheless, sail technology advanced steadily, so that feebler and feebler gusts could be used, even gusts that were in the wrong direction.
By the 1850s, Yankee clippers were the speediest and most beautiful vessels the world had ever seen. The steamships that replaced them were larger and, eventually, faster, but they were also uglier, dirtier, and noisier and always greedy for a steady stream of fuel. Sailing ships (except for pleasure vessels) have now disappeared from the world’s oceans, but humanity faces another ocean today, an infinitely vaster and emptier one. Our water ocean spans tens of thousands of miles, but the ocean of outer space stretches for billions of trillions of miles.
Now, with fossil fuels growing scarce, there is new interest in cutting this umbilical again. Research into renewable power includes experiments in ultramodern sails for cargo ships and even cruise liners at sea.
We’ve begun the navigation of outer space with the equivalent of steamship races—the use of raw power, incredibly noisy power. Rockets. Nothing else will do, perhaps, to break through Earth’s atmosphere and the lower reaches of its gravitational field.
Once a ship is in space, however, and is moving through a vacuum in orbit about Earth, is there anything quieter? Gentler? Better? More important, is there a way to move out there without being tied forever to an earthly supply of fuel? A wind would fit the bill, certainly, but outer space is a vacuum. What is there in space that can form a wind?
Two things, actually—at least in the neighborhood of a star like our sun. As my esteemed friend Arthur has told you, the sun emits two types of radiation: high-energy charged particles—mostly protons and electrons—and, of course, light. The first of these flows, the stream of high-speed particles, is what’s referred to as the “solar wind.” The name may be misleading, especially for the purposes of this book, as this is not the wind that drives a solar sail. The charged protons of the solar wind do possess momentum, and this momentum can be transferred to other charged objects in space. But we won’t be discussing it much here.
The second “wind” in space is the sun’s torrential output of light, which also possesses momentum, and which exerts a minuscule pressure on anything it strikes. Like the solar wind, it grows weaker with increasing distance from the sun. As it turns out, sunlight itself is more than a thousand times stronger than the solar wind from our sun.
A good example of these two streams at work can be seen during the active phase of a comet. A comet, as it approaches the sun, is partially vaporized. The rocky dust frozen in its outer layer then rises to surround the still-frozen nucleus in a haze. The tiny grains have large surface area per mass and so get swept away by light pressure alone. This dust is swept outward from the sun by sunlight to form the long “dust tail.” There is a second comet tail as well, the ion tail, which consists of charged particles interacting with the protons and electrons of the solar wind, shining like neon lamps across millions of miles.
Scientists have long known about these two outward forces from the sun. However, they seemed so weak that it was long before serious consideration was given to how they might somehow prove useful. Except for the predictions of a few visionaries, it wasn’t until the advent of spaceflight that plans began to be made to use these winds of space.
So far, of the two, it is sunlight which has received the most attention. Because light interacts with matter on a shorter scale than charged particles do, we can manipulate light more easily than the charged particles. Radiation pressure is much weaker than the sun’s gravitational pull unless you mimic the dust grains . . . increase your surface area compared to mass. This can be done by spreading a thin collecting area—a reflecting sail—over vast tracts of space.
Fortunately, the free-fall conditions of space mean you don’t need a lot of mass to support your reflector. You don’t need the heavy masts or sturdy canvas sails of an earthly sailing ship. The result is a gossamer wonder, the lightsail, which has inspired this book.
While lightsails cannot do the grunt work, the heavy lifting from ground to orbit, they do appear to offer some wondrous opportunities for getting around once you’re up there. What’s more, they also appear to be cheap. By that I mean they may open the door for smaller groups to play a role in space than just the club of big national governments that now run the show.
Many grand ideas have been proposed for how photon sails might help us maneuver in, and make use of, interplanetary space. I will leave the experts to tell you of them.
I just want to mention, before I stop, one special application of light pressure—as a way to reach the stars. There are ways of converting the ordinary light of the sun into a laser beam, which is a wave of “coherent” light. This is light in which all the waves are the same length and move in the same direction. Whereas ordinary sunlight spreads out rapidly, so that far from the sun its pressure weakens into utter uselessness, a laser beam spreads out only very slightly, so its pressures, and its ability to move a ship, can remain constant over a very long distance.
A laser beam from Earth could drive such a ship through space by solar energy, burning no fuel and never running out of energy. Such a ship could be driven to the nearest star, Alpha Centauri, in just a few years.
It might seem that such a laser beam could drive the ship only outward, never to return; but with the use of still larger sails and ingenious ways of extending electrically charged systems, such ships could be brought to a halt in the neighborhood of Alpha Centauri and might even be made to take up the return journey. The requisite technology is a bit beyond us today, but a hundred years from now it may not be, and sailing vessels more magnificent than any we have ever seen may navigate distances vaster and emptier than anything our ancestors would have dreamed of.
This concept, and many others, await you in the pages ahead, as eminent scientists and gifted writers take you on a tour of the future. In fact and fiction, in essays and stories, you will catch glimpses of a tomorrow filled with wonder and possibility, as we and our descendants spread our sails and head forth, the wind at our backs, to the stars.
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Isaac Asimov says: “My life is incredibly dull since I do nothing but write and have done nothing but write for fifty years. All I have to show for it is a number. Leaving quality aside as something that is moot, I have published to date 434 books, and have some 25 or 26 in press. I have written thousands of short pieces, both fiction and nonfiction, some, but not all, of which are included in the books. I might add that, dullness or not, it has been a very happy life.”
And let me add that Isaac has been a friend for a very long time. Thank you for the typically incisive introduction, Isaac. Now it’s my turn.