Project Solar Sail

by Arther C. Clarke

  A buzz of people surrounded the elevator. Arms reached out to embrace him, and he almost collapsed in their grasp. He looked frantically around. The men with the embryos must have gone to one of the other elevators into another part of the wheel. Ramis stood on his tiptoes and called out, wondering what he should do. “Wait—the sail creatures! They are my only way back! I need to tell you about the wall kelp—”

  One man stepped up to him, pushing his way through the crowd. “Curtis Brahms, acting director of Orbitech 1.” He seemed to be out of breath. He shook Ramis’s gloved hand. “Welcome to our colony. I hope you brought a miracle with you. We had only . . . unpleasant options left.”

  Ramis studied the man, trying to quell the urgency he felt. “The wall kelp is a chance for you to survive.”

  “It’s been taken to a safe place. You can speak to our scientists after we’ve shown you your quarters. You’re in the same boat with the rest of us, I’m afraid. You’ll be here awhile.”

  The boy tried not to think of the Aguinaldo, of the many years he would have to live here before the other sail-creature embryos reached maturity.

  But a ripple of optimism ran through the crowd. Though he knew the bad times weren’t over, somehow he felt the birth of a new spirit of hope.

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  Kevin J. Anderson has sold eight novels and more than a hundred short stories. His first novel, Resurrection, Inc., is a cross between hard science fiction and Gothic horror; it was a final nominee for the Bram Stoker Award. His second and third novels, Gamearth and Gameplay, are the first two books in a fantasy trilogy with Jules Verne and Dr. Frankenstein as main characters. Three of his upcoming novels are collaborations with Beason; all concern scientific issues and their effects on society.

  Doug Beason is the author of several non-sci-fi techno-thriller novels, including Honor, Assault on Alpha Base, and Strike Eagle. Doug holds a Ph.D. in physics and was an assistant professor of physics at the U.S. Air Force Academy before his current job, heading up a high-energy plasma physics laboratory in Albuquerque.

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  So far we have kept to the solar system while talking about the future of lightsails. But some are never satisfied with the simple, or the obvious . . . or with playing in the backyard. Might we someday set sail to the stars themselves? Robert Forward and Joel Davis think it’s very likely indeed.

  Lightsails to the Stars

  By Robert L. Forward and Joel Davis

  It may not be tails of fusion fire or faintly glowing trails of Star Trek anti-matter propulsion that propel humanity’s initial interstellar probes to other stars. Our robot pathfinders to Proxima Centauri may well travel as Columbus did to the New World: by sail. The first space probe to another star may be a payload attached to a gossamer-thin aluminum sail some four kilometers wide and weighing a ton. Or it may be a kilometer-wide, semi-intelligent aluminum mesh weighing all of 20 grams.

  Such spacecraft may sound more like fantasy than hard fact. But they may end up being more practical than Buck Rogers-type rockets. Interstellar travel is not a trivial endeavor. We really have no conception of how much empty space exists between the stars. We measure distances and travel times on a human scale: the distance from home to the office, the time it takes to travel from London to Paris. And talking with relatives in Perth is practically instantaneous, even though they are nearly halfway around the world.

  The universe is big. Many of Earth’s billions have never traveled more than 40 kilometers from their place of birth. Only twenty-seven men have traveled as far as the moon, 10,000 times farther or nearly 400,000 kilometers away.

  The Voyager 2 spacecraft recently sped past the planet Neptune, currently the most distant planet in the solar system: 10,000 times farther than the moon or more than four billion kilometers away. But the nearest star to our solar system, Proxima Centauri, is 10,000 times more distant still at 40 trillion kilometers distant, some 4.3 light-years. Voyager 2 will leave the solar system at a speed of 58,000 kilometers per hour and will take over 80,000 years to cover 4.3 light-years. A flight system capable of traversing interstellar distances during a human lifetime must therefore, be several orders of magnitude better than even our best present interplanetary flight systems.

  This epic enterprise may still lie several decades in the future—or it could, with considerable effort, be only around the corner of the millennium. In fact, the United States of America is currently engaged in a multi-billion-dollar research effort to produce one of the major components of an interstellar propulsion system—a high-power laser. The U.S. Department of Defense considers it part of the Strategic Defense Initiative (SDI), the so-called Star Wars project. But any science fiction aficionado will recognize that a superpowerful laser is a dandy tool to literally blow an interstellar probe out of our solar system and out to the stars.

  In the last twenty-five years, humanity has begun developing the technology for interstellar travel. And it does exist, for at least one interstellar propulsion scheme called nuclear pulse propulsion. This is really just a fancy name for throwing hydrogen bombs out the back of your spaceship and letting the explosions push you to Sirius (or wherever) and back. The earliest work on nuclear pulse propulsion was done at the U.S. government’s Los Alamos National Laboratory in the late 1950s. The researchers wanted to devise a way of getting to Mars and back by 1968. The proposed spaceship was called Orion. It would have been powered by hundreds of tiny hydrogen bombs ejected out the back of the ship. As each bomb exploded, its debris would hit a huge “pusher plate” at the rear of the ship. The impulse from the explosion would be transferred via giant shock absorbers to the rest of the vehicle. Orion would thus be “kicked” across space to its Martian destination.

  No nuclear-pulsed spaceship ever reached Mars in 1968; Orion remained on the drawing boards. In that very year, however, the visionary British-American physicist Freeman Dyson modified the Orion proposal and turned it into a manned interstellar space probe. Dyson’s version of Orion would weigh some 400,000 metric tons and carry 300,000 one-ton fusion bombs. The interstellar Orion could also be built as a two-stage ship, so it could slow down as it neared its target star. The first stage of such a ship, Dyson calculated, would have to weigh 1.6 million metric tons. The 20,000-ton payload would include hundreds of men and women, along with everything necessary to maintain them and their descendants.

  For Dyson’s interstellar Orion would not be a fast boat to Alpha Centauri. The fusion devices would explode behind the ship at a rate of one every three seconds, accelerating the ship at a constant 1 gravity to 1/30 the speed of light in just ten days. At that speed (10,000 kilometers per second, or 36 million kilometers per hour) it would take 130 years to reach the nearest star. None of the original crew would live to reach their destination, since relativistic time-dilating effects are negligible at 1/30c.

  Orion is the only form of true interstellar transport which could have been built and sent on its way in the last decade. The reasons Orion was not built and launched were (1) the Nuclear Test Ban Treaty, which prohibits the explosion of nuclear devices in outer space, and (2) a monumental lack of interest on the part of the political, engineering, and scientific communities. The latter reason is the more important of the two. If there were sufficient interest in building Orion, mutually agreeable and acceptable ways could be found to sidestep or modify the Test Ban Treaty.

  The technology to send (reasonably) fast space probes to other stars will soon be within our reach. These interstellar probes will be powered by photon pressure—from a microwave laser (i.e., a maser) in one case, and a visible-light laser in the other.

  The idea of using a maser to propel an interstellar probe originated with Freeman Dyson. In 1984 one of us (R.L.F.) coupled that concept with the use of advanced computer technology. The result is called Starwisp. A wire hexagonal mesh sail 1 kilometer in diameter weighing only 20 grams would make up the entire spacecraft. At each of the 10 trillion wire intersections of the mesh would be a tiny microcirc
uit. These semi-intelligent chips not only would work as a computer element in a 10-trillion-component parallel-processing supercomputer, but would be light-sensitive, and function as tiny pinhole cameras.

  The propulsion system for Starwisp would not be on board the space probe, but would stay “at home.” It would consist of a 20-gigawatt (20 billion watts) microwave beam from an Earth-orbiting solar-power satellite. Starwisp, because it would be so delicate, would be fabricated in space at a point beyond the orbit of Mars. There it would unfold like a delicate rose in deep space. The microwave beam would be turned on and focused on the sail by a special kind of lens called a Fresnel zone lens.

  The lens would be huge, 50,000 kilometers wide or four times the diameter of Earth. It would be a complex of wire mesh rings alternating with empty rings. The radii of the rings would be adjusted so that all the path lengths of the microwaves passing through the empty rings would be in phase at the focal point of the Fresnel zone lens.

  The focused microwave beam would accelerate Starwisp by photon pressure, somewhat like terrestrial wind pushing a sailing ship. The 10 trillion microcircuits, like electronic halyards, would adjust the conductivity of the wires in the mesh to maximize reflected photon power. Accelerating at 115 times Earth gravity, Starwisp would reach one-fifth light speed (60,000 kilometers per second or 216 million kilometers per hour) in just one week.

  Seventeen years later, Starwisp would be three-quarters of the way to Proxima Centauri. The next part of the mission would begin as Mission Control turned the microwave beam on again. A powerful pulse of energy would be sent streaming through the target star system, arriving (at the speed of light) as Starwisp arrived (at 0.2c) four years later. The beam, though spread out, would still be strong enough to “turn on” Starwisp’s 10 trillion microcircuits. The circuits would use the wires of the mesh sail as microwave antennas to collect the energy of the beam. Each circuit would adjust its built-in internal clock to the phase of the microwave beam. Then, acting as the photoreceptors in the retina of this artificial “eye,” the 10 trillion semi-intelligent chips would analyze the infrared, visible, and ultraviolet light they would sense coming from objects in the Proxima Centauri system.

  At a velocity of 60,000 kilometers per second, Starwisp would make the fastest flyby in history. It took Voyager 2 about one week to pass through the system of Saturn’s major moons and rings, a distance of about 8 million kilometers. Starwisp would race through the inner Proxima Centauri system (say 9 billion kilometers, or the distance across the orbit of Neptune) in just forty hours. During that time the energized superchips of Starwisp would produce twenty-five high-resolution images per second, close to frame rates for television. Using the timing information from the microwave beam from Earth, the mesh would configure itself as a directional antenna and beam the data back to Earth.

  Four years after the flyby, Starwisp would be almost a light-year beyond Proxima Centauri. But its data stream would just be arriving at Earth, where computers would turn the numbers into photos to dazzle a waiting world just a quarter-century after Starwisp’s launch.

  If some interesting planets were seen by Starwisp, the next step would be to send a more massive probe with a much better optical imaging system and an array of other sophisticated instruments. This would require a more powerful system, a laser-pushed lightsail. We call it Starlite.

  The idea for a Starlite-type probe is somewhat older than that for Starwisp. Konstantin Tsiolkovsky talked about something like it as early as 1921. Detailed technical studies appeared in 1958 and 1959 (about the time Los Alamos researchers were designing the Orion H-bomb ship). However, the assumption in these studies was that the photon pressure would come from sunlight, not a laser. The first laser was “turned on” in 1960 at the Hughes Research Laboratories, where one of us (R.L.F.) was working. He realized that lasers could be used to propel a lightsail over interstellar distances, and he published his first paper on the subject in 1962. Several other papers have appeared since then, and the idea has been considerably refined.

  Like Starwisp, Starlite would be quite large. For a one-way flyby of Proxima Centauri, a Starlite probe would have a sail 3.6 kilometers in diameter, made of aluminum film about 16 nanometers (or 160 angstroms) thick. An aluminum film with this thickness would reflect about 82 percent of the light hitting it, allow about 4.5 percent of the light to pass through, and absorb the remaining 13.5 percent. The sail plus attached probe would mass about 1,000 kilograms or a metric ton.

  Propulsion would come from the photon pressure of a 65-gigawatt laser. The laser could be in Earth orbit, like the power system for the Starwisp probe, or in close orbit around the sun, where the solar flux is higher. Examples of power systems for such a laser include carbon dioxide electric discharge lasers at 10.6 micrometers wavelength, or solar-pumped iodine lasers at 1.315 micrometers wavelength. Building such a laser would not be easy; but work leading to such a device is already being done by contractors for the American SDI project. A Fresnel zone “lens” 1,000 kilometers wide, located at a “station-keeping” point 15 a.u. (between the orbits of Saturn and Uranus), would focus the laser beam onto the Starlite sail.

  A 65-gigawatt laser would accelerate Starlite at 0.36 meters per second squared, or 4 percent of Earth’s gravity. Three years of continuous acceleration would give Starlite a velocity of 11 percent of light speed. The probe would be 0.17 light-years from the sun at that point. The spot size of the laser would be 3.8 kilometers, only 200 meters wider than the sail itself, when it was finally turned off. Starlite would reach Proxima Centauri forty years after launch, taking twice as long to get there as Starwisp. This is despite the fact that Starlite would use a laser three times as powerful. The difference, of course, is in the mass of the probe itself—20 grams versus 1,000 kilograms. The more heavily instrumented Starlite probe, however, could do a much better job of finding planets that humans could visit in person. And larger versions of Starlite could take them there.

  Consider, for example, the following scenario for a round-trip journey via laser-pushed lightsail craft to the star Epsilon Eridani, 10.8 light-years from Earth. For this mission we will use a super-Starlite vehicle: the Starlite Special. The sail will be 1,000 kilometers wide, the same as the Fresnel transmitting lens. The vehicle will mass about 75,800 metric tons. If we want to get the crew to Epsilon Eridani and back before they die of old age, they’ll have to move at relativistic velocities. That will allow the Einstein time-dilation effect to kick in, as well as get them to their destination relatively quickly. A laser or cluster of lasers outputting a beam of 43,000 terawatts (TW) will accelerate the Starlite Special at one-third Earth gravity. In 1.6 years the vehicle will travel about 0.4 light-years, and will reach a cruise velocity of 150,000 meters per second—50 percent of the speed of light. It will thus take the Starlite Special just twenty years to get to Epsilon Eridani.

  Now we mustn’t forget Einstein. This velocity will cause a 13 percent increase in the mass of the lightsail craft and a concomitant time-dilation effect for the crew! It will also have a Doppler-shift effect on the laser beam’s frequency and energy. To provide a constant one-third G acceleration, therefore, the laser power will have to increase to a formidable 75,000 TW by the end of the acceleration phase. No one said it would be easy or cheap to get human beings to the stars! About 10.4 years before the craft arrives at its destination the laser banks will again be turned on, sending a “slug” of light energy racing through space to Epsilon Eridani.

  The lightsail for the Starlite Special is itself special. It will be made in three concentric segments: an outer “decelerator stage” the full 1,000 kilometers wide; an inner “rendezvous stage” 320 kilometers wide; and an innermost “return stage” 100 kilometers wide. When the Starlite Special is 0.4 light-years from Epsilon Eridani, the outer decelerator stage sail will be cut loose from the other two sail stages that compose the rendezvous sail. The latter will be turned around so its reflective surface will be facing the now-detached dece
leratory ring sail. The slug of laser light will hit the decelerator ring sail and bounce back to strike the rendezvous sail. The result, as far as the interstellar craft and its crew are concerned, will be the same as if it were being slowed down by a multi-terawatt laser beamed from Epsilon Eridani!

  Deceleration time will be the same as acceleration time, 1.6 years. Thus the travel time to Epsilon Eridani will be 23.2 years by Earth clocks, and 20.5 years by shipboard clocks.

  After our intrepid crew has spent a few years (let’s say five) exploring the Epsilon Eridani system, it will be time to head back home. Now the return-stage sail, the innermost 100-kilometer-diameter part of the rendezvous sail, will be detached. The outer 320-kilometer-diameter ring sail will be flipped around so its reflective surface now faces Sol system. A third 1.6-light-year-long slug of laser light, fired from the sun-orbiting laser 10.8 years earlier, now pours through the Epsilon Eridani system and strikes the reflective surface of the detached ring sail. The light bounces back and hits the reflective surface of the return sail. Both sails are accelerated out of the Epsilon Eridani system, but the return sail and its attached crew module head back home toward Earth at one-third G acceleration.

  Of course it might be possible for the crew to make their own laser for the return voyage. But for the sake of argument, we assume here that they’ll rely on power from home all the way.

  Twenty years later, the Starlite Special will approach the solar system at 50 percent light speed. It will be brought to a halt with a final burst of laser energy. The crew will have been away from earth for about fifty-one years. They will have aged about forty-six years.