Current proposals for human exploration of Mars call for numerous automated missions before we send the first crew. Both the United States and Soviet Union contemplate a series of robotic explorers—as many as six rover and sample-return missions might be performed to help select landing sites of greatest scientific interest and practical benefit. One concept for making this series of missions more economical is having a regular interplanetary transport which continuously shuttles back and forth between Earth and Mars. Sails would be ideal for such an “interplanetary shuttle.”
A three-quarter-million-square-meter-class solar sail (850 meters or one-half mile on a side, or the equivalent heliogyro) could be used for a high-performance Mars sample-return mission. This is the same size sail designed for the cancelled Halley’s Comet Rendezvous Mission at the JPL. For Mars, the landed mass could be as high as three tons, sufficient to include an atmospheric entry vehicle, a lander with sample-gathering equipment, and an ascent rocket to return the sample to the sail for return to Earth. The entire interplanetary shuttle weighs about 1.6 tons, or about as much as a mid-size automobile.
A possible intermediate step between the World Space Foundation’s Engineering Development Mission and a true interplanetary shuttle is the proposed Asteroid Rendezvous Mission (ARM).
If we select carefully, there are a few opportunities where a single chemically propelled spacecraft can rendezvous with one asteroid, and then leave at a precise time to go on to the next. But such a spacecraft would be so dominated by the need for fuel and oxidizer that it would look like a propellant-tank farm, with little room left for instrumentation. On the other hand, even a modest solar sail can get to any asteroid. And if it can get to one, it is perfectly equipped to go on to another, and another, and so on, stopping at each long enough to perform a thorough reconnaissance. The itinerary can even be changed along the way if new discoveries or changing priorities suggest new targets.
Among the first asteroids to be visited will be those in Earth-approaching orbits around the sun. At just the right times, a few are even easier to reach in propulsive terms than the surface of the moon. Nearly all are accessible to a small solar sail. (Many were discovered through the World Space Foundation’s Asteroid Project, in conjunction with the NASA-sponsored Planet Crossing Asteroid Survey and International Near-Earth Asteroid Survey at JPL. Principal investigator Eleanor Helin and her team travel nearly every month to Palomar Mountain to search for “fast-moving objects.” During 1989 her team discovered five new comets and over 250 new asteroids, including nine of the rare Earth-approachers.)
Originally the Foundation’s Engineering Development Mission (EDM) sail design was done with ARM in mind. It would measure about 100 feet on a side, giving a sail area of 800 square meters, or 8,600 square feet, sufficient to go to the moon and escape Earth orbit, although its subsystems are not designed for an interplanetary cruise.
Responding to the possible challenge of the Columbus Quincentenary Solar Sail Race (see the chapter titled “A Rebel Technology Comes Alive”), the foundation upgraded the EDM design to give it greater maneuverability, a 3,000-square-meter (32,000-square-foot) sail area, and interplanetary capability. This sail could go to Mars or a succession of asteroids, trading simplicity for a longer trip time.
In either its original 800-square-meter form, or in its upgraded 3,000-square-meter Race Vehicle form, the Engineering Development Mission could be the first solar sail flown. Either way, the intent is to learn enough to go on and build larger sails for future exploration needs.
Supply Lines for Human Explorers
The first expedition crew to travel to Mars will almost certainly use chemical rockets. For crew members, speed will be essential, because every day requires more oxygen, food, water, and even a “short” trip can last over two years.
However, just as mountaineering expeditions sometimes use air-dropped supplies or caches put in place earlier, the crew of a Mars mission really need not carry with it everything they need. In fact, everything required from the moment the crew arrives at Mars, including the propellant for their return home, should be already there before they even set out—delivered by “slow freighter” prior to their departure from Earth—so they can be sure that essential supplies are waiting there, in good condition.
In research performed for NASA’s Lewis Research Center, Dr. Robert Frisbee of the JPL has shown that dramatic savings can be achieved using solar sail freighters. (This was based on earlier work by Robert Staehle, Eric Drexler, and John Garvey.) In the least complex of these cargo vessels, a sail two miles on a side and weighing 19 tons hauls 32 tons of supplies to Mars in 4.2 years, returning empty in two years. This appears to be largest sail which could be assembled on the ground and launched aboard the Shuttle or a Titan IV. The payload mass was selected to match the capability of an upgraded space shuttle, and happens to be about the maximum load carried by an eighteen-wheeler tractor-trailer. And since most of the expense would no longer go into hauling propellant around, many more supplies could be sent to Mars, ensuring an adequate safety margin when the crew arrives, in case of any emergency. If the excess isn’t used, it can be left on the surface or in orbit above Mars, to tempt forth later visitors from Earth.
A fleet of ten to twenty such vessels would be sufficient to carry the supplies for the first expedition. Then these same spacecraft, continuously shuttling between Earth and Mars, could carry the equipment to build a permanent outpost on the Red Planet. Dramatic improvements in trip time and a reduction in fleet size might also be possible, using the metal-only sails proposed by Eric Drexler and others.
Mars is not the only destination which can be served by such cargo vessels. As pointed out earlier, they might form the basis for early commercial shipping. Higher-temperature metals or highly radiative coatings to provide cooling are necessary for trips to Venus or Mercury. One can even envisage solar sail vessels being used to save Earth from a threatening collision with a small asteroid! If a collision threat were known far enough in advance, only a gentle nudge of a few centimeters per second would be required to avert disaster.
(The last occasion an object large enough to do substantial damage collided with the Earth was 1908, above the remote Tunguska region of Siberia, and we were lucky that time. Heard as far away as London, the explosion leveled a forest. The late Nobel laureate, Luis Alvarez, theorized that a much larger asteroid collision might have resulted in the extinction of the dinosaurs about 67 million years ago. Another Tunguska-like event, next time in a more populated world, could be as devastating as a nuclear bomb.)
The Commercial Clippership
Because a lightsail will use reflected sunlight, or perhaps laser beams, it needs to appear to its light source as a nearly rigid mirror. To our ground-bred intuition, this suggests something fairly heavy, but of course we know a sail must be gossamer-thin to be effective. To rigidize a sail we must either build a supporting structure or spin the sail so that centrifugal force maintains its rigidity.
An obvious shape for a spinning sail would be a flat disk, with the entire surface spinning. Another alternative is the heliogyro, invented by helicopter expert Richard MacNeal, which is appropriate since it is like the blades of a helicopter without the helicopter. Sunlight sets them spinning, which rigidizes an efficient structure to catch more sunlight.
A more critical question than shape is how to control a sail. For a square sail, control is achieved by small sails, called vanes, which act very much like ailerons on an airliner to create differential torques to turn the sail. Disk sails are difficult to control, requiring either gas jets or a very complicated center-of-mass/center-of-pressure offset system in the middle of the sail. The heliogyro is controlled in a better understood manner, similar to a helicopter.
Even more touchy than shape and control is the packaging of a large sail for launch, and then its deployment in space. When large sails can be manufactured in space it will be much simpler to construct and use different types of designs.
R. Gilbert Moore has proposed one technique of essentially blowing very large bubbles of thin plastic, coating the inside with reflective aluminum, and then cutting away the excess material. If two bubbles are blown of equal size with a common wall, that common wall will be flat and could serve as a sail. If the bubble substrate is chemically composed to break down into gasses after a week or so of exposure to the sun’s ultraviolet radiation, only the metal will be left behind, making a very lightweight sail.
Eric Drexler has proposed various other techniques to build such lightsails in orbit. (See his article in this volume.) And John Garvey of McDonnell Douglas looked at the use of a sail-construction machine running along a tether tied to NASA’s space station. There appears to be no theoretical limitation on the size of sails built in space.
Solar Sailing and Environmental Protection in Space1
Virtually every time mankind has entered a new frontier, we’ve assumed humans are too insignificant to cause widespread harm to the vast reaches spread before us. Imagine early settlers from Europe standing on a gentle rise in the North American Great Plains, contemplating bison as far as the eye could see, or cursing hours of unexpected darkened sky resulting from migrations of passenger pigeons. It must have seemed inconceivable that the actions of people could endanger either species. However, we have found to our chagrin just how fragile the Great Plains and forests were. Do similar hard lessons await us in space?
Awareness of space as a natural environment, which could be subject to human damage, is not common in the field of astronautics. Of course we have discovered no non-terrestrial life in the solar system. One may ask, therefore, why anyone should bother to protect the space “desert” environment. The answer is threefold.
First, obviously, we have not actually proven the nonexistence of extraterrestrial life in the solar system. In fact, a close examination has only been made in two very small spots on the surface of Mars, where the results were inconclusive. While the conditions for life on Mars now are not good, they apparently once were. We infer this from the evidence of running water in the past. Thus there may have been past life, and we don’t want the evidence destroyed before we can study it properly.
The second answer is that within the next century, people may begin to inhabit space themselves, and it is certainly undesirable to alter the space environment in any way which renders it less habitable for our descendants.
In a third sense, simple prudence demands that we take care, rather than tamper with any natural environment before having a reasonable understanding what the consequences might be. Environmental destruction of newly inhabited frontiers has often resulted from a simple lack of forethought. Frontiersmen often simply do not consider the long-term payoff of environmental protection. Bearing this in mind, solar sails suggest advantages compared to other propulsion techniques, because nearly all other systems require the expulsion of mass from the vehicle. For every 1 kilogram of payload launched out of Earth orbit, say on a trajectory to Mars, approximately 9 kilograms of typical chemical propellants must be expended. For small scientific payloads such as those launched in the last three decades, the quantity of propellant released in low Earth orbit is meaningless.2 However, looking twenty to fifty years ahead, moving the material to manufacture one modest space settlement (such as proposed by Dr. Gerard K. O’Neill of Princeton University) from the asteroid belt to Earth could require on the order of twenty thousand million (2 x 1010) kilograms (1 kg = 2.2 pounds) of expelled reaction mass.
A propulsion technique frequently suggested for moving massive objects (such as asteroids) is the so-called “mass driver,” which operates by accelerating small, solid chunks (pieces of the asteroid, for example) to high velocity with a linear-induction motor. But one unfortunate side effect would be a vast number of dangerous “pellets” orbiting the sun, each permanent and capable of doing considerable damage in a collision. When we talk of building just one settlement, the disruptions may not be particularly large. (Similar statistics applied to the first few buffalo hunters.) But when we contemplate building 100 settlements, using 1-kilogram pellets, then there are two million million (2 x 1012) of them.
Solar sails offer an environmentally attractive alternative to rockets and mass drivers. No reaction mass or propellant need be carried. The sail is a totally reusable propulsion system with a useful life of perhaps decades. There is no by-product to remain in orbit as a hazard, and sails are very unlikely to explode.
Bryan Palaszewski’s article tells how ion engines share some of these advantages. However, even these “clean” engines might have environmental impact. Routine activities in low Earth orbit could inject quantities of mercury, cesium, argon, or other propellants into the magnetosphere. We don’t know if this would be benign, or possibly affect geomagnetic storms or the Van Allen radiation belts, with unknown consequences for the lower atmosphere.
To be fair, it is not clear whether solar sails can function well between 1,000-kilometer and 20,000-kilometer altitudes. Turning rates required to reorient a sail toward the sun during each orbit may prove operationally difficult that low. Experience gained during the first engineering development mission should help answer this question. As Palaszewski points out clearly, solar sailing and ion drives both have definite limitations and trade-offs.
While not a universal space-propulsion technique, solar sailing does seem likely to play a major role in some future space-transportation markets because of its economic attractiveness. When compared with several rocket techniques requiring mass expulsion, solar sailing compares favorably with regard to its impact on the space environment. Environmental impact in space is only a dimly perceived concept at this time, but a historical perspective suggests that considerable long-term regret can be avoided through near-term awareness of possible environmental sensitivities of space itself.
Concluding Remarks
Finally, one of us (Robert Staehle) would like to take a moment for a comment or two:
First, I want to thank all of the contributors to this fund-raising volume, whose generosity goes beyond anything I have words for. Bob Cesarone, whose idea this was, David Brin, who made it happen, and Arthur Clarke, who made it a success; all three have our special gratitude.
It was Arthur’s inspiration, especially in the story, The Wind from the Sun, which introduced me to solar sails. His factual writings prepared me for a tough technical curriculum at Purdue University, and all his writing inspired me to follow my strongest calling to contribute whatever I could to the art and science of space exploration.
We would be remiss without acknowledging the support of the Charles A. Lindbergh Fund, which has generously helped the Solar Sail Project. One of their criteria for funding is that there be a potential environment benefit, which we hope has been shown. Other corporate and institutional supporters include the JPL; Hughes Aircraft Company; Radio Amateur Satellite Corporation (AMSAT); the law firm of Silvestri & Massicot; Morton Thiokol Corp.; United Technologies Corporation’s Chemical Systems Division; E.I. du Pont de Nemours & Co.; the University of California and their California Space Institute; Societé d’Astronomie Populaire de Toulouse; Technische Universität München; and Pasadena City College.
Also critical to our progress have been Solar Sail Project subscribers and associates of the World Space Foundation. All Solar Sail Project staff members deserve mention, though they are simply too numerous. They have graciously donated seemingly endless hours of dedication and expertise beginning in 1979, and many will continue to do so. Among the most important are Jerry Wright, the project’s originator and first director, Chauncey Uphoff, Jim French, Mark Bergam, Emerson LaBombard, Hoppy Price, Dallas Legan, and Gabe Gabriel. Sharlyn French was always cheerful and efficient dealing with so many editorial changes. Other key individuals have included Richard Dowling, Phil Hatten, Kristan Lattu, and the artists Julian Baum (for the cover) and Carter Emmart. Many others have supported the foundation in a variety of innovative and dedicated ways—accountants, a
ttorneys, educators, writers, artists, photographers, editors, secretaries, managers, and all the other talents necessary to make an organization run.
(Having said all of this, I must nevertheless offer one important caveat. In any diverse assemblage of talent like the authors contributing to this book, there are bound to be healthy differences of opinion. One should therefore, expect that opinions expressed in the individual chapters belong to the authors themselves and do not necessarily represent opinions of other authors, the editors, or officers of the World Space Foundation.)
Things are definitely looking up. Even without the Columbus Regatta (though we hope it flies) and operating on a shoestring, we believe we should be able to fly a test sail soon enough that every person who bought this book will be able to point up into the night sky and say, “I helped make that happen.”
Even after that, it will be a long step from this first mission to the interplanetary shuttle, commercial cargo vessels, and interstellar probes. But we believe this is the place to start, within the modest resources available to visionary individuals, corporations, and other sources.
We acknowledge the help of everyone who purchases this book. This does make a difference. If you wish to do more, please write to us:
Project Solar Sail, World Space Foundation, Post Office Box Y, South Pasadena, CA 91031-1000, USA.
Thank you.
Robert L. Staehle, President
World Space Foundation
P.S. To all of those who join the foundation by sending in the coupon at the back of the book, this promise: your name will be part of the cargo of our first solar sail.
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