Project Solar Sail
Page 10
For the past decade, the foundation has been committed to the development of an engineering test vehicle to demonstrate that solar sail spacecraft are deployable, controllable, feasible.
The Solar Sail Project was started originally as a NASA enterprise by Jerome Wright, discoverer of a solar sail rendezvous trajectory with Halley’s comet. Although NASA eventually canceled the proposed Halley mission in 1986, development continued through the private efforts of the foundation’s Engineering Development Mission.
The EDM is a precursor of more ambitious endeavors. If the design is shown viable during a first launch, a more advanced and higher-performance vehicle may be constructed to rendezvous with an asteroid. As more experience is gained, a high-temperature sail may be constructed for close polar orbit of the sun. Either project would demonstrate the enormous potential of solar sails for accomplishing missions in the inner solar system.
The World Space Foundation is a nonpolitical foundation dedicated to supporting and conducting space research and development, funded by tax-deductible subscriptions and cooperative efforts with other organizations. At the end of this volume, readers will learn how they can find out more.
A Regatta Past the Moon?
An exciting suggestion has been proposed to the aerospace community, under the auspices of the American Institute of Aeronautics and Astronautics, for a solar sail race similar to the one in Arthur C. Clarke’s story “The Wind from the Sun.”
The race would take place in 1992, the quincentennial of Columbus’s discovery of the new world, and entries from all countries are being sought. Many teams have filed an intent to respond to the AIAA request for proposals, even though the entries, race rules, and qualification requirements were unclear as this book went to press.
Races have traditionally stimulated performance improvements that often are reflected later in products offered for mass consumption. For instance, automobile racing has a long history of transferring technology first to sports cars and then to the general public. Competition also stimulates a kind of short-term excitement that allows onlookers to participate.
And there are esthetic reasons for racing. There is something of the hope that springs eternal in the realization of the effort, and people identify with the long hours and hard physical effort required to excel, the grueling hours of rigorous effort. When the race is on, almost everyone can identify with the driver.
But racing can also have dangerous consequences. The Stanley brothers thought that all the advertising their steamer needed was to win the race to Pike’s Peak every year. For many years the Stanley steamer held the world’s land speed record at 142 miles per hour. Then one year it flew off the ground and nearly killed the driver. Reliance on the race probably cost the Stanley brothers their business.
Many solar sail enthusiasts are also concerned that a race may perpetuate an attitude among some aerospace managers that solar sailing is an amateur or “playboy” technology. The effect would be especially damaging if the race got lots of publicity, then fizzled.
Robert Staehle voices some of the concerns. Imagine that in 1901, an international commission organized an airplane race. Suppose the prospect of international competition whipped up popular fervor and national pride. Reports of the short hops claimed by a couple of bicycle mechanics from Ohio would be considered amateur, and the commission would probably have said, “These short flights are not worthy of such a race. Let’s fly across the Channel.” The result could easily have been three or four piles of wood and canvas at the base of the white cliffs of Dover, which would have set back aviation many years.
But these concerns aside, a well-organized and carefully thought-out race might be effective if it does not place unrealistic strain on designs and participants. Publicity and excitement might attract real investment capital from the private sector. It is here that the solar sail will certainly flourish, not as racecars but as the magnificent and highly profitable ocean liners of the future. But if it takes a race to get things started, then, by all means hoist sails!
The Future of Lightships
All this is not to claim that solar sails will soon make rockets obsolete. No lightship we can envision could climb through Earth’s atmosphere and surface gravity into space. Also, current-technology sails are probably too slow to be useful for transporting astronauts.
But high-tech sails of the future, like those suggested by Drexler, Forward and Dyson, could someday make the trip from Earth to Mars in a few months and would not be subject to the delays of ballistic trajectories, or waiting for the planets to line up for return to Earth. In the long run, people, too, may ride “sunjammers.”
Even current-technology sails would be highly useful as cargo haulers. Foundation president Robert Staehle has described how lightship freighters might assist a manned mission to Mars. Sent four years ahead of the astronauts’ takeoff, these cargo robot sloops would carry all the supplies for the astronauts’ return trip (including bulky water and food and fuel) plus two Mars landing vehicles and a space station equipped for a long stay in Mars orbit. (The astronauts would have to live there for more than a year before the alignment of the planets allowed them to return.)
Furthermore, says Jerome Wright, “The sail freighter wouldn’t be used up after one trip, but could provide that kind of service for twenty or thirty years. A single sail shuttle might cost $20 million and in a decade save billions.”
As discussed in other essays and stories in this volume, some contend that lightships could open up the vast mineral wealth of the asteroids. At least seventy thousand of these rocky “minor planets” circling the sun are more than a mile in diameter, large enough to be interesting yet small enough not to foul a lightship’s rigging with gravity or atmosphere. Some might provide badly needed water ice and carbon. And we already know that many other bodies contain rich accumulations of rare metals. The solar system might someday flow with great commercial shipping lanes, with lightships bringing ore back to Earth, or to manufacturing operations in Earth orbit.
In a tempting speculation, an asteroid or two “could fill the factories of Earth with raw materials,” according to Eric Drexler of the Foresight Institute. “The metal on a typical midsized asteroid probably contains $1,000 per ton worth of platinum-group metals: platinum, iridium, osmium, and palladium. These are strategic metals needed by the West. Over ninety percent of the world’s supply of platinum now comes from the Soviet Union and South Africa.”
Drexler wants to develop sails that would have a “couple of dozen” times more acceleration than those currently envisioned. The trick (described in more detail elsewhere in this volume) is to make sails of thin aluminum film alone, without the heavy plastic backing planned for the EDM mission. The plastic is what allows the sails to be folded for transport off the Earth, so pure aluminum sails will have to be manufactured in space. Drexler contemplates sails as thin as a ten-millionth of an inch could be so produced out in weightless vacuum.
The lightship has one limitation yet unmentioned so far: that of the solar system itself. Lightships may be fine for visiting the sun and asteroids and inner planets, but by the time one passes Pluto, the sun starts looking like just another bright star. Lightship travel beyond that is out of the question—or is it?
There’s always someone with a scheme, and this time it’s Robert Forward, a retired senior scientist with Hughes Research Laboratories in Malibu, California, and also a successful science fiction author.
“Right after the laser was invented, I realized its beam brightness was greater than that of the sun and that if you had a big enough one you could push sails to the stars,” Forward says. So he conceived of a laser drawing its energy from the sun with the power of 65,000 of today’s bigger nuclear power plants, floating in space near Mercury. By passing the beam through a lens made from rings of thin plastic sheeting, measuring six hundred miles across and located between Saturn and Uranus, the laser could maintain its focus tens of light-years away. The nearest star
, Alpha Centauri, is only four light-years away, and so the laser could keep pushing all the way there, sending a one-ton sailing ship there in only forty years.
The scheme seems incredible. And indeed, Forward says he explored the idea originally as a means of transporting characters to a nearby star in one of his science fiction novels. But he also recently filled ten pages of a technical journal with equations proving the feasibility of his idea. And in his essay later in this volume, he makes a convincing case that we may yet see vast laser-driven sails, putting forth for the stars.
Notes
1 Of course, today we’ve also seen, in dramatic terms, what can happen when individual self-interest goes too far. The ecological destruction going on around us testifies that there must be some socially accepted limit placed on human greed. Changes in public awareness and government action that reflects that awareness are both causing us to change some of our worst habits.
These new, more enlightened attitudes will have to follow us into space. For although the riches awaiting us out there are vast and seemingly inexhaustible, that was exactly how the forests of North America appeared to our ancestors only two centuries ago!
Of course, we know of no “ecosystems” in space—no life forms or food chains that might be disrupted by pollution. But that’s no guarantee we can’t do harm out there if we aren’t careful. In fact, today NASA and other agencies are worried about debris from old rockets and satellites causing hazards in low Earth orbit.
Solar sails offer a special opportunity, then, to take good habits with us to a new frontier. As discussed in the article by Staehle and Friedman, they are inherently nonpolluting, relying on what may be an infinitely renewable resource—starlight.
They are also delicate things, which won’t work if we fill space with too much garbage! In effect, investing in solar sails means we intend to tread lightly on the new frontier—even as we head out there to live and learn and get rich.
2 Although Tsiolkovsky and Tsander deserve primary credit, more information has recently become available from Ron Muir of San Jose, California, and Sergei Golotyuk of the U.S.S.R., who point out the 1881 science fiction story “Adventures Extraordinaires d’un Savante Russe” by Georges LeFavre and Henri de Graffigny contains the concept of solar sailing “though not in a quite clear form.” Furthermore, a 1913 story, “Upon the Ethereal Waves” by B. Krasnogorsky, describes a viable solar sail. (There are even hints that solar pressure propulsion was mentioned by Jules Verne!)
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Chauncey Uphoff has been involved in the space program since the early 1960s, when as a student at the University of New Hampshire, he worked on one of his professor’s satellite tracking projects. (One of his duties was to adjust the receivers to measure the Doppler shift of the signal from Sputnik IV.) Born in 1940, he was fortunate enough to be in the right place at the right time to be among the first professional aerospace engineers. While at JPL, he won the NASA exceptional service medal for pioneering work on the gravity-assisted satellite tour portion of the Galileo mission and was instrumental in the decision to study the solar sail as a potential deep-space propulsion system.
Jonathan Post is a graduate of the California Institute of Technology. He has worked for JPL and is now with a prominent aerospace firm helping lay plans for the twenty-first century. He is a well-respected poet and a member of the Science Fiction Writers of America.
Argosies of Magic Sails—Excerpts from “Locksley Hall”
by Alfred Lord Tennyson
Many a night from yonder ivied casement, ere I went to rest,
Did I look on great Orion sloping slowly to the West.
Many a night I saw the Pleiads, rising thro’ the mellow shade,
Glitter like a swarm of fire-flies tangled in a silver braid.
Here about the beach I wander’d, nourishing a youth sublime
With the fairy tales of science, and the long result of Time;
When the centuries behind me like a fruitful land reposed;
When I clung to all the present for the promise that it closed:
When I dipt into the future far as human eye could see;
Saw the Vision of the world, and all the wonder that would be;
Saw the heavens fill with commerce, argosies of magic sails,
Pilots of the purple twilight, dropping down with costly bales;
Heard the heavens fill with shouting, and there rain’d a ghastly dew
From the nations’ airy navies grappling in the central blue . . .
Ion Propulsion:
The Solar Sail’s Competition for Access to the Solar System
By Bryan Palaszewski
A solar sail has grace and potential simplicity to capture the imagination of engineer, scientist, and layman alike. But another type of propulsion technology has equal elegance. It is called ion propulsion, and this innovative technology may accomplish some things more cheaply and easily than any other way of moving about in space. In fact, some call ion technology the “friendly competitor” with solar sails.
Exploring the Solar System
Cost-effective exploration of the solar system requires two major breakthroughs. First, access to low Earth orbit must be made cheaper and easier. Getting off the planet’s surface demands huge amounts of energy to be applied in a short time. For the near future, rockets remain the only way to provide that first push off the Earth.
But even if there are major advances in launch capabilities, that won’t be the answer alone. A lot can be accomplished in low Earth orbit, but to really use space, we’ll need a second breakthrough, getting much farther out to where the real riches wait.
Why Ion Propulsion?
With current space propulsion systems, the cost can be very high, and it will only get higher when we begin planning complex multipart endeavors, like sending people to Mars, or returning to the moon. Many flights of large boosters will be needed to lift vehicle components into orbit. But even more expensive will be the propellant (fuel and oxidizer) needed by space tugs and other vehicles out there.
Also, because so many launches may be needed, the length of time to assemble a space vehicle will be extended, adding to the cost and complexity of each mission.
Why is it so hard to get from place to place out there? To understand movement in space, the idea of Newton’s Third Law must be understood. Simply stated, any action produces an equal and opposite reaction. As we throw mass (propellant) away from a spacecraft or any object, the object will move in the opposite direction. This is the case for solar sails, which “bounce” sunlight one way and recoil the other. It is also true for chemical and ion rockets, which carry their own propellant and achieve a change in orbit by flinging mass away from themselves.
The performance level of this process is called specific impulse. In most cases, the higher the specific impulse of a rocket engine, the lower the mass of propellant needed to accomplish a mission. The specific impulse of an ion engine can be four to twenty times higher than that of a chemical propulsion system, and that of a solar sail approaches infinity. (Since there is no propellant, one might say that a sail’s “equivalent specific impulse” is determined by its mass and the total mission velocity it accumulates over a lifetime.)
So it makes sense to compare these two revolutionary technologies, and explore how each of them might help us explore and develop space.
Flight Paths and Trajectories
“The easiest way to get from point A to point B is to go in a straight line.” But this adage was developed before the advent of interplanetary travel. For more than a generation, space planners have gotten used to the limitations of orbital mechanics. You blast toward a distant planet with one huge rocket boost, drift in a long curving orbit, then match velocities at the other end with another rocket blast. With chemical rockets, the energy to leave orbit is spent quickly and the spacecraft coasts the rest of its way. Ion rockets, on the other hand, burn faintly but continuously, for a very long time.
; To leave the orbit of a planet, the low thrust of an ion propulsion system limits acceleration, so the escape path is a long, spiraling one, taking as many as hundreds of days. But once the spacecraft is out orbiting the sun (in heliocentric space), ion propulsion can give a fast trip, especially to the outer planets. This is because the drive can keep on pushing, long after a chemical rocket would have burned out. Quicker voyages can make future human exploration a possibility rather than a gleam in the eye of space engineers.
How Ion Propulsion Works
With an ion drive, the mass we throw away from the spacecraft is in the form of ionized atoms, accelerated by electric fields in a machine called a thruster. These ions reach very high velocities: 20,000 to 100,000 meters per second, or more. For most missions within the solar system, 30,000 to 60,000 m/s provides the “best” (but not necessarily the shortest) trip times, or the lowest mass starting in low Earth orbit (LEO).
Several propellants can be used in ion thrusters. In the past, mercury and cesium (liquid metals) were used, but these brought unwanted problems. So inert gases are now the propellants of choice: xenon, krypton, and argon.