This is a transcript of a "Science Forum" which I conducted for the (now dead) magazine Science Fiction Age. Rather than have a science column, SF Age ran the "Science Forum", where we had a conversation with several scientist science-fiction writers on a subject of interest to science fiction fans. In this particular forum, I was the interviewer, so for the most part you won't see me popping in with opinions; instead I pretty much tried to keep to the background, asking a new question every time the conversation seems to slow down. The interview was conducted at the "Symposium on Practical Robotic Interstellar Flight" held at New York University.
An earlier Science Fiction Age forum talked about Faster Than Light starflight.
The voyage to the stars has been a dream of science fiction writers, and indeed of humanity, for centuries. But the stars are distant, and the theories of Einstein appear to forbid traveling at speeds exceeding light. Science fiction writers often postulate some sort of magical future scientific advance that will allow us to get around Einstein's barrier, and reach the stars in a few days (as was discussed in the Science Fiction Age Science Forum in November). Yet some scientists aren't willing to wait for magic, but are planning and engineering clever new ways to get to the stars now, without warp drive.
SF writer, scientist and Science Fiction Age Science Forum regular Geoffrey A. Landis attended the recent scientific conference "Practical Robotic Starflight: Are We Ready?", sponsored by the Planetary Society, where some of the leading scientists gathered to discuss exactly this topic. After the third day of the conference, he caught up with three attendees who are both scientists and science fiction writers: David Brin, Robert Forward, and Jonathan Post, to discuss for Science Fiction Age the possibility of interstellar travel without magic. Three-time Hugo winning writer David Brin is also a physicist; among his scientific publications is one of the most complete studies of the Fermi paradox in the Quarterly Journal of the Royal Astronautical Society. Space scientist Jonathan Vos Post was the mission planning engineer on the Voyager Uranus Interstellar Mission, responsible for planning the Miranda fly-by and other items. Dr. Robert Forward is equally prominent in both science fiction, where he is known for combining rigorous scientific backgrounds with free-wheeling speculation, and in the field of astronautics, where as a senior scientist for Hughes Laboratories, he led studies of advanced propulsion for future space missions.
LANDIS: How do science fiction writers treat interstellar flight?
BRIN: Typically with warp drive, or other technologies that are essentially magic, that assume some physics far beyond what we know today. I do see encouraging signs in recent years. Science fiction writers are starting to use such space warps less. They aren't going cold turkey, but they're cutting down, like going from a three pack a day habit to one pack a week or so.
FORWARD: I spend most of my time trying to figure out a new way to get to the stars that is at least plausible, and doesn't involve too much magic. I try to live with the limitations of the fact that it takes time to get up to anywhere near the speed of light, and time to travel between the stars, even at nearly the speed of light. Because that is what the human race has to face: you can't accelerate at more than 3 gees for long periods of time. To get up to anywhere near the speed of light requires traveling at one gee for a year, or three gees for a third of a year, or--in Timemaster, for instance, I had my hero going at thirty gees, in an emergency situation, breathing liquid, and suffering for months in order to get up to near the speed of light. That was part of the story; that was part of the realism. The mechanism I used was like magic--it happened to be negative matter--but I still had the problem that you can't move people at high accelerations, and they don't live very long compared to the time it takes them to travel between the stars. You can use the difficulties of real space flight positively in science fiction to generate plot possibilities.
POST: There must be at least one hundred ways to get to the stars using physics that we already know. There's a lot of engineering to be worked out, but a hundred ways to the stars using physics that we know. There must be many more ways to the stars using physics that we don't yet know, without having to violate Saint Albert Einstein.
FORWARD: Saint Einstein, as you say, also gave us miracle; he gives us the possibilities of space warps and time machines, faster than light travel. I'll stick with him. He's our prophet.
BRIN: Still, those are only fun speculations for now. Einstein's main lesson is that the speed of light is a tough traffic cop. It does not allow us, in a story, to have the heroine kiss her sweetheart, go off and save the universe, and then come back home in time for dinner. On the other hand, there are many phenomena of relativity that make great story images. If you can approach the speed of light, you begin to experience some of the strange effects of special relativity, such as time dilation. This is illustrated by the well known twin paradox, where one twin leaves the Earth, goes off at relativistic velocity, and comes back the same day, as far as he's concerned, but a hundred years have passed on the Earth, and his brother's in a retirement home.
POST: Robert Heinlein wrote one of the first good twin paradox novels, Time for the Stars. One twin comes home to marry the grand-daughter of the other twin.
BRIN: As you get close to the speed of light, you don't perceive any change in your own mass, but as far as the rest of the universe is concerned, you've gotten a hell of a lot heavier, so it get harder and harder to push yourself that added little bit. What Forward had done, and Charles Sheffield did in One Man's Universe-- was to take away one of the problems of relativistic travel by assuming some now-unknown energy source could power your spaceship. They deal in different ways with the problem of how to accelerate past three gees. These are actually wonderfully weird aspects of nature, and it is too bad that other authors have avoided dealing with them for so long by being suckered into the sweet temptation of a warp drive that lets us treat a spaceship like an airplane.
POST: Albert Einstein played the violin; he was clearly an artist. As a science fiction author, he had great backgrounds, but lacked plot and character. The bongo-playing saint of physics, my late co-author Richard Feynman, suspected that there might be an infinite number of natural laws to this universe. If so, there might be an infinite number of ways to travel to the stars.
LANDIS: What was the biggest obstacle that plans for starflight face?
FORWARD: The difficulty of paying for it. We have plenty of ideas for going to the stars. Antimatter rockets aren't that far away. The other rockets, such as fission and fusion rockets, ended up being quite slow. If you had to, you could go that way, but it would take you hundreds of years, it would take you generations.
BRIN: Or send small robots
FORWARD: Yes, that's right. Which gave us another option: if it's too costly to send a human crew, how small can we make a spacecraft? and get it up to speed? and still have it do something interesting when it gets there?
Jordin Kare summarized efforts on making spacecraft instruments smaller and smaller. He described a series of three generations of instruments to take pictures, basically cameras. The camera shrunk from breadbox-sized, to something about the size of a dental-floss canister, and finally to the size a sugar cube. But it was stuck with this huge barrel of a lens, which you need to take a good picture of a planet from somewhere in the solar system. We were stuck by the laws of optics. We couldn't make a camera which weighed a picogram, or a milligram--we could make the electronics that small, but we couldn't make the light collecting apparatus that small.
BRIN: Unless, of course, we find a way to do something like inflatable lenses, or these smoky solids, aerogels.
LANDIS: How much more ambitious would a star probe be than earlier probes such as Voyager?
BRIN: Dr. Rich Terrell of JPL came up with an allegory to show how great the distances beyond Pluto are. He poured the contents of a salt container onto a table, and said it would take two hundred such containers to make a billion grains of salt. There are five hundred billion stars in the galaxy. If you then spread the salt out to scale, as thinly as stars are spread out, here in the periphery of the galaxy, the nearest salt grain to the one you held in your hand would be seven miles away. We already have an interstellar space probe, Voyager. It's leaving the solar system all right, but on this scale, it's departing at the rate that grass grows.
POST: We have four interstellar space probes right now, Pioneer 11, Pioneer 12, Voyager 1, and Voyager 2. And they cost hundreds of millions of dollars each. One of the questions at this conference is, what does it cost us to go to the stars? The stars are our destiny, but can we pay the bill? The largest cost I heard in the last few days was quadrillions of dollars, enough to make every man, woman, and child on this planet a millionaire. The lowest cost I heard was ten million dollars for an interstellar robotic probe, assuming of course that we have already exploited the solar system to get hydrogen from Jupiter and Uranium from the asteroids. Ten million means that a 21st Century yuppie could go to the stars. The truth is somewhere in between. One thing that people seem to agree on is that if we are going to use uranium or plutonium at all, better to use it as far away from home as possible.
Right now a lot of people are worrying about asteroids or comets that might hit the Earth, the way Jupiter was recently bombarded. It's a legitimate thing that even the Congress of the United States admits should be investigated. Arthur C. Clarke suggested that the way to find bits of rubble in the solar system that might smack in to us is to make a one thousand megaton bomb, maybe even a one million megaton bomb, go around to the far side of the sun from Earth, and then blow up the enormous superbomb. The sun will shield us from the radiation, but the solar system will be bombarded with very intense microwaves, and we will look for the reflected microwaves, and be able to spot anything bigger than a golf ball in the solar system. I would just like to add to Arthur C. Clarke's remarkable proposal that you get the same effect by exploding several thousand smaller atomic bombs or hydrogen bombs, and drive a spacecraft, the so-called "Orion" spacecraft, out of the solar system at the same time. You not only save the Earth from collision, and get rid of old bombs, but you also get someone, or something, out to the stars.
FORWARD: Actually, it's not a bad idea, because the one interstellar vehicle that we could have built twenty years ago was the atomic-bomb-powered Orion vehicle.
BRIN: I have heard that there are enough warheads in the arsenals of the United States and the former Soviet Union that, if we beat them all into plowshares and used them all for Orion ships, that we could send a mass equivalent to the United States Navy to Mars.
LANDIS: What are some of the other possibilities for ways that we might get to the stars with real technology that we know today?
BRIN: All sorts of possibilities have been discussed. At the opposite end of the spectrum from the image of a fiery antimatter rocket, was the idea of sending a little "Starwisp" spacecraft streaking out of the solar system. This was originally Bob Forward's notion; a broad, very light sail that takes a focused microwave beam to drive this little one-ounce spacecraft. The microwave beam sends it hurtling across the starscape.
FORWARD: One of the newest ideas is one that Dana Andrews, Bob Zubrin, and Geoffrey Landis have proposed, particle-beam propulsion. The basic idea is to have a particle beam generator, stuck to an asteroid (because you can't use it on the Earth, the atmosphere gets in the way, and once it gets firing it has a lot of recoil, so you have to put it on something heavy). So you take an asteroid and you build your particle beam generator, and you beam both positive particles and negative particles out into space--
BRIN: You make your vehicle a hoop.
FORWARD: Right, a hoop. And you put current through it to make a strong magnetic field, and when the charged particles come they hit the magnetic field, and they give the magnetic field a push, and it gives the wire a push, and the wire gives the spacecraft a push, and so that's the way you get up to speed.
BRIN: This is a variant on the idea of sending a microwave beam--Forward's Starwisp--or of hitting a solar sail with a laser.
FORWARD: Beamed power propulsion.
BRIN: What all three of these ideas have in common is that you can send a ship out that doesn't have to carry its own energy, doesn't have to carry its own fuel. Because the biggest problem in approaching the speed of light is that you not only have to accelerate your own ship, but you have to accelerate the fuel that you're going to use for later acceleration. So people have been swinging over to this idea that the best way to reach the stars is to have a home base shoot power out to you, whether by particle beams, lasers, or microwaves.
FORWARD: I think that, after years of study, it's now very obvious that, if you want to go to the stars, don't use rockets. You have to use something else. Beamed power is one way.
The beauty about this engine is that, unlike some of the ideas I've had where you push it with lasers or microwaves, is that when you enter the target solar system, you can use it as a drag brake against the solar wind to slow down and come to a stop, without doing anything fancy except re-energizing the magnetic loop.
POST: In fact, any kind of interstellar craft can use a magnetic sail to brake. So we're talking about hybrids, good ideas that combine two other good ideas.
The ancestor of the magnetic sail was the interstellar ramjet of Robert Bussard. Many people played with that concept, which uses interstellar hydrogen as freely-available fuel, but the scoop to collect the hydrogen seems to produce more drag than thrust. James Stephens of JPL tried to patent the magsail first, under the name 'loopsat.' When I worked for Dana Andrews on Boeing's 1981 survey of advanced propulsion, I tried to hybridize huge superconducting loops with ion drives, and considered trajectories through the Earth's magnetotail, the Jovian magnetosphere, and the Io Flux tube. Bob Zubrin deserves credit as father of the magsail--he derived the essential equations--but the idea has many grandfathers, and clearly Bussard is the great-grandfather.
BRIN: So, what do we have so far? We have the original idea to leave the solar system using chemical rockets, illustrated by Voyager and Pioneer, with the wonderful plaques that are supposed to tell alien collectors where they came from. You all know the joke: Voyager is discovered by an alien species that recovers the record that was sent out with Jimmy Carter's voice on it, and Beethoven, and Chuck Berry, and what is their first message back to Earthlings? ÔSend more Chuck Berry.'
In fact, if Voyager ever does someday end up in some intelligent species' museum that species will be us. Because any time in the next ten thousand years, the world closest to Voyager will be ours. And we keep getting faster and faster, so, as Dr. Forward pointed out, in any given generation, the next generation will be able to go to the stars faster. The question is, why launch now, when the next technology will speed past anything we send now? Will it ever make sense to launch?
Then, the next generation thought about using nuclear rockets, the Orion motor that beat into plowshares nuclear bombs. Then there was some talk of making a pulse-propulsion rocket from inertial confinement fusion, where little hydrogen pellets would be squashed by laser beams into imploding and giving fusion power. There was some discussion of using antimatter, which Dr. Forward has been a leader at learning how to obtain and prepare. Most recently the consensus appears to be moving toward the three technologies that Bob just summarized: laser-pushed lightsails, microwave-pushed lightsails, and the more recent, wonderfully elegant particle-beam pushed magnetic sail.
POST: It may be premature to give up on rockets completely. They're a mature technology; we've been flying them since the 11th century. The most efficient rocket is an Ôautophage,' a rocket that eats itself. My paper at this conference suggests building spaceships out of frozen hydrogen, which is first a structural material, then a radiation shield, and finally is used as fuel.
FORWARD: The thing that's new is that we're beginning to push the minimum size of the spacecraft down below a ton. And that's important too. Because before now it used to be a ton or more, plus propellant, plus engine, and now we're beginning to realize that we can make a payload that's at least tens or hundreds of kilograms that can do a lot. I think that Clementine taught us that.
POST: Or even less. I think that the ideal payload will be the size of a bacterium. It gets to the next star system, reproduces itself from available interplanetary dust, then builds bigger and bigger gadgets from asteroidal material, including planetary landers, communications gear to report to Earth, and then makes an Interstellar Spacecraft factory to send probes outward into the universe.
LANDIS: When does it seem likely that we might be able to send the first human interstellar probes?
FORWARD: I think that even with the new ideas that we have about making spacecraft smaller, and making the propulsion system more efficient, and involving less mass and equipment in space, when we go through the numbers, we still end up with a very expensive life-cycle cost. I think we need more new ideas.
BRIN: Would you say that this endeavor is fundamentally dependent upon us becoming an interplanetary-traveling species, so that we're comfortable in near-Earth space, moving around the asteroids; so that we have those technologies and we're ready to build the lasers in space?
FORWARD: I think that what we need is a manufacturing capability, people up in space, building things from space resources, collecting space energy. We're going to need a lot of energy to do any of these jobs, even for sending small probes, and it would be better to do it out there. That means that we're going to have to have a full-fledged space manufacturing capability, which we're going to use for other things.
BRIN: Many other things, such as using space resources to become so rich that our civilization won't begrudge those kooks like us who want to send little robots to the stars. In the end, our exploratory spirit feeds from the same fundamental vigor as liberalism, free enterprise, and environmentalism, and that is the wealth and the luxury of free time to dream. Only people who don't worry about where their next meal is coming from worry about saving the whales, about feeding poor children, or about flying to the stars. So the long range goal of visiting other planets is ultimately tied up with the serious business of saving this one.
LANDIS: We've talked about how and why. The next question is clearly, where do we go?
POST: Although the Alpha Centauri system is closest, at 4.3 light years, some people like Barnard's Star, 6 light years. There are three good solar-type stars at 11 to 12 light years-- Epsilon Indi, Tau Ceti, and 61 Cygni.
There are also much closer targets in great favor. A few hundred to a few thousand astronomical units is the inner edge of the Oort cloud of comets, which would be very interesting to examine up close. And 550 Astronomical Units from the sun, a telescope can use the gravitational lens effect of the sun. If we have a true interstellar destination, we can send a precursor probe in exactly the opposite direction, and view the target through the sun's focusing of that target's radiation. Or, there may be brown dwarfs or black holes within a light year of the Earth.
LANDIS: Any final comments?
POST: The final question is, why go to the stars? My answer is, developing the technology will open the solar system to us. My slogan is, we want the stars, but we'll start with Mars. The rewards will be nothing less than infinite knowledge, infinite power, infinite wealth, and immortality.