From julian_hiscox@micro.microbio.uab.edu Wed Jun 5 09:21:38 1996 Date: 5 Jun 1996 10:24:25 -0500 From: Julian Hiscox To: "Dr. Julian Hiscox" Subject: Marbugs- Vol.3. No.3. Mail*Link¨ SMTP Marbugs: Vol.3. No.3. MARSBUGS: The Electronic Exobiology Newsletter Volume 3, Number 3, 4th June, 1996. Special Issue: Interstellar travel. Co-editors: David Thomas, Department of Biological Sciences, University of Idaho, Moscow, ID, 83843, USA, thoma457@uidaho.edu. Julian Hiscox, Microbiology Department, BBRB 17, Room 361, University of Alabama at Birmingham, Birmingham, AL 35294-2170, USA, Julian_hiscox@micro.microbio.uab.edu. MARSBUGS is published on a weekly to quarterly basis as warranted by the number of articles and announcements. Copyright exists with the co-editors, except for specific articles, in which instance copyright exists with the author/authors. E-mail subscriptions are free, and may be obtained by contacting either of the editors. Contributions are welcome, and should be submitted to either of the two editors. Contributions should include a short biographical statement about the author(s) along with the author(s)' correspondence address. Subscribers are advised to make appropriate inquiries before joining societies, ordering goods etc. ---------------------------------------------------- INDEX 1) EDITORS INTRODUCTION 2) ON THE REALISM OF INTERSTELLAR TRAVEL 3) INTERSTELLAR PROPULSION RELATED MATERIAL AND SOCIETIES -Eds. ---------------------------------------------------- 1) EDITORS INTRODUCTION We are pleased to present an issue devoted to interstellar travel. In the past year or so a number of extra-solar planets have been discovered and verified. Although planets orbiting within the habitable zones of such stars have not been inferred, within the next few years the technology may become developed enough to allow this. Therefore, interstellar travel is of extreme importance when one considers that the only way to actually sample extra-solar planets is to go there and visit them with either robotic or crewed space craft. We a very pleased to present our featured article by Gerald Nordley, an astronautical engineer and author with degrees in physics and systems management. We have also included a section that contains material on interstellar propulsion that might be of interest. ---------------------------------------------------- 2) ON THE REALISM OF INTERSTELLAR TRAVEL by Gerald David Nordley With the pioneers and voyagers on their way out of the solar system, no one seriously disputes the possibility of slow interstellar travel, at least for automatic spacecraft. But objections continue to be raised against _fast_ interstellar travel, i.e. starships that travel close to the speed of light. The usual purpose of such arguments is to make interstellar communications through radio or lasers seem like the only real possibility. (Well, obviously, if sentient beings can go, or send surrogates, nearly as fast as light to some place where they can converse in real time; they might prefer to do this rather than wait decades for an answer on the radio--leaving not much radio traffic to hear.). Therefore, some people with vested interests in radio SETI often emphasize the futility of such interstellar physical commerce. The late Dr. Barney Oliver was one such person, and he delighted in making calculations of the incredible size and impossibly large energy requirements for any reasonably feasible rocket designed to go between the stars. His mathematics were elegant and his formulae quite accurate, and yes, indeed, an interstellar rocket designed to approach the speed of light is a very daunting technological proposition. While storing its propulsive energy as antimatter might make rockets approaching half the speed of light barely feasible for a Solar-system scale civilization, flights of such rockets would seem to be necessarily awe inspiring and infrequent. But recent work in interstellar propulsion makes arguments concerning rockets increasingly irrelevant, and the energy argument alone ignores the implications of the cybernetic revolution already underway. Let's take energy first. The kinetic energy of a 1000 ton (1 E6 kg) starship moving at 87% the speed of light (gamma = 2) is about 9 E22 joules. The total non-food energy consumed by human civilization today is something like 1E 21 joules--about a hundredth of that. If we are talking about getting the spacecraft up to that speed and back down by rockets, many times that energy is needed to move the reaction mass required, depending on exhaust velocity, efficiency, and the weight of the engines. (Even for photon rockets--while the photons don't "weigh" anything, the energy needed to generate them must have an inertial mass, in some form, of E/c^2, where E is the total energy needed to generate all the photons required for the mission. The photon rocket must carry that mass.) Here we have to note that, lacking a breakthrough (like "The Kubota Effect"--see Analog, May '96) in physics to make antimatter for rockets, several hundred times more solar (or other) energy must be consumed to run the accelerators needed to make the antimatter than one gets back in annihilating that antimatter. But while interstellar travel energy requirements are huge compared to what we can generate on Earth today, they are not particularly impressive in comparison to what might be generated with the fusion of deuterium taken from a giant planet's atmosphere, or in comparison to the sun's output. Practical fusion remains to be seen, but the ability to collect the necessary solar energy is easily demonstrated. Once we make self-replicating robots, they can also make solar power collection stations. With exponential growth, a few decades should suffice to put the needed power infrastructure in place. For instance, a the end of two years, a system that each year produces a copy of itself and a one-gigawatt solar power collection satellite (from moon or asteroidal material) has made three powersats and three copies of itself. At the end of n years, there are 2^n production systems and 2^n - 1 power satellites. In fifteen years, the energy production would be on the order of magnitude of Earth's current total. In thirty years, the system would be producing 34,000 times as much and doubling that each year--until the solar system environmental impact authorities start getting nervous about the number of asteroids being consumed or the size of the (new, artificial) holes in Mercury or the Moon. Obviously, a real system won't produce exactly one replica and exactly one gigawatt powersat in exactly one year. Also, I doubt such a system would be fully autonomous--some human supervision, especially early on, will likely be involved. But this calculation should give a good idea of the power of robotic systems and their exponential growth to get the energy needed for starflight. It is not necessary to try to anticipate the future engineering details nor the trades concerning which conversion systems are most appropriate, how big the robots should be, and so on. The work can be done. Simple solar power conversion systems have already been built--they will get better, simpler, and more efficient with time. Machines making parts of machines are an increasingly relevant part of daily life in these times, as are robotic assemblers. What robotic production does to conventional notions of cost is worth a paper in itself--and a serious current issue regarding the displacement of labor--I'm afraid I tend to snicker a little when people start talking about interstellar travel in conventional economic terms. But, clearly, the energy requirements are such that robotic production of power producers is needed, and, in kind of an anthropic principal argument, the civilizations that achieve that will have developed the technology to deal with any conceivable energy and material shortages along the way. The distribution of such largess remains, of course, a significant cultural issue--the robots are coming and we'd best start dealing with it--humanely, I hope. But we shall have to have solved that issue before we are ready to go to the stars (or it will solve us!). In projecting the future, as always, the difference between science and engineering is very important. Natural laws do impose asymptotic limits on what engineering can do--but current practice (such as the state of the art in robotics), is not a valid limitation on future engineering capability. The huge energy numbers still have a lot of emotional impact in the negative direction. Dr. Oliver, for instance, dismissed the possibility of self replicating robots as "handwaving arguments" when we discussed this following our papers in the 1986 IAF conference in Brighton. I think he understood full well that their potential advent was a monkey wrench in the argument for a radio-only linked galactic civilization--that they could destroy the energy objection to interstellar travel and make extraterrestrial radio transmissions less likely to find. At the time we were discussing getting energy to make antimatter for rockets, but there are much better alternatives. Indeed, it is probably time to forget about interstellar rockets. It is increasingly clear that the way to do interstellar travel is to leave the fuel tanks and heavy engines at rest and push the spacecraft with beams of light or particles. Photons of light are still somewhat wasteful starship-pushers at low velocity--a reflected photon caries away a lot of energy until the spacecraft is moving fast enough to downshift its frequency significantly. But the velocity of incoming physical particles can be chosen to leave the reflected particles dead in space--with no kinetic energy left. If so, except for some radiation and alignment losses, all the energy went into the starship. Thus the theoretical minimum energy required is simply the kinetic energy of the vehicle at its cruising velocity--though a reasonable person would apply a factor of ten to this, for system inefficiencies. Yes, the beams have to hit the spacecraft--but there are all sorts of ways of arranging that they do this. And yes, the process can't be 100% efficient--but it can come close in principal, especially with particles. Dr. Robert L. Forward has described in several papers and articles how to accelerate and decelerate a photon propelled system. The reflector materials must perform near their physical limits. For first time missions, big (asteroid-sized or larger) Fresnel-type lenses floating in space are needed to focus light from huge lasers across interstellar distances to staged deceleration reflectors. The engineering is advanced, but the physics works. In a particle beam propulsion system, much smaller lasers along the beam route could push an occasional errant propulsion particle back to its path toward the starship. Or the particles could make use of oncoming microtechnology to be smart enough to steer themselves, homing in on the spacecraft reflector with tiny laser-diode photon rockets, or, as Forward suggested to me, by using tiny mirrors to reflect a steering beam from the ship, to one side or another. Depending on the size of the particles accelerated, the particle accelerators themselves could be mass produced versions of today's linacs, or much longer versions for much heavier particles (or pellets). To slow a particle beam reflecting starship down at the target system, a trail of the appropriate kind of mass needs to be laid out in front of the oncoming starship. For first time missions, this could be done by a colony of small self-replicating robots sent ahead. The initial robots would decelerate more slowly, with photon, magsail, or nuclear rocket decelerator systems to the target system's asteroid belt or Oort cloud. Of course, if a co-operating technical civilization is found, they could provide the deceleration beam. Reflectors can be shiny thin films for photons, magnetic mirrors for particles (which are first blasted into plasma as they approach the starship), charged metal plates, or even ultra-thin sails designed to run before an artificial particle wind. And many other variants, of course. The upshot of these "new" techniques is that rocket calculations (and, sadly, I started out in this field by doing them, too) are turning out to be as relevant to the question of interstellar travel as calculating the supply of helium for balloons is to the question of intercontinental air travel. Indeed, the gain in velocity and efficiency for beam supported propulsion is so great that I am beginning to wonder whether rockets will be anything but auxiliary propulsion by the end of the next century--even for solar system travel. It's trains versus horses. For comparison, the 9 E22 joules of energy mentioned above packaged in a rocket with a final mass of 1000 tons and an exhaust velocity of 0.1 c (optimistic even for nuclear fuels) would get the rocket up to just over 0.25 c--and this neglects practical considerations such as the mass of the engines needed to give meaningful acceleration to a rocket with an initial mass of 173,000 tons. Rocket engines, like car engines, scale with power. Accelerating a thousand tonne ship at one gravity or so takes a lot of power, which means heavy engines, which means more power and more fuel, which means more powerful engines, which. . . . Well, even to approach this performance requires the rocket to consume its engines for reaction mass along the way, as its need for thrust decreases. By comparison, beamed propulsion is technologically conservative. The large lasers, particle accelerators, beam cooling and steering (or self-steering micropellets), solar power satellites, asteroid mining--taken individually the necessary technologies are fairly reasonable extensions of work already in progress. Beamed propulsion doesn't need new science nor as yet problematical engineering breakthroughs such as efficient antihydrogen production, or D-He3 fusion reactors. All the necessary elements, except the software for the robots to make the energy collectors, have already been made on small scale. Progress in software is proceeding by leaps and bounds, and it is the one area least affected by government funding cuts or private investment restrictions. If the parts are all there, the systems can, at least in principal, be built. Unfortunately, when one puts them all together, the systems that can be made of these pieces generate conceptual barriers many minds. That, however, is something for the sociologists and psychologists. I think I must leave this with the observation that intuition and judgement are honed by experience, and in matters that are necessarily outside experience, negative intuition is often (and sometimes hilariously) worthless. I'll also note that much of physics (particularly orbital dynamics) delights in being counterintuitive. As with any "new" idea (the seminal work, by Forward on lightsails and Singer on pellet beams, goes back at least 16 years), beamed propulsion is taking a while to sink in. It isn't an idea with a lot of emotional appeal. Beam supported propulsion systems (like railroads) require a substantial infrastructure in addition to the vehicle and part of the resistance to the concept (beyond vested interests) is, I suspect, that beamed propulsion surrenders the romance of a self-sufficient spacecraft which (once built and fuelled!) can go where its crew wants to go. Beam riding spacecraft are much more restricted in their destination--not very "Star Trek." Whatever the reasons, there are as yet few mentions of beamed propulsion in the interstellar propulsion literature compared to rockets. But one can't rule out interstellar travel just because the propulsion systems are unfamiliar or the cornucopia of energy involved implies social changes that, as we said back in the 60's, blow one's mind. Yes, there is always the wild card of some new physics that allows one to bypass the speed of light by throwing the power switch on the right piece of equipment. However it is one thing to propose engineering that will approach the limits of nature after a reasonable amount of work and is quite another thing to propose devices that require a significant recalculation of those limits--if not a complete revision of physics as we know it. Dyson spheres, space stations, genetic engineering and robots--yes; but warp drive, Star Trek's "Q," time travel, or switch-on gravity in spaceships--not very likely. But since beamed propulsion doesn't need new physics, I see nothing at all unrealistic about spacecraft made by humans (or others) travelling at speeds approaching (but not passing) the speed of light--the only question is when. My best guess right now is about 100 years--given all of humanity's other priorities. But if we really had to do it soon, I suspect we could get at least probes up to that velocity in about 50 years, with starships shortly thereafter. Ad astra! (Note: The above is a slightly expanded and edited version of a response to email concerning one of the late Dr. Oliver's papers on the difficulties of interstellar travel. For further reading, check interstellar propulsion-related articles in the last three or four years of the Journal of the British Interplanetary Society and the references in Bob Forward's latest non-fiction book: Indistinguishable from Magic. --G.D.N.) Gerald (G. David) Nordley is an astronautical engineer and author with degrees in physics and systems management. After retiring from the Air Force, he started writing as a second career, and has sold some 30 pieces of short fiction and non fiction to various markets. Novels are in work. His novella "Martian Valkyrie" appeared in the January 95 Analog and he has a short story "The Kubota Effect" in the May 95 _Analog_ . Upcoming are two novellas: "Fugue on a Sunken Continent," based on the CONTACT world of Epona and planned for the November issue of Analog, and "Messengers of Chaos," a lunar murder mystery to be published in a future issue of Asimov's. ---------------------------------------------------- 3) INTERSTELLAR PROPULSION RELATED MATERIAL AND SOCIETIES -Eds. Material related to interstellar propulsion: Books: The Starflight Handbook. By Eugene Mallove and Gregory Matloff. 1989. John Wiley & Sons, Inc. New York. ISBN 0-471-61912-4. $27.95. The Millennial Project: Colonizing the Galaxy in Eight Easy Steps. By Marshall T. Savage. 1992. Empyrean Publishing Ltd. ISBN 0-9633914-8-8. $24.94. ISBN 0-9633914-9-6. $18.95. Infinite in All Directions. By Freeman Dyson. 1988. Harper and Row, New York (Perennial Library Edition). ISBN 0-06-091569-2. $11. Cosmos. By Carl Sagan. 1980. Random House, New York. ISBN 0-517-12355-X. ~$30. Journal of the British Interplanetary Society Issues (recent): Back issues may be obtained by contacting the society at: BIS@CIX.COMPULINK.CO.UK Electric Propulsion (Part 2). v49, no5. May, 1996. Practical Robotics for Interstellar missions (Part 2). v49, no4. April, 1996. Practical Robotics for Interstellar missions (Part 1). v49, no1. January, 1996. Electric Propulsion (Part 1). v48, no12. December, 1995. Exobiology (Part 4). v48, no11. November, 1995. Space Missions and Astrodynamics (Part 2). v47, no11. November, 1994. Interstellar Studies. v41, no11. November, 1988. Interstellar Studies. v39, no11. November, 1986. Also, Ad Astra: v7, no4, four articles on interstellar travel. (See National Space Society for details). Societies: The British Interplanetary Society, 27-29 South Lambeth Road, London SW8 1SZ, UK. BIS@CIX.COMPULINK.CO.UK (Monthly Spaceflight or JBIS included in membership). The Planetary Society, 65 North Catalina Avenue, Pasadena, CA 91106-2301, USA. tps@genie.geis.com (One main bimonthly, also optional The Mars Underground News and Bioastrononomy Newsletter). National Space Society, call 1-800-543-1280. (Includes one bimonthly magazine). ---------------------------------------------------- End. Marsbugs, Vol.3. No.3.