We do a change of pace today as we move from the UFO Community to the mainstream ( sort of ) of Paul Gilster’s Centauri Dreams and Tau Zero’s discussion of real life interstellar propulsion methods and starflight.
Here Robert Bussard’s ramjet is linked with Robert Forward’s idea of laser-powered starflight and a more efficient method of vehicle acceleration – and more importantly ‘deceleration’ at the appointed destination:
Many of the interstellar concepts I write about in these pages take on a life of their own. After the initial brainstorming, the idea gets widely enough disseminated that other scientists take it on, looking to modify and improve on the original concept. That’s been true in the case of solar sails and the more recently devised ‘lightsails,’ which use beamed energy from a laser or microwave source to drive the vehicle. We continue to study magnetic sails — ‘magsails’ — and various nuclear options like the inertial confinement fusion that powered Daedalus and perhaps Icarus. Sometimes insights arise when ideas are grafted onto each other to create a hybrid solution.
The idea I want to examine today, a hybrid design combining a Bussard-style interstellar ramjet with laser beaming — exemplifies this mix and match process. Working with Daniel Whitmire, A. A. Jackson, a frequent commenter and contributor here on Centauri Dreams, pondered the various issues the Bussard ramjet had run into, including the difficulty in lighting the proton/proton fusion reaction Bussard advocated early in the process. Writing at a time not long after he had finished up a PhD in relativistic physics (at the University of Texas), Jackson conceived the idea of beaming energy to the spacecraft and discovered that the method offered advantages over the baseline Bussard design. The laser-powered ramjet is a fascinating concept that has received less attention than it deserves.
Image: Physicist and interstellar theorist Al Jackson, originator of the laser-powered ramjet concept.
Bussard’s ramjet, you’ll recall, lit its fusion fires using reaction mass gathered from the interstellar medium by a huge magnetic ram scoop, which itself has proven problematic given the drag issues such a scoop introduces. The other way to power up a starship using an external source of energy is to beam a terrestrial or Solar System-based laser at the departing craft, which has deployed a lightsail to draw momentum from the incoming photons. Jackson and Whitmire found the latter method inefficient. Their solution was to beam the laser at a ramjet that would use reaction mass obtained from a Bussard-style magnetic ram scoop. The ramjet uses the laser beam as a source of energy but, unlike the sail, not as a source of momentum.
Running the numbers and assuming all photons transmitted by the laser will be absorbed by the ship, the authors discovered that the laser-powered ramjet (LPR) is superior to the baseline Bussard ramjet at low velocities, while superior to the laser-pushed sail at all velocities. The Bussard design becomes the most efficient of the three at velocities equal to and above about 0.14 c. The laser-powered ramjet, then, solves at least one of the Bussard vehicle’s problems, the fact that it has to get up to a significant percentage of lightspeed before lighting its fusion reaction. LPR propulsion could be used up to 0.14 c, with the vehicle switching over to full interstellar ramjet mode to achieve high efficiency at relativistic velocities.
The laser-powered ramjet offers other advantages as well. Think back to some of Robert Forward’s laser sail concepts and you’ll recall the problem of deceleration. With the sail powered by a laser beam from the Solar System, it’s possible to reach velocities high enough to take you to the nearest stars in a matter of decades rather than centuries. But how do you slow down once you arrive? Conceiving a manned mission to Epsilon Eridani, Forward came up with a ‘staged’ solution in which the sail separates upon arrival, with the large outer sail ring moving ahead of the vehicle and reflecting beamed laser energy to the now smaller inner sail, thus slowing it down. It would be so much easier if the beam worked in both directions!
But with the laser-powered ramjet, a round trip can be made using a single laser beam because the beam is being used as a source of energy rather than momentum. Jackson and Whitmire showed that the efficiency in the deceleration phase of the outbound journey as a function of velocity is the same as for the acceleration phase. And on the return trip, the energy utilisation efficiency is more favorable in both the acceleration and deceleration phases because the ship is traveling into the beam. In fact, the laser-powered ramjet is superior to both the laser sail and the Bussard ramjet even at high fractions of the speed of light when traveling into the laser beam.
Let’s go over that again: Jackson and Whitmire’s calculations focus on the energy utilisation efficiency parameter, showing that the laser-powered ramjet is superior to the laser sail at all velocities, whether the ship is receding from the beam or approaching (moving into the beam). The LPR is also superior to the Bussard ramjet at velocities less than about 0.14 c when receding from the beam, and superior to the Bussard design at all velocities when approaching. Add to this that the LPR concept requires no onboard proton-burning reactor — the authors assume the use of Whitmire’s ‘catalytic’ ramjet using the CNO (carbon-nitrogen-oxygen) cycle — and that the LPR’s power requirements are less than those of the laser sail.
As this talk is more ‘mainstream’ than usual, the idea of spotting interstellar craft incoming to this Solar System is easy to spot, given the power output of the craft. Any invasion would be highly visible.
But the UFO phenomenon excludes that – no incoming craft are visible until they are already in the atmosphere.
So is wormhole technology being used, or are other folding door type technologies ( if one can call such things technology ) being utilized?
Paul Gilster of Centauri Dreams continues the discussion of the below light-speed seeding of Intelligence in the Galaxy from the paper that Robert Freitas wrote in the 1980s and the prospect that such an intelligence ( or future “human” descended intelligence ) could seed the Galaxy over a period of 1,000,000 years:
It was back in the 1980s when Robert Freitas came up with a self-reproducing probe concept based on the British Interplanetary Society’s Project Daedalus, but extending it in completely new directions. Like Daedalus, Freitas’ REPRO probe would be fusion-based and would mine the atmosphere of Jupiter to acquire the necessary helium-3. Unlike Daedalus, REPRO would devote half its payload to what Freitas called its SEED package, which would use resources in a target solar system to produce a new REPRO probe every 500 years. Probes like this could spread through the galaxy over the course of a million years without further human intervention.
A Vision of Technological Propagation
I leave to wiser heads than mine the question of whether self-reproducing technologies like these will ever be feasible, or when. My thought is that I wouldn’t want to rule out the possibility for cultures significantly more advanced than ours, but the question is a lively one, as is the issue of whether artificial intelligence will ever take us to a ‘Singularity,’ beyond which robotic generations move in ways we cannot fathom. John Mathews discusses self-reproducing probes, as we saw yesterday, as natural extensions of our early planetary explorer craft, eventually being modified to carry out inspections of the vast array of objects in the Kuiper Belt and Oort Cloud.
Image: The Kuiper Belt and much larger Oort Cloud offer billions of targets for self-reproducing space probes, if we can figure out how to build them. Credit: Donald Yeoman/NASA/JPL.
Here is Mathews’ vision, operating under a System-of-Systems paradigm in which the many separate systems needed to make a self-reproducing probe (he calls them Explorer roBots, or EBs) are examined separately, and conceding that all of them must be functional for the EB to emerge (the approach thus includes not only the technological questions but also the ethical and economic issues involved in the production of such probes). Witness the probes in operation:
Once the 1st generation proto-EBs arrive in, say, the asteroid belt, they would evolve and manufacture the 2nd generation per the outline above. The 2nd generation proto-EBs would be launched outward toward appropriate asteroids and the Kuiper/Oort objects as determined by observations of the parent proto-EB and, as communication delays are relatively small, human/ET operators. A few generations of the proto-EBs would likely suffice to evolve and produce EBs capable of traversing interstellar distances either in a single “leap” or, more likely, by jumping from Oort Cloud to Oort Cloud. Again, it is clear that early generation proto-EBs would trail a communications network.
The data network — what Mathews calls the Explorer Network, or ENET — has clear SETI implications if you buy the idea that self-reproducing probes are not only possible (someday) but also likely to be how intelligent cultures explore the galaxy. Here the assumption is that extraterrestrials are likely, as we have been thus far, to be limited to speeds far below the speed of light, and in fact Mathews works with 0.01c as a baseline. If EBs are an economical and efficient way to exploring huge volumes of space, then the possibility of picking up the transmissions linking them into a network cannot be ruled out. Mathews envisages them building a library of their activities and knowledge gained that will eventually propagate back to the parent species.
A Celestial Network’s Detectability
Here we can give a nod to the existing work on extending Internet protocols into space, the intent being to connect remote space probes to each other, making the download of mission data far more efficient. Rather than pointing an enormous dish at each spacecraft in turn, we point at a spacecraft serving as the communications hub, downloading information from, say, landers and atmospheric explorers and orbiters in turn. Perhaps this early interplanetary networking is a precursor to the kind of networks that might one day communicate the findings of interstellar probes. Mathews notes the MESSENGER mission to Mercury, which has used a near-infrared laser ranging system to link the vehicle with the NASA Goddard Astronomical Observatory at a distance of 24 million kilometers (0.16 AU) as an example of what is feasible today.
Tomorrow’s ENET would be, in the author’s view, a tight-beam communications network. In SETI terms, such networks would be not beacons but highly directed communications, greatly compromising but not eliminating our ability to detect them. Self-reproducing probes propagating from star to star — conceivably with many stops along the way — would in his estimation use mm-wave or far-IR lasers, communicating through highly efficient and highly directive beams. From the paper:
The solar system and local galaxy is relatively unobscured at these wavelengths and so these signaling lasers would readily enable communications links spanning up to a few hundred AUs each. It is also clear that successive generations of EBs would establish a communications network forming multiple paths to each other and to “home” thus serving to update all generations on time scales small compared with physical transit times. These various generations of EBs would identify the locations of “nearby” EBs, establish links with them, and thus complete the communications net in all directions.
Working the math, Mathews finds that current technologies for laser communications yield reasonable photon counts out to the near edge of the Oort Cloud, given optimistic assumptions about receiver noise levels. It is enough, in any case, to indicate that future technologies will allow networked probes to communicate from one probe to another over time, eventually returning data to the source civilization. An extraterrestrial Explorer Network like this one thus becomes a SETI target, though not one whose wavelengths have received much SETI attention.
SETI as it is set up now does not concentrate its observations or detections on possible physical artifacts, just radio transmissions at certain frequencies.
Personally I think advanced civilizations (cultures?) would be evolved more than the mere “biological”, but would be cybernetic in nature and thus would be beyond “god-like” in nature and would’ve figured out a way past the light-speed barrier.
That would put the possiblity of old fashion radio transmission on the back burner, other than the construction of radio “beacons” as proposed by the Benford Brothers.
Interstellar Galactic Federations and Empires not withstanding, Einstein’s Special Theory of Relativty still rules.
However, Paul Gilster posts on his blog Centauri Dreams that below light speed colonization of the galaxy can have a normal, more organic method of colonizing the galaxy by human, or alien intelligences:
Imagine a future in which we manage to reach average speeds in the area of one percent of the speed of light. That would make for a 437-year one-way trip to the Alpha Centauri system, too long for anything manned other than generation ships or missions with crews in some kind of suspended animation. Although 0.01c is well beyond our current capabilities, there is absolutely nothing in the laws of physics that would prevent our attaining such velocities, assuming we can find the energy source to drive the vehicle. And because it seems an achievable goal, it’s worth looking at what we might do with space probes and advanced robotics that can move at such velocities.
How, in other words, would a spacefaring culture use artificial intelligence and fast probes to move beyond its parent solar system? John Mathews ( Pennyslvania State) looks at the issue in a new paper, with a nod to the work of John von Neumann on self-reproducing automata and the subsequent thoughts of Ronald Bracewell and Frank Tipler on how, even at comparatively slow (in interstellar terms) speeds like 0.01c, such a culture could spread through the galaxy. There are implications for our own future here, but also for SETI, for Mathews uses the projected human future as a model for what any civilization might accomplish. Assume the same model of incremental expansion through robotics and you may uncover the right wavelengths to use in observing an extraterrestrial civilization, if indeed one exists.
Image: The spiral galaxy M101. If civilizations choose to build them, self-reproducing robotic probes could theoretically expand across the entire disk within a scant million years, at speeds well below the speed of light. Credit: STScI.
But let’s leave SETI aside for a moment and ponder robotics and intelligent probes. Building on recent work by James and Gregory Benford on interstellar beacons, Mathews likewise wants to figure out the most efficient and cost-effective way of exploring nearby space, one that assumes exploration like this will proceed using only a small fraction of the Gross Planetary Product (GPP) and (much later) the Gross Solar System Product (GSSP). The solution, given constraints of speed and efficiency, is the autonomous, self-replicating robot, early versions of which we have already sent into the cosmos in the form of probes like our Pioneers and Voyagers.
The role of self-replicating probes — Mathews calls them Explorer roBots, or EBs — is to propagate throughout the Solar System and, eventually, the nearby galaxy, finding the resources needed to produce the next generation of automata and looking for life. Close to home, we can imagine such robotic probes moving at far less than 0.01c as they set out to do something targeted manned missions can’t accomplish, reaching and cataloging vast numbers of outer system objects. Consider that the main asteroid belt is currently known to house over 500,000 objects, while the Kuiper Belt is currently thought to have more than 70,000 100-kilometer and larger objects. Move into the Oort and we’re talking about billions of potential targets.
A wave of self-reproducing probes (with necessary constraints to avoid uninhibited growth) could range freely through these vast domains. Mathews projects forward not so many years to find that ongoing trends in computerization will allow for the gradual development of the self-sufficient robots we need, capable of using the resources they encounter on their journeys and communicating with a growing network in which observations are pooled. Thus the growth toward a truly interstellar capability is organic, moving inexorably outward through robotics of ever-increasing proficiency, a wave of exploration that does not need continual monitoring from humans who are, in any case, gradually going to be far enough away to make two-way communications less and less useful.
Paul calls robotic networks “organic” in the way they might grow, but there is a commenter on the post who disagrees with it and I might agree with that.
But that doesn’t discount a more “cybernetic” approach in which the combination of machine with organic technology is the more “natural” extension or evolution of intelligent lifeforms.
I would look for rigidly constructed organic molecular structures in the interstellar medium as materials for Bracewell Probes.