Centauri Dreams
Imagining and Planning Interstellar Exploration
Detection of Pebbles in a Circumstellar Disk
Not long ago we looked at a new paper from Alan Boss that modeled interactions in young protoplanetary disks (A Disruptive Pathway for Planet Formation). The idea here is that as dust grains and larger objects bump into each other on the way to forming planetesimals, a mechanism must exist to keep them from spiraling into their star. Boss’ models show explosive phases in young stars that lead to gravitational instabilities of the sort needed to scatter these small objects outward and preserve their prospects for forming into planetesimals, and perhaps one day, planets.
Watching infant solar systems form is akin to studying embryology in animal species, a chance to understand the myriad interactions that affect growth and set it in particular directions. Now we have work out of the University of St. Andrews, recently presented at the National Astronomy Meeting in Llandudno, Wales, that announces the discovery of a ring of small rocks circling the star DG Tauri, a 2.5 million year old object some 450 light years from Earth.
Image: An artist’s impression of the belt of ‘pebbles’ in orbit around the star DG Tauri. The inset is a close up view of a section of the belt. Credit: J. Ilee. Adapted from original work by ESO/L. Calçada/M. Kornmesser, ALMA (ESO/NAOJ/NRAO)/L. Calçada (ESO).
The work of Jane Greaves and Anita Richards (University of Manchester) is based on data from the e-MERLIN array of radio telescopes centered in Jodrell Bank (Cheshire) and extending over southern England to form an interferometer, giving it the resolution of a single large telescope. The instrumentation proved up to the considerable challenge, as Greaves relates:
“The extraordinarily fine detail we can see with the e-MERLIN telescopes was the key to this discovery. We could zoom into a region as small as the orbit of Jupiter would be in the Solar System. We found a belt of pebbles strung along a very similar orbit – just where they are needed if a planet is to grow in the next few million years. Although we thought this was how planets must get started, it’s very exciting to actually see the process in action!”
The observations, as this Royal Astronomical Society news release explains, were made at a wavelength of about 4.6 cm. They revealed a signature that requires chunks of rock at least a centimeter in size. This is a useful finding, for as we’ve seen in the work of Alan Boss, we’re in the process of tuning up our computer models of protoplanetary disks and their interactions. Now we can identify, at least in some systems, the location of pebble-like material that will one day accrete into larger objects. A deeper analysis of young disks should emerge from all this.
Image: An e-MERLIN map of the star DG Tauri. The yellow and red areas show what is thought to be a ring of pebble-sized clumps in orbit around the star. Credit: J. Greaves / A. Richards / JCBA.
Studying the results will be, among others, a group called the Planet Earth Building Blocks Legacy e-MERLIN Survey (known by its fitting acronym — PEBBLeS). The team plans to extend studies like this to a number of stars that are in the process of forming their own solar systems. Right now we’re using equipment sensitive to regions as small as Jupiter’s orbit, but the logical goal is to move in five times closer to witness the formation of planets like our own. The researchers believe that upgrades to e-MERLIN and the coming capabilities of the Square Kilometer Array will make such observations possible.
The Exploratory Imperative
If you’re a long-time reader of this site, you doubtless share my fascination with the missions that are defining our summer — Dawn at Ceres, Rosetta at comet 67P/Churyumov-Gerasimenko, and in the coming week particularly, New Horizons at Pluto. But have you ever wondered why the fascination is there? Because get beyond the sustaining network of space professionals and enthusiasts and it’s relatively routine to find the basic premise questioned. Human curiosity seems unquenchable but it’s often under assault.
‘Why spend millions on another space rock?’ was the most recent question I’ve received to this effect, but beyond the economics, there’s an underlying theme: Why leave one place to go to another, when soon enough you’ll just want to go to still another place even more distant? The impulse to explore runs throughout human history, but it’s shared at different levels of intensity within the population. I find that intriguing in itself and wonder how it plays out in past events. The impulse is often cited as a driving motif that has pushed human culture into every corner of the planet, but it comes in waves and can lie fallow until new discoveries bring it to the fore.
Back when I was writing Centauri Dreams (the book), I looked at the Conference on Interstellar Migration, which was held in 1983 at Los Alamos. This was a multidisciplinary gathering including biologists and humanists along with physicists and economists, and a key paper there was the synergistic work of Ben Finney (an anthropologist) and Eric M. Jones (an astrophysicist). Called “The Exploring Animal,” the paper argued that evolution has produced an exploratory urge driven by innate curiosity. The authors considered this the root of science itself.
It was probably the Los Alamos conference that introduced the theme of Polynesia into interstellar studies, the idea being to relate the settlement of the far-flung islands of the Pacific to future missions into the interstellar ocean. From Fiji, Tonga and Samoa and then, in another great wave, to the Marquesas, Hawaii and New Zealand, using double-hulled dugout canoes with outrigger floats, these explorers pushed out, navigating by ocean swells, the stars, and the flight of birds. Finney and Jones call this the outstanding achievement of the Stone Age.
Here’s an excerpt that puts the view succinctly:
The whole history of Hominidae has been one of expansion from an East African homeland over the globe and of developing technological means to spread into habitats for which we are not biologically adapted. Various peoples in successive epochs have taken the lead on this expansion, among them the Polynesians and their ancestors. During successive bursts lasting a few hundred years, punctuated by long pauses of a thousand or more years, these seafarers seem to have become intoxicated with the discovery of new lands, with using a voyaging technology they alone possessed to sail where no one had ever been before.
And to me, this resonates when you see something like this:
Image: This map of Pluto, made from images taken by the LORRI instrument aboard New Horizons, shows a wide array of bright and dark markings of varying sizes and shapes. Perhaps most intriguing is the fact that all of the darkest material on the surface lies along Pluto’s equator. The color version was created from lower-resolution color data from the spacecraft’s Ralph instrument. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute.
Are Finney and Jones right that there is an ‘intoxication’ in the discovery of new lands? It’s certainly a sense I have, and most of the people I deal with in aerospace clearly have it. But note as well the fact that these bursts of expansion into the Pacific were also marked by long pauses, thousand-year breaks from the outward movement. We can see this as a process of consolidation, I suppose, or maybe a broader cultural fatigue with the demands of exploration. Are we entering a similar era in space, a time of reflection and retrenching before the next great push? If so, we have no good guidelines on how long that period may last.
In his new book Beyond: Our Future in Space (W.W. Norton, 2015), Chris Impey likewise speculates on the urge to explore. Humans do it differently from animals, after all — we are the only species that moves over large distances with a sense of purpose and organization, sometimes for reasons that have little to do with the availability of resources. Like Finney, Impey cites the Polynesian example, seeing it as driven by a mixture of culture and genetics.
Are there specific genes that come into play in at least parts of the population to make this happen? Because we all know that for some people, exploration is not an option — not even an interest. Most of Europe stayed home when the ships that would open up the Pacific and the Orient to western trade and expansion set sail, and remained at home while emigrants took to the seas to settle elsewhere. Even so, in every era, the exploratory impulse seems firmly planted, which is why even dangerous missions always find no lack of volunteers.
Impey is interested in certain developmental genes that he believes give us advantages over other hominids. In particular, he focuses on a gene called DRD4, which controls dopamine and thus has influence on human motivation and behavior. A variant of this gene known as 7R produces people more likely to take risks, seek new places and explore what is around them. About one person in five carries DRD4 in its 7R form. Impey notes that the 7R mutation occurred first about 40,000 years ago as humans began to spread across Asia and Europe.
Impey doesn’t go so far as to call this an ‘exploration gene,’ but he does think that carriers of 7R are more comfortable with change and are better problem solvers. From the book:
Even if they’re only present in a fraction of the population, the traits that favor adventurousness are self-reinforcing. If the 7R mutation has slightly higher frequency in a population that migrates, that frequency will increase in a finite gene pool. Mobility and dexterity are enhanced as they are expressed. The most successful nomads will encounter new sources of food and new possibilities for enhancing their lifestyle. The best users and makers of tools will be spurred to come up with new tools and novel applications of existing tools. The fulcrum of this feedback loop is our one attribute that’s unparalleled: a big brain.
Is DRD4 7R the source of the internal fire that drives our explorations? It’s a pleasing thought, for it implies that despite periods of pullback, the exploratory impulse is ever outward, for it exists as part of the template of our species. I doubt we can pin curiosity and migration down to this one salient, but our past does imply there is something within us that accounts for our restlessness. Whatever that something is, we find it reinforced in the great explorations of our time. And it seems to be strong enough to survive the periods of retrenchment and apathy that sometimes punctuate our efforts, and that tend to get lost in the big picture of history.
Yesterday’s post asked whether we were nearing the end of an era with the flyby of the last of the ‘classical’ planets. The answer is yes, but the beauty of eras ending is that they have successors, and our inherent human curiosity is not something that can be long suppressed. The Voyagers have already begun the bridge between the interplanetary era and the interstellar, and New Horizons will soon enough follow. Keep in mind the words of Andre Gide when New Horizons swings about to take images of Pluto and Charon receding in the night: “Man cannot discover new oceans unless he has the courage to lose sight of the shore.”
End of an Era in Planetary Exploration?
While both Alan Stern and Glen Fountain admitted to having anxious moments over the weekend when New Horizons went silent, it became clear at yesterday’s news conference that those moments were short and quickly subsumed with ongoing duties. Stern is principal investigator for New Horizons, and the man most closely identified with making the mission a reality, while Fountain is project manager for New Horizons at JHU/APL in Maryland. It was Stern who pointed out that the spacecraft has been in safe mode a number of times already.
Nine times, as a matter of fact, since launch, although as of yesterday we are back in the realm of normal operations. So the circumstances were not unfamiliar even if this safe mode came so close to destination that it raised inevitable concern and a flutter of worry on Twitter. Stern said he was in the control center six or seven minutes after getting the call that something was wrong. It also turns out that this was the first safe mode occurrence in which the backup computer was involved. So what exactly happened? Here’s Fountain’s explanation:
On the third of July we were preparing for the main event, with encounter mode starting on the 7th. That means loading those commands that are sequenced to get observations from the 7th of July to encounter and on through July 16th. This is a single command load that was to be put on the primary as well as the backup computer.
We had already loaded to the backup and on the 4th were loading to the primary computer, while at the same time taking data we had not been able to get down to the ground and compressing it, to free up the rest of the recorder for all the other data. So we were doing multiple things on the processor at the same time, and as we were doing the compression, the computer couldn’t handle the load. The processor said it was overloaded. The spacecraft then switched to the backup computer and went into safe mode, exactly as it should have.
Thus a timing conflict in the spacecraft command sequence is the culprit. Stern said that the data loss was minimal, with all science operations for Sunday and Monday lost as well as part of Saturday. Altogether, New Horizons lost 30 observations out of a total of 496 scheduled to be made between July 4 and the end of the close approach (the last of these occurring two days after the flyby). Because the team weighs the value of observations according to how close to the planet they are made, these lost sessions aren’t critical, and Stern figures the mission has experienced zero impact in terms of its highest priority science.
“We wish this hadn’t occurred,” Stern added,” but as PI, I can tell you that this is a speed bump in terms of the total return we expect from this flyby. We’re looking forward now to getting back to data collection. Pluto and Charon are already surprising us with their surface appearance.”
Science observations resume today at 1234 EDT (1634 UTC). Meanwhile, we have images captured by the Long Range Reconnaissance Imager (LORRI) between July 1 and 3, which were released immediately after the news conference.
Image: The left image shows, on the right side of the disk, a large bright area on the hemisphere of Pluto that will be seen in close-up by New Horizons on July 14. The three images together show the full extent of a continuous swath of dark terrain that wraps around much of Pluto’s equatorial region. The western end of the swath (right image) breaks up into a series of striking dark regularly-spaced spots, each hundreds of miles in size, which were first detected in New Horizons images taken in late June. Intriguing details are beginning to emerge in the bright material north of the dark region, in particular a series of bright and dark patches that are conspicuous just below the center of the disk in the right image. In all three black-and-white views, the apparent jagged bottom edge of Pluto is the result of image processing. The inset shows Pluto’s orientation, illustrating its north pole, equator, and central meridian running from pole to pole. Credit: NASA/JHUAPL/SWRI.
The image below adds color to the July 3 LORRI image using data gathered by the Ralph instrument, which performs visible and infrared imaging and spectroscopy.
Be sure to read Dennis Overbye’s fine essay in the New York Times on what New Horizons’ flyby of Pluto means in the larger scheme of planetary exploration (you also get the benefit of a video Overbye produced). I owe a lot to Dennis Overbye, whose Lonely Hearts of the Cosmos (HarperCollins, 1991) reignited my long-standing ambition to move from technology writing into astronomy and astrophysics. In the Times essay, he sees New Horizons as the beginning of the end of at least one phase of human exploration.
Beyond the hills are always more hills, and beyond the worlds are more worlds. So New Horizons will go on, if all goes well, to pass by one or more of the cosmic icebergs of the Kuiper belt, where leftovers from the dawn of the solar system have been preserved in a deep freeze extending five billion miles from the sun…
But the inventory of major planets — whether you count Pluto as one of those or not — is about to be done. None of us alive today will see a new planet up close for the first time again. In some sense, this is, as Alan Stern, the leader of the New Horizons mission, says, “the last picture show.”
We are at the edge of a vast sea, as Overbye notes, the one that separates us from the stars. Innumerable explorations await us as we learn more about the Solar System and push outward into the Kuiper Belt. But what a poignant moment as we realize that of the nine ‘classical’ planets (let’s not argue about ‘dwarf’ planets for now), there will never be another moment when one swims into focus for the first time. Let’s cherish the week ahead, and hope that one day humans will experience a similar kind of moment with a new planet in a new solar system.
New Horizons: A ‘Timing Flaw’ Scare Resolved
You get to expect the unexpected when writing about space probes, but somehow what New Horizons did to my weekend completely blind-sided me. Running a routine check of email before (I thought) sliding into the rest of a relaxing work break, I found messages about the glitch on the Pluto-bound spacecraft. Sunday turned into an all-screens-on exercise in checking multiple feeds and waiting for news.
The problem with New Horizons brought to mind a short story I wrote many years ago about an unmanned probe sent to Epsilon Indi on a 90-year journey. The probe is within a month of encounter when all goes silent and Earth controllers can only wait to see what happens.
The point of the story (it was called “Merchant Dying,” published in Charlie Ryan’s Aboriginal Science Fiction in the July/August 1987 issue) was that spacecraft going to another star are going to need autonomous repair capabilities we can only dream of today. New Horizons is a long way out, but we can still work with it through the Deep Space Network, and a check this morning shows DSN’s 70-meter Canberra dish working New Horizons as I write. But space is teaching us all about backup computers and autonomy one step at a time.
The ‘anomaly’ occurred on July 4 and led to a loss of communications with Earth. New Horizons’ autonomous systems were able to switch to the critical backup computer while placing the spacecraft in ‘safe mode’ and re-starting communications. Emily Lakdawalla reports here that the New Horizons Anomaly Review Board met at 1600 EDT yesterday to analyze the situation. The subsequent NASA statement was reassuring, and I’ll quote its latest update in its entirety:
NASA’s New Horizons mission is returning to normal science operations after a July 4 anomaly and remains on track for its July 14 flyby of Pluto.
The investigation into the anomaly that caused New Horizons to enter “safe mode” on July 4 has concluded that no hardware or software fault occurred on the spacecraft. The underlying cause of the incident was a hard-to-detect timing flaw in the spacecraft command sequence that occurred during an operation to prepare for the close flyby. No similar operations are planned for the remainder of the Pluto encounter.
“I’m pleased that our mission team quickly identified the problem and assured the health of the spacecraft,” said Jim Green, NASA’s Director of Planetary Science. “Now – with Pluto in our sights – we’re on the verge of returning to normal operations and going for the gold.”
Preparations are ongoing to resume the originally planned science operations on July 7 and to conduct the entire close flyby sequence as planned. The mission science team and principal investigator have concluded that the science observations lost during the anomaly recovery do not affect any primary objectives of the mission, with a minimal effect on lesser objectives. “In terms of science, it won’t change an A-plus even into an A,” said New Horizons Principal Investigator Alan Stern of the Southwest Research Institute, Boulder.
Adding to the challenge of recovery is the spacecraft’s extreme distance from Earth. New Horizons is almost 3 billion miles away, where radio signals, even traveling at light speed, need 4.5 hours to reach home. Two-way communication between the spacecraft and its operators requires a nine-hour round trip.
Status updates will be issued as new information is available.
So we’re less than ten days out from Pluto/Charon and a knuckle-whitening moment seems to have passed with little loss of data. With observations re-starting on Tuesday, here’s imagery from July 1, with the inset showing an enlarged Pluto. Features as small as 160 kilometers are visible at this point (credit: NASA/JHUAPL/SWRI). Onward and outward…
Update: More news this afternoon, as per NASA:
NASA will host a media teleconference at 3 p.m. EDT (19:00 UTC) today to discuss the New Horizons spacecraft returning to normal science operations after a July 4 anomaly. The mission remains on track to conduct the entire close flyby sequence as planned, including the July 14 flyby of Pluto…
Audio of the teleconference will be streamed live at http://www.nasa.gov/newsaudio
The Spacecoach Equation
My view is that the spacecoach, the subject of renewed discussion below by Brian McConnell and a design he and Alex Tolley have created, is the most innovative and downright practical idea for getting crews and large payloads to the planets that I’ve yet encountered. It’s low-cost and uses ordinary consumables as propellant, dramatically revising mission planning. Brian and Alex have continued refining the concept, as explained below in Brian’s essay on a modified version of the rocket equation. Have a look and you’ll see that planning long duration missions or missions with larger crews becomes a much more workable proposition because more consumables translate into more propellant. Could the spacecoach be our ticket to building a space-based infrastructure, with unmistakable implications for even deeper space?
by Brian S McConnell
The spacecoach, first introduced here in Spaceward Ho! and A Stagecoach To The Stars and on spacecoach.org, is based on the idea of using consumables waste streams, such as water, CO2 and gasified waste, as propellant in solar powered electric engines. The idea is to turn what is normally dead weight (and a lot of dead weight on a long duration mission such as to Mars) into propellant. This in turn leads to dramatic reductions in mass, and thus mission cost, because a ship that uses waste from consumables as propellant no longer needs an external stage weighing several times as much to push it to its destination. (If you or your colleagues are working on electric propulsion systems and have test data and citations to share see below)
Image: The spacecoach. Credit: Rendering by Rüdiger Klaehn based on a design by Brian McConnell.
To understand the impact this has, we developed a modified version of the rocket equation that leads with the crew consumable requirements for a given mission, and then calculates the level of engine performance required to fly the mission using only consumables waste streams (mostly water and carbon dioxide) as propellant. This, in turn, yields a minimum mission cost, as no surplus propellant is required, so the mission cost is reduced to the cost to deliver the crew and consumables to the starting point (while the ship itself is reusable so its construction and launch cost can be amortized across many missions).
The rocket equation, shown below, predicts the ship’s delta-v (change in velocity), as a function of specific impulse (a measure of engine performance) and the ship’s mass ratio (starting mass divided by ending mass).
The spacecoach equation, shown below, predicts the minimum exhaust velocity (or specific impulse) required for a cost optimized mission as a function of its delta v and consumables budget.
For programmers, this can also be written in pseudocode as:
Let’s consider a ship that has a 40,000 kg hull mass when empty that is being resupplied for a trip to the Martian moons from EML-2 (Earth Moon Lagrange point 2). With low thrust propulsion this requires a delta-v of roughly 18 km/s roundtrip. The ship has a six person crew, with a 15 kg/person-day budget for water, food and oxygen. The mission is expected to last 600 days, so the consumables budget is 54,000kg.
According to the equation, the engines will need to achieve an exhaust velocity of 21 km/s, which equates to a specific impulse of about 2,100s, assuming 100% of the waste streams are reclaimed (if engines can be made to work with gasified waste, even solids such as trash should be usable as propellant). If we assume that some percentage of the consumables waste streams (e.g. solid waste) cannot be used, say 20%, the engines will need to operate at a specific impulse of 2,900s. This is within the performance envelope of Hall effect thrusters, as well as several other electric propulsion technologies. If the engine performance is not quite good enough, that’s ok, the ship would just be loaded with more water than the crew really needs to compensate for this, or could even support a larger crew. This will increase costs a bit above the minimum possible cost, but also provide safety reserves above what the crew is projected to need.
Next, let’s compare the mass budget for a similar ship using chemical propulsion (e.g. LOX + methane). This mission requires much less delta-v as the ship can exploit the Oberth effect (aka powered flyby) when departing Earth, and on arrival at Mars. To give the chemical ship a further advantage, we’ll assume it uses aerobraking for Mars capture and for Earth return. So the round trip delta-v in that scenario is roughly 8 km/s. The downside is the engine specific impulse is much lower, about 360s for oxygen + methane. Plugging this into the rocket equation results in a propellant mass budget of almost 820,000 kg, over twenty times the mass of the empty hull. This can be optimized by shedding mass, such as waste, spent stages, etc, but not by a great deal without making compromises in terms of consumables, payload, etc (and we’ve already given the chemical ship a big advantage by assuming it can use aerobraking extensively to minimize propulsive delta-v).
Compare this with the spacecoach, where the consumables are the propellant. It would require the delivery of only 54,000 kg of consumables. This is 1/15th what is required for the conventional mission, and should lead to comparable reductions in overall mission costs. Meanwhile the mission itself is much simpler and less risky (all low thrust propulsion, no chemical rockets with catastrophic failure modes, no high G maneuvers, no aerobraking, plus the option to add more crew and/or consumables with little penalty).
The savings come from two sources. Because the consumables are the propellant, there is no need for external propellant. This effect is amplified further because electric engines have much higher exhaust velocities than chemical rockets so even the relatively small consumables mass needed by the crew is sufficient to propel the ship (if electric engines operated at a specific impulse comparable to a chemical rocket, you’d need ten times more water than the crew would consume).
And it gets even better. This is counterintuitive, but it is actually easier to plan for long duration missions with larger crews and high delta v (Ceres, Venus and the Asteroid Belt for example). This is because more consumables = more propellant = higher delta-v given the same engine performance, whereas in a conventional ship you get into a vicious circle of mass incurring more mass. Running the numbers for a 6 person, 1000 day mission to Ceres (delta v : 26.5 km/s roundtrip from EML-2), the consumables budget is 90,000 kg, and the required engine specific impulse is again in the 2000s, which suggests that a ship capable of reaching Mars will be capable of reaching Ceres due to the larger consumables budget.
And speaking of Ceres, it is an enormous water reservoir. While early spacecoaches would be supplied entirely from the Earth, developing the ability to extract water from low gravity sites like Ceres, and possibly the Martian moons, will be a priority as it will reduce the need to launch water from Earth, and thus further reduce operating costs, but even without in situ resource utilization, spacecoaches will be an order of magnitude cheaper to operate, and will be capable of reaching destinations like Ceres that simply cannot be reached by humans using chemical propulsion.
While it takes people a while to see the implications of this (the thinking about how to design a spacecraft is pretty ingrained), the math is pretty straightforward and suggests that order of magnitude cost reductions for interplanetary missions, with greatly expanded range, will be possible with this approach.
If you are working on electric propulsion technology, we are compiling data about the relative performance of different technologies and propellants, especially as it relates to the use of water and waste gases, to provide the community with an easy to search repository of SEP test data and citations. This data will be made available at spacecoach.org as well as on github. If you’d like to submit test data and citations, you can use this form. Contact Brian McConnell at bsmcconnell@gmail.com for more information.
Thoughts on DE-STAR and Laser Sailing
Last week we looked at DE-STAR (Directed Energy Solar Targeting of Asteroids and Exploration), an ambitious program for developing modular phased arrays of kilowatt class lasers. The work of Philip Lubin (UC-Santa Barbara), DE-STAR is envisioned as a way to scale up a space-based system for asteroid mitigation. And in a new NIAC grant, Lubin will study an off-shoot called Directed Energy Propulsion for Interstellar Exploration (DEEP-IN) as a way of driving tiny ‘wafer’ probes on interstellar journeys. Reading about these ideas, Jim Benford responded with the comments below. A plasma physicist and president of Microwave Sciences (Lafayette, CA), Dr. Benford’s work on microwave beaming to sailcraft has included laboratory experiments at JPL with brother Greg that I’ve written about in these pages. Here are his thoughts on DE-STAR’s beaming methods and the issues they invoke.
by James Benford
The calculations presented by the DE-STAR group are basically a revisit of the work of Bob Forward, which was published over 30 years ago. Several other workers have formulated beam-driven sail scaling in the past, so this new group is revisiting that work.
From inspection of their papers, it appears that the DE-STAR group is producing concepts for very low mass, fast sailship probes using a method several others have explored previously: They get very small, light probes by putting all the effort into making them small, at the expense of having an enormous beaming aperture. The very large aperture is not emphasized in their papers, but the smallness of the sail is.
The “realistic” DE-STAR 4 design, for example, has an aperture of 10 km on a side. So the aperture is 330 times larger in linear dimension than the sail, 100,000 times larger in area.
Looking at it from the point of view of cost, such designs are far more expensive than alternatives where the aperture and sail are not so very different in size. (Beamer system cost scales inversely with the sail area.) So the choice of very large aperture drives the capital cost of the system very high; I estimate between 10 and 100 trillion dollars, depending on economies of scale. The operating cost is low because the required power is lower. I would recommend that the DE-STAR group look at cost-optimized methods, which are much more realistic and therefore much more likely to be built.
Image: Testing carbon sails at JPL. Here a carbon disk sail lifts off of a truncated rectangular waveguide under 10 kW microwave power (four frames, 30 ms interval, first at top). Credit: James and Gregory Benford.
Another problem with small sails is that if you do try to launch all of them simultaneously as a constellation, on a beam much larger than a single sail, the sails will not ride on the beam. They will drift off to the side due to perturbations. There are an extensive series of papers on the program I managed in sail acceleration, spin and stability from 1999 to 2005 (two of these are listed below). They describe beam-riding analytically, computationally and experimentally.
Image: Carbon-carbon sail used in sail stability and spin experiments. SAIL diameter is 5 cm, height 2 cm, mass 0.056 gm. Experiments and simulations show that such conical sails can ride a microwave beam very stably. Credit: James Benford.
So it’s a pretty solid understanding, based on internal reflections on the underside of the sail, as follows:
- The sail must be non-flat, with a conical cone the best shape with the apex pointing away from the beam;
- The diameter of the sail must be of the same order as the transverse dimensions of the beam; and
- Spin helps stabilize beam-riding craft.
This cannot be avoided or all is lost — the sail tumbles. For more, see:
“Stability and Control of Microwave Propelled Sails in 1-D”, Chaouki T. Abdalla, Edl Schamiloglu, James Benford and Gregory Benford, Proc. Space Technology and Applications International Forum (STAIF-2001), Space Exploration Technology Conf, AIP Conf. Proc. 552, ISBN 1-56396-980-7STAIF, pg. 552, (2001).
“Experimental Tests Of Beam-Riding Sail Dynamics”, Gregory Benford, Olga Goronostavea and James Benford, Beamed Energy Propulsion, AIP Conf. Proc. 664, pg. 325, A. Pakhomov, ed., 2003.
Follow with RSS or E-Mail
