Aurora / Explore / The Science
Interstellar Travel
The Challenge
In creating the Aurora Trilogy, we had to tackle a fundamental plausibility challenge: Without making magical assumptions about faster-than-light travel, how (and more importantly why) would anyone go to the extreme lengths of establishing a colony in another star system?
It can be hard to wrap your head around the distances involved. You may know that the nearest star system, Alpha Centauri, is 4.367 light-years away. The next closest, Barnard’s Star, where our trilogy takes place, is 5.978 light-years away. To put that in perspective, as of late 2022 the space probe Voyager 1, after 46 years of continuous flight, was approximately 23.5 billion kilometers from Earth, which marks the farthest a spacecraft has ever gone—and that’s just one four hundredth of one light-year.
For another comparison, the fastest a spacecraft has ever traveled is 163 kilometers per second, which was achieved by the Parker Solar Probe on November 20, 2021. At that speed, you could get from New York to Los Angeles in just 24 seconds—but it would take 11,000 years to get to Barnard’s Star.
Three Possible “Hows”
Most science fiction involving interstellar travel handles the huge distances involved by assuming humans have invented faster-than-light drives of some sort. There are Warp Drives (Star Trek), Hyperdrives (Star Wars), FTL Drives (Battlestar Galactica), and Holtzman Effect drives that fold space (Dune). Unfortunately, the mathematics of Special Relativity is pretty convincing. And constructing wormholes that can safely ingest a ship and spit it out somewhere else may be even more far-fetched.
Another possibility is “generation ships,” which are designed to travel at conventional velocities for more than a generation. The father of modern rocketry, Robert Goddard, may have been the first to write about the idea in 1918 in his essay "The Ultimate Migration," in which the passengers traveled for centuries in hibernation. Konstantin Tsiolkovsky, who originated much of modern astronautic theory, first discussed a true multi-generation ship, which he called Noah’s Ark and which would travel for thousands of years, in his 1928 essay "The Future of Earth and Mankind." There have been many modern sci-fi novels making use of the same concept. The problem they all must address is why? What would motivate people to go on a journey they and their children would never see to the end? Often the answer is that Earth is facing an apocalypse.
We chose a third way to reach the stars—with advanced fusion engines that achieve one fifth of light speed, not coming close to breaking any rules of physics, and not requiring that passengers sacrifice all for a journey they will never complete.
We do employ a limited form of human hibernation, but that is mainly because we think, based on current research, that this is a very plausible, even likely scenario for 200 years from now. Even so, the books do not depend on hibernation to make the story plausible.
The Why
Even if humans develop the technology to travel to the stars, why would they go? It’s not hard to imagine sending robotic probes for the sake of exploration, though even this would be controversial, not just because of the expense but also because it would take decades for the first results to get back to Earth. Would you support your government spending hundreds of billions of dollars to snoop around a star, knowing you wouldn’t see any results for 36 years? Well, you read sci-fi, so maybe you would, but not everyone would agree.
So if we assume the trip will take 30+ years, why do it? Of course, examples abound of explorers willing to leave home for many years: the Vikings, the Spanish missionaries, the French trappers, and before them the brave souls who crossed Siberia to reach North America or those who crossed the Pacific to colonize the Polynesian Islands. But those people didn’t need a trillion dollars to finance their trip, even after accounting for inflation.
Of course, magical, inexpensive, faster-than-light travel would answer the question. If going to Barnard’s Star was as quick and affordable as plane travel, lots of people would go, some just to visit, others to live there, knowing they could stay in easy contact with Earth.
But we assume that traveling to Barnard’s Star takes about 30 years and is enormously expensive, so much so that it requires public investment by the world’s international powers. So the question remains, why would they do it?
There would need to be some compelling economic justification—something to make it worth the investment. Our solution was stath, an element that is so effective at shielding fusion engines that it revolutionizes space travel. See here for a discussion of the science behind this not-yet-discovered element, and here for a discussion of its role in the story.
Fusion
Fusion Engines vs. Fusion Power Plants
Since the completion of the first hydrogen bomb based on fusion in 1952, it has been widely assumed that fusion would eventually provide a cleaner, safer, and inexhaustible source of electric power. When one of us was taking physics classes at the University of Wisconsin in the 1970s, the joke was that fusion was the energy of the future and always would be. Now, 50 years and trillions of dollars later, scientists have sustained for a fraction of a second a fusion reaction that produced slightly more power than it consumed.
The world is still at least decades away from cost-competitive fusion power, and there are some good reasons to question whether it will ever be a viable power technology on Earth. Fusion efforts to-date suffer from three core problems. First, they use the “easiest” D-T reaction, fusing deuterium and tritium. While deuterium is relatively abundant in seawater, tritium is not found in nature, and so it must be “bred” in the reactor. This is a potentially messy and complex process that has yet to be tested on a large scale. Many fusion reactions other than D-T exist but they are all harder to achieve. Second, any actual operating power plant will require substantial energy input and produce large amounts of unproductive energy. Achieving total net reactor power (as opposed to the current goal of net reaction power) remains a distant dream. And finally, it is likely that the cost of construction and operation of a fusion plant per watt produced could exceed the cost of current fission plants, which are already not cost-competitive.
Some of these problems could be solved by employing fundamentally different fusion processes than those used today, but they are likely more than a century from being practical for generating electrical power. Long before then, we believe humans will have developed the space-mining infrastructure (https://arxiv.org/ftp/arxiv/papers/1810/1810.03836.pdf) to make space-based solar power (SBSP) a far less expensive solution to Earth’s energy needs.
For outposts farther from the sun than Mars, SBSP may not be a practical power source (incident solar flux on Jupiter, for example, is about 4% that on Earth), and fission or fusion solutions will be required. This will also be true around Barnard’s Star, where the total power output of the star is less than 0.4% that of our sun.
Fusion Engines Before Fusion Power Plants?
Progress on fusion power plants, though slow, could lead to rapid progress on fusion engines. Once scientists are able to reliably produce sustained reactions, progress on ignition and containment technology will be fairly rapid. While it may be a century or more before economically viable electric power plants come online, there are reasons to believe that fusion engines will be viable much sooner:
First, the bar is low. There are basically two types of rocket engines: chemical rockets that can generate large thrust only for a short period of time, and electric ion thrusters which generate tiny amounts of thrust but for a long time. Some modern launch systems use both, but generally cannot get ships to a velocity of more than about 11 kilometers per second (40,000 km/h).
(Interesting side note: The Parker Space Probe once traveled 163 kilometers per second, the fastest a spacecraft has ever traveled. It achieved this not with its engines but with a gravity assist by flying very close to the sun. It is scheduled to go even faster in another swing in 2025.)
Fusion engines could theoretically accelerate ships to over 60,000 km/s, but they only have to perform at 0.1% of their potential to do 5 times better than our best propulsion systems today. In other words, even the earliest, most primitive fusion engines would be game changers.
Furthermore, outer space provides four “resources” not available to terrestrial power plants. First is an easily accessed high-quality vacuum, which requires a massive investment to produce down on Earth. Second is thermal insulation, which reduces the cost and mass of achieving and maintaining cryogenic temperatures for the lasers and superconducting magnets that will almost certainly be major parts of any fusion system. Finally, space provides a less expensive repository for some of the unwanted by-products of fusion reactions. Space has its own challenges, such as disposing of waste heat. But on balance, once fusion reactions can be sustained, fusion engines should advance to usefulness much sooner than fusion power plants.
Types of Fusion
Stars are often described as fusing four hydrogen atoms to form one helium atom under immense pressure. In fact, that specific reaction, while theoretically possible, almost never happens. Hydrogen does get converted to helium, but through a complex variety of fusion reactions, each different from the others.
The scientists developing fusion reactors have a choice of many fusion reactions, each involving different atoms, temperatures, and pressures, and each producing different amounts and types of energy.
Virtually all current reactor research is based on fusing deuterium (D) and tritium (T). This is because D-T fusion is the easiest to do—it requires the least compression and temperature of all fusion reactions.
However, D-T may not be the best long-term solution. While D and T are both isotopes of hydrogen, only D is found in nature. T is unstable and must be manufactured in the following process: D-T reactors, which produce a lot of neutrons, will use them to bombard blankets made of lithium; the neutrons will hit lithium atoms and convert them to tritium, which will be extracted from the blanket and fed back into the reactor. The process is mainly still theoretical and has yet to be tested in a reactor. Current fusion experiments are done with small quantities of tritium recovered from conventional fission-based nuclear power plants—a procedure that is not commercially sustainable. The lithium blanket/tritium recovery process, if it can be made to work, promises to be complex and expensive.
Another problem with the D-T reaction is that 80% of its energy output is in the form of high-velocity neutrons. The lithium blankets must not only use them to generate tritium, but must also capture their energy by converting it into heat, which will then be used to generate electricity. This is not optimally efficient, but more importantly, the neutrons will pass through the structures and equipment that contain and manage the lithium process, causing damage by dislodging atoms and generating some hydrogen and helium “bubbles” in the metal. New materials and alloys may be required to avoid unacceptable replacement and maintenance costs.
Finally, the lithium in the blankets is consumed as it is converted to tritium and has to be replenished. It is not clear, given rising demand for lithium for use in batteries, whether the world’s supply of lithium will be adequate.
So while D-T fusion is the easiest reaction to ignite, it may not be the best solution in the long run. But what is learned about the technologies needed to get D-T working will be useful for all the other reactions, including the two leading alternatives, deuterium-deuterium (D-D) and deuterium-helium3 (D-He3). Note that D-He3 is technically incorrect nomenclature—it should be D-He-3 or D-3He, but D-He3 is easier to follow.
D-He3 is the next easiest reaction to ignite after D-T. It has several things going for it. First, it generates few neutrons. Its product is a helium ion and a proton, both of which are charged particles whose paths can be directed by a magnetic field to do useful work, either by inducing an electric current to generate electric power or heat, or by being directed out a rocket nozzle for thrust. The downside is that He-3, like tritium, is not found naturally on Earth. There is some on the surface of the moon in very small concentrations. However, it is found in abundance in the outer clouds of the gas giants, and is in many ways an ideal fuel for space engines. So D-He3 is what we propose as the fusion reaction for planetary ships.
D-D is attractive because of the abundance of deuterium and is the third easiest process to ignite after D-T and D-He3, but it has significant drawbacks. First, unlike the other fusion reactions, D-D actually has two possible outcomes. Half the time (at random) it produces He3 plus a neutron, while the other half of the time it produces a tritium atom and a proton. This makes utilizing the reaction more complex. Second, its energy output is lower than D-T and D-He3. Third, like D-T, it produces a lot of those annoying neutrons.
However, the T and He3 produced by D-D can each be used as fuel in secondary processes, and will create more of the kind of energy needed for rocket propulsion than either the D-T or D-He3 reactions alone. This would be a more complex and probably much larger engine, and will take many more years to develop than D-T and D-He3, but it might be optimal for large interstellar ships. Early iterations might reasonably be expected to be available by the 22nd century, when the first interstellar probes are launched in the Aurora universe, and mature, optimized versions by the mid-22nd century, when the huge interstellar transports and power plants to run the colony tubes are needed for our story.
Stath
Introduction
Statheros is the name we gave to element 126, which, as of 2023, is not known to exist in nature, nor has it ever been created in a lab. However, as has been the case for elements 116, 117, and 118, it is expected to be created in labs sometime in the future. Element 126 has been assigned a placeholder name and symbol: unbihexium, Ubh. By convention, it will be assigned its “real” name after it has been created or discovered.
(Interesting side note: Although the name unbihexium is widely used in classrooms and textbooks, the name is mostly ignored among the small community of scientists who work theoretically or experimentally on superheavy elements, who usually refer to it as element 126, or E126.)
What interests scientists about element 126 in particular is that it is expected to reside in what is called the “island of stability” of the periodic table (see below). In the Aurora universe, once element 126 is created in the lab in the late 21st century, it is given the name statheros, which is Greek for “stable.”
Stability
All elements have isotopes. Isotopes of an element have the same number of protons but varying numbers of neutrons. Isotopes of the same element usually have almost identical chemical properties (chemistry is based mainly on the number of electrons, which are related to the number of protons, not neutrons, and thus the same for all the isotopes). However isotopes can differ markedly in certain physical attributes, in particular stability.
The nuclei of the most common isotopes of elements up through lead are stable, meaning they are not known to decay. For example, 99% of the carbon in the world is carbon-12, which has 6 neutrons. The remainder is almost all carbon-13, which is also stable. But there are trace amounts of carbon-14, which has a half-life of 5,730 years. In other words, every 5,730 years, half of the carbon-14 spontaneously decays. In its case, one of its neutrons converts to a proton and it becomes nitrogen (this process is the basis of carbon dating, used in anthropology to determine the age of organic material).
Many heavy elements, such as uranium, have dozens of isotopes, each with a different half-life (and thus each with a different degree of radioactivity). For example, the second most stable isotope of uranium, 235U, has a half-life of 703.8 million years. That may seem like a long time, but it means that since the Earth formed 4.5 billion years ago, more than 98% of it has decayed into something else, so only trace amounts remain. The most common form of uranium, 238U, has a half-life of 4.468 billion years.
As atoms get even heavier, their half-lives tend to get shorter. As an example, element 100, fermium, was first created in 1953. Its most common isotope, 257Fm, has a half-life of 100.5 days. The most recently created—elements 116, 117, and 118—have half-lives measured in milliseconds. This is why we do not see them in nature—they disappear too fast.
But nuclear physicists predict that there is something they call the “island of stability” in the periodic table, where some isotopes of certain superheavy elements may have much longer half-lives. The theory is that certain “magic numbers” of protons and neutrons may have stable nuclei.
The as-yet-undiscovered element 126 with 184 neutrons is one of the combinations considered to be a top candidate for the island of stability. As noted above, this element has been given the temporary name unbihexium, symbol Ubh, and when it is first created (or discovered), it will be assigned a permanent name. We have given it the name statheros, or stath for short.
Models vary concerning the likely chemical properties of Ubh. It could be similar to plutonium (element 94), because it is expected to have eight valence electrons surrounding a noble gas “core” (an inner cloud of electrons). Plutonium is one of the densest elements, almost twice as dense as lead (element 82). In the novels, we speculate that stath, which is one and a half times plutonium’s atomic weight, has roughly twice its density, which is what makes it good for fusion engine shielding (see below).
Interestingly, stath is so dense that if you were holding a teaspoon of it, it would be heavier than a billiard ball (a pool ball).
Historical attempts to find or create
Unbihexium has attracted attention among nuclear physicists for many years. An experiment at CERN in 1971 tried to synthesize it, unsuccessfully. It is currently thought that the method that has been used to create other superheavy elements (fusion-evaporation) may not work for some yet-to-be-created elements, including 126, and that a new technique will be required.
It has long been speculated that superheavy elements may have been formed during various stages of stellar evolution, such as supernovae and neutron star mergers. If so, and any of them had long half-lives (in the range of at least millions of years), traces of them could still be around.
In 1961, Polish-Australian astronomer Antoni Przybylski was having difficulty categorizing the star HD 101065, an oscillating star 356 light-years from the sun. He discovered that it had a unique spectrum, with low amounts of iron and nickel but high amounts of heavy elements.
The star, which was subsequently renamed Przybylski’s Star, has been confirmed to have an overabundance of exotic elements, with some controversial speculation that it may contain superheavy elements, including unbihexium.
In 1976, American researchers studying unexplained primordial damage in minerals speculated that the cause could be the decay of superheavy elements, including unbihexium. There were several searches for them in nature. Tom Cahill of UC Davis claimed his group detected alpha particles and X-rays with the right energies to cause the damage observed, supporting the presence of unbihexium. This has been the subject of some debate and has not been confirmed.
Some of the above work focused on searching for 228Ubh, but several studies have suggested that 354Ubh (as in the novels) may be a better candidate.
Shielding
Fusion engines generate two kinds of radiation: neutrons, which are particles, and gamma and X-rays (we’ll just call them X-rays for the rest of this discussion), which are high-energy photons. The emitted neutrons are captured and reused in later steps of the fusion process. But ship equipment, crews, and passengers require shielding from X-rays. Before stath, standard shielding was composed of tungsten alloys. Tungsten was used due to its high density—about 2.5 times that of steel and 1.7 times that of lead. Higher density means the atoms are packed closer together, so it will stop more photons per unit volume. Less volume saves mass on a ship because other ship structures can then be smaller.
Stath is about twice as dense as tungsten, so just on that basis, it would substantially reduce the size of the shielding and yield savings in ship mass.
But stath has another, larger advantage over tungsten. To explain this, we first have to talk about X-ray reflection. In general, X-rays penetrate radiation shielding and get absorbed, dissipating their energy as heat in the shield material. This is a problem in space because that heat can’t dissipate off the ship via conduction or convection; it can only dissipate by radiation, which is slow and dependent on surface area. Therefore, ships have radiators, which are large in terms of both mass and volume. Without them, the thermal energy of the ship would increase continuously until systems stopped working.
But what if the X-rays could be reflected rather than absorbed? Then the heat wouldn’t be absorbed either, and most of those radiators wouldn’t be needed. It turns out X-rays can be reflected, but just barely. X-rays can reflect off tungsten and other heavy metals only when they hit at a very shallow angle, like bullets ricocheting off a wall. This angle, called the critical grazing angle, is 0.5 degrees for tungsten. This is a very small angle, and means that to be useful as a reflector, tungsten shields would have to be extremely long (and therefore more massive), and the part of the ship being shielded would have to be very far from the engine. The extra structure involved would offset the savings from eliminating the extra radiators.
Stath, due to its tight “packing” (which is what gives it its high density), has a staggeringly high critical grazing angle of 12 degrees, much higher than any other known material. This means that stath shields can be set up to deflect the X-rays, absorbing very little heat and saving all that radiator mass. The X-rays are bounced into space where they fade away as the square of the distance.
The savings for a specific ship depend on its size and design, but in general, the dry mass savings for a typical interplanetary ship is about 45%.
Stath Value Proposition
Delta-V
Rockets don’t have a fuel economy rating or range the way a car does. That’s because once you get your rocket going, it just keeps going until it runs into something (like an atmosphere or asteroid) or until you turn on your engine to slow it down or change direction. The most important metric for any ship is its delta-V, or ΔV, which is a way of measuring how much a rocket with a full load of fuel can change its speed before running out of fuel.
Everything a ship does in space, whether it is simply transporting goods from A to B or engaging in combat with another ship, involves increasing velocity and decreasing velocity. ΔV determines how much of that it can do. ΔV is like a budget for rockets: they have to spend it wisely to achieve their goals in space.
ΔV varies based on ship design and fuel efficiency. Key factors are ship mass, fuel capacity, and exhaust velocity (how fast the exhaust comes out of the nozzle). There’s a formula for ΔV called the rocket equation:
ΔV = Ve * ln (Mi / Mf)
where Ve is exhaust velocity, Mi is initial mass including fuel, and Mf is final mass after all the fuel has been expended. A typical interplanetary ship might have a 30-ton engine, 30-ton hull and fuel tanks, 100-ton payload, 200 tons combined shielding/radiators (note: radiators unrelated to the shielding are included in the hull), and carry 500 tons of fuel. So Mi = 30+30+100+200+500 = 860 tons and Mf = 860-500 = 360 tons.
The small fusion engines in interplanetary ships can generate an exhaust velocity (Ve) of 330,000 meters per second. Plug that into the equation and this ship has a ΔV = 287,000 m/s. In other words, it can do some combination of acceleration and deceleration up to a total of 287,000 m/s before it runs out of fuel. For example, it could accelerate to 143,500 m/s in one direction and then flip around and slow back down to 0 by the time it reached its destination. It could also simply accelerate in one direction all the way to 287,000 m/s, but then it wouldn’t have any fuel left to slow down, which wouldn’t be very useful—unless you wanted to use the ship as a weapon, which we call a Javelin.
That same basic ship design, but with stath shielding, only needs 8 tons of shielding and 2 tons of radiators. In the above equation, Mi becomes 670 and Mf becomes 170, yielding a ΔV = 452,000 m/s, a revolutionary 57% improvement.
Market Price Projection
When Pamex was planning its colony, it needed to project the market value for stath, which would determine what it would pay Aurora.
In the example above, the market price for that pre-stath 360-ton ship was about $400 million. Pamex’s studies showed the expected price for the stath-shielded ship—the one that would perform 57% better—would yield a $580 million price, or $180 million more. Subtracting 20% for the shipyard’s margin yielded an expected wholesale price for stath of $18 million per ton, or $18,000/kg.
Barnard's Star
History
Barnard’s Star (BS) is a red dwarf named for American astronomer Edward Emerson Barnard. Although Barnard is often credited with discovering the star in 1916, it had actually been photographed as early as 1888. What Barnard did, by comparing older and newer photos, was measure its proper motion as 10.3 arcseconds per year relative to the sun, the highest known for any star. It was in honor of this discovery that the star was named for him.
Red dwarf stars, as the term implies, are small. They are also cooler than our sun (hence the red color) and therefore dim. In fact, while 50 of the 60 stars closest to Earth are red dwarfs, none are visible to the naked eye. The smallest red dwarfs are 7.5% the mass of the sun—anything smaller is unable to fuse hydrogen and is called a brown dwarf. Exoplanets have been found around many red dwarfs.
Interestingly, Barnard also recorded the first observation of a Trojan asteroid around Jupiter in 1904, although he did not know that was what it was. Trojans in the Barnard system play a major role in the novels.
Edward Emerson Barnard.
Photo of staff of Yerkes Observatory in Williams Bay, Wisconsin, where E.E. Barnard was the leading astronomer. Barnard is in the middle back row with the white hair and mustache. There’s another notable figure five to the right of Barnard: Albert Einstein, who was presumably just visiting, since he was not an observational astronomer. May 6, 1921.
Characteristics
The Barnard System is the second closest star system to Earth after the Alpha Centauri System, which consists of three stars—Alpha Centauri, Beta Centauri, and Proxima Centauri, approximately 4 light-years away.
Barnard’s Star is a red dwarf, the smallest and coolest kind of star. Red dwarfs are by far the most common type of star in the Milky Way, at least in the neighborhood of the sun, but because of their low luminosity, individual red dwarfs cannot be easily observed. From Earth, not one red dwarf is visible to the naked eye. Red Dwarfs are estimated to make up three-quarters of the stars in the Milky Way.
A photograph of Barnard’s Star taken by the Hubble Space Telescope in 2022.
Barnard’s Star is small and dim—14% of the mass of the sun (Sol), 18% the diameter, and only .04% as bright.
Barnard's Star size comparison.
It has a high velocity relative to Sol, 90 kilometers per second laterally and 110 km/s radially (total vector is 140 km/s). So it is getting 110 km closer to Earth every second.
Currently, in 2023, BS is 5.958 light-years from Earth. Radio communications would take 5 years and 350.4 days.
Primordial—book 1 of the Aurora Trilogy—takes place in 2280. By then, Barnard’s Star will be 892,133,352,000 km closer, or 34.44 light-days (110 km/sec * 3,600 sec/hr * 24 hr/day * 365.25 days/yr * 257 yr). So in 2280, communications will “only” take 5 years and 315.8 days.
Barnard's Planets
In 1963, Dutch astronomer Peter van de Kamp, a professor at Swarthmore College, made headlines when he announced he’d detected a planet about the size of Jupiter orbiting Barnard’s Star. He couldn't actually see the planet itself, though; rather, he’d detected a “wobble” in BS’s position that he attributed to the gravitational tugging of a large planet.
This was a momentous event. At this point in history, no exoplanets (planets outside our solar system) had been discovered, although their existence was widely assumed. Van de Kamp’s purported discovery made him one of the most famous astronomers in the world.
More than a decade later, however, further studies proved that the wobble van de Kamp had recorded was actually in his telescope, not in the position of Barnard's Star. (The first true discovery of an exoplanet—which was based on the same wobble methodology—did not occur until 1992.)
In November 2018, a candidate super-Earth known as Barnard’s Star b was reported to orbit BS. It was believed to have a minimum of 3.2 Me (Earth masses) and orbit at 0.4 AU, roughly the same distance as Mercury. However, work presented in July 2021 and 2022 cast doubts upon the existence of this planet.
Artist’s conception of the surface of Barnard’s Star b, a planet that would be 3.2 times the size of Earth at the same distance from BS as Mercury is from our sun. However, more recent studies question whether this planet really exists.
As of this writing in 2023, it is unlikely that a planet has been detected around BS. However, current planet-detecting technology is not good enough to detect small planets or even large ones that are not in close orbit to the star. This is true even for a star as “close” to Earth as BS.
Existing astronomical data rules out some possibilities (i.e, if they existed, we’d have already detected them), including anything Earth-size or larger in the habitable zone, and anything more than 10 Earth masses within a 730-day orbit.
In the novels, BS has five planets plus various clouds of asteroids and comets. All fit within the constraints imposed by the best current data. Aurora orbits the as-yet undiscovered outermost planet, the small gas giant Moira, 2.1 AUs from BS.
At Moira’s orbit, Barnard’s Star would be .0004 times the sun’s apparent brightness in visual light, but its total emissions are 9 times its visible light output, the difference being mostly infrared. Moira is 2.3 times as far away from BS as Earth is from the sun. So BS’s visual brightness from Moira would be 0.0004 / (2.1^2) = .00009 times as bright as the sun from Earth. But it would generate .0035 / 2.1^2 = 0.00067 times the amount of infrared. The apparent brightness of the sun from Neptune is 1/900th the brightness of the sun from Earth. From Aurora, Barnard’s Star would look about 8% (.0816) as bright as our sun does from Neptune.
Everything else about Moira is speculation and is discussed here.
Lagrange Points
Pamex, China, and India agreed in the Treaty of Bangalore, following the Shipyard War, that their three colonies would be established in the Barnard System at the Moira-Pogo Lagrange points, respectively L4, L5, and L1.
Lagrange points, named after Joseph-Louis Lagrange (1736 - 1813), are the locations in space where a small object, like a satellite or asteroid, can stay in a fixed position with respect to two large orbiting bodies. At Lagrange points, the gravitational forces of the two large objects and the centripetal force of the smaller object are balanced.
There are five Lagrange points for any pair of large astronomical objects, labeled L1 through L5. The image below illustrates the normal arrangement. In this image, the two large objects are the star and the planet on the right. L1 is between the two objects; L2 is on the other side of the planet; L3 is on the other side of the star; and L4 and L5 are at 60 degrees ahead of and behind the planet along its orbit around the star. In the Moira/Pogo system, the arrangement would be the same except that the two large bodies are the planet and moon.
Lagrange points in a star system.
Lagrange points have varying levels of stability. L1, L2, and L3 are in “unstable equilibrium,” meaning that an object right at the Lagrange point is stable, but a small perturbation one way or another will make it “fall” toward one of the two larger objects. It is analogous to being on top of a hill—if you get nudged away from the very top, gravity will start pulling you down the mountain. Despite this, L1-3 are attractive “parking” places because they require very little fuel to stay in place—just a little nudge back in place if you start to drift.
The L4 and L5 points are in stable equilibrium, meaning that when an object is slightly displaced, it will tend to return to the starting point. The analogy is to being at the bottom of a bowl—when you get pushed up and away from the center, gravity will push you back to the center.
Trojan Asteroids
When you learn about the asteroids in the Sol system, it’s usually in the context of the “asteroid belt between Mars and Jupier,” and you may envision a band of rocks between those two planets where another planet might have once been. In fact, asteroids are plentiful throughout the Sol System, as the image below shows. Current estimates are that in the belts there are over one million asteroids larger than one kilometer in diameter and many millions, perhaps hundreds of millions, of smaller ones. This image may give the impression that there are dense clouds of asteroids, such that if you flew a spaceship through them, you’d need to swerve and dodge like the Millenium Falcon in The Empire Strikes Back. However, on average, asteroids are more than a million kilometers apart—an illustration of the vast distances in space.
Asteroids in the inner Solar System.
An understanding of Lagrange points helps explain the location of many asteroids. An asteroid that finds itself in one of the L1-3 points will tend to “fall off the mountain” (as discussed in the Lagrange section above). Hence, no natural asteroids are found at the L1-3 points. On the other hand, when the Solar System formed, some asteroids collected naturally in the area of the L4 and L5 Lagrange points.
There are at least several million Jupiter L4 and L5 asteroids, with more than 10,000 larger than 2 km wide. The first of these were discovered in the early 1900s by E.E. Barnard and were named after specific heroes from the Trojan War. But over time, all asteroids at L4-5 locations anywhere came to be called trojans. Nine have been discovered at Mars, 28 at Neptune, two at Uranus, and even two at the sun-Earth Lagrange points.
Trojans can also exist at Lagrange points for planet-moon systems, and Saturn has trojans in the Saturn-Tethys and Saturn-Dione Lagrange points.
Barnard's Star is only 14% of the mass of our sun, and the trilogy takes place around its largest planet, a gas giant named Moira with a single, large moon, named Pogo. Because of the star’s small size and the relatively large mass of Moira and Pogo, we posit a large population of trojans in the Moira-Pogo Lagrange points. These are where the colonies have gathered most of their materials in the 83 years since founding.
Colony Rotation
Aurora consists of two counter-rotating tubes lined up end to end.
The question of whether and why the tubes need to counter-rotate is complicated.
The most rigorous study of a theoretical cylindrical space colony was led by Princeton professor Gerard K. O’Neill in 1976. You will often see it stated that O’Neill and his students wanted a pair of counter-rotating cylinders to prevent instability. This may seem to make intuitive sense, possibly because it matches the way helicopters work—they can’t have just a single rotor or their bodies would start spinning, so they must have either a vertical tail rotor or counter-rotating main rotors, as shown in the images below respectively.
Tail rotor of a Sikorsky S-61, which counteracts the torque effect of the main rotor and keeps the body from spinning.
Kamov Ka-50 of the Russian Air Force featuring a distinctive coaxial rotor. The two rotors rotate in opposite directions, which prevents the body of the helicopter from spinning, and precludes the need for a tail rotor.
But this issue for helicopters has nothing to do with O’Neill cylinders, since the whole cylinders are moving.
It is also often stated that O’Neill needed counter-rotating tubes to avoid the odd Dzhanibekov Effect, which has its own fascinating history—its “discovery” by Soviet cosmonaut Vladimir Dzhanibekov was kept classified by the USSR for 10 years (www.youtube.com/watch?v=1VPfZ_XzisU). This effect applies to spinning objects in freefall when they rotate around the “intermediate” of their three axes. It can be observed in spinning books and tennis rackets, and is often called the tennis racket effect. But an O’Neill cylinder, due to its symmetries, does not have an intermediate axis and is not subject to it.
O’Neill’s design actually needed counter-rotating cylinders to cancel out the gyroscope effect so they could easily be reoriented, because its giant hinged mirrors needed to be continuously pointed at the sun as the cylinders revolved around the Earth. The gyroscope effect would make them resist being turned. O’Neill went further and designed a clever scheme that used changes in rotation rate and distance between the connected cylinders to turn the whole system through 360 degrees without rockets, in effect using the counter-rotating tubes as reaction wheels.
In the timeline of our novels, Earth began construction of its first small (800 meters in diameter, 2,400 meters long) O’Neill cylinder in Earth orbit in 2071, after humans spent several decades learning to mine, refine, and manufacture resources from near-Earth asteroids, and when it was clear that space-based solar power would require thousands of workers and their families to be housed in orbit. They immediately abandoned O’Neill’s mirrors as impractical, opting instead to derive their power and light from the same space-based solar power arrays that were feeding power to the planet. The design was for a gyroscopically stabilized 2.7-rpm single cylinder, generating ⅔ g and housing 6,000 people for up to two years at a time.
Over the next century, more and larger cylinders were constructed, both in and beyond Earth orbit, and many became real settlements, not just long-term housing. Much was learned about self-supporting ecologies, habitability, internal and external transportation, construction, and defense against natural and military threats.
The first settlement to be built in the asteroid belt required the flexibility to move around to avoid errant asteroids and to periodically change orbit to new areas of the belt, and so counter-rotating tubes were reintroduced to cancel the gyroscope effect. But with no need to align mirrors toward the sun, the two cylinders could be placed end-to-end, which greatly simplified the connection mechanism and also made it easier for passengers and vehicles to move between the cylinders, and for ships to transition to and from the launch platform.
When the time came in the mid-22nd century to design the colony that would become Aurora, an almost perfect model— twin cylinders, each 8x23 km, almost totally self-supporting—had already been built in Sol’s asteroid belt.
Artificial Gravity
The tubes rotate once every two minutes and 6.9 seconds, or 0.4728 rpm (when characters in the book describe this they usually round off to 0.5 rpm). This rotational speed is set to simulate gravity equal to that on Earth’s surface. It is “simulated” because it is not caused by gravity. There are two ways to understand how this works. The simplest is to imagine spinning a rope over your head with a rock attached to the end, like David’s slingshot. The faster you spin it, the harder the rock at the end of the rope pulls against your hand. While the O’Neill cylinder is not spinning at the end of a rope, the effect of anything that is on the inside of the tube is the same as the rock at the end of the slingshot.
Another way to think about it is to look at the forces at work. When you stand on the ground on Earth, its mass generates the gravity that pulls you towards its center. If the planet had less mass, it would pull less and you would feel lighter. An O’Neill cylinder, while large for a manmade structure, has an insignificant gravitational pull (technical note: you are actually inside most of its mass so it wouldn’t have any effect anyway). But the tube is spinning, so the surface of the cylinder (and you when you're standing on it) have a velocity, which is easily calculated: the circumference of the tube is 2𝜋r and r is 4,000 meters, so the circumference is about 25.12 kilometers. The tube takes 126.9 seconds to make a full turn, or 28.37 turns an hour. So any point on the tube will travel 25.12 kilometers 28.37 times in one hour. Thus, the velocity is approximately 713 km/h (443 mph).
You don’t sense this forward velocity because everything—the “ground” and air—is moving with you at a constant speed. But you do feel it. Your body has momentum and like any mass in motion it wants to go straight ahead. If you were on the outside of the tube, that is exactly what would happen—you would go flying in a tangent at 713 km/h, straight off the tube.
But you are inside the tube. Your body tries to go straight, but the tube curves up and keeps that from happening, constantly pushing you up. You feel this as a force against your feet, which at this exact tube size and spin rate equals the force of gravity on Earth. (Purists argue that it should be called simulated weight, not gravity, and they’re probably right, but it doesn’t matter for this discussion, and in the books we just refer to full “gravity” as 1g.)
If you were to travel toward the center of the tube, the spin rate would stay the same, but the radius would decrease, so this force would too. The simulated gravity would decrease in direct proportion to your altitude. Midway between the ground and the center—at 2,000 meters above “ground”—you experience one-half g. When you reach the very center, your velocity is zero, and you are weightless.
If you were on a much smaller O’Neill cylinder—say 40 meters across—you would actually notice the gravity difference between your feet (1g at the floor) and your head (0.89g at 180 cm above the floor). This would have noticeable vestibular effects every time you move.
If the tube were a bit larger, say 200 meters, the vestibular effects would be less pronounced, but you would still experience some unusual, possibly disorienting effects when you move from ground level to the center of the tube or back down. For example, if you travel from the center to the ground at 25 meters per second (90 kp/h or 56 mph), you would gain gravity at the rate of .25g’s, and it would feel like you were accelerating despite going down at a constant velocity. But with Aurora's 8,000-meter diameter, you would have to be traveling at very high speeds before this particular effect would be noticeable. Coriolis forces, though, are another matter, as we explain below.
The Coriolis Effect
Residents of rotating habitats experience Coriolis effects, in which objects appear to move in non-intuitive ways. It is called the Coriolis effect rather than the Coriolis force because it does not actually change the motion of objects. It just changes the way objects appear to move to observers who are rotating with the habitat. It is sometimes called a virtual or fictitious force.
If you live on a planet like Earth, Coriolis effects are noticeable in the weather and airplane flight paths, but are generally imperceptible in everyday life. This is because Coriolis effects are proportional to rate of rotation, which on Earth is just once per 24 hours. The Auroran cylinders have a rotation rate of 30 times per hour, 720 times that of earth, so the effects are more pronounced. If you were on a much smaller rotating habitat, e.g., a 100-meter-wide space station that spun at 4 rpm, the Coriolis effects would be strong enough to disrupt many ordinary activities. Aurora’s spin rate is low enough that the effects are not noticeable to most people most of the time.
On Earth, as you move north or south from the equator, the ground’s sideways velocity decreases from 1,670 km/h to zero at the poles. For example, at the 40-degree latitude of Denver or Rome, it is 1,280 km/h. The Auroran tubes are not spheres so they have no poles or equator. But the sideways velocity in Aurora decreases from 713 km/h at the ground (see calculation above in "Artificial Gravity") to zero at the center. Imagine a thin wire in the exact center of the tube. It will still be turning once every two minutes, but the wire’s surface will barely be moving.
This is where the Coriolis effect comes in. If you want to travel from the ground to the Axle, you will need to gradually decrease your sideways velocity from 713 km/h to zero, otherwise you will rapidly curve off in the spinward direction of the tube. Any ascending entity has to lose roughly 18 km/h of velocity per 100 meters, and conversely, any descending entity has to gain 18 km/h per 100 meters. If this is done too rapidly, riders will suffer from motion sickness, because the sideways acceleration sensed by the vestibular system will not match what the body perceives from its other senses.
Everyone will have a different level of sensitivity to this effect, based in part on age and habituation, so they get a V-rating that hoppers use to adjust their velocity and flight paths. The shortest distance between any two points on the tube (unless they are directly north/south of each other) will be in the air, but hoppers seldom fly exactly that path.
Elevators, for both ships and pedestrians, do not have the luxury of adjustable flight paths. They adjust seat angles to partially compensate, but in general just go slower than they otherwise would, Some people are unable to use them without experiencing symptoms.
Coriolis effects make themselves apparent in other ways as well. Many sports are significantly affected. A golf ball hit off a tee at 230 km/h will have a significant curve that will vary with direction, speed, and height. To a lesser extent, the same will be true for a batted baseball and a kicked soccer ball.
Access to the Tubes - Spinning Up and Down
Pedestrians and hoppers need to be able to move between A and B Tube, which spin in opposite directions. And ships need to move from the hangar (Ascension Station) to the stationary launch/landing platform and back.
Let’s say you take an elevator or a hopper from the ground level up to the Axle. Because you’re now at the center of the tube, gravity has pretty much disappeared. But you’re still spinning. So you can’t just walk or fly across the platform to the other tube.
The key to the transfer process is the Spinner, a cross-shaped platform with four arms that are each 60 meters long. Two arms serve ships while the other two are split between pedestrians and hoppers. The “tops” of the arms are for departures, the “bottoms” for arrivals.
The Spinner spends 55 seconds synched with the rotating tube, during which departing people and vehicles “park” themselves, while the arrivals disembark. The tube makes half a turn per minute, so during this time, to those on the Spinner, the launch platform appears to be slowly rotating (there are expansive windows that look out into space). Fifty seconds after opening, the gates close. Five seconds later the Spinner starts turning and matches the rotational speed of the platform, then attaches to it, and ships, hoppers, and people embark and disembark. Fifty-five seconds later it detaches, then matches rotation with the tube, and the process repeats. Relative velocities are small at this radius, so these changes in spin have little effect on the almost weightless passengers.
Pedestrians
Pedestrians going from A Tube to B typically take an elevator to the staging area in front of one of the Spinner arms. Because they’re less than 50 meters from the middle of the Axle, they’re in microgravity. A pen dropped from shoulder level will take about 5 seconds to land at their feet.
They use vertical poles to pull and push themselves to a spot in a cab on one of the Spinner arms. They naturally stand such that their feet point to the end of the Spinner arm, their heads toward the Spinner center. They then strap down because once the Spinner aligns with the Platform, they will be in true zero-g.
In front of them they can see outside the tube, the platform slowly rotating. They may also see ships landing or taking off. As the Spinner spins them down, the view outside stops rotating.
At the platform, the few who are staying at Ascension Station—typically those who work there—disembark. The rest stay in their cabs, which slide on a track across the platform to the other tube. The distance is 3,300 meters. With their feet pointed back toward A Tube, each cab is gently accelerated at 0.2 g. This lasts for 37 seconds, at which point they’re traveling about 266 km/hr. Then they coast for 6 seconds as the room is rotated so their feet face B Tube, and the room is decelerated at the same .2g until it comes to a stop at the B Tube Spinner. This trip takes 80 seconds. Finally, the Spinner spins them back up so they can enter B Tube.
Hoppers
Hoppers fly though the Axle and land in a staging area while they wait for the Spinner. The “parking spaces” are stacked vertically in a ferry cab, with the hoppers’ canopies toward the Axle center and their undersides facing the “ground.” This way they are aligned with the microgravity. Like the pedestrians, they can see a rotating view of the launch/landing platform outside the tube.
When the Spinner is stopped and in position, the ferry cab is shifted across to it. The Spinner slows down to align and lock in place with the stationary platform. The ferry then slides across the platform to B Tube, where the hoppers are spun back up in the opposite direction.
Ships
In many sci-fi movies, ships land on space stations by flying in through huge doors and landing in cavernous hangars. To do this with a rotating station, the ship would have to match the station’s speed and rotation. In practice, such maneuvering would be complex and dangerous, so it would require a lot of room, and all that air handling is expensive and time consuming. Thus, from a safety, cost, and time standpoint, it makes more sense to have a spacious, zero-g, stationary landing platform, and then run the ships through relatively small airlocks and into the hangar.
Ascension does have some airlocks that feed directly out to space for launching small military fighters, as well as limited capability to land them as well, in case there’s an attack on the platform. The Navy practices these maneuvers regularly.
Ships take elevators up from Ascension Station and are then slid to the Spinner and placed on the arms perpendicular to the ones used by hoppers and pedestrians. After being spun down, ships are rolled into airlocks and then out onto the platform in vacuum. They take off with small reaction thrusters, and only ignite their fusion engines when they are well away from the platform and other ships to avoid damaging them.