Section 4.14: Later Program Phases
The remaining program phases (4D, E and F; 5C, D, and E; 6A and B) are too undeveloped at present to devote full sections to them. For now we will gather our early ideas about these program phases here in one place, pending further concept exploration work.
Phase 4D - Main Belt and Trojan Development
The first known asteroid, 1 Ceres, happened to be discovered on the first day of the 19th Century - 1 January 1801. Through that century 462 more were discovered, and by 1951 the count had reached 2,158. Since then, larger telescopes, electronic sensors, and automated analysis have greatly increased the known population. It reached 28,000 by 1995, 280,000 by 2005, 750,000 by 2017, and is still rapidly increasing (Figure 4.n-1). Their locations were originally concentrated in what we now call the Main Belt between Mars and Jupiter. Asteroids are now known to exist all over the Solar System, from inside the orbit of Mercury to far beyond Neptune. By count, the largest number are still located in the Main Belt, but this may be observational bias. Ones that are farther from the Sun are dimmer, so we tend to only find the larger ones. Ones that are closer to the Sun are hard to see due to interference by the Sun itself, and the fact we are looking at their unlit side.
Although asteroids occur everywhere in the Solar System, for program purposes we divide them into four regions by distance from the Sun, with a separate phase for each. This is due to variations in solar flux, temperature, and other environment parameters, and differences in average composition. These features will drive different designs for each region. Development of the Main Belt and Trojan region is an extension of work in the Inner Interplanetary region of Phase 4C, which starts earlier because it is closer to Earth. Both regions have objects with a range of orbit eccentricities (Figure 4.n-2). So each object varies in distances from the Sun and orbits as a whole overlap, making the region boundaries fuzzy. We set an inner limit for this region just beyond Mars at a semi-major axis of 1.8 AU, where the density of Main Belt asteroids significantly increases, and the outer limit at 5.4 AU, where the density of the Jupiter Trojan group falls off. This is an arbitrary choice, but it includes a very large number of smaller bodies in similar environments. Therefore we can develop a shared set of designs across the region.
The region includes the Main Belt asteroids, with a core region between 2.1 and 3.3 AU where their density is highest. It also includes the Hilda Group which are in 3:2 resonance with Jupiter. Their orbits are between 3.7 and 4.2 AU from the Sun. The final major group are the Jupiter Trojans which occupy the Lagrange regions ahead of and behind Jupiter. They share the same average distance from the Sun as Jupiter, in the range of 5.2 ± 0.15 AU. There is a relatively small percentage of the total region population that doesn't fall into any of these major groups. The region does not include Jupiter itself and orbits within 20 million km of the planet (see Phase 5D, below).
Historically, asteroids and comets were regarded as separate classes of objects. We now know some objects are actually former comets which have lost most of their volatiles, and now look like asteroids. Some objects identified as asteroids are still releasing vapor, notably including the largest one, Ceres. It is therefore reasonable to consider all small bodies in the region as a single class, with a range of compositions and solar distances. We will use the name "asteroids" for all of them, since they are by far the largest in number. Objects traditionally called short period comets, with semi-major axes between 1.8 and 5.4 are included under our asteroid heading, but make up only 0.1% of the total population.
Asteroid sizes range from 945 km in diameter for 1 Ceres, which is now counted as a dwarf planet, down to Interplanetary Dust of sub-millimeter scale. The dust component is short-lived and does not account for much of the mass in the region. About one million tons/second of solar wind particles flow through the region at high velocity, but the flow is very diffuse, on the order of 1 nanogram/km2/s. Total mass in the region is about 3 x 1018 tons, which is equivalent to 16 million years of Earth's total current mining output. All of it is available to determined mining efforts, because the low gravity on the asteroids creates low subsurface rock pressures. About half the total mass is in the four largest objects: 1 Ceres, 4 Vesta, 2 Pallas, and 10 Hygeia.
Asteroid compositions vary considerably due to differences in their formation and history (DeMeo, 2015). Between spectroscopic observations, and examination of meteorites, many of which are fallen pieces of asteroids, we can identify a number of composition groups. However, only a few asteroids have been visited by spacecraft, so detailed verification of their compositions is yet to be done in most cases.
Velocity to reach orbit from the largest body, Ceres, is only 270 meters/second, or 860 times less kinetic energy than from Earth. So materials from these asteroids are easy to export once you are near them. The main energy cost is changing orbit around the Sun to reach them. Solar power is available 100% of the time in the region, except shadowed areas around and on the asteroids. Intensity varies from 31 to 3.4% of that near Earth. Ambient temperature varies from 244 to 217K (-29 to -56C) for black objects, and less for lighter colored ones. Travel time from Earth is typically years using least energy trajectories, with high to lethal radiation levels for unprotected people. Communication time from Earth varies from 13 to 120 minutes round-trip, including a relay when needed to avoid a direct path through the Sun.
This region is nearly devoid of spacecraft at present, so most uses are in the future. Abundant raw materials of diverse composition, and adequate amounts of energy when concentrated, will enable mining as an early activity. Materials would be shipped to previous regions at first, which are more developed and have higher solar intensity for further processing. When it makes sense to do so, seed factories can help bootstrap a full range of local industry, and eventually large scale habitation. There is enough total raw materials and energy in this region to support a full civilization.
The largest object in the region is the dwarf planet 1 Ceres. Equatorial orbit velocity is 359 m/s, and equatorial rotation velocity is 94 m/s. So to reach orbit requires 265 m/s net. This velocity can be reached by a mild steel centrifuge, and easily reached with any advanced material. Therefore bulk material launch from any other Main Belt asteroid, all of which are smaller than Ceres, does not require any rocket propulsion. A 1-g Skyhook would be 7 km in radius for Ceres, and allow crew and equipment to be landed and take off from at low acceleration, and a cost of 0.5% of net mass flow in reaction mass to maintain orbit. So surface access for any asteroid should not be difficult. For the smaller bodies, the operation is closer to docking in zero gravity than landing from orbit.
For smaller asteroids staying on the surface will be more of a problem than getting on and off. For example, the 35th largest asteroid by diameter is 9 Metis, which has an equatorial radius of 170 km and a mass of 1.47 x 1019 kg. This gives a surface gravity of 0.034 m/s2 (0.34% of Earth). The rotation period is 5.08 hours, which give a rotation velocity of 58.4 m/s and a centrifugal acceleration at the equator of 0.020 m/s^2. So the net effective gravity is only 0.014 m/s (0.14% of Earth gravity). Indeed, the orbital velocity is 76.0 m/s, so it only takes 17.6 m/s ( 39 mph ) added velocity to reach orbit. Therefore humans or low speed machinery can toss things into orbit, and a firm anchoring method will be needed to not have equipment move accidentally.
The inner parts of the region have enough sunlight for solar panels to produce power directly. In the outer portions, solar panels will benefit from reflectors to increase the light intensity. Concentrating reflectors can produce high temperatures at all distances, either for industrial processes or warming habitats. Increasing amounts of reflectors are needed as you get farther from the Sun, but they are inherently low mass in a zero gravity environment with no weather. Note that the total amount of solar energy available in this region is no larger than for the Inner Interplanetary region, and equal to the total output of the Sun, which is 3.83 x 1026 W. It is the same total solar flux, only more spread out as it increases in distance. The difference is the Main Belt & Trojan region has more raw materials available than the inner region.
Asteroids are generally covered in a mixture of rocks and dust of varying sizes. This is the result of repeated impacts over their life and gravitational attraction of loose orbital material. In fact, some asteroids are so low in density that they must be "gravel piles", with no solid central body. The loose material makes surface mining easy, but at the same time most asteroids are small. The rocks and dust are easily disturbed and can become a hazard to mining and production operations. So attention has to be given how to carefully remove materials without too much disturbance. They are then moved elsewhere by a tug, or to a nearby processing plant out of range of any dust clouds created. For larger operations, an inflatable or assembled shell can surround the whole asteroid, keeping dust contained. Processing equipment can then be attached to the outside of the shell, and materials delivered continuously until the asteroid is consumed. Because dust and debris is contained, more vigorous mining methods can be used. Mining and processing methods should have been developed earlier for the Inner Interplanetary region due to similarity in asteroid sizes and types. The one difference is the larger size of some bodies in the Main Belt and Trojan region, making their gravity significant enough to matter in design.
Habitats for this region can start with unmodified designs from the previous regions, except with the addition of reflectors for increased power and keeping warm. Early units can be delivered whole from inner regions and moved gradually into this region over time. With continued access to nearby asteroids for supplies, there is no need to deliver them all at once. Once in place at a good location, such as orbiting Ceres, an early habitat can grow by making and assembling structural parts for larger habitats, then a series of shells of increasing size. Ceres is in the middle of the densest region of the Main Belt, so supply trips to nearby asteroids with different composition will be relatively easy. This makes it a good candidate for starting large-scale development of the region.
The same transport methods can be used in this region as for the Inner Interplanetary region. The main difference is adding reflectors to solar panels, or larger reflectors to thermal power units, to make up for the lower solar intensity. Centrifugal transport hubs are somewhat more efficient for injecting bulk cargo to transfer orbits, because they do impulse transfers rather than spiral orbits. If a large asteroid absorbs the reaction force, they also don't need propellant to send cargo on their way.
The first service functions in the region will be communications, scientific exploration, and prospecting, to locate and define available resources in detail. Other services are to be determined later
Phase 4E - Outer Interplanetary Development
The Outer Interplanetary region is the third such region to begin development, after the Inner Interplanetary (Phase 4C) and Main Belt and Trojan (Phase 4D) ones. It is the next in physical distance, covering orbits with semi-major axes from 5.4 to 50 AU. It excludes the major planets Saturn, Uranus, and Neptune, their moons, and an orbital region around each, which are assigned to Phase 5E. Only a few spacecraft have reached this region by 2017, and most were directed at the major planets and Pluto, so it is largely unexplored. Most of our information to date comes from astronomical observations on or near Earth.
The first object found in this region was the dwarf planet 134340 Pluto, in 1930. As of late 2017 the known population has grown to about 275 in the Centaur group (Figure 4.n-3), and nearly 1800 in the Kuiper Belt group (Figure 4.n-4) beyond Neptune. The Centaurs have orbits between or cross those of the four Gas Giants, including Jupiter. This tends to make their orbits unstable and short-lived. The Trans-Neptune group as a whole spends most or all of their time farther than Neptune, so their orbits are more stable. The large number of objects in the inner part of the Trans-Neptune group, from 30-50 AU, are referred to as the Kuiper Belt. There are also about 20 known Trojan objects whose obits are tied to the outer Gas Giants, mostly Neptune, and about 240 short- and long-period comets in the region.
For our purposes we group asteroids and comets in the region together as forming a continuous range objects with varying orbits and compositions. Comets are distinguished by sometimes coming close enough to the Sun to actively lose gas and dust. Historically this made them easy to spot. But at other times they are as inactive as asteroids, which keep more consistent distances. Current telescopes have a hard time finding inactive objects in the region which are smaller than 10 km in diameter, so our count is incomplete and continues to grow.
About six of the known objects in the region (Pluto, Makemake, Haumea, Orcus, Quaoar, and Varda) are large enough to be considered dwarf planets, and about 675 are estimated to be larger than 100 km in diameter. Total mass in the region is estimated at 240-600 x 1018 tons (4-10% of Earth), which is a very large amount of total available material. Except for the deeper parts of the largest bodies, most of this is theoretically accessible. Due to generally low gravity and density, the subsurface pressures are not too high for mining operations, and the interior temperatures are probably not too high to be a problem. The Centaurs are likely to be of mixed composition. Since their orbits are unstable, they originally came from elsewhere, where the conditions of their formation were different. Water ice and carbon compounds have been detected on several of them.
The entire region beyond Jupiter is outside the Frost Line, the distance in the original Solar Nebula where water ice could condense. Therefore water is common in the region, and other ices, like methane, ammonia, and nitrogen, are present in the outer portions, where the local temperatures were cold enough for them to also condense. Since the Solar System's formation, the opaque Solar Nebula has dispersed, and the Sun has gradually brightened, increasing temperature at a given distance. So surface materials which were originally stable have since evaporated. They can survive to the present deeper within objects. Changes in orbit since their formation will also have affected what remains in these objects. The larger bodies have undergone impact heating during formation, and radioactive heating afterwards. This causes them to separate into layers by density. Nominally this would be metallic and rocky material towards the center, and icy material towards the surface. Smaller impacts and exposure to solar ultraviolet and other radiation may have modified the surface layers. Since very few of these objects have been explored close-up, we can only speak in generalities at present. Much more exploration and prospecting will be needed before we can start to use the materials from this region, and begin local development.
Escape velocity from Pluto is 1.2 km/s, and less for smaller objects. This is well within the reach of mechanical transport. The minimum velocity to reach the outer parts of the region from Earth is near Solar System escape, or 12 km/s. Such orbits will take over 60 years, so faster transport using more energy is desirable. The dominant energy cost in using the region is then first reaching it. Gravity assists and advanced propulsion will be needed to access the region in reasonably short times, or a lot of patience.
Available total solar power is the same as for the previous two regions, being the total output of the Sun. The intensity per area, however, is low, from 3.4% to 0.04% of Earth orbit values. This would require large reflectors to increase intensity, or using nuclear or other power sources instead. Ambient temperatures are very cold, from 217 to 70K for black objects, and lower for lighter colored ones. Travel time from Earth will typically be many years. Unprotected radiation levels are high to occasionally lethal for people, and damaging over long periods for equipment. Round-trip communications time is 1.2 to 13.6 hours on a direct path, and slightly higher if a relay is needed to avoid the Sun.
This region is likely too far to do much beyond scientific exploration with present technology. When civilization has expanded through the previous regions, and better technology is available, the first use is likely to be mining the large sources of raw materials. They would be brought back to inner regions, where there is more energy density to process them and use them for other activities. The combination of low temperature and sorting by density makes the various ices the most accessible early resource. Use of this region is far enough in the future that technology is likely to improve dramatically in unexpected directions. So any concepts we present for this region should be considered very preliminary and likely to change.
We don't expect a lot of production besides mining in this region until technology significantly improves. Ices like water and nitrogen are very useful to people, and available in large amounts in the region. So mining and transport to the inner regions is a possibility once there is enough demand. Transport would be slow, taking many years, so there is an incentive to set up a "pipeline" of cargo in transit, with vehicles at each end to set it on course and collect it at the end. The cargo can travel unattended in between, saving on vehicle time. Once the pipeline is filled, then cargoes arrive on a regular schedule. If fusion is well developed, a fusion-based economy may develop, with full production and habitation. We don't see a strong reason to live this far out, rather than staying in the warmer and brighter inner regions, but such reasons may develop.
A challenge for the Kuiper Belt and farther regions is supplying enough solar energy to operate. Civilization on Earth consumes about 2.7 kW/person, and we would expect a higher number for space locations, both due to higher standard of living, and the need to do artificially things handled by natural processes on Earth. Let us assume 20 kW/person is needed, system mass is double that for the ISS, or 150 tons/person, and half is devoted to solar collection. If magnesium-aluminum reflectors 1 micron thick are used to concentrate sunlight, they will have a mass of 2.4 tons/km2. So we are allowed a maximum of 31.25 km2 of reflectors/person. For a net power of 20kW at 1/3 efficiency, we need 60 kW of sunlight. At Earth, solar flux is 1.361 kW/m^2, so we need 44 m2. Since we are allowed 711,500 times this area, and solar flux falls as the inverse-square of distance, we can provide sufficient solar energy out to 843.5 AU, a surprisingly large distance. Beyond this, operations would limited to low power situations, or require other sources, like nuclear or beamed energy.
Due to weak sunlight in this region, we expect that nuclear powered propulsion, and gravity assists from the larger bodies, would be major ways to get around. If nuclear fusion has not been sufficiently developed, fission would be the only available nuclear source. There is a finite known supply of suitable radioactive elements on Earth and the Moon. To supplement them, artificial radioactives can be produced near the Sun, where abundant energy can power accelerators to convert non-radioactive starting materials. If nuclear fusion is well developed, there is abundant hydrogen in the region from which fusion fuels can be extracted. As distance increases from the Sun, orbit velocities, and so the required orbit velocity changes, decrease as the square root of distance. Solar flux decreases faster, as the inverse square of distance. So solar sails become will become less effective as a transport method than for closer regions.
We note a few features about 136108 Haumea, a large object in the outer part of the region. Haumea is massive enough to be in hydrostatic equilibrium, and therefore is classed as a dwarf planet. However, the short rotation period (3.9155 hours) means it is not round, but rather ellipsoidal, with a long axis about twice that of the short axis. Circular orbit speed at the long ends is ~527 m/s, while the tips themselves rotate at ~428 m/s. So only ~99 m/s velocity change is needed to land or take off from it, one of the lowest numbers for a large Solar System object. If Haumea retains any sort of atmosphere, it would tend to be in a wedding-band shaped ring around the short axis. Gravity would also vary significantly from the long ends to the short axis.
Phase 4F - Scattered, Hills, and Oort Development
The vast space beyond the Kuiper Belt is the fourth and last interplanetary region to begin development. It includes orbits with semi-major axes from 50 AU to the limits of the Sun's gravitational dominance, which we set at 100,000 AU. Although it covers a huge range of distances, it is a small range in energy when measured from Earth, covering the last 2% relative to reaching solar escape. Only four spacecraft have entered this region by 2017, after completing their primary missions closer in, with a fifth to enter it in a few years. So nearly all of our information comes from observations on and near Earth.
Long-Period and Near-Parabolic Comets, whose orbits are large enough to be counted in this region, have been seen since ancient times. Determining their orbits are in fact so large had to await the development of orbital mechanics and better telescopes. They are easily seen when close to the Sun. They emit large amounts of gas and dust when heated, creating a coma and tail which can extend millions of kilometers. At large distances, they are cold, dark, and inert, and therefore much harder to find. So the first object in this region that wasn't an active comet, (48639) 1995 TL8, was not discovered until 1995, and that one because it is relatively large - about 350 km in diameter for the primary and 160 km for its satellite.
The known population of objects in the region includes 100 long-period and 420 near-parabolic comets, and 440 Scattered Disk Objects, whose orbits lie entirely beyond Neptune and are therefore relatively stable. Four of these have maximum distances greater than 2000 AU. This places them in the Hills Cloud, whose orbits range from 2000 to 10,000 AU in aphelion, beyond which is the Oort Cloud, which extends to 100,000 AU in semi-major axis. We have indirect evidence for a large population in the outermost areas, based on the known near-parabolic comets. Our ability to detect such objects is currently limited to ones which are within about 80 AU of the Sun. So we can only find objects from this region whose closest orbit point (perihelion) is in the 50-80 AU range and are currently at that the near end of their orbit. Since orbit speeds are lower at higher distances, the objects spend most of their time too far to see. We therefore expect to find many times more objects in the future.
The total mass is poorly known at present, but is estimated to be 4-80 times that of Earth, which is a vast reservoir of materials. This total includes a suspected, but as yet undiscovered, 9th planet with a mass possible in the range of Neptune's (~15 x Earth's mass). Since comets are from this region, and their evaporating gas and dust is easy to observe, we have a reasonable idea of compositions in the region, even though we can't directly observe most of it. It is most likely a mixture of water, other ices, complex carbon compounds, and some heavier mineral grains. Solar energy is quite weak in this region, below 0.04% of that near Earth, and ambient temperatures are below 70K down to near 2.7K. Travel time with current propulsion technology is many years to centuries. Round-trip communications time ranges from 14 hours to 3 years.
We don't have enough information about objects in this region to make detailed plans, and they are too far away to access with current technology. So anything beyond science and exploration are deferred to a future time when increased needs and better technology exist. When that time comes, though, there is a very large reserve of materials from the region that can be put to use.
We showed under Production for Phase 4E that enough solar energy is available even to 1000 AU from the Sun to sustain production and habitation. Beyond that, nuclear or beamed energy sources would likely be needed. Production beyond materials extraction must remain speculative at present.
To get transport times to the region to reasonable levels, very high energy propulsion would be needed, such as nuclear fusion. Since the light elements needed for fusion are common in these outer regions, this could be self-fueling once set up. Unfortunately, fusion is not yet a viable technology, so transport that uses it remains speculative at present.
Phase 5C - Venus and Mercury Development
Making use of Venus
My approach would be to turn some metallic asteroids into orbiting sunshades to block the Sun, and let the atmosphere cool down. A cooler atmosphere would have a smaller scale height (variation of pressure with altitude), which means the high points, like Ishtar Terra, would be even lower pressure and temperature. Once conditions get tolerable, you can start building on the high points, and expand over time to lower altitudes.
The currently hot surface can't hold on to CO2, but it is well known that certain minerals, like [Peridotite](https://en.wikipedia.org/wiki/Peridotite) can absorb CO2 and form carbonates. Depending what minerals exist on Venus' surface, this may happen automatically as the planet cools down, or we could accelerate it artificially.
Once the planet reaches a suitable average temperature, we would keep some of the sunshades in place to control it. They don't have to stay dumb shades, we can replace them with active solar panels, to drive industry in orbit, and beam power to the surface.
While the planet is cooling, we can scoop-mine the fringes of the atmosphere from orbit. It is primarily CO2 + 3.5% nitrogen. That can supply an oxygen-nitrogen atmosphere + carbon for various purposes, to supplement asteroid materials from elsewhere for orbital colonies. Since solar flux is 90% higher at Venus, there is plenty of power to run colonies and industry in orbit.
Phase 5D - Jupiter System Development
Depleting Jupiter's Radiation Belts - Placing sheets of material in the dense parts of the radiation belts may intercept enough of the particles to lower radiation levels there.
Phase 5E - Outer Gas Giant Development
Phase 6A - Nearby Interstellar Development
This region begins with orbits whose semi-major axis is 100,000 AU or more from the Sun, where our star's gravity is no longer dominant. For design purposes we set an arbitrary outer limit of 20 light-years from the Sun. If we can reach that distance, and restock/rebuild our equipment locally, then later projects can travel farther in increments of 20 light years using the same designs. The 40 light-year sphere surrounding our Sun in turn makes up the nearby portion of the Solar Neighborhood (Bovy, 2017), the part of the Milky Way galaxy around our star. The Nearby Interstellar region excludes other stars, the objects which orbit them, and the orbital regions of gravitational dominance surrounding them. The Phase 6A region volume therefore resembles the solid portion of Swiss or Emmentaler cheese (Figure 4.n-5), with scattered holes around stars not included. Natural or artificial objects which are moving fast enough not to be tied to any star, and are more than half the region boundary from any star (i.e. 50,000 AU for the Sun) are counted as interstellar. Otherwise they are considered temporary members of a stellar system.
The open space between stars includes a number of components. The most massive of these include Sub-Brown Dwarfs, which formed the same way as stars by the collapse of a gas cloud. They do not have enough mass for deuterium fusion, and therefore are not stars. Their masses range from 1-13 times Jupiter's (MJ). The lower bound is set by not having enough mass to collapse, and the upper bound by sufficient mass to initiate fusion, making them stars. Several such free-floating objects have been detected, which are not orbiting a larger brown dwarf or regular star. The other route to forming large interstellar objects is a planetary system which forms around around a larger star. Later gravitational interactions can eject some of the objects into interstellar space. The larger lost objects are called Rogue Planets, and can range from the same upper limit as sub-brown dwarfs (13 MJ) down to a lower limit where they don't have enough mass to assume a round shape. This is somewhat below 1000 km in diameter, at which point they are no longer considered planetary size. Rogue planets are distinguished by having a higher concentration of heavier elements than sub-brown dwarfs. This is due to more of the heavier elements tending to condense into planets, and the lighter ones tending to be blown away by the parent star.
Simulations of the history of the Solar Nebula (Shannon, 2014) indicate that about 80% of the original small bodies within 40 AU of the Sun were ejected into interstellar space. With over 3,500 confirmed Exoplanets by late 2017, we now know that formation of planetary systems is common around stars (see NASA Exoplanet Archive). So if the same ejection process happened for other stellar systems, then interstellar space should be filled with a large population of many objects from many stars. The largest of these objects would qualify as the rogue planets previously mentioned, but their size distribution should continue down to dust-sized particles. We define dust particles as those less than 1 mm in size. Their compositions would include comet-like icy objects and asteroid-like rocky objects, according to where they originally formed and later events in their history. Due to the relative velocities of their parent stars and the age of the Milky Way, objects currently within 20 light-years could have started anywhere in the galaxy, and even from outside it.
Only a few of the largest objects, in the Sub-Brown Dwarf range, have been detected by their infrared glow. Smaller objects will have rapidly cooled to ambient interstellar temperatures, and would have nearly no light reflected from nearby stars, making them extremely difficult to detect with current instruments. Therefore the population of these smaller objects is nearly unknown at present, and only roughly estimated from losses by our own Solar System. A possible future method for finding them is to use natural gravitationally focused light from stars, or artificial lasers, to scan around a region looking for reflections. Moving the scanner along interstellar paths would then build up a map of object locations. More investigation of this concept is needed to determine if it is feasible, and other detection methods should be pursued.
In addition to larger objects which formed independently, or were ejected from stellar systems, the Interstellar Medium between stars contains gas, dust, particles, and electromagnetic radiation which vary in density and temperature by location, and hypothetical Dark Matter and Dark Energy. We don't yet understand what the "dark" components are or how to use them. They are of scientific interest, but they can be ignored as far as our program is concerned. The Sun is presently moving through a region of slightly higher gas density called the Local Interstellar Cloud (Figure 4.n-6). It will continue to do so for the next 10-20,000 years. The local cloud has a gas density of about 0.3 atoms per cubic centimeter, or 1 gram per 564 km cube. This does not count any larger objects.
Stellar energy sources are too small in the region for practical use, except possibly along lines of gravitationally focused starlight. Ambient temperatures will mostly be close to the cosmic background temperature of 2.7K. Travel time is many years with known technology, and depends on future improvements to reach useful engineering time scales. Round-trip communication time will range from 3 to 40 years from Earth, and up to 80 years across the region. Stellar radiation is generally not a factor in this region, but cosmic radiation still is.
We don't yet know enough about the material and energy resources in the region to propose economic uses. Likely the region will be used for fast travel by self-contained vehicles to reach other star systems, or slow travel by permanent colonies, who use local resources as they go. The great distances from the Sun will likely detach any industries from regular trade with the rest of civilization. Science, exploration, and seeding interstellar colonies are possible future activities.
We don't know enough about resources in this region to consider gathering raw materials. Therefore the only production we can consider is aboard transport vehicles.
Interstellar transport can be divided into slow and fast types. The slow type is on the order of stellar velocities (5-500 km/s) using large habitats with large material reserves and fusion power as an energy source. These habitats subsist on the cometary clouds around stars and rogue objects between stars. When they get close enough to a selected star, they can enter orbit and travel with it. Travel times between stars would be 3000 years or longer. Fast interstellar puts much more energy into transportation, to reach higher velocity and shorten time to a destination. Possible methods include fusion powered engines and beamed power from the Sun. Rather than a large habitat with a full range of civilized activity, fast habitats operate more like ships on Earth, with a crew dedicated to reaching a destination and maintaining operations.
Phase 6B - Nearby Exostellar Development
Interstellar travel and planets around other stars have been explored in Science Fiction for a number of decades. The authors of such works can assume whatever transportation methods and planetary environments are needed for their purposes. Engineers considering development projects can draw ideas from fiction, but are limited to actual technologies and places places to implement our ideas. The last phase of our program to be started is development of the regions around other stars than the Sun. It logically follows Phase 6A, which is concerned with the regions between stars, since we must travel through such regions to reach other stars. It also follows all the earlier phases which are dedicated to various parts of our Solar System.
Like Phase 6A, we limit this one arbitrarily to within 20 light-years of the Sun. This includes a sufficiently large sample of stars and their attendant systems to address our designs to, and is large enough to identify problems caused by sheer distance. Development farther than this should be able to reuse most of the same designs. If new designs are needed, they can be handled by adding one or more additional phases at some later time.
The excluded regions are assigned to Phase 6B, which has more in common with the earlier phases close to our Sun than the spaces between stars.
Possibly the most significant feature of the region is that the stars which make it up are all in relative motion to each other, with an average velocity of 50 km/s. This motion is in addition to the general rotation of the Milky Way galaxy, which is about 225 km/s in this area. There are about 105 Nearby Systems within 20 light years, including our Sun. Given their average velocity, they will travel 20 light-years in 120,000 years, so the nearby population will change about every 1150 years on average. Current technology will require much more than this time to reach 20 light-years. So future plans need to take account of the motions of the stars and changes in the nearby population.
People assume that a "starship" will be a metal can with big engines on the back. Imagine colonizing a long period comet, one of the ones that came from the Oort cloud, and is heading back out there. Comets are made of a mix of ices (water, methane, ammonia, CO2, etc) and rocky materials. If there is not enough metals, get one of the metallic asteroids to match orbits with it. Then build your colony out of the materials there. Comets range in size up to 50 km in diameter, so there is plenty of stuff to build with.
The Oort cloud is many times the distance of the Earth from the Sun, and the velocity needed to get the comet to leave the Sun and head for another star is very small. All the ices have some amount of hydrogen, and thus deuterium, which means if you know how to build fusion reactors, you have power for a long long time. It will be a long trip, but you have a whole city worth of space to play in, with occasional side trips to other comets in the Oort cloud.
There are an estimated trillion comets out there, some will be along your route, more or less. The average spacing is something like 6 AU, about the distance to Jupiter. So you can in theory seed other comets as you pass by with new colonies. If some people feel like it, they could head back to the Sun, the velocities are low enough to do that.
The requirements for this kind of slow star travel are fusion power, and knowing how to build permanent habitats in space.
Gravitational lensing occurs around every massive object. In fact, measuring the bending of light during a solar eclipse 100 years ago was the first proof of relativity theory. For the Sun, the light bent from all sides comes to a focus at distances greater than ~540 AU. The focus is not to a point, but rather a radial line. This is because photons that miss the edge of the Sun by a larger distance are bent less, and thus focus farther away. Every star in the sky therefore produces a line of focused light on the other side of the Sun, and thus we refer to them as Starlines. Every other star also produces a pattern of starlines surrounding it, forming a network of lines filling interstellar space. These lines may prove useful for power and propulsion for distant missions.
(to merge) Phase 6 is the last major phase of our program, and is far enough in the future that we can only speculate about it in general terms. Interstellar space, the cold regions between stars, is not much different from the environment of the outer parts of the Oort Cloud in Phase 4F. We know very little as yet about equivalents to the Oort Cloud around other stars, or wandering objects not attached to stars. We have better information about planets and disks around other stars. Their parent stars tell us where to look, and the stars themselves provide data about the planets from Doppler shifts and transits. The number of discovered planets is growing rapidly, from none before 1988 to about 2000 by the end of 2015. We expect there to be smaller objects in systems with planets, but we can only see them directly if they form a thick enough disk.
The key difference that warrants a new phase is the extreme distances in this phase. This mostly breaks the ability to deliver things from, and communicate with, the Solar System. Expansion of civilization to these regions would require high self-sufficiency in transport and good enough seed factories and starting materials to enable growth without assistance. The Sun acts as a gravitational lens, with a focus around 800 AU from the Sun, in the Scattered Disk region. Placing telescopes directly opposite a star of interest would allow much more detailed observations than otherwise possible, because of the 2 million km optical diameter of the Sun.
- Exostellar Features
We define Exostellar regions as those surrounding individual stars or multi-stellar systems, of which there are 85 within 20 light years, with 127 stars. The size of the regions are scaled to the square root of the system mass divided by Sun's mass, times 100,000 AU. This accounts for their region of gravitational dominance and any cometary cloud they have. Four of these systems currently have known planets, but ten more are suspected, and data is incomplete at present. Two of these star systems, epsilon Eridani and Tau Ceti, have known circumstellar disks. More study is needed with better telescopes before any attempt to plan travel to these stars.
- Economic Uses
Due to extreme distance, the only economic uses we see for now are science, exploration, and seeding independent colonies.
- Exostellar Transport
Transport between stars is covered under Phase 6A in the previous section. Travel within a given stellar region would use the same technologies as around the Sun, with modifications for available energy sources.
- Exostellar Production
As mentioned earlier, we would want to observe the nearby stars in more detail by using the Sun as a giant gravitational lens. Following that would likely be robotic probes to more closely examine whatever is found around these stars. A self-bootstrapping seed factory approach should work at other stars, since energy and matter are the same everywhere. However the details will depend on what resources are available.
Phase 6C - Farther Interstellar Development
and extends in theory to the edge of the accessible Universe. In practice it extends to whatever travel distances are possible with future technology.