Section 4.8: Phase 4 - Orbital Development (continued)
The remaining parts of Phase 4 (4D, E and F) 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.
The earlier parts of Phase 4 cover developing the Low Orbit (Phase 4A) and High Orbit (Phase 4B) regions around the Earth, and the Inner Interplanetary region (Phase 4C) around the Sun to a distance of 1.8 AU. They were described previously in sections 4.5 to 4.7 of this book. Those three regions are the closest and easiest to reach from Earth, so we expect them to start development first. The remaining three orbital regions cover successively farther regions from the Sun: Phase 4D - Main Belt and Trojan, Phase 4E - Outer Interplanetary, and Phase 4F - Scattered, Hills, and Oort. These are farther in the future, so our concepts are less developed for them.
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 visible since ancient times. Determining that their orbits were in fact so large had to await the development of orbital mechanics and better telescopes. Active comets 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. When far from the Sun, 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 (as of late 2017) 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. Their name comes from being scattered by the major planets out of closer orbits in the Solar Nebula where they formed. Four known objects 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. This is presumed to be a large reservoir of objects scattered farther away, and limited by closely passing stars in the cluster where the Sun formed. Beyond this 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 orbits of known near-parabolic comets. Oort Cloud objects are far enough from the Sun to be affected by galactic tides and passing stars and massive gas clouds. These forces sometimes send them close to the Sun, where we see them as active comets.
Our ability to detect all these distant objects is currently limited to the larger ones which are presently within about 80 AU of the Sun. So our discoveries over the last 20 years come from objects in the region whose closest orbital distance (perihelion) is less than 80 AU, and which happened to be near that minimum distance. Since orbit velocities are lower at greater distances, the ones with highly elliptical orbits spend most of their time too far to see. The ones with more circular orbits which stay more than 80 AU from the Sun can't be found at all today, and neither can most of the ones less than 15 km in diameter. We therefore expect to find many times more objects in the region as our instruments improve.
Total mass in the region 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 possible mass 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. Roughly 4% of the population would have originated from inner parts of the Solar Nebula, and therefore be more rocky or metallic than volatile compounds. Solar energy is quite weak in the 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. These plus the required orbital energy to get there make reaching and working in the region difficult, despite the large amounts of material likely to be there.
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.