# Space Transport and Engineering Methods/Electric Propulsion

Phase 5 in general covers the larger Solar System bodies besides Earth, the moons and rings which many have, and the orbital regions where their gravity dominates over the Sun's. The large gravity wells around these planets require extra transport systems and energy to navigate. Sunlight is blocked sometimes in closer orbits, at night on surfaces, and always below the surface. Some of the planets and moons have atmospheres and trapped radiation belts, and all the larger ones have significant surface gravity. All of these conditions are different enough from the orbital regions in Phase 4 to need design changes. So we assign development of the larger bodies to a new major phase. The various planets and their surrounding regions are also different from each other, so Phase 5 is divided into five parts. They are in approximate order of difficulty and start times, and tend to start after the respective orbital regions where each body is located. They operate in parallel with all the other phases once started.
The first two parts, Phase 5A: Lunar Development and Phase 5B: Mars Development, cover the easiest to reach bodies from Earth. Concepts for these regions are developed enough we devoted two previous sections of this book to them - 4.9 and 4.10. Concepts for the remaining three are less developed and presented below. Phase 5C covers the hot planets Venus and Mercury and their surroundings. 5D covers Jupiter and the varied bodies and difficult environment around it. Lastly, Phase 5E treats the outer three Gas Giants, and the objects and cold environments around them. The Mars region requires slightly more velocity to reach than Venus, but Venus has a much deeper gravity well and no moons. So it is harder to reach anything useful at Venus, and conditions are much worse, so we place it and Mercury after Mars in our sequence. The Moon and all the major planets in Phase 5 have been visited by spacecraft, sometimes many of them, so we have reasonably detailed knowledge about them.

## Phase 5C - Venus and Mercury Development

This phase includes development of the two major planets which are closer to the Sun than Earth, and orbits around them with semi-major axes less than 600,000 km for Venus and 100,000 km for Mercury. These are distances to which reasonably stable orbits are possible against the Sun's influence. Venus and Mercury have both been seen since ancient times, although it was not until the Copernican Revolution that they were understood to orbit the Sun. They are more difficult to reach than Mars. Added to this are the high temperatures for both, and high pressures for Venus. They combine to place this phase after 5B Mars Development in start time. It also comes after Phase 4C Inner Interplanetary Development, since you must travel through that region to reach the inner planets.
The similarity in size and orbit to Earth made Venus popular in Fiction until the literally hellish temperatures and lack of water were confirmed in the 1960's. Primary interest then shifted to Mars as the next most earth-like planet. Mercury has also received someAttention in Fiction, but less popular interest because it was known to be too hot and too small to be like Earth. In contrast, our program considers all of the Solar System's resources to be useful, limited only by the difficulty in accessing them.

### Region Features

Venus is the brightest planet as seen from Earth. It's orbit is nearly circular at 0.723 ± 0.005 AU, coming closer to Earth than any other major planet. Since 1961, a total of 43 Spacecraft Missions have attempted to reach Venus in some way, with 27 considered at least partly successful. It is 18.5% lower in mass and 5% smaller in radius, making it a near-cousin of Earth in overall size. However the atmosphere has a reference pressure 90.8 times higher than Earth, of which 96.5% is carbon dioxide, 3.5% nitrogen, and some trace gases. Combined with 90% higher solar flux, this results in an extreme Greenhouse Effect, raising surface temperature to 737 K (462 C). This is nearly hot enough to visibly glow red hot if you could survive the surface conditions to see it. The black-body temperature in sunlit orbits is 468 K (195 C), 75 degrees warmer than for Earth. Objects of other colors are similarly warmer. Surface gravity is 8.87 m/s$^2$, or 9.55% lower than Earth. While surface conditions are hostile, moderate pressures of 0.5 times Earth and temperatures around 27 C exist at 55 km altitude. These can be reached by tall or floating structures.
Mercury orbits closer to the Sun and is therefore harder to reach from Earth. Only Two Spacecraft Missions have visited it so far, with a third planned to launch in late 2018. It is only 5.5% of Earth's mass and 38% in radius, making it the smallest inner planet. The orbit is significantly eccentric, varying from 0.307 to 0.467 AU from the Sun. The axis is very nearly perpendicular to the orbit plane, so some polar craters can have permanently shadowed areas. The surprising result is despite being close to the Sun, these parts of Mercury can be as cold as 100K (-173 C). Conversely, the subsolar point at perihelion can reach 700 K. Since Mercury has nearly no atmosphere, heat is more easily radiated to space. So the highest temperatures are slightly less than Venus, despite being closer to the Sun. Sunlit orbital temperatures range from 575 to 710 K (302-437 C) for black bodies, and are lower for other color objects. Mercury's surface gravity is 3.7 m/s$^2$, the same as Mars despite being considerably smaller. This is because it is 33% denser, making it the second densest planet after Earth.
Direct transfer orbits from Earth to Venus and Mercury need about 2.5 and 7.5 km/s insertion velocities. These can be lowered by gravity assists. Escape velocities are 10.36 and 4.25 km/s from their surfaces and 1.041 and 0.664 km/s from the outer edge of their regions. Orbital velocities are 29.3% lower in each case. Orbit velocity from the surface of Venus is a theoretical number, since the thick atmosphere would impede direct launch. Orbit periods in the region range from 90 minutes to 59 days around Venus, and 85 minutes to 15.5 days around Mercury, depending on orbit size. Travel times from Earth by direct transfer orbits are 4.8 and 3.5 months respectively. Gravity assisted trajectories need less propulsion, but take longer. Round-trip communications times from Earth vary from 4.3 to 30 minutes for Venus, and 9 to 25 minutes for Mercury. Communication times within their regions are less than 8 and 1.3 seconds respectively.
Both Venus and Mercury rotate slowly, taking -243.02 and 58.65 days with respect to the stars. Venus is negative because it rotates opposite the orbit direction, unlike Earth. Their solar days are 116.75 and 176 Earth days, respectively. The low equatorial rotation velocity makes a negligible difference in taking off or landing on these planets. The lack of atmosphere and long solar night on Mercury produces large temperature swings. Venus' thick atmosphere has impeded some observations, but topography has been mapped by radar from orbit. We know less about global surface composition, but landed instruments indicate at least two types of basalt are present, and radar mapping indicates extensive Volcanic Activity. Mercury's Geology is influenced by a large iron core, volcanism, and extensive impact features. The surface has a primary composition of 40% Oxygen, 25% Silicon, 11% Magnesium, 6% Aluminum, 4% each Calcium and Iron, and 2% Sulfur, with some variability by location. This makes it more similar to the Earth's mantle than our crust.

### Development Projects

The inner planets present some difficulties in reaching them, and from local conditions, but they have an advantage in large amounts of energy for production and other activities. This comes from 1.9 times Earth's solar flux at Venus, and 4.6-10.6 times at Mercury. To see how much an advantage this is, it takes 7 to 12 km/s velocity change to travel from near Earth to the Venus and Mercury orbital regions. High efficiency electric propulsion will use 0.15-0.27 kg of propellant/kg cargo without gravity assists, using 263-474 MJ/kg of solar array energy. If the cargo is also solar arrays, or equivalent thermal power generation, they will produce an additional 13.75 and 91.5 MJ/kg/day in added energy output, and repay the extra energy to get them there in 19 and 5.2 days respectively. Space system lifetimes are nominally 15 years, so energy-intensive processes highly favor going closer to the Sun. When added to the material resources of these planets, we have enough reason to consider development.
    Orbital Development
Near-term development of Venus and Mercury would begin in orbit by delivery of ready equipment and supplies from previous phases, and bootstrapping further growth using starter sets of production equipment. Solar energy is available 60-100% of the time in orbit, and intensity is high, so it should enable rapid growth. Materials would initially come from asteroids in the Inner Interplanetary region, then supplemented with mining Venus' atmosphere and the polar regions of Mercury. Long-term development includes rotovator-type spaceports to make surface access easier, and building sunshields to terraform the planets below.
Orbital Production - Since Venus and Mercury appear to have no natural moons, we would like to use nearby asteroids as a resource for orbital development. There are a relatively small number of known asteroids near the orbits of those planets (the two inner circles on a Plot by the Minor Planet Center). It is not clear if this is from low scattering efficiency and short residence time for asteroids starting from more distant orbits, or the difficulty in spotting small objects in the Sun's direction when looking at their partly unlit sides. If nearby asteroids are insufficient, or the wrong composition, materials can be imported from better-supplied interplanetary areas with the help of planetary gravity assists. Venus' atmosphere is a ready source of carbon, oxygen, and nitrogen, assuming scoop-mining from orbit is developed. Mercury is small enough to directly throw bulk material to orbit, and the polar regions have tolerable natural temperatures. Equipment that needs to operate at Earth-like temperatures can be protected by sun-shields when local conditions are too hot.
Orbital Habitats - Habitats with artificial gravity and sufficient thermal and radiation shielding can be built in previous regions which are more developed, and transported to orbit around Venus and Mercury. This transport can be gradual, with modifications made as the conditions change, and supplies extracted as needed en-route. This approach can enable substantial occupancy from the start. Most of their design should be unchanged from previous orbital phases.
Orbital Transport - Orbital transport requirements include destinations within each orbital region, access of the planets below, and trade between the two planets and with elsewhere. The early needs can be met by extending Inner Interplanetary transport that was already operating. Later systems would be based in the local orbital regions. The previous systems would include solar-electric propulsion and planetary gravity assists. Solid materials are easier to extract from Mercury, and atmospheric gases from Venus. So trade is likely between those planets.
Increased gravity as you get closer to the Sun implies more energy is needed to change orbits and reach Venus and Mercury. Solar flux increases faster than velocity changes, and this introduces the possibility of solar sails as effective transport method in the inner regions. For example, reflected sunlight provides 15.5 Newtons/km^2 at Venus, and a 1 micron Magnesium-Aluminum sail material would mass 2400 kg/km^2. This generates 558 m/s/day acceleration for the bare sail. This is reduced by the remaining structure and cargo mass, and angling the sail to control thrust direction. This acceleration is comparable to that for electric propulsion including solar array mass near Earth. The advantage of solar sails is they do not consume propellant. Their disadvantage in the outer Solar System is low solar intensity makes them very slow. Electric propulsion is also quite viable closer to the Sun, and a combined system is possible to take advantage of the reduced propellant from the sail and wider thrust angles from the electric engine. In the long term, rotovator-type orbiting spaceports can assist with reaching the surface or escaping from Mercury and Venus, once enough traffic exists to justify their construction.
Orbital Services - [TBD]
    Surface Development
Near-term development of the inner planet's surfaces is impeded by their generally hostile conditions. The exceptions are the polar areas of Mercury, and high altitudes in Venus' atmosphere, where conditions are more moderate. For full development, some level of terraforming is desirable, as discussed below. Higher latitudes on Mercury get less sunlight per area as a function of Sun angle, and terrain features or artificial reflectors can provide protected areas that are cooler. Either floating structures, or ones supported by towers, can reach moderate conditions at higher altitudes over Venus. It is not obvious which will be more practical, or if surface development on Venus should wait until some level of terraforming is accomplished. In the long term, if the surface conditions can be made more tolerable, primarily with sunshades, then large scale access to the surface would encourage development there. The combination of larger sources of raw materials plus continuous high energy available from orbit would make them attractive locations, at least for industry.

### Concept Details

    Terraforming Venus and Mercury
Venus and Mercury are mostly too hot, and Venus has too much atmospheric pressure, to use their surfaces in their natural state. A long-term project would be to modify these conditions to make them more Earth-like, a process known as Terraforming. Terraforming Venus has been considered by various means. Our approach assumes that asteroid and Lunar mining in previous phases becomes well developed. Material extracted from these sources is turned into many orbiting sunshades, which block most or all of the sunlight reaching the planets now. The shades would be in medium orbits around the planet and be facing the Sun while crossing in front of it. The remainder of their orbit they orient themselves so as to counteract the light pressure which would otherwise push them out of the desired orbit. To do this effectively they may need to be reflective on at least one side. Early sunshades can simply block the Sun, but later ones can incorporate solar collectors and other equipment to make use of the available energy.
The minimum mass to totally block the Sun can be estimated by the cross section of Venus and assuming 1 micron thick shades. This would only require 0.115 km$^3$ of material. Due to the shades being in orbit more than this is needed in practice, so we can use 1 km$^3$ as a more reasonable estimate for that planet. The largest metallic asteroid in the Main belt has 6 million times this volume, so there is more than enough material available for such a project. Mercury has only 16% of Venus' cross section, and needs correspondingly less reflector area, but the solar intensity is much higher trying to dislodge them. So it is not clear what the relative difficulty will be. Since Mercury has no atmosphere, sunshades installed on the surface may be a better approach.
Blocking the Sun for Venus would allow the atmosphere and surface to cool. A cooler atmosphere has a shorter Scale Height, over which pressure changes by a factor of e. Therefore high altitude terrain, like Ishtar Terra will preferentially see lower pressures and temperatures. Construction of towers on these high points would additionally lower pressure and temperature, making them the earliest places to occupy, with expansion to other areas as conditions improve. Some minerals, like Peridotite, can capture carbon dioxide, which makes up 96.5% of Venus' atmosphere. If such minerals exist on or near the surface of the planet, natural or accelerated capture may lower pressures further. Landers on Venus have detected basaltic-type surface compositions, so the right types of minerals may be present. Much further work on the Geology of Venus is needed before the feasibility of carbon capture is determined.
At first, more shading or entirely blocking the Sun would accelerate cooling the planet. Afterwards, the level of shade can be adjusted to maintain desirable temperatures. Mining the atmosphere from orbit using scoops can begin long before the planet cools. Since the atmosphere is nearly 0.01% of the entire mass of Venus, this mining it is unlikely to make a significant difference in the terraforming process. Even so, the carbon, oxygen, and nitrogen extracted this way have lots of uses. The shades can start out as simple reflectors, but as the orbital region develops, they can be gradually replaced by solar collectors and habitats.

## Phase 5D - Jupiter System Development

Jupiter is one of the wanderers of the night sky known from ancient times. The discovery in 1610 of four moons going around it, rather than Earth, helped establish the modern Heliocentric (Sun-centered) model of the Solar System. Today we recognize Jupiter as the most massive planet in the Solar System, 317.8 times Earth, with 0.066 more in the moons and ring system. This is 2.5 times the mass of all the other known planets combined. Effectively it is a miniature solar system of its own. Included in the Phase 5D region is the planet, 69 Known Moons, a thin Ring System among the four innermost moons, and orbits with semi-major axes within 20 million km of Jupiter. The outer irregular satellites orbit farther than this, but we also include them as part of the Jupiter System.
This phase follows 5C: Venus and Mercury because it is more difficult in several ways. Jupiter orbits 5.2 times farther from the Sun than Earth. With the addition of navigating its gravity well, more total propulsion is needed to reach its resources. At the same time, solar flux is much weaker, and parts of the region have high radiation levels. The phase also follows 4D: Main Belt and Trojan, because Jupiter is at the outer edge of that region and sits between the Trojan clusters which lead and follow it. So it is a small step from the Trojans to developing the outermost Jupiter moons. For previous planets like Mars we divided development into orbital and surface projects. For Jupiter, we divide it into three parts. The Outer System includes the irregular satellites and orbits larger than 2 million km. The Inner System includes the four large moons, four smaller inner ones, the rings, and orbits closer than 2 million km. Finally comes Jupiter itself, but that development would be far in the future due to extreme difficulty in reaching it.

### Region Features

    Jupiter
Jupiter's orbit ranges from 4.95 to 5.46 AU from the Sun, with a period of 11.86 years. Travel from Earth by direct transfer orbit nominally takes 2.73 years. It has been observed by astronomers since the invention of the telescope, and Visited or Orbited by 9 spacecraft, including one currently active, and one atmospheric probe. It is a Gas Giant with approximately the same overall composition as the Sun, and therefore does not have a well-defined surface. The radius to the visible clouds is 71,492 km at the equator and 69,911 at the poles, giving it a total surface area 121.9 times the Earth. The difference in radius is due to the rapid rotation period of 9.925 hours, and that it is mostly not a solid body. The Upper Atmosphere is about 90% hydrogen, 10% helium, 0.3% methane, and small amounts of ammonia and other trace gases. Temperatures are 112 K (-161 C) at the 10 kPa level and 165 K (-108 C) at the 100 kPa level (about 1 Earth atmosphere). Conditions reach 2.5 MPa pressure and 430 K (157 C) temperature at a depth of 132 km below the 100 kPa level, which is where the Galileo probe stopped transmitting. Therefore there is a level where temperature and pressure are not too different from Earth, but getting and surviving there would be very difficult.
Solar flux at Jupiter varies from 4.1 to 3.35% of that at 1 AU, with an additional 0.2% variation across the orbital region. Black body temperature in the region averages about 173 K (-100 C). Escape velocity from low orbit is 59.5 km/s and orbit is 42 km/s. Since the equator rotates at 12.6 km/s, the required velocity to reach orbit or enter the atmosphere is 29.4 km/s. This large difference between orbit and the atmosphere makes accessing the planet itself very difficult. Round-trip communications time from Earth varies from 1.06 to 1.85 hours.
    Inner System
The inner system extends from the lowest stable orbits to ones with semi-major axes of 2 million km. Orbit periods range from 2.97 hours to 18.28 days. It includes the four large Galiean Moons (Io, Europa, Ganymede, and Callisto), which orbit 0.422, 0.671, 1.07, and 1.88 million km from Jupiter in nearly circular orbits. The first three have 4:2:1 resonant orbit periods. Four smaller moons have orbits between 127,000 and 222,000 km in radius, with a thin ring system among them. Their depth in Jupiter's gravity well makes access difficult.
The large moons range from about half to twice Earth's Moon in mass. This is enough to be useful for gravity assists, making travel within the Jupiter System easier. They are 90-150% of the Moon's diameter, with a combined surface area of 232.8 million km^2, or 1.56 times the land area of Earth. All four are tidally locked to Jupiter, so their days are equal to their orbit periods of 1.77, 3.55, 7.15, and 16.7 Earth days. To reach them unassisted from beyond the Jupiter System takes 3.4-7.15 km/s of velocity change. Their escape velocities range from 2.0-2.75 km/s, and surface gravity from 1.23-1.8 m/s$^2$ (12.5-18.4% of Earth). The Galiean moons are large enough to have stable orbit regions around each, but have negligible atmospheres. This makes transport to and from their surfaces easier.
Io's surface is composed of silicates, sulfur, and sulfur dioxide. Tidal heating makes it the most volcanically active body in the Solar System. Europa is covered in water ice with a probable water ocean underneath. Ganymede's surface is about 2/3 areas with high water ice content, and 1/3 darker areas with clays and organic material. Callisto has 25-50% water ice on the surface, with hydrated silicates, carbon and sulfur dioxides, and possibly ammonia and organic compounds detected. Surface temperatures of the large moons range from 70-165K, except for volcanic hot spots on Io, and vary mostly by latitude and how close they are to Jupiter, and how much reflected light they get on the near side.
    Outer System
The outer system includes orbits from 2-20 million km in semi-major axes, and 61 known irregular moons. They have much higher inclinations and eccentricities than the inner satellites. Only eight are more than 10 km in diameter, with the remainder being smaller. The last two were discovered in 2016 and 2017, so it is likely there are more which are less than 3 km in size to be discovered. 51 of the irregular moons have orbits 20-28.57 million km in size. This is beyond the orbital region we defined for the Jupiter System, but since they are bound to the planet we include them as part of it. Escape velocity from the edge of the region is about 3.5 km/s. Total mass is over 8,000 trillion tons, mostly from Himalia, the largest outer moon. Total available solar energy in the outer system is 63 billion TW, or 3 billion times civilization's current energy use.
By size, the largest feature of the Jupiter System is the Magentosphere, a cavity in the solar wind created by the planet's strong magnetic field. It extends up to 7 million km towards the Sun, and as far as Saturn's orbit in the other direction. It is filled with highly conductive plasma and contains complex current flows driven by the rotation of the planet's magnetic field with the planet. The magnetosphere traps high energy particles in a belt concentrated between 280,000 and 775,000 km from Jupiter's center. Without shielding, radiation levels are high enough to damage electronics and are lethal to people. This is in addition to the normal solar and galactic radiation present in the Solar System.

### Development Projects

We expect development of the Jupiter system to start from the outer edge, and work inwards. Moving closer requires increasing transport energy, and later dealing with very high radiation levels. So it makes sense to start with the outer orbits and moons that are easier to reach. Resupply and support stations can be set up there to prepare for later development of the inner system. The outer areas of Jupiter are also easier to reach from the Main Belt & Trojan areas, which would have previously started development. Equipment and starting supplies can be delivered from these already developed regions.
Together the Galilean moons are over 60% of Mars' mass, and 130 times that of the Main Belt and Trojan region. They differ considerably from each other and from sources in previous phases. The vast and varied source of materials makes eventual development of the inner system worthwhile. It is complicated by high radiation levels and added velocity required as you get closer to Jupiter. The added work required will delay their development until sufficient needs exist. Jupiter itself is much harder to reach than the bodies orbiting it, so any direct uses besides gravity assists is deferred until much later.
    Outer System
All of the outer system moons have irregular orbits, are somewhat smaller than the Jupiter Trojans, and their spectra show similarity to some asteroid types. So it is likely they started as captured asteroids. Later collisions fragmented those asteroids to create the current groups with related orbits. The irregular moons have the same average distance from the Sun as the Jupiter Trojans. So reaching them and starting to use their resources should be an extension of previous work in Phase 4D, using the same designs. Although solar energy is weak in this region (3.15-4.3% of the 1 AU intensity), lightweight reflectors should be able to concentrate it to usable levels. Going from the surface of Himalia, the largest outer moon, to orbit around it should only take 50-60 m/s, and the remaining moons will need less velocity. So mining and delivery to processing plants should be easy. The Main Belt and Trojan region has much more total material and energy resources than the outer Jupiter System. So the likely reason to develop this area is a step towards the much larger resources of the inner moons.
    Inner System
The inner Jovian system has very high natural radiation levels from trapped particle belts. Early development would therefore depend on bulk and active shielding, and remote control from more distant orbits that have lower radiation levels. The unprotected radiation levels affect electronics as well as living things, and would be rapidly lethal to people. We don't know if there is a practical way to permanently change the radiation levels in the long term, due to the strength and size of Jupiter's magnetic field.
Production - Growth of local production would likely follow the usual path of mining first, then seed factories to bootstrap other industries. The smaller outer moons can be an early source of propellants and supplies to support growth, with local sources developed over time. Orbit velocity for Ganymede, the largest moon, is 1,938 m/s, and it is less for the other major moons. An electric catapult can therefore throw cargo directly into orbit for processing in full sunlight, or transport to other destinations. Water is widely available for propellant throughout the system. Rocky and metallic materials may need to be imported from the surrounding regions, depending on the composition of the moons. Large reflectors would be a desirable early product to generate power and heat.
Habitation - Because of the high radiation levels close to Jupiter, we expect most of the habitation in the region to be located farther out. When people are needed, they can occupy heavily shielded habitats and do as much as possible by remote control.
Transport - Transport from previously developed regions would include a mix of electric propulsion, spaceport acceleration, and gravity assists. People and the items they use would travel in shielded habitat modules. High thrust propulsion would be needed for early landings on the large moons, while later transport can use spaceport structures.
Services - Early services include science, exploration, and communications. Later service industries are [TBD].
    Jupiter
We don't anticipate much development of Jupiter itself in the near or mid-term. The very high velocity difference from orbit to the atmosphere requires over five times as much energy as escaping the Solar System from Earth's orbit. There are also much easier sources of the gases in Jupiter's atmosphere in the outer Gas Giants and cold regions of Phase 4F. We will keep this heading as a place-holder for the long-term future, and as a spur to finding new concepts.

## Phase 5E - Outer Gas Giant Development

This phase includes development of the outer three giant planets: Saturn, Uranus, and Neptune (SU&N). It also includes 103 known moons, a number of which are large, and three ring systems, one of which is famously prominent. There are likely to be more moons that are currently too small and dim to be found. Finally, the phase includes orbits with semi-major axes up to 20, 12, and 12 million km around the three planets. Some of the moons orbit farther than these distances, but we include them in the phase because they orbit their respective planets. Saturn is easily visible to the naked eye, and has been known since ancient times. Uranus is marginally visible without equipment, and Neptune is about six times too dim, so they were only identified as planets in 1781 and 1846. Saturn has been visited four times by spacecraft, including deploying a lander to its largest moon, Titan. Uranus and Neptune have only been flown past once each, by the Voyager 2 spacecraft. Due to their distance and only a single brief visit each, our knowledge of these planetary systems is less complete than for Saturn.
Phase 5E starts after 5D Jupiter System for several reasons. The farther planets need more transport energy to reach, but at the same time solar energy is weaker for propulsion and other needs. Travel times are significantly longer, a number of years by most methods, and temperatures are very cold. It also starts after 4E Outer Interplanetary because these planets are located within that larger region, which must be crossed to reach them.

### Region Features

    Saturn
Saturn's orbit ranges from 9.04 to 10.12 AU from the Sun, with a period of 29.46 years. It is 95.16 times Earth's mass, with an equatorial radius of 60,268 km, and a polar one of 58,232 km, making it the most flattened major planet. Like Jupiter, it is a gas giant, and does not have a well defined surface. Instead the size is measured where atmospheric pressure equals Earth at sea-level. The upper atmosphere is about 96.3% hydrogen, 3.25% helium, 0.45% methane, and trace amounts of ammonia and other gases. Temperatures range from 84 K (-189 C) at the 10 kPa level to 134 K at the 100 kPa level, and 270-330 K (-3 to 57 C) at the 1-2 MPa levels. There is a complicated set of cloud layers with different compositions at these heights. Gravity varies significantly from 12.06 m/s$^2$ at the poles to 9.04 m/s$^2$ at the equator, due to the flattened shape and rapid rotation of the planet every 10.55 hours. These values are approximately equal to Earth's (9.81), but there is no easy way to support yourself in the atmosphere to experience it. The planet is tilted 26.7 degrees to its orbit. Solar flux in the Saturn region is 0.95-1.26% of the 1 AU intensity, except when blocked by the planet, moons, rings, or Titan's atmosphere. Total available solar energy averages 18.6 billion TW in the region.
Saturn has a complex system of 62 known Moons and multiple Rings. The total system mass is about 1.4 x 10$^{23}$ kg, or 1.9 times Earth's Moon. Seven of the moons are large enough to have become round by self-gravity. The largest, Titan, holds over 96% of the mass orbiting Saturn, with an atmosphere containing 95% nitrogen and 5% methane at a pressure of 147 kPa, about 45% higher than Earth. The moons can be grouped into the inner Regular Moons, which have low inclinations and are closer than 1.5 million km in semi-major axis, and the outer Irregular Moons that are more than 3 million km from the planet and have higher inclinations and more eccentric orbits. The larger moons are generally covered in water-ice, and Titan also has hydrocarbon lakes with methane and ethane. The rings are 99.9% water ice.
Titan has an orbit period of 15.945 days and is tidally locked to Saturn, so the day is the same length. It requires 2.3 km/s to match orbit from outside the Saturn system, and escape velocity is 2.64 km/s. Surface gravity is 1.35 m/s$^2$. Total surface area is 83 million km$^2$, or 56% of Earth's land. Surface temperature is 94K (-179 C).
    Uranus
Uranus travels between 18.33 and 20.11 AU from the Sun, with a period of 84.02 years. It has 15.91 times the Earth's mass, with a radius of 25,559 km at the equator and 25,362 km at the poles. Uranus is an Ice Giant, which is mainly composed of elements heavier than helium. It still has a thick atmosphere of lighter elements, the upper layers being 83% hydrogen, 15% helium, 2.3% methane, and traces of other compounds. Temperature at the 100 kPa level is 76 K (-197 C), rising to about 330 K (57 C) at the 10 MPa level, which is 300 km lower. There are multiple cloud layers in between. Nominal surface gravity is 8.69 m/s$^2$, with about 0.25 m/s$^2$ reduction at the equator from its -17.24 hour rotation period. The period is negative because Uranus is tilted 98 degrees to its orbit. Solar flux in the Uranus region is 0.245-0.30% of the 1 AU value when not blocked. Total available solar energy averages 1.67 billion TW in the region.
Uranus has 27 known Moons plus a set of narrow Rings. The five major moons orbit from 129,000 to 583,000 km from the planet. They are 470 to 1575 km in diameter, with a combined mass of 9.1 x 10$^{21}$ kg, or 12.4% of Earth's Moon. Water ice has been detected on all five moons, and carbon dioxide or carbonate minerals on some of them.
    Neptune
Neptune's orbit is quite circular, staying between 29.81 and 30.33 AU from the Sun. It takes 164.8 years to complete an orbit, so it only completed one orbit since discovery in 2011. It is slightly more massive than Uranus, at 17.15 times Earth, and nearly the same size, with radii of 24,764 km at the equator and 24,341 at the poles. Like Uranus, it is an ice giant, with the bulk of its mass being elements heavier than helium, but with a thick atmosphere containing nearly 80% hydrogen, 19% helium, 1.5% methane, and trace gases. The atmosphere at the 100 kPa level is 72K (-201 C), reaching 273 K (0 C) at the 5 MPa depth. Nominal surface gravity is 11.15 m/s$^2$, decreasing about 0.29 m/s$^2$ at the equator from the 16.1 hour rotation period. The planet is tilted 28.3 degrees to its orbit. Solar flux in the Neptune region is nearly constant at 0.108-0.113% of the 1 AU value, except when blocked by something. Total available solar energy averages 679 million TW in the region.
Neptune has 14 known Moons and five Rings. Only one moon, Triton is large enough to be spherical, at 2702 km diameter and 2.14 x 10$^{22}$ kg mass (29% of Earth's Moon). The remaining moons are only 0.4% of Triton's mass combined. Triton is thought to be a captured Kuiper Belt object because of its retrograde orbit. It is larger and more massive than Pluto, a Kuiper Belt object whose orbit crosses that of Neptune. Triton has a thin nitrogen atmosphere, and the surface appears to be ~55% solid nitrogen, ~25% water ice, and ~15% carbon dioxide ice. It currently orbits 355,000 km from Neptune with a period of -5.877 days. Since it is tidally locked, this is also the day length. Reaching Triton's orbit from outside the region requires 1.8 km/s velocity change, and escape velocity from the surface is 1.455 km/s. Surface temperature is only 38 K (-235 C),

### Development Projects

None of the giant plants have accessible solid surfaces, and their atmospheres are mostly hydrogen, making floating systems difficult. They also require high velocities to reach or take off from. So any development of the planets themselves would be very limited. The majority of development would then use the many moons and rings for materials, and either orbital or surface locations in the three systems. Development would likely start as an extension of Phases 4E and 5D, as Jupiter and the inner Centaurs become developed enough to be staging points for sending equipment and supplies to the farther planets. It would then work inwards around each planet in succession. As with other phases, early development would focus on extracting materials to be used in more developed regions. This phase involves large distances and relatively difficult environments, so a significant amount of research and development will be needed before it can start. Prior development in multiple preceding phases should allow enough time and experience for that to happen.
    Production
Due to low solar flux, either large, lightweight reflectors would be needed to bring it to useful levels, or alternate energy sources like nuclear. Orbit velocity around Titan is 1.86 km/s, making gas mining particularly easy. The difference between orbit and equatorial rotation velocities for SU&N are 15.2, 12.5, and 13.9 km/s. This is possibly low enough for mining their atmospheres from orbit.
Helium-3 has been proposed as a low radiation fusion fuel. Fusion in general has not yet been solved, and the He-3 - He-3 reaction is harder than the Deuterium-Tritium one that is the main target of research. If the technical issues are overcome and a need develops, Uranus and Neptune have the highest ore concentrations of Helium in their atmospheres, and thus the He-3 isotope. At the same time, fusion reactors would be available to make trips to these planets in reasonable time. Such uses are far enough in the future that technology is likely to change dramatically in unexpected directions. So it is too early to make plans for projects like this, but we can note the possibility for long-term development.
    Habitation
All three of the outer giants have magnetic fields which create trapped high energy particle belts. They are weaker than those around Jupiter, but likely still a hazard to unprotected people and electronics. Since Jupiter is a worse case, designs should already be available from the previous phase, but more work is needed to understand the severity in different parts of the respective outer regions. Even outside the radiation belts, some solar and cosmic radiation is present generally throughout the Solar System, requiring a moderate level of protection.
    Transport
Direct transfer ΔV departing from Earth's orbit to the SU&N regions are 10.3, 11.3, and 11.7 km/s, the latter not far from Solar System escape at 12.3 km/s. To match interplanetary orbit with the regions when arriving requires 5.45, 4.65, and 4.05 km/s. Orbit velocities at the edges of the regions are 1.377, 0.695, and 0.755 km/s, and low orbits around the planets are at 25.0, 15.0, and 16.5 km/s. So destinations within the region require a variable additional ΔV. Reaching the regions from Earth can use planetary gravity assists, and all three of outer giants and their larger moons have enough mass to help at arrival and later orbit changes. The larger moons can probably support stable enough orbits for orbiting spaceports, which can lower the velocity to get to and from the surface.
Low orbit periods for SU&N are 250, 180, and 155 minutes, and are 2.89, 3.44, and 3.16 years at the edge of their regions. Transit time between points in the region will vary beyond these limits depending on the transport methods used, and needs for orbit phasing and inclination changes. Direct transfer orbits from Earth will take 6, 16, and 30 years to reach the regions. These value are high enough that for people and equipment you would want to do something useful during the trip, or use faster transport systems. For bulk cargo you might use slower but more efficient routes with gravity assists.
The power source for outer system transport is still to be determined. Nuclear fuel has an energy content of ~80 TeraJoules/kg, while solar panels near Earth produce around 80 GigaJoules/kg. The relative output then depends on the conversion efficiency of nuclear reactor fuel energy to useful output, and the mass ratio of the reactor system to the fuel load. Fissionable elements are relatively rare compared to silicon used for solar panels or aluminum/magnesium used for concentrating reflectors. So for large-scale energy demands, the solar sources have better material supply. However, in distant regions where sunlight is weak, nuclear approaches may have the advantage. Although reactors emit harmful radiation, most parts of the Solar System are filled with harmful radiation anyway. The same shielding that protects people and equipment can do it from both sources.
    Services
Early services are likely to include science, exploration, and communications. Later service industries are [TBD]. Round-trip communications times from Earth to the SU&N regions are 2.19-3.12, 4.78-5.87, and 7.96-8.71 hours. If communications are to be maintained when Earth is in opposition, a relay link will be needed, which slightly increases the maximum time.