# Space Transport and Engineering Methods/Interplanetary

The space between the major planets and moons has traditionally been viewed as devoid of use. They were to be gotten through as quickly as possible to reach the "real" destinations like the Moon or Mars. This view is incorrect and obsolete. Open space has always been known to have a large and constant flux of solar energy. At the Earth's distance from the Sun this is 1361 MW/km$^2$, or about one large nuclear plant, and there are 281 quadrillion square km around the Sun. From 1990 to late 2017, known Near Earth Objects alone in interplanetary space have grown from about 180 to 17,000, and we continue to rapidly find more. So there are plenty of available resources, and with new ideas on how to make use of them, widespread use of these regions should be possible.
The available energy flux, and quantity and type of materials varies significantly with distance from the Sun, as do local environment parameters. Therefore we divide development of interplanetary space into four phases (4C, 4D, 4E, and 4F) by distance, and discuss them separately. This one, Phase 4C - Inner Interplanetary Development, is closest both to the Sun and the Earth, so it comes first in sequence among the four. To date, only a few scientific missions have explored the region, and it hasn't been otherwise been used except for travel going elsewhere.
We begin concept exploration for this phase by describing the region's features in terms of the environment and available resources. We then survey industry categories to identify potential future activities in the region, and what will drive projects to implement these activities. These are combined into a development approach, and an initial list of projects by time and function. Later work will link the projects to each other and other parts of the program. Projects for which we have produced more details and calculations have that information included as the last major part of this section. An output of the concept exploration work is identifying what preparatory research and development is needed for this phase. This information is supplied to the earlier Phase 0I - R&D for Inner Interplanetary Development, since it must be completed before it can be used in this phase.

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### 7.3 - Phase 4C: Inner Interplanetary Locations

    Inner Interplanetary Features
These locations are detached from the Earth's dominant gravity and orbit the Sun instead, although they may pass close to the Earth at times. They range from as close as equipment can function near the Sun to 1.8 AU, which is just beyond Mars' greatest distance from the Sun and where the Main Asteroid Belt starts. It excludes the four inner planets (Mercury, Venus, Earth, and Mars) and close orbits around them. Solar power is available 100% of the time in these orbits, but the intensity varies from 31% to many times that near Earth, depending on Solar distance. Ambient temperature correspondingly varies from very hot to 244K (-29C) for dark objects, less for bright or reflective ones. Travel time from Earth can range from months to years depending on orbit and propulsion method, and whether gravity assists from the planets are used. These save fuel, but usually require extra time. Solar and cosmic radiation are a moderately high background, with occasional flares/solar particle events that are much more intense, up to lethal human levels without shielding. Ping time ranges from a few seconds for orbits crossing near Earth, to over 45 minutes at 1.8 AU on the far side of the Sun from Earth, by way of a relay satellite. The Sun interrupts direct communication to the opposite side.
As noted in section 1.0, there are over 13,500 known asteroids closer than 1.3 AU, and several thousand more out to 1.8 AU. The largest is over 30 km in diameter, with about 100 times the mass of all the rock ever mined on Earth. So total material resources are very large. Asteroid orbits vary in size, are typically not circular, and somewhat tilted with respect to Earth's, so the energy required to reach a particular one varies. Timing matters also, since everything moves at different speeds in solar orbits. Efficient travel depends on your vehicle and the target being at the same place at the same time. The composition of asteroids vary across about a dozen spectral classes, indicating different chemical compositions. Only ten asteroids of the size we might mine have been visited by spacecraft. That does not count Vesta and Ceres in the Main Asteroid Belt. So most of our knowledge is telescopic and from examining meteorites that have fallen to Earth.
    Economic Uses
There are not that many spacecraft currently in this region. They are mostly scientific probes in transit to other planets, or stationed at the Earth-Sun Lagrange points 1 and 2 (ESL-1 and ESL-2). Future use is likely to start with asteroid mining and delivery to high Earth orbit with electric tugs. Most known asteroids in this region are too large to move as a whole. This is more because we can't find very small ones than because they don't exist. So mining would involve scraping material or grabbing a boulder from the surface of these larger objects. Prior to mining, a prospecting mission should visit multiple candidate asteroids and find out what they are made of in detail. As high Earth orbits become more developed, they can start to send equipment and seed factories to this region in addition to mining tugs. Since raw materials and full-time solar energy are available, the seed factories can grow into full scale factories and produce habitats, vehicles, and whatever else is needed.
    Interplanetary Transport
The main transport system in this region is slow but efficient electric tugs. They can haul large loads of rock relative to their mass, up to 1000 tons for a 10 ton vehicle and 23 tons of fuel, but this depends on the orbit destination and velocity changes needed. They can also move crew habitats faster with a lighter load. Chemical rockets are used when fast velocity changes are needed, and solar sails may be effective in moving things even more slowly, but with no propellant use, once large lightweight reflectors can be made in orbit. Over time, a network of "transfer habitats" are built up. These are stationed in repeating orbits to particular destinations, and save having to move crew habitats each time for multiple trips. Centrifugal platforms can also be built over time to both provide comfortable gravity and fast velocity change with efficient propulsion. The platform mass serves as energy storage which can be transferred to payloads. Since the rotating structure can be relatively massive, it depends on large amounts of traffic to justify it economically, and production of high strength materials in orbit.
    Interplanetary Production
Worldwide energy use on Earth is about 18 TeraWatts, and includes mining, processing, and manufacturing about 2 million kg/s of materials. Thus the energy intensity of Earth civilization is 9 MJ/kg on average. We will double this to allow for recycling of materials in space, and add 8 km/s of orbit velocity change, requiring 300 MJ of electric tug power. That covers a reasonable amount of interplanetary orbits. Transportation is thus the dominant energy use for new materials that need delivery. A space solar panel today produces 177 W/kg at the Earth's orbit, and produces the required 318 MJ in 20.8 days. Given an average life in space of 15 years, their total energy output is 260 times that needed to transport their own mass in raw materials and run the rest of civilization, including making replacement panels. Concentrating reflector and nuclear power sources are not yet developed enough for space to do calculate energy return ratios. They may turn out better or worse than solar panels, but as long as we have one known energy source with a high return ratio, we can base space industry on it.
The same process of bootstrapping production in high orbits can be used in interplanetary space. This starts with mining for export, then simple products made locally, and gradually bootstrapping to more complex ones. As distance increases, fewer raw materials would be brought from the Moon, and more from nearby asteroids. These asteroids are of different types, which provides a reasonable variety of materials to work with. If we restrict ourselves to within 20 degrees of the ecliptic plane, to maintain access to the planets and keep velocity changes lower, we have access to 1/3 of the Sun's total energy, or 1.3 x 10^26 Watts. This is 7 trillion times our current energy use, a number so large it is hard to imagine it could not sustain civilization.
With a network of systems developed around Earth and the Moon, the next step is to extend the network out to Mars and the Main Asteroid Belt. A similar process of setting up seed factories and adding facilities is followed as before.

### System Concept

The system concept has the following main parts:
* Use free flying electric thruster powered vehicles to reach new locations that have useful resources.
* Set up seed factories in each location to build up industrial capacity, including more ships and seed factories for the next location.
* Produce fuel, life support supplies, and habitats at each location so they can be occupied permanently.
* Build more Skyhooks to provide fast velocity changes, but get the benefit of electric propulsion efficiency.

### High Earth Orbit Skyhook

In a previous step we defined a Low Earth Orbit (LEO) Skyhook with a tip velocity of 2400 m/s. Since its orbit velocity is 7474 m/s, at the top of its rotation it could release a cargo at a total of 9,874 m/s. At an altitude of 1,226 km, or a radius of 7,604 km from the Earth's center, with a Standard gravitational parameter of 398.6 x 10^12, we get an escape velocity of 10,239 m/s. Thus our LEO Skyhook is only 365 m/s short of reaching escape velocity. The LEO Skyhook can thus deliver cargo to an elliptical transfer orbit with a semi-major axis of the orbit of 54,278 km, and thus a high point of 100,952 km.
A circular orbit at that altitude has a velocity of 1,987 m/s, and the transfer orbit arrives at 743 m/s. The difference is 1,244 m/s. A tip velocity of 1,500 m/s or more would allow injection to Mars and Main Asteroid Belt transfer orbits, and any closer transfer orbits to the Moon and low inclination Near Earth Asteroids. A High Earth Orbit (HEO) Skyhook can therefore serve as a launch platform to any desired inner Solar System destination orbit by selecting the radius, and thus velocity, and the time, which gives the direction, of release. This location is outside of the Earth's radiation belts, but it is also unprotected by the Earth's magnetosphere from solar and cosmic radiation. So human habitats will need radiation shielding. Such a high orbit is relatively easy to reach from the Moon or NEO's, so bulk matter for shielding will likely be brought from one of those sources.
The construction sequence would start with fetching Near Earth Orbit asteroid material and placing it in High orbit, and delivering equipment from Earth. Once a processing plant, factory, and habitat are set up, carbon from carbonaceous asteroids is used to make carbon fiber for the Skyhook. Initial velocity capacity would not be as high, and so more vehicle propulsion would be needed, but as the Skyhook grows, it can reach a wider range of orbits. Momentum changes are not free, so a substantial power supply and thruster set will be needed at the Skyhook. But since those do not have to be carried along with the vehicles who are getting their orbit changed, the propulsion can be as large and heavy as needed.

### Inclination Stations

Near Earth Objects have a limited range of Solar orbit velocities in the ecliptic plane by definition. They also have a range of orbit inclinations, which results in a velocity component when crossing the ecliptic. The Inclination Station is a second Skyhook located in the vicinity of Earth, such as at one of the Lunar Lagrange points, and oriented perpendicular to the ecliptic plane. Therefore it can deliver cargo to and from inclined orbits to reach NEOs with less fuel and mission time, while the first HEO Skyhook operates in the ecliptic plane to reach Mars and the main Asteroid Belt. Additionally, flybys of planets can be used to further change orbit inclinations and reach other groups of asteroids. The Station itself will react to the average of cargo orbit changes. It will therefore need some propulsion to maintain position, but that will use less fuel than each vehicle doing it's own orbit maneuvers separately.
Given sufficient traffic, it may make sense to have other Inclination Stations set up at different tilts to generate different combinations of ecliptic plane and inclination velocity changes. The velocity difference between the various high orbit Skyhooks will be very low, and release from less than their full radius will be sufficient to get between them. Since trip times between them will be short, the first one can have the bulk of the habitat and production facilities. The later ones can serve mostly as transit hubs, and not be as built up.

### Transfer Habitats

Transfer orbits to NEOs or Mars will require trip times measured in months. For human passengers there is the risk of exposure to radiation, and also a need for food and life support. If you expect to make multiple trips, it makes sense to have habitats permanently in transfer orbits to those destinations. Then the mass of the shielding and greenhouses does not matter that much, as they are not moving once set up. The passengers would use a small vehicle between the HEO Skyhook and the Transfer Habitat when it passes near Earth, then ride in the habitat until it is near the destination, and then again use a small vehicle for arrival. Since only the passengers and cargo need to change velocity the total mass transferred per mission is greatly reduced.
All objects in the Solar System are in motion relative to each other, and transfer orbits only line up properly when, for example, the Earth and Mars are in the right relative positions. Thus a network of multiple Transfer Habitats in different orbits will be needed to deliver passengers and cargo to the right destinations at the right time. The source materials to build the Habitats would come mostly from whichever asteroids are already in the closest orbits. Depending on size, the NEO would be mined for materials, or if small, moved entire to the desired transfer orbit.
Optionally a transfer habitat would have a Skyhook attached to it to enable added delta-V for arriving or departing vehicles. This also provides an artificial gravity environment for the habitat. If that is not provided, then part of the habitat would be rotating to create artificial gravity. Whether to use a Skyhook or not will take more detailed analysis of the complete transportation network.
When the habitats are not ferrying passengers to Mars, they are exploiting Near Habitat Asteroids. Just like there is a set of asteroids whose orbits are "Near Earth", and so easy to reach for mining purposes, there will be a different set which are close to any given Transfer Habitat orbit. So you can busy yourself producing fuel, setting up manufacturing, and eventually have a space city there, just one that happens to get close to Mars periodically. When it does, you drop off humans and accumulated hardware at Phobos, where you are also building up facilities, and thence onward to Mars itself.

#### Orbit Characteristics

Since we are starting from Earth, we want the Habitat to pass by Earth on a regular basis. If we set the orbit period to be 1.50 years, it will do so every second orbit. The orbit will be an ellipse, and the long axis will be 2.62 AU. If the near point of the orbit is at Earth (1.00 AU), then the far point will be 1.62 AU, which is slightly past the average distance of Mars (1.52 AU). A velocity change of 3,340 m/s beyond Earth orbit is required to reach this orbit. This comes from a combination of the HEO Skyhook, Lunar gravity assist, propulsion on the transfer vehicle carrying crew and cargo between points, and possibly a Skyhook at the Transfer Habitat. The Habitat will align with Mars once every 7.5 years, and a velocity change of 4,440 m/s is needed at that end to match orbit. Again this would use a combination of propulsion, including Mars gravity assist. On early trips the transfer vehicle would need to do more work, but later a Mars orbit Skyhook can take up more of the velocity changes.
Since once per 7.5 years is not very often, you can place multiple habitats in a given orbit track, and use multiple orbit tracks spaced equally around the Earth's orbit at their near points. This will give more frequent opportunities to go back and forth from Earth to Mars. For 80% of their orbit cycle the habitats would would be doing mining and construction around the habitat, accessing asteroids in nearby orbits. The other 20% of the time they would also be carrying passengers and cargo for the trips to and from Mars, when they happen to line up. There may be a better arrangement of orbits and habitats to increase the fraction of time they can be used as a ferry service.