Section 4.6: Phase 4B - High Orbit Development
High Orbit Features
Our program defines the High Earth Orbit (HEO) region as orbits between 2700 km average altitude and the limit of the Earth's dominant gravitational influence, or Hill Sphere, at 1.5 million km. Where it would not be confused with high orbits around other bodies, we will often shorten the name to "High Orbit". Other sources describe this region as Medium Earth Orbit (2,000 to 35,000 km), High Earth Orbit (above 35,000), and Synchronous Orbit as the boundary between them. We are more concerned with the energy to reach the region and the local environment conditions for design purposes. They are similar enough to treat it as one region. Although it covers a large range of distances from Earth, it only represents the upper 25% of energy between the Earth's surface and escape. That's because gravity is an inverse square force and weakens rapidly as you increase distance. All of the region is in vacuum, and most of it is outside the Earth's magnetosphere and exposed to the solar wind. Solar energy levels in the region are also within 2% of that near Earth, so conditions in general are similar across the region.
This region completely surrounds the Low Orbit region (Phase 4A), and in turn is embedded in the Inner Interplanetary region (Phase 4C). The Moon is the most prominent object in in the area. It has it's own region embedded in the High Orbit one, with a radius of 35,000 km from the Moon's center and travels along with it. It also has its own program phase (5A) for development. Due to the Moon's gravity field, transport to and from the Lunar region requires additional energy, and local environment conditions are different, especially on the surface. Therefore for design purposes we assign it to a separate region.
High orbits are in sunlight 85-100% of the time, reaching the highest values when farther from the Earth and Moon. Temperature is determined mostly by the Sun and the cold Cosmic Background, but at the lower altitudes the Earth contributes a significant amount of reflected light and infrared heat. Orbit periods range from 2.5 hours to 7 months, so travel times by the most efficient routes can be long. Direct paths can be much faster, 12 days or less, at the expense of additional energy. Ping time varies from as little as 25 ms, which is not difficult, up to 10 seconds, which has a large impact on voice, real-time control, and electronic data. The upper part of the Earth's radiation belts, solar, and cosmic radiation create high to dangerous levels for people, without added shielding. Energy resources are abundant in this region, but material resources are low in their natural state. The Moon and Near Earth Asteroids can supply materials with fairly low transport energies.
The most popular satellite orbit, geostationary, at 35,000 km altitude, is in this region. This orbit has a period of 24 hours, which matches the Earth's rotation. Therefore satellites stay above a fixed ground location, and ground antennas can be stationary rather than having to track satellite motion. Synchronous orbit is in the outer fringe of the radiation belts, so manufacturing and human habitation tends to want to be higher up. Delivery, refueling, and maintenance of high orbit satellites is the main current market in this region. Likely the next step is supplying fuel and other supplies back to low orbit and for early interplanetary locations. Future industries are numerous, but depend on bringing costs down to affordable levels. This would happen incrementally as production for early markets bootstraps to larger levels. The total solar flux through this region is 500 million times what our civilization uses in 2015, and just the Moon can support a billion years of mining at the whole world's current rate. A small fraction of these resources can make our civilization sustainable for a very long time.
Current and Near-Term Projects
High Orbit Production
High orbits have abundant solar energy. It can be converted to electricity by solar panels or thermal generators, and used directly for heating using concentrating reflectors. Modern space solar panels and reflectors are very light weight relative to their power output, because they don't need to withstand gravity or weather. An initial stock of these power sources is enough to get early production going. Later expansions would be mostly self-built. The simplest product of all is radiation shielding for human crew. This only requires some crushing and sorting, then packaging into suitable containers around crew modules. Shielded modules allow extended crew stays in high orbit. The crews can operate a satellite maintenance and refueling station, and assist with early materials processing and production. To some extent the crew will be helped by remote control from Earth.
Next in difficulty are water and carbon compounds, from Carbonaceous-type asteroid. This requires 200-300C heat, which reflectors can supply, and a container and condenser to capture the vapors. Water and carbon can be chemically reformed to Oxygen and Hydrocarbons, which is a common high thrust rocket fuel. This is useful when transporting people through the radiation belts or for landing on the Moon. Water, carbon, air mined in low orbit, and possibly rock for soil can supply greenhouse modules, so that crews can produce their own food and recycling life support.
A higher temperature furnace can melt metallic asteroid pieces, add carbon to make steel, and then cast into basic shapes. With a supply of basic metal shapes, a seed factory that includes machine tools can then start making parts for additional machines. Basalt fibers made from Lunar basalts, and carbon fibers made from asteroid carbon compounds are very high strength. Other products would be vapor-deposited reflector sheets and parts for radiator panels. These are combined with high concentration solar cells from Earth to supply electricity at lower launch mass than complete panels. The same parts can be used to make furnaces and cooling systems for thermal processing of materials. Early production would therefore use a mix of pre-made processing equipment, like furnaces, and a growing set of equipment made on orbit. These will output a growing range of products, starting with fuel and other bulk supplies, and basic construction materials. Orbital industry can transition from importing modules and other station parts to building them locally, then exporting habitats to other destinations.
Ultimately large, comfortable space habitats can be built as permanent living space. These can be grown in layers, like an onion. Each layer adds a new compartmentalized pressure shell outside the previous ones. The outer few shells are in vacuum, and provide radiation, meteor impact, and thermal shielding. Inwards of that are pressurized areas with storage and mechanical equipment. Then comes living quarters and a central open space. As new layers are added, items are moved outwards to fill the larger space. Compared to building a large habitat all at once, this spreads the construction cost over time, and the habitat is only expanded when extra space is needed.
High Orbit Habitation
High Orbit Transport
Since high orbits are low in materials, they must be imported from elsewhere. The current method for transport from Earth uses a rocket to reach low orbit, then another rocket or electric propulsion to reach higher orbits. Electric propulsion is about ten times more fuel-efficient than chemical rockets, and is being used more in recent years. Electric thrusters require large amounts of solar power to operate, but efficient and lightweight solar panels have been developed in the last few decades (see NREL Efficiency Chart, Dec 2015 but frequently updated). However they are low thrust, and would expose unprotected humans to high radiation levels while slowly crossing the radiation belts. So some transport will have to be by alternate methods. Large amounts of propellant obtained in space relieves the penalty of doing this.
Bulk materials mined from Near Earth Asteroids are not time- or radiation-sensitive. They can be transported entirely by electric thrusters on tugs that make multiple trips. Since part of the product from these asteroids is more fuel for the tugs, the transport becomes self-sustaining once started. A tug can return about 750 times its hardware mass over a 15 year working life, while consuming about 17 times its mass in fuel over the same period. Tugs can also deliver hardware and finished products to other orbits as needed. The Moon is small enough that bulk materials can be tossed directly into orbit by an electric centrifuge. At 50% efficiency and 50% duty cycle from lunar night, a solar panel can power throwing 1000 times its own mass per year for 15 years. If the centrifuge is not too massive relative to the loads it throws, the overall mass return ratio is high. From low Lunar orbit, electric tugs take over and deliver the materials for processing. We want to source raw materials from both the Moon and Near Earth Asteroids, because they have different compositions.
High Orbit Services