§ 3.1 Overview -- Transport of Earth, Lunar and Asteroidal Materials
The main barrier to space development is the cost of getting there. For example, the current Space Shuttle cost is around $12,000 per kilogram, and near future rockets are almost certainly not going to be able to deliver for less than $2000/kg.
That is why we are looking at using materials already in space -- getting materials from asteroids near Earth and the Moon. After all, the settlers of America didn't bring from Europe all their bricks, wood, cement, food, water, etc.
This section explains the differences in transportation costs between delivery from Earth, the Moon, and asteroids near Earth, both theoretically and by practical engineering methods. Different transportation technologies are compared, with an emphasis on practical, near term solutions.
There are two factors that determine the economics of space transportation:
The relative weight of these two factors varies, depending on origin and destination.
The effort required to transport a particular payload from one point to another in space is often compared in terms of "delta-v" -- the total velocity change you would need to impart in order to go to a different destination. It's not necessary to completely understand what this means, but it is important to understand that this reflects a kind of theoretical minimum, somewhat as if the payload were fired from a gun without the need to launch any fuel propellants for use later. In reality, as the delta-v gets higher, the economics get worse much quicker due to fuel and fuel transport needs.
Some delta-v's are given below, in kilometers per second (km/s) and miles per hour (mph):
Getting off of Earth's surface and into low Earth orbit requires so much fuel that rockets must lift off with massive amounts of fuel for later in flight. For example, the Space Shuttle, when sitting on the launch pad, is about 5% cargo, 85% fuel propellants, and 10% vehicle. It's like launching a skyscraper building. The stresses and engineering demands on these vehicles, in addition to safety and economic reliability standards, are so extreme that the costs of manufacturing and operating rockets are exorbitant for launching materials off of Earth's surface.
In comparison, the Apollo lunar lander's fuel tank to launch the astronauts off of the Moon was so small that you could sit on it like a chair, and made up a small corner of the spacecraft. Even in the future, with a heavy lift rocket from the Moon, the stresses will be substantially less than for launch from Earth.
Asteroids have practically no gravity. If your rocket engine fails during launch, there's practically no chance you could crash back down. The vehicle and fuel propellants can be low throughst, low spec, and very small. The equipment for deploying to the asteroid can be configured in low Earth orbit and "docked" to the asteroid, e.g., by a deflating airbag, ratchet-locking springs, or other nonrocketry means if preferred. (For sending a cargo to the Moon, it must be packaged, landed and deployed in the Moon's gravity.) Retrieving asteroidal material is like moving from one orbit to another, which gives tremendous benefits in flexibility, reliability and safety.
Notably, the Shuttle goes only to low Earth orbit. If the cargo in the Shuttle bay is a communications satellite headed for geostationary orbit (a high Earth orbit), as is commonly the case, then the Shuttle's cargo bay consists of one third satellite and two thirds additional fuel propellant. The Shuttle serves mainly as the first stage to get the satellite off the ground and above the atmosphere. Once in orbit, the payload leaves the Shuttle and an interorbital vehicle takes over.
Launch vehicles and the vast majority of interorbital vehicles to date have all been based on chemical rocketry, whereby a fuel is mixed with oxygen and the "controlled, continuous explosion" produces the throughst. For oxygen-hydrogen rocketry, the oxygen makes up between 86% and 89% of the fuel mix.
On the moon, oxygen is easily extracted from lunar soil. Hydrogen is scarce on the Moon except at the poles where it is abundantly available as extremely cold ice. Elsewhere on the Moon, water needs may compete with fuel needs. Rocketry based on powdered metal fuels mixed with oxygen has been developed but needs to be adapted to the lunar mission.
Many asteroids are rich in both oxygen and hydrogen, as well as carbon for alternative hydrocarbon fuels which are more storable and easier to handle. All of these elements are easy to extract from asteroids, using a simple solar oven, for example.
One study put together a design for an initial mission to retrieve asteroidal material. This study determined that a 100 ton spacecraft launched to and assembled in low Earth orbit (and consisting mainly of the fuel propellants to get to the asteroid), could come back to Earth orbit with 10,000 tons of asteroidal material if it gets hydrogen and oxygen from water at the asteroid to use as fuel propellant. This spacecraft is simple and does not involve any radical new technology.
Excess fuel propellants will be one of the first products from asteroidal and/or lunar materials. This will enable rockets on Earth to launch only payloads to low Earth orbit. Separate, "interorbital vehicles" fueled by asteroidal and lunar propellants at fuel stations in high Earth orbit, can come down to bring satellites up to higher orbits, e.g., geostationary orbit, as well as deliver fuel propellants for stationkeeping and maneuvering in any orbit. Rockets on Earth can triple their useful payloads by not launching fuel propellants for the interorbital vehicle.
No longer will satellites be lost in space due to rocket stage failures or decommissioned due to exhaustion of stationkeeping propellants. Lifetimes can be extended, and indeed obsolete satellites can be sold to less developed countries ... though this is getting a bit off the topic of transportation.
Also covered in this section are non-rocketry techniques which are starting to emerge today, as well as techniques further down the road as space becomes more industrialized. However, a near-term project can be based entirely on proven transportation technology.
The one technology that is most likely to emerge for asteroidal materials is the steam rocket -- not making a fuel propellant but simply heating water from the asteroid to produce steam for interorbital propulsion, as discussed in a section of this section.