The International Space Station will be a large facility housing many people, and that adds up to high resupply demands. This provides an opportunity for the lunar community to get into the business of selling lunar resources in Earth orbit early in the development of lunar settlements. While we're shipping lunar oxygen to commercial passenger rockets in Earth orbit, we can also resupply the ISS with lunar material.
Water and Power
For instance, water. Even with the water loop as closed as they can get it, ISS will need a lot of resupply water. If we can extract oxgyen from the moon, put it into tanks made from lunar material, and sling it off the moon into Earth orbit, and do all that for less cost than ISS can resupply itself from the ground, we'll be in business. The space station will then only need to ship up hydrogen to burn with the oxygen to have an abundant supply of water. Perhaps we can sell them a power generation plant to take advantage of the energy released in the process.
Other Products of Lunar Industry
We can extrapolate this one concept to having the lunar community providing a host of other supplies to facilities in Earth orbit. The catalog of moon goods expands along with industry on the moon; it all depends on which lunar industries are able to produce a surplus of goods.
The Transportation Problem
Today, using the Space Shuttle costs NASA on the order of US$10,000 per pound to move goods from Earth's surface to low Earth orbit.
Some estimates go even higher, but there is a certain phoniness to this number. Every time NASA launches a Space Shuttle, they launch neartly 200,000 lbs of Orbiter along with the payload. The Orbiter provides an incredible wealth of resources that go with the payload: the crew, living accommodations for the crew, the construction facilities of the payload bay and its robotic arms, the airlock and other EVA support equipment, and of course the Orbiter's ability to return things from orbit to the Earth.
A better model for resupplying the International Space Station is today's big expendable boosters. Supplies will be delivered in large lots of 40,000 pounds or more per launch, with each launch costing the taxpayers anywhere from $200 million to $800 million.
These huge parcels of supplies should be no problem for lunar industry once they a large mass driver built; throwing a 20-ton payload into a transearth trajectory will be routine work. The challenge is to be able to deliver the goods, starting from raw lunar material through processing the material and transporting it to rendezvous with the International Space Station, for less than the same services cost from Earth.
Another advantage to International Space Station is that this scenario saves the Shuttle fleet for carrying things we cannot reasonably supply for the moon any time soon, such as experiment racks, scientific equipment, and crewmembers. As time goes on we will see other manned spacecraft launching from Earth's surface, but it will be a long time before the available fleet's has enough excess capacity that this won't be an important factor in resupplying facilities in Earth orbit. People will always have better things to do with their expensive capital equipment than shipping bottles of water into space.
Unfortunately, we probably will not be in a position to resupply the space station's propellant needs; at least not for quite some time. NASA's current plan is to ship up a whole new propulsion module when the reboost rockets run out of gas. However, the ability to evolve is inherent in the space station's design; so even this will change -- if the lunar community can make a propulsion module entirely from lunar material (and perhaps imported asteroidal material) they will be able to fabricate replacement propulsion modules for the space station.
Here, the beauty of the space station design comes into play -- the only connections between the propulsion module and the space station are the mechanical attachments to the station structure and some electrical connectors. These things can be made entirely of lunar material; all lunar industry needs is the ability to do the precision machining required to assure the parts will fit, and the quality control processes required to make parts for any manned space facility.
Dawn of the Lunar Economy
So we can see that as assembly of the International Space Station begins later this year, we even now see the beginning of a bright economic future for people living in the moon and beyond. If we need it on the moon, someone will need it in Earth orbit; and the moon's advantage of being able to deliver goods to Earth orbit for a fraction of the cost of Earth-based rockets puts lunar industry in an excellent competitive position throughout the foreseeable future.