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Perhaps a mass driver atop one of the great volcanoes in the really thin "air" of high martian altitude might be able to launch cargos without to much drag/friction/heat. Only problem is launch windows to Earth only come every 2.13 years. Better to launch stuff to Mars orbit or a martian Lagrange point, then use a rocket for flight back to Earth-Moon space to sell things for $$$$. If the mass driver doesn't work, it is still possible to use SSTO vehicles to orbit things mined on Mars, since Mars orbital V is so low, and it will be possible to mine the moonlets Deimos and Phobos which have negligible gravity wells.
Mars surface area=50million sq. mi. A Bernal Sphere 2 miles wide has an interior surface area of 12.5 sq.mi. It would take 4 million of these space colonies to have as much area as Mars. If we built 10+ decks in the sphere sort of like an onion, maybe we could have 125 sq. mi. We'd still need 400,000 of them. Since it will take 1,000 years at most to completely terraform Mars, we'd have to build 400 Bernal Spheres every year for 1,000 years to rival Mars. I doubt that it would be possible. I'm all in favor of space tourism, energy and manufacturing, resorts on the Moon and orbital hotels, for millions of space tourists every year; but Mars has the most potential to provide a second home for humanity. Mining Deimos and Phobos to build O'Neill cylinders, bernal spheres, toruses etc. may be easier than mining Earth's Moon. We might see the martians build more space cities in orbit and in the asteroid belt than will ever be built by terran civilization.
Can't do anything about the sea cement, but I'm sure OTECs will someday be more efficient. As for flooding the market, a larger population in the future=more demand. The Millenial Project is interesting, though flawed, but something great might eventually come out of it. If the OTECs just broke even with electricity generation, the cold nutrient rich seawater pumped up from the depths could allow profitable aquaculture. My proof? None. I just like the idea.
Molten core fission reactors for rocket engines are theoretically possible.
If you have the technology to successfully mine Mars and it's moons you might as well go for the gusto and target NEOs as well. Some of those NEOs after all are very rich in valuable materials and you'd only need the same type of equipment you mine the Martian moons with.
This is true, however, the companies that do this work will have a limited amount of captial. The question is-Where do we put our money for the best and fastest return?? Have to please the stock holders. Can't be like a dot.com turned dot.gone. I think Demios and Phobos are the place to start. Of course, businesses have to be operating in Earth-Moon space who want our products. It's never been Mars or the Moon with me, but Mars and the Moon and more. After we build up business on the moonlets we can go for an asteroid, probably a stony-iron to get platinum group metals. Once we get our mines on Deimos and Phobos working we can fuel ships and build SPSs in Mars orbit to supply energy to colonists on the surface, and other stuff. Anyhow, that's my reasoning for sinking money into the moonlets first and mining the NEOs later.
Actually, we could do alot on the Moon and in Earth orbit similar to O'Neill's visions if transportation to LEO came down in price, but eventually the Moon and Earth orbital space will need more CHN and metals like copper if there are ever really going to be L5 space colonies and lunar tourism on a large scale. Most people talk about bringing in materials from NEOs, but I think Mars would be better. Mars has enough water to export some hydrogen. More importantly, launch windows to or from Mars occur every 2.13 years. Some of those NEOs are in such wild orbits that launch windows might be had every 5 to 10 years or worse. On Mars you have an atmosphere to provide protection from radiation, reasonable temps, and easily accessed CHN and some decent gravity to work in. The absence of gravity on an NEO could be more of a hindrance than a help, but who knows? Phobos and Deimos are also excellent resources. If we're lucky they will contain CHN because they seem to be C-type and maybe some other useful elements. If not, then they will still be made of silicates that will serve as an oxygen source. I don't see much use in mining an NEO when I can mine Mars and benefit from the moonlets, sell materials to colonists in Earth-Moon space, make cash and colonize Mars.
"All the universe, or nothing!"
PHOBOS/DEIMOS MINES INC.
Specializing in the sale of rocket propellant to 1) cycling station taxis near Mars 2) ships bound for the Main Belt or Saturn 3) ships bound for Earth 4) landers descending to Mars 5) any ship leaving Mars orbit to anywhere else 6) satellites, orbital tugs, EVA pods working in Mars orbit. We also sell construction materials and gases to SPS and colony builders in Mars orbit. Given the mere 2 kps needed to reach cis-lunar space with our chemically propelled aerobraking freighters that don't violate any laws regulating the aerobraking of nuclear rockets in Earth's upper atmosphere, and the low cost of fuel and LOX from Phobos&Deimos, we have become the biggest exporter of hydrogen, carbon, nitrogen and other elements found on the Mars moonlets and surface of Mars to colonies in HEO, LEO hotels, mines and resorts on Luna. Copper, zinc and other valuable metals mined on Mars are launched inexpensively by mass drivers on the slopes of Pavonis Mons. Our ships haul them to cis-lunar space to fuel industry there. Without Mars, man could not have colonized the Moon and Earth orbital space. Our cutting edge technology made it happen. Now trading on Wall Street at $100 per share. Buy now!!!