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Extensive NASA and independent studies have found that all the necessary materials for space colonization are available from the Moon and near-Earth objects, and that solar energy is plentiful in space. No fundamental scientific breakthroughs will be required to build space colonies - though they definitely couldn't hurt - the challenge is mainly a matter of engineering and cost. Numerous space colonies have been designed at various levels of detail, but all are prohibitively costly in terms of today's economics. However, we can expect the situation to change with improvements in technology. Many see space travel and colonization as humanity's destiny.
The two largest obstacles to building space colonies today are the health risks of microgravity and high launch costs.
Living in a microgravity environment is tough. Frequent exercise is needed to maintain muscle mass, some people suffer from "space sickness," similar to sea sickness, one's face puffs up, and most space visitors suffer from uncontrollable flatulence. All crumbly foods are forbidden, lest they float around and get into electronics and create a difficult-to-clean-up mess. For long-term space habitation for large numbers of people, microgravity makes no sense. A true space colony would need to rotate to create artificial gravity. Thus, most space colonies will probably be cylinders or toruses.
A space colony the size of a small city, around 2,000 inhabitants, would need to weigh around a million tons and be about 1 km (0.6 mi) long to have sufficient room for laboratories, machine shops, fire stations, police stations, private residences, gardens, pools, storage for food and water, radiation shielding, docking facilities, a robotics bay, and a small hospital - basically, enough amenities to make living in space tolerable and safe. By comparison, the Empire State Building weighs 370,000 tons, is 381 m (1,250 feet) tall, and has enough office space for 22,000 employees.
The two main challenges of building a million-ton structure in space are the construction work, which would be done best by robots capable of building other robots from raw materials, and getting all that material into orbit, which would be too expensive if all launched from the Earth's surface. At today's prices, launching a million tons of material out of the atmosphere would cost several trillion US dollars. The trick is to use Near Earth objects or materials launched from the surface of the Moon, where gravity is much lower and launches a lot cheaper. The largest Near-Earth Asteroid, 1036 Ganymed, is 31 km (19 mi) across and has a mass of 3.6 × 1013 tons, enough to build thirty million small space colonies.
Because of all the dangerous work involved, robots would take the lead in construction, landing on a Near Earth object, ripping off chunks of it, guiding it into Earth orbit, and processing it into materials for scaffolding. Once a sealed torus or cylinder is created, pumped full of oxygen, and set rotating, human workers could construct the inside to their liking. Most structural materials and technology could be built on-site from raw materials.
No one really knows when a space colony will become possible, but it seems like the growing space tourism industry will definitely hurry things along. At least two companies have already announced plans to build space hotels, and one company - Bigelow Aerospace - already has a test module in orbit.
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