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While NASA is planning to send the first human explorers to Mars sometime in the 2030s, several non-governmental organizations, including at least one private company (SpaceX), are looking at more aggressive timelines starting in the 2020s. They are also setting a higher bar: rather than sending astronauts for a limited-duration mission, the goal is establishing a permanent human settlement.

A commercial space transportation industry is growing rapidly. Private companies are competing for launch contracts for government, military and private customers, and prices are falling quickly. NASA is turning its human spaceflight attention from low Earth orbit to the Moon and beyond. Several companies are even thinking about building settlements on Mars.

Fast forward a few years from now: Several space companies have successfully and competitively reduced the cost of launch into low Earth orbit (LEO) by more than a factor of 10. The commercial space industry is thriving. There are soaring numbers of satellite launches, astronaut ferries to and from space stations, and forward movement in the human and robotic exploration of the inner solar system. How might all these spacecraft get around? The answer: Fueling stations in the sky!

Some companies are working on different concepts and strategies to reduce the cost of sending material into low Earth orbit, or LEO, but for destinations beyond, there remains an essential problem of propellant mass that these technologies do not address: that is, even if the rocket is reusable, if all the propellant needed for the entire journey is taken along from Earth, there is a tremendous disadvantage, because most (for a round trip to Mars, more than 99 percent) of what is lifted into orbit is propellant. Is this a fundamental limitation?


Urban Mobility

How are we moving people from point A to point B?
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Is it cheaper to buy a kilogram of European white truffles, or send a kilogram of rocks into low Earth orbit (LEO)? The answer may surprise you: while the average cost of sending a kilogram of anything into orbit varies considerably (anywhere from approximately 5,000 to 45,000 US dollars), the truffles actually cost less, if one assumes a representative launch cost to LEO of 20,000 US dollars per kilogram. Lowering the cost of space access has been one of the major goals of the space transportation industry, but what technologies can help to reduce the cost of access to space?