Future Mobility

Fueling Stations in the Sky

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!

As discussed in a previous blog post, the cost of sending propellant up into space along with a spaceship is very expensive for missions with destinations well beyond LEO, because the mass of propellant is so much greater than the spacecraft itself. For a journey to Mars, the ratio is roughly 10:1, but where else can this propellant be obtained than the Earth?

Depending on its chemistry, most common rocket propellants only need simple starting ingredients for manufacture: water, carbon dioxide and perhaps nitrogen. Water can be turned into hydrogen and oxygen, a very effective rocket propellant. (Note that in space, an oxidant – usually oxygen – must be supplied along with fuel, because there is no air in which to burn. Thus all rocket propellants are comprised of both fuel and oxidant.)

Adding carbon dioxide could provide a wide range of hydrocarbon fuels, including methane, methanol, ethylene – even kerosene. Most are familiar fuels on Earth, and they can also be used in rockets. Nitrogen provides still other possibilities, including hydrazine (a fuel) and nitrous oxide and nitrogen tetroxide (oxidants). The main issues are complexity of the chemical conversion process, energy requirements and storability.

It turns out that at least some of these basic chemicals are available on the Moon, Mars comets and some asteroids. The idea is to mine the raw materials in one or more of these places and manufacture propellant in space, then deliver it back into Earth orbit. Once a spacecraft arrives in orbit, it “tanks up” at a waiting fueling station and is on its way.

While at first glance it might seem like a crazy idea to ship propellant millions of miles across space, one has to realize that space transportation is VERY different from transportation on Earth. Because there is no friction in space, it often requires less energy to ship something a long distance, if the material originates on a body with low gravity relative to the Earth.

Energy requirements for future spaceships

For instance, the Moon’s gravity is about 1/6 that of Earth, meaning someone who weighs 200 pounds (about 90 kilograms) on Earth would only weigh 33 pounds (15 kilograms) on the Moon. Getting a spacecraft off the Moon and into orbit similarly requires about 1/6 the energy (technically, change in velocity or “delta-v”) as getting it into orbit from the Earth.

While it does require more energy to get back to LEO from lunar orbit, the overall energy requirement is roughly half that of launching it from the Earth’s surface. Serious companies have now proposed exactly this idea as a way to provide commodity-priced hydrogen and oxygen propellant for future spaceships.

While it requires almost as much energy to send material back from Mars as it does to launch it from Earth into orbit, making propellant on Mars for the return trip still makes a lot of sense, again due to the lower gravity (38 percent of Earth). So it is likely that along with fueling stations in orbit around the Earth, there will be stations on the surface of Mars or in Mars orbit to service routine spaceflight departing from (or refueling at) the planet.

Sending material into Earth orbit from asteroids or comets might require even less energy, because the gravity of these objects is almost negligible. If orbiting objects with the right composition can be found and reached, we might be turning them into propellant (plus other products – asteroids may also contain valuable and even precious metals, as well as rock minerals and perhaps volatile chemicals), ushering in a thriving space propellant logistics industry. The next “oil boom” would not be based on oil, and it may well be in the sky!

What do you think about the idea of fueling stations in orbit or on other planets? Share your thoughts in the comment section below.

Please note that this article expresses the opinions of the author and does not reflect the views of Move Forward.