5 Green Technologies for Interplanetary Space Travel

It takes a lot of resources to put a vehicle into space. Not all of those resources are harmless. Hydrazine, used in rocket fuel, is a powerful propellant. But it's also toxic and corrosive. Organizations like NASA are now looking into green propellant alternatives to hydrazine.

Ideally, the new propellant would be less hazardous to handle than current rocket fuel, reducing the costs of organizing a space voyage. It should also break down into harmless components, eliminating the risk of polluting the environment.

Wishing for a green alternative to hydrazine doesn't make a new propellant magically appear. That's why NASA has invited companies and organizations to present technological demonstrations of alternative propellants. In February 2012, NASA announced that it would accept proposals until the end of April. A winning proposal could earn up to $50 million.

Reducing the environmental impact of launches is a big job. To launch a space shuttle into orbit, NASA used two solid rocket boosters, each carrying 1 million pounds (453,592 kilograms) of propellant. The shuttle itself carried an additional half-million gallons (1.9 million liters) of liquid fuel [source: NASA].

Listing all the challenges related to transporting humans safely to another planet could fill up a book or three. But one of the toughest problems to solve has everything to do with weight. The heavier a spacecraft is, the more fuel it needs to escape the Earth's gravity.

A journey to another planet would last for several months. Assuming you're either going to set up shop on a new planet or plan a return trip, you'll need plenty of supplies to keep you alive. Those supplies have weight and volume, requiring more fuel to get you up in space in the first place.

One potential solution to this problem is to build a space elevator. Here's how it works: We put something with a lot of mass in geosynchronous orbit around the Earth -- that means it will stay in orbit above a fixed point on the planet's surface. Then we attach a cable between the orbiting mass and an anchoring point on Earth. Now all we have to do is build an elevator that can climb the cable out into space!

It sounds like science fiction, but many engineers and scientists are working on building space elevators. Compared to launching a rocket into space, a space elevator is a bargain. The elevator could take equipment and even humans into space. Once there, we could assemble spaceship pieces and build a craft in space itself. There's no need to launch the craft from Earth because it will already be in orbit.

Once you're in space, whether by launching a rocket or departing a space station, you'll need some way to propel your spacecraft toward its destination. That may require you to carry an on-board fuel source. Ideally, you'll have an efficient system so that you don't have to dedicate too much space to carry fuel. One potential solution is fusion.

Fusion is the method by which the sun generates energy. Under intense pressure and heat, hydrogen atoms smash into each other and form helium. Hydrogen has a single proton and helium has two of them. During this process in which two hydrogen atoms fuse together there's a release of neutrons and energy.

But there's a big problem -- we haven't figured out how to use fusion to generate power in a reliable and sustainable way. The process requires incredible amounts of heat and pressure. Just generating the conditions necessary for fusion can require a great deal of energy all on its own. The goal is to reach a point where we can initiate fusion and keep the process going while we harvest energy. We're not there yet.

If we ever do get there, fusion may be a good choice for powering spacecraft. We could harvest a great deal of energy from a comparatively tiny amount of fuel. Fusion could generate the power necessary to operate thrusters to allow for in-flight adjustments as we fly our way to the next planet over. But whether fusion is a practical option remains to be seen.

Another alternative to blasting toward distant planets using rocket thrusters is to sail there. But what good are sails in an environment that doesn't have wind? Enter the solar sail!

Solar sails use the sun as an engine. The sun emits photons -- the basic units of light. We know that photons act as both waves and particles. Photons can seem insubstantial to us here on Earth but they exert a force on objects as they come into contact with them. This includes solar sails.

A solar sail is made of an ultrathin mirror that stretches across a large area. As photons strike the mirror, they exert a force and push against the sail. The sail is hit by billions of photons -- enough to push the sail and anything it might be tugging along through space.

At first, traveling in a vehicle pulled by a solar sail would be pretty dull. You wouldn't have a great deal of initial thrust like you do with a rocket. But the power of those photons can't be denied, and your spacecraft would continue to accelerate well beyond the point a thruster could manage. Not only do you not have to worry about fueling your spacecraft for interplanetary travel, you'll also reach your destination faster!

Solar sails could work well in space, but they aren't designed to get a craft off a planet's surface. For that, we'd still have to either use rockets or construct the spacecraft while in orbit. And a solar sail might be able to get us to another planet but without other means of leaving our new world we'd be stuck there. But for a one-way trip to another planet, a solar sail could be just the thing -- and you never need to worry about running out of fuel.

Propelling a spacecraft to get us to another planet is just one challenge. Another is making sure we have the resources to stay alive on board our spacecraft while we make our way to our destination. Even a visit to a nearby planet would require months of travel. With weight and space at such a premium, how do you determine how much water to bring and how do you manage it?

To say that every drop of water aboard a spacecraft is precious is an understatement. On board the International Space Station there are systems that recycle 93 percent of the water used [source: NASA]. The processes purify water so that it may be used repeatedly, reducing the need to send up more water from Earth.

That means gray water -- the waste water produced after cleaning dishes, clothes or even people -- can be made into drinking water again. But that's not all! Even sweat and, yes, urine are processed. Everything is filtered out and only pure water remains.

The waste water moves into a distiller. The distiller rotates in order to simulate gravity -- otherwise contaminants in the liquid wouldn't separate. Water passes through a filtration system that uses materials like charcoal and chemical compounds to bond with contaminants, letting only the water pass through.

A long space flight won't have the chance to pick up more water along the way. Conserving every drop possible will be a necessity. And some of that technology may even find its way into systems down here on Earth.