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5 Green Technologies for Interplanetary Space Travel

What sort of green technologies might we rely upon to get us to distant planets? See more green science pictures.

On July 20, 1969, four days after launching into space, the Apollo 11 command and service module Columbia landed on the Earth's moon. People watched televisions and tuned radio stations to follow the dramatic landing. It was the culmination of years of hard work and training. Designing a vehicle capable of transporting humans to the moon and back to Earth safely was a challenge.

The Columbia returned safely to Earth on July 21, 1969. The entire mission lasted 195 hours, 18 minutes and 35 seconds -- that's a little longer than eight days. The distance from Earth to the moon back in July 1969 was approximately 222,663 miles (358,342 kilometers). That might make your daily commute seem insignificant, but it's still just a hop, skip and a jump away compared to a visit to a neighboring planet.

A trip to Venus, Earth's closest neighboring planet, would require you to cross 0.6989 astronomical units of space on average. That's just less than 65 million miles or around 104.5 million kilometers. And conditions on Venus aren't ideal for a getaway -- the surface temperature on the planet is 460 degrees Celsius (860 degrees Fahrenheit). A better vacation bet is a trip to Mars or one of its moons, but they're even further away.

With these vast distances in mind, it's important to come up with efficient systems that use as few resources as possible. Otherwise, getting off the ground could become a problem. By its very nature, interplanetary travel needs to be green to work. We've got five technologies, not listed in any particular order, that could help humans reach the astounding goal of setting foot on another planet.

Green Fuel
A cyrogenic chamber designed to test propellants
A cyrogenic chamber designed to test propellants
Courtesy NASA

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].

Space Elevators
A space elevator may become an alternative to blasting rockets into the atmosphere.
A space elevator may become an alternative to blasting rockets into the atmosphere.
Courtesy LiftPort Group

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.

Solar Sails
A four quadrant, 20-meter solar sail system is fully deployed during testing at NASA Glenn Research Center's Plum Brook facility in Sandusky, Ohio.
A four quadrant, 20-meter solar sail system is fully deployed during testing at NASA Glenn Research Center's Plum Brook facility in Sandusky, Ohio.
Courtesy NASA

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.

Water Recycling
Skylab 3 pilot Jack R. Lousma takes a hot bath. This water would need to be recycled for further use to conserve resources and storage space.
Skylab 3 pilot Jack R. Lousma takes a hot bath. This water would need to be recycled for further use to conserve resources and storage space.
Courtesy NASA

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.


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Author's Note: 5 Green Technologies for Interplanetary Space Travel

Green technology and interplanetary space travel may seem like a strange combination, but it makes sense. Green technology is all about finding environmentally-friendly and efficient ways to achieve goals. Interplanetary travel by necessity requires efficiency and safety. It's fun to imagine crossing the galaxy in a spaceship kitted out with replicators and holodecks, but it's a safe bet that our early days of space travel will be more about making every effort count.

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More Great Links


  • Brauenig, Robert. "Rocket Propellants." Rocket and Space Technology. 2008. (March 28, 2012)
  • Brumfiel, Geoffrey. "Experts Fuss Over Cost Of Nuclear Fusion Research." NPR. July 27, 2010. (March 27, 2012)
  • Carter, Lynn. "If Mars is only about 35-60 million miles away at close approach, why does it take 6-8 months to get there?" Curious About Astronomy. Cornell University. May 2003. (March 28, 2012)
  • Coffey, Jerry "How Long Does it Take to Get to Mars?" Universe Today. June 4, 2008. (March 28, 2012)
  • Environmental Protection Agency. "Hydrazine." January 2000. (March 27, 2012)
  • Heussner, Ki Mae. " One-Way Mars Mission: Would You Boldly Go?" ABC News. Jan. 13, 2011. (March 28, 2012)
  • Irvine, Dean. "Express lift to the stars." CNN. Dec. 4, 2006. (March 28, 2012)
  • Jones, Willie. "A Fusion Thruster for Space Travel." Ieee Spectrum. August 2011. (March 27, 2012)
  • LaMonica, Martin. "A reality check on nuclear fusion at MIT." CNET. June 29, 2011. (March 27, 2012)
  • NASA. "Green Flight Challenge." (March 29, 2012)
  • NASA. "Recycling Water is not Just for Earth Anymore." Nov. 17, 2008. (March 28, 2012)
  • NASA. "Space Shuttle and International Space Station Frequently Asked Questions." Feb. 24, 2008. (March 28, 2012)
  • NASA. "Water on the Space Station." Nov. 1, 2000. (March 28, 2012)
  • Park, Robert. "Voodoo Science." Oxford University Press: Oxford, U.K. 2000.
  • Saeta, Peter N. "What is the current scientific thinking on cold fusion? Is there any possible validity to this phenomenon?" Scientific American. Oct. 21, 1999. (March 28, 2012)
  • Smithsonian. "Apollo 11." National Air and Space Museum. (March 27, 2012)
  • Steitz, David and Newton, Kimberly. "NASA Seeks Proposals for Green Propellant Technology Demonstrations." Feb. 8, 2012. (March 27, 2012)
  • The Space Elevator Reference.
  • Wolfram Alpha. "What is the distance between Earth and Mars?" (March 28, 2012)
  • Wolfram Alpha. "What is the distance between Earth and Venus?" (March 28, 2012)

Wolfram Alpha. "What was the distance between Earth and the Moon in July, 1969?" (March 28, 2012)