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Chemical rockets have done a lot for humanity.
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Liquid hydrogen and liquid oxygen engines powered the Space Age, but they can only get us so far.
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If we want to travel to deep space and beyond without waiting for the planets to align,
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we're going to have to come up with some new propulsion systems.
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The problem with chemical rockets is the fuel is heavy, and for all that weight, it's not very efficient.
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The fuel's energy is limited to what's stored in the chemical bonds,
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and after 90 years of research, chemical rockets aren't going to get much better.
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Plus, once that candle's lit, it uses all its fuel in one burst
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and then coasts the rest of the way to the destination.
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If the kids are yelling in the back, you can't turn this rocket around, you know what I'm saying?
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Well, you could orient it that way with hydrazine engines like the ones satellites use,
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but those don't actually have enough thrust to do much more than orientation.
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Whatever way the chemical rocket was pointing, that's where you're going.
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So to explore deep space, NASA is looking past the energy stored in chemical bonds.
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Another propulsion technology that's been around since the 60s is just showing promise in the last 20 years: ion engines.
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Ion engines work by accelerating charged atoms, like xenon ion, through a magnetic field, and out the back of the spacecraft.
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The fuel is lightweight and provides a low amount of thrust over a very long period of time,
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so in theory, they're great for long-term deep-space exploration.
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Early engines, though, destroyed themselves as the ions eroded the walls and it's hard to have a long-term mission with a short-term engine.
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Engineers have finally cracked the puzzle by diverting the magnetic field around the walls, to stop the ions from bombarding it, and recent missions
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like the Dawn Space Probe, sent to the asteroid Ceres, used ion engines to power it once it was out of Earth's orbit.
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Another novel idea is getting rid of onboard fuel altogether, and letting the Sun push the craft along.
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That's the principle harnessed by solar sails, and they're exactly what they sound like:
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they're sails as large as a football field, and 40-100x thinner than a sheet of paper.
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When fully unfurled, they catch the sun's light, and away they go.
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Despite photons having no rest mass, they do have energy, which means they do carry a tiny amount of momentum.
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When they bounce off the craft's surface, they impart their momentum.
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So, an enormous sail out in space can take advantage of the practically endless stream of photons from the Sun,
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accelerating for as long enough light keeps hitting it.
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The Japanese space agency JAXA launched the first solar sail IKAROS in 2010,
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and one month after it unfurled, JAXA reported the craft was accelerating due to photonic pressure.
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(Just don't go too close to the Sun, IKAROS.)
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Solar sails work well when they're close enough to the Sun,
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but out past Mars, the power of the Sun fades, making solar sails impractical past that point.
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If you can't wait for an ion engine or solar sail to pick up speed, there is the nuclear option...
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literally. Some scientists have proposed using fusion or efficient propulsion systems to get humans to Mars in one month instead of seven.
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Proposals vary, with some using nuclear reactors to generate plasma
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that's then accelerated with a magnetic field -- like an ion engine on steroids.
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Others would use hydrogen atoms that are forced together by collapsing lithium rings around them to generate pulses of fusion.
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This method could provide the same amount of energy as four litres of rocket propellant with an amount of fuel as big as a grain of sand.
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But these ideas have a lot of hurdles, and are still a long way off.
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Then again, so are other planets,
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and if we're going to send humans to any of them,
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we prefer the journey be as fast as possible.
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We'll still need chemical rockets to get us out of Earth's orbit.
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But once we develop these technologies, who knows where we go from there?
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Hey! While you're here, check out this next video at NASA's Jet Propulsion Laboratory
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which shows how the design and manufacturing of future deep space exploration is sometimes based on origami.
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