Liquid hydrogen and liquid oxygen engines powered the Space Age, but they can only get us so far.

If we want to travel to deep space and beyond without waiting for the planets to align,

we're going to have to come up with some new propulsion systems.

The problem with chemical rockets is the fuel is heavy, and for all that weight, it's not very efficient.

The fuel's energy is limited to what's stored in the chemical bonds,

and after 90 years of research, chemical rockets aren't going to get much better.

Plus, once that candle's lit, it uses all its fuel in one burst

and then coasts the rest of the way to the destination.

If the kids are yelling in the back, you can't turn this rocket around, you know what I'm saying?

Well, you could orient it that way with hydrazine engines like the ones satellites use,

but those don't actually have enough thrust to do much more than orientation.

Whatever way the chemical rocket was pointing, that's where you're going.

So to explore deep space, NASA is looking past the energy stored in chemical bonds.

Another propulsion technology that's been around since the 60s is just showing promise in the last 20 years: ion engines.

Ion engines work by accelerating charged atoms, like xenon ion, through a magnetic field, and out the back of the spacecraft.

The fuel is lightweight and provides a low amount of thrust over a very long period of time,

so in theory, they're great for long-term deep-space exploration.

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.

Engineers have finally cracked the puzzle by diverting the magnetic field around the walls, to stop the ions from bombarding it, and recent missions

like the Dawn Space Probe, sent to the asteroid Ceres, used ion engines to power it once it was out of Earth's orbit.

Another novel idea is getting rid of onboard fuel altogether, and letting the Sun push the craft along.

That's the principle harnessed by solar sails, and they're exactly what they sound like:

they're sails as large as a football field, and 40-100x thinner than a sheet of paper.

When fully unfurled, they catch the sun's light, and away they go.

Despite photons having no rest mass, they do have energy, which means they do carry a tiny amount of momentum.

When they bounce off the craft's surface, they impart their momentum.

So, an enormous sail out in space can take advantage of the practically endless stream of photons from the Sun,

accelerating for as long enough light keeps hitting it.

The Japanese space agency JAXA launched the first solar sail IKAROS in 2010,

and one month after it unfurled, JAXA reported the craft was accelerating due to photonic pressure.

(Just don't go too close to the Sun, IKAROS.)

Solar sails work well when they're close enough to the Sun,

but out past Mars, the power of the Sun fades, making solar sails impractical past that point.

If you can't wait for an ion engine or solar sail to pick up speed, there is the nuclear option...

literally. Some scientists have proposed using fusion or efficient propulsion systems to get humans to Mars in one month instead of seven.

Proposals vary, with some using nuclear reactors to generate plasma

that's then accelerated with a magnetic field -- like an ion engine on steroids.

Others would use hydrogen atoms that are forced together by collapsing lithium rings around them to generate pulses of fusion.

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.

But these ideas have a lot of hurdles, and are still a long way off.

Then again, so are other planets,

and if we're going to send humans to any of them,

we prefer the journey be as fast as possible.

We'll still need chemical rockets to get us out of Earth's orbit.

But once we develop these technologies, who knows where we go from there?

Hey! While you're here, check out this next video at NASA's Jet Propulsion Laboratory

which shows how the design and manufacturing of future deep space exploration is sometimes based on origami.

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