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There are a lot of ways to get around in space, from using plain old sunlight to making super-hot plasma. We’ve talked about a lot of propulsion methods over the years, and now, it’s time for some highlights!

Hosted by: Hank Green

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Links to Original Episodes and Sources:

Using Sunlight to Propel Spaceships

Thrusters That Eat Teflon! | Pulsed Plasma Thrusters

The Future of CubeSat Propulsion

The VASIMR Engine: How to Get to Mars in 40 Days

Photonic Propulsion: Mars in 3 Days?

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If you want to travel the universe, well, you gotta figure out how to travel the universe. How to get your spacecraft from point A to point B. And it's definitely not as simple as pointing your rocket at the stars and hitting go.

Depending on how big your mission is and where you're going, there are a lot of ways to get around in space, from using plain old sunlight to making super hot plasma. We've talked about a lot of propulsion methods over the years, and now it's time for some highlights.

So we'll start with what is maybe the simplest method: solar sails. Basically, you just open your big sail up and let sunlight push you around except we don't normally think of sunlight as being able to push anything. abd so i will let reid share more with a video from 2015

In 2010, the Japanese Icarus spacecraft became the first probe to successfully propel itself through space using nothing more than light from the sun. That's because it had a solar sail, which is just like a regular sail except that it uses light to push itself along instead of wind.

Icarus is still out there orbiting the sun, but back here on Earth scientists are preparing for new missions with solar sails. Even though we didn't really start using them until a few years ago, most of the physics behind solar sails were worked out in the early decades of the 20th century. That's when physicists who studied relativity and quantum mechanics--like Albert Einstein, Max Planck, and Louis de Broglie--showed that photons that make up light have some weird properties. It might help to think of photons like little massless, sizeless tennis balls that are also waves at the same time, because photons act as both particles and waves.

Okay, so photons aren't really like tennis balls, but they kind of work in similar ways when it comes to momentum. When a tennis ball hits something like a chain link fence, it transfers momentum to the fence, which pushes the fence in the direction the ball was moving. Then the posts pull the fence back, which is why it rattles and swings back and forth. 

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Light pushes solar sails in pretty much the same way, but there are no posts to keep them in place. Light hits the sail, some of the momentum is transferred, and the sail is pushed in the direction the light was moving.

There is one big difference, though: photons don't have mass, and tennis balls do. And an object's momentum is usually defined as its mass multiplied by its velocity. Since 0 times anything is still 0, massless photons seem like they shouldn't have any momentum to transfer.

Those physicists in the early 20th century, though, showed that photons manage to have momentum without any mass. Instead, a photon's momentum is proportional to its frequency--how quickly it oscillates back and forth as a wave--instead of to its mass. 

That means that different colors of light have different amounts of momentum. Blue light has a higher frequency than red light, so photons of blue light have more momentum than photons of red light and would push a solar sail more.

When light pushes, it's called radiation pressure, and it's something that space scientists have been using and accounting for since the early 1960s. Like when they were designing the Mariner 4 Mars probe, for example, which used radiation pressure to stabilize itself and to make sure it stayed on course.

Because they compensated for the push from sunlight, Mariner 4 became the first probe to ever return pictures of another planet from deep space when it sent back images of Mars in 1967.

Radiation pressure is also being used to keep the Kepler Space Telescope stable. We've talked about Kepler before. Its main mission was to find planets around other stars by staring at them for years at a time. To do this while orbiting our sun, Kepler neederd to be able to rotate slightly every once in awhile so that it could keep an eye on the same patch of sky while it moved. But in the vacuum of space, friction won't stop you from rotating once you've started. So Kepler had what are called reaction wheels, which would basically spin in the opposite direction to stop the telescope from rotating out of control.

In 2013, when two of these reaction wheels stopped working, though, it looked like Kepler's days were numbered. But in 2014, the team working on the project was able to use the sun's radiation pressure to change where the telescope was facing. 

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And it's still working more than a year after the fix, all thanks to light's ability to push.

It's one thing to keep a ship stable, but for radiation pressure to be a practical way of propelling a spacecraft, solar sails have to be huge, like tens or hundreds of square meters, because there needs to be room for a lot of photons to hit it. To give you an idea of how small a photon's momentum really is, it takes about 2 million billion trillion blue photons to match the momentum of a single well-served tennis ball.

But, following the example of Icarus, some of the missions planned for the next few years are going to use solar sails. The Planetary Society, a group whose founders include Carl Sagan and whose current CEO is Bill Nye, is planning a mission for 2016, that they call LightSail-1, which will orbit around Earth with a solar sail big and reflective enough to be seen from the ground. And, NASA's near-Earth asteroid scout mission, slated for July 2018, is going to test out a solar sail on its way to scope out near-Earth asteroids.

Both of these missions are going to be unmanned, but there's no reason why solar sails can't be used on missions with humans on them someday, moving through space using only the pushing power of light itself. 

Hank: LightSail-1 was successful by the way, though when we filmed this video that I'm talking to you from right now in 2020, that asteroid scout mission still had not happened yet. Maybe next year. Now solar sails are pretty straight forward, but that's definitely not true of all propulsion systems. Like if you're using pulsed plasma thrusters to get around, you're feeding your spacecraft Teflon. Here's more from Caitlin. 

Caitlin: While traveling in space, one of the hardest things to do is stop, or change direction. Without anything to push against or friction to slow things down, spacecraft need to do all the hard work of changing their speed or path. And sometimes they do that in ways you would never expect, like by vaporizing Teflon.

They're called pulsed plasma thrusters and they can use the same stuff that's on your frying pan to make spacecraft zoom around the universe.

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