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  • We are prisoners on Earth. The Universe taunts us, by showing all the places we can't ever visit.

  • However, if our species wants to have a long term future, we have to escape our prison.

  • But what is keeping us here in the first place?

  • Turns out, we owe the universe a debt that is 4.5 billion years old.

  • *Intro*

  • Everything with mass in the universe attracts every other thing with mass.

  • We call this phenomenon 'gravity'.

  • The closer you are to a big chunk of mass, the stronger the attraction or the more you're pulled

  • in.

  • This effect traps us on earth.

  • We can imagine this as being prisoners in a gravity prison, or a gravity well.

  • It's not a literal well, but a handy concept to understand how this works.

  • Being in a gravity prison means that you owe gravity energy.

  • But how can you owe energy?

  • Because in our universe, things don't want to change their speed or direction.

  • To convince them to move, you have to expend energy.

  • Billions of years ago, the gravitational attraction of trillions of trillions of dust particles

  • orbiting our sun caught them together until they formed a planet.

  • This process used energy and created the gravity well we're now a part of.

  • The deeper you are inside the gravity well, the more energy you owe gravity.

  • If you don't find a way to get enough energy, then you aren't able to leave no matter what you do.

  • Because your atoms were once part of the dust that the universe expended energy on to get to this place.

  • Ok. Hmm...Let's summarise all of that again.

  • Objects in the universe don't like to move. You have to convince them to do so with energy.

  • Gravity used energy to convince the parts of our planet to move together.

  • This created a gravity prison in the process, trapping us .

  • To escape, it we need to repay it with energy.

  • Ok. How do we do that?

  • To get into space, we need to go through a complicated process of exchanging energy.

  • For this purpose, we build a negative potential energy repaying machine.

  • Known By their more boring name 'rockets'

  • Rockets work by using some of the most energetic chemical reactions humans know about

  • to basically explode fuel in a controlled way.

  • This converts chemical energy into kinetic energy.

  • The exhaust of the reaction is directed outwards,

  • and pushes the rocket away from Earth.

  • By expanding a lot of energy, we are increasing our gravitational potential energy.

  • Which is a complicated way to say that we are paying back our energy debt to gravity.

  • But it's actually a lot trickier than that.

  • When you burn fuel to get into orbit, you lose lots of energy to heat, exhaust and atmospheric drag

  • ,so you actually need much more.

  • And you can't just pile a huge amount of radioactive, really explosive, dangerous fuel close to your

  • payload and detonate it.

  • You need a controlled burn which is complicated and makes your rocket very heavy

  • ...which means it has more mass.

  • The more mass something has, the more energy you need to convince it to move,

  • so you need more fuel to lift up your rocket.

  • But,if you need more fuel, that means you need more rocket to carry that fuel!

  • But, this makes your rocket heavier thus requiring more fuel which requires more rocket to carry that

  • new fuel and so on.

  • At the end of this madness, you need closer to a 100x the weight of your payload to launch

  • Ariane 6, for example (the european rocket) will weigh 800 tons and should be able to

  • transport 10 tons into geostationary transfer orbit or, 20 tons into medium earth orbit.

  • But a rocket can only produce so much thrust, so there is a maximum weight, after which it just won't

  • take off.

  • If you add too much weight, it won't lift off. So, you can't just build bigger and bigger fuel tanks

  • This is the tyranny of the rocket equation and it means space flight will never become easy.

  • But wait, it gets worse.

  • Getting to space is still not good enough- you're still inside the gravity prison at the edge of space and

  • will crash back to Earth.

  • Staying in space is much harder than getting there.

  • To get to a stable position, where it can stay for a while, a rocket has to reach low Earth orbit.

  • To do this, you need a lot of kinetic energy which means going extremely fast

  • at an altitude of about 100 km, this is 8 km/s

  • 28,000 km/h is fast enough to travel around Earth in 90 minutes.

  • Here, we can use a trick.

  • Instead of flying straight up, we can go sideways!

  • Earth is a sphere.

  • So, if you're going sideways, fast enough (even though you're falling towards Earth) the ground will curve

  • away beneath you.

  • So, as long as you're above the atmosphere (about 100 km up) you'll be able to stay up there in orbit.

  • This is what the ISS does (falling around Earth, expending energy form time to time) to stay

  • fast enough.

  • If we look at orbits in scale, we see that near Earth orbit is laughably close to Earth.

  • To deploy, for eg Satellites, on leave for other planets, requires another round of energy debt repayment.

  • Getting to orbit is the most difficult part of space flight for us right now.

  • For example: If we want to send a rocket to Mars, half the energy is necessary just to get into orbit

  • and the other half for the 55 million km to Mars.

  • Therefore, to be as effective as possible, rockets aren't built in one giant piece.

  • Instead, we use multistage rockets. We don't need to carry an empty fuel tank, so rockets drop it.

  • Rockets today, shed their boosters and main stage as they ascend with each successive stage being it's own

  • fully contained rocket, complete with its own engine and fuel.

  • Ok. So this is why getting to space is hard.

  • If you feel all of this seems really complicated, don't worry. It's literally rocket science!

  • This video was made possible in part by a sponsorship by Airbus Safran Launchers

  • and Arianespace.

  • Who are getting their new Ariane 6 rocket ready to launch into space in 2020.

  • You can learn more about the rocket here.

  • And as always, if you like what we do please consider supporting us on Patreon.com.

  • It really helps us out a lot!

  • If you're craving more space stuff now: here's a playlist for you.

We are prisoners on Earth. The Universe taunts us, by showing all the places we can't ever visit.

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