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  • SciShow Space is supported by Brilliant.org.

  • [ ♪ Intro ]

  • No matter what your mission is, one of the biggest obstacles to getting to space is fuel.

  • Obviously, you need some to get off the ground.

  • But fuel is also heavy, so if you want to lift a full rocket,

  • you're going to need more of it.

  • But that just makes everything heavier, so then you need even more fuel.

  • And on and on.

  • This is a part of what keeps us from sending probes to lots of different places in one trip:

  • Moving around just takes so much heavy fuel.

  • But back in the 1980s, scientists found a loophole,

  • a way of touring the solar system on almost no fuel at all.

  • It's called the Interplanetary Superhighway.

  • These days, moving between objects in the solar system, like going from Earth-orbit to Mars,

  • generally requires what's called a Hohmann transfer.

  • You fire your rocket and speed up until your orbit around Earth gets so big that it crosses Mars's orbit around the Sun.

  • Then, once you pass near Mars,

  • you use your rocket to slow down and officially begin to orbit the Red Planet.

  • All that speeding up and slowing down burns a lot of fuel,

  • but Hohmann transfers are one of the most efficient ways of getting between any two bodies in space.

  • Then, there are also gravity assists, which can be even more efficient.

  • In them, a craft speeds up by using a different planet's gravity to slingshot past it without burning much fuel.

  • They're used all the time to get probes far from Earth.

  • But if a spacecraft needs to orbit somewhere after a gravity assist,

  • it still has to slow down when it gets there.

  • And, again, that takes a lot of fuel.

  • So in the 1980s, a few scientists tried approaching things a little differently.

  • By thinking of gravity more like a landscape than a slingshot,

  • they stumbled upon the Interplanetary Superhighway.

  • Its entrance and exit ramps are Lagrange Points:

  • Five regions where gravity and rotational forces balance.

  • They exist whenever one body orbits another, and they're creatively named L1 through L5.

  • If you stick a spaceship at a Lagrange point, it will stay there,

  • because gravity doesn't pull it more in one direction than another.

  • But not all Lagrange points are made equal.

  • Some points are stable: In other words, if you're there and you move away a bit,

  • gravity tends to pull you back, kind of like a ball at the bottom of a valley.

  • Other points are unstable: If you move away a little, gravity keeps pulling you farther out.

  • Spacecraft at these points are like balls on a hilltop:

  • They'll stay if they're delicately balanced or if you hold them,

  • but the slightest breeze will send them speeding downhill.

  • Normally, getting around the solar system requires accounting for and navigating all these hills and valleys,

  • burning a bunch of fuel along the way.

  • But instead of just dealing with all these Lagrange points,

  • the Interplanetary Superhighway puts them to work.

  • See, there are five Lagrange points for every stable orbit,

  • the Moon around the Earth, the Earth around the Sun, each of Saturn's moons around Saturn,

  • you name it.

  • So they're everywhere, although they aren't always in the same place.

  • The points always follow the less massive object around its orbit,

  • and that creates a constantly changing landscape of gravitational hills and valleys throughout the solar system.

  • In other words, the distribution of gravitational forces in the solar system changes over time:

  • Sometimes, a spot might be mostly pulled to the Sun, sometimes mostly to a planet,

  • and sometimes neither or both, when a Lagrange point passes by.

  • The metaphor of hills and valleys just helps all of us, professional and non-professional scientists,

  • get a better grip on that abstract idea.

  • Occasionally, in space, some hills pass near each other or even overlap.

  • And when they do, space travel can get really efficient.

  • For example, if you're on a gravitational hill that overlaps a valley,

  • you can just roll right down into it.

  • Or if you're on one hilltop that overlaps another, you can just move from summit to summit.

  • When Lagrange points from different orbits get near each other,

  • it takes almost no effort to get from one to the next.

  • That means, if you're willing to wait for the right overlaps, you can hop from orbit to orbit,

  • say, from Earth to Mars to Jupiter and beyond, with almost no fuel.

  • That's the idea behind the Superhighway:

  • It uses this ever-evolving network of Lagrange points

  • to let you to travel between orbits practically for free, no fuel required.

  • Officially, all these moves between points are known as low-energy transfers,

  • and they have been used in a couple of missions.

  • One of the better known ones was a successful last-ditch effort to save a Japanese lunar probe in 1991,

  • where there wasn't enough fuel to salvage the mission the conventional way.

  • Unfortunately, though, we can't use these transfers for every mission,

  • because it can sometimes take years or even millennia for Lagrange points to favorably overlap.

  • But if we were willing to wait, we could send a probe out somewhere in space,

  • where it could collect data for a while and wait for a good overlap.

  • Then, when one popped up,

  • it could glide over and collect data somewhere else until another alignment appeared.

  • The craft could wander through the solar system almost indefinitely,

  • inspecting whatever happened to get close enough and teaching us a lot along the way.

  • Admittedly, it would be a pretty different type of space exploration than what we're doing now,

  • but it could be a promising option for a future long-term mission.

  • And it would all be thanks to the Interplanetary Superhighway.

  • But what if I want to explore the universe now?!

  • Luckily I can always pretend to be an astronaut from the future

  • with this Brilliant.org quiz on interstellar travel.

  • If you liked this episode, I think you'll really enjoy the whole Astronomy course on Brilliant,

  • and this quiz addresses a lot of the challenges of space travel we've been talking about.

  • Right now, our friends at Brilliant are offering the first 200 SciShow Space viewers that go to

  • brilliant.org/SciShowSpace a 20% discount on an annual premium subscription.

  • You'll sharpen your math and science skills,

  • and maybe help humanity get one step closer to traveling the interplanetary superhighway.

  • So, get going!

  • Time's a-wastin'!

  • [ ♪ Outro ]

SciShow Space is supported by Brilliant.org.

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