[ ♪ 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 ]