The Tyranny and Treachery of Rockets
We’re trapped here on our planet: there’s a debt of energy that we need to pay back if we want to ever travel to another world. To leave Earth we need to escape what’s called the “gravity well” — a dimple in the fabric of space and time created by the very large mass of the Earth.
A representation of Earth’s “gravity well”. Image by Mysid via wikimedia
To escape this well we need to pay a toll, and the only denomination it accepts is speed. You need to go fast enough to not only make it to space (100km up), but also fast enough to escape the pull that gravity has on us. Getting to space can be done with an X-15 jet plane, while getting out of the gravity well of Earth requires going quite a bit faster. For more on this difference see this great xkcd explainer.
What about satellites?
Satellites manage to stay in space by going sideways really fast. Anything that orbits around Earth is actually still trapped by gravity, and is constantly being pulled towards the centre. However, their sideways speeds mean that satellites are always “missing” the surface of Earth; they instead follow a circular path around Earth. Satellites like the International Space Station have really only paid back 10% of their energy debt — they aren’t free to jet off and visit other planets or solar systems as they please.
So why is it so hard to escape?
The answer comes down to some fairly simple physics (conservation of momentum), some moderately complex chemistry (what fuel you choose), and some pretty extreme engineering (how to design your rocket to get the most out of your fuel, and the physics) — click here for more on all these points. Unless you want to strap a nuclear reactor onto the bottom of your rocket, something most astronauts are rightly leery about, there’s only so much energy you can wring out of chemical reactions.
Space Shuttle Columbia taking off: 95% is filled with fuel! Image by NASA via wikimedia
Honestly, the chemistry and engineering are a bit beyond me, but the even the physics is sobering. Getting heavy things to start moving fast means giving them a big push. Getting a rocket into space takes a lot of fuel. If you light this fuel up all at once on the launch pad, you get what is generously called a huge chemical bomb, destroying the rocket and anyone nearby: we need to carry this fuel with us and burn it on our way out of the gravity well of Earth.
Sadly, the many tons of fuel required weigh… many tons. To get the fuel going up and out of the gravity well with the rocket you will need more fuel. This extra fuel you need will itself weigh quite a bit, so you need more fuel. And that fuel will also need more fuel. And so on.
The limits that the rocket equation sets out are rock-solid — unless our entire understanding of physics and chemistry is overturned we will still have to pay the toll. The simple equations of physics promise so much: put in enough energy and you can do anything! The kicker is in the details; escaping Earth is much harder than it seems. This is the tyranny and the treachery of rockets.
Get me out of here!
Is all hope lost? Will we never escape Earth without burning a huge amount of fuel? Stay tuned for my next post where I’ll go through some methods for making escape more affordable!