Let’s use particle accelerators to propel our spaceships!
Wait, back up, why should we even try to reach Alpha-Centauri? Because the fate of the human race depends on it. Sure, we have five billion years to escape the death of the sun, but we gotta move sometime. Otherwise we’ll just happily inhabit this solar system until our star becomes a red giant and boils away our oceans and continents. Humans tend to be excellent procrastinators, and any kind of deadline over a few months is easy to ignore, but it’s worth starting to think about now.
The single biggest barrier to interstellar travel is the unimaginably vast screaming void of space. Our nearest neighbouring solar system is four light years away. To put that into perspective, light travels three hundred thousand kilometres in a second. Now imagine how far it can travel in the time between federal elections. Nope, you couldn’t, because numbers like that break our primitive brains.
So what’s the big deal? Just strap on a giant rocket and blast it until we get there. Well, that’s what NASA did to get to the moon, and it took three days to get there at eleven kilometres per second. Travelling at those same speeds to Alpha-Centauri would take one hundred thousand years. On top of that, the amount of fuel/propellant needed would be absurd.
Ion engines are used by lots of satellites and spacecraft nowadays. They exert a gentle push by accelerating charged particles out of the engines using electromagnets. Small amounts of thrust in space is fine; there’s no friction, so a little adds up over time. The problem with them is that they still consume propellant. Even if we could develop ion engines with more than just the puny thrust they have now, any deep-space craft, or deepcraft, would still need loads of extra mass for propellant.
This is where particle accelerators, made famous by the Large Hadron Collider, come in. Essentially they are ion engines, but on steroids. They hold the key to the most efficient engine possible: a relativistic mass thruster. According to Einstein’s special theory of relativity, the closer an object gets to the speed of light, the heavier it gets. Travelling at 99.995% of light speed means you weigh one hundred times more. This extra mass comes from pure energy – the energy needed to accelerate it so damn fast. Think about what that means for the weight allowance for propellant if you could launch it out of your thruster at those speeds. If you’re multiplying the effectiveness of your propellant by one hundred, you only need one hundredth of it to get the same effect.
Let’s say you want to get to our nearest star in twenty-two years. One year at the beginning will be spent accelerating to 20% of the speed of light. The last year will be just slowing down again. This is a gross simplification, as the two stars are moving about as well and aren’t just fixed points in space. There are no fixed points in space anyway, that was the whole point of relativity. Two decades sounds long, but it’s within a human lifetime, and still leaves middle aged astronauts the rest of their lives to do their thing.
To simplify things, let’s use relativistic kinetic energy. Kinetic energy is just the measure of energy it takes to get an object travelling at that speed. The relativistic just refers to the correction that Einstein applied to the formula to account for all those zany things that happen to time, space and mass close to the speed of light. To be absurdly optimistic, assume the deepcraft is only one hundred metric tons. This means that when the year of acceleration is over, and it’s travelling at 60,000 kilometres per second, the relativistic kinetic energy is 185 million million million joules (1.85xE^20 joules). Now that’s a vast amount of energy. It would take the world’s biggest nuclear reactor six years to generate that many joules.
You can use the Tsiolkovsky rocket equation to find out how much propellant you’d need to get a deepcraft flying at these speeds. Using the Apollo F-1 engines, the mass multiplier is twenty orders of magnitude. In other words, you would need something close to the mass of the Earth itself in rocket fuel to get our humble craft to Alpha Centauri. If we were to use the ion engines that can be found in modern spacecraft, you’d need approximately 640 tons of propellant for 100 tons of payload when you account for both speeding up and slowing down.
Here’s the best part of using particle accelerators instead. To find out how much propellant was used to accelerate the deepcraft, you just plug in a different velocity into the equation to get the propellant mass: just a pitiful 21 kilograms to get up to speed and the same amount again to slow down. If this mass is ejected over a period of a year, you end up with a steady stream of 0.67 milligrams per second. So depending on how much you know about particle accelerators, this will seem like either a tiny amount, or a crazy huge amount. The goal in this case is to maximise the amount of material you push out, so the design would have to be different. Ring accelerators like the LHC are great for speeding things up, but lose energy around the curves, so the best choice is a linear (straight line) accelerator called a linac. Even gigantic ring accelerators will use a small linac, often only tens of metres long, as an initial injector. Perhaps a single powerful beam is optimal, or maybe a large grid of hundreds of smaller accelerators – working out the maths for that is beyond this author’s skills. One nice bonus is that using superconducting magnets will be easy – to cool them to the temperatures needed, you only need to fail to heat them in space and they will naturally drop to a few degrees above absolute zero.
So far this all sounds great, but the biggest challenge will be the power consumption. This kind of drive is almost the equivalent of an electric rocket, in that it mostly needs power to run. How much power? 6 gigawatts, at the very least. To put that into perspective, Earth’s biggest nuclear power plant generates around one gigawatt of power, and it certainly weighs far more than one hundred tons. So then does that mean a relativistic thruster is science fiction? No, because any engine will still require this kind of power source, whether conventional rocket or ion engine, but also have the added inefficiency of extra energy needed for carrying all that propellant around. One possible solution is some kind of antimatter reactor – it’s the most efficient energy storage form, since it turns matter into pure energy. Unfortunately that kind of power source is totally theoretical, as the amount of antimatter the entire planet can currently produce is nowhere near enough.
Rather than produce that energy onboard, what if we were to store that energy in some kind of battery? Don’t scoff just yet – not the lead-acid kind of battery that lives in your car. How about a superconducting wire loop? This isn’t science fiction – terrestrial power stations actually use this form of energy storage for output smoothing, and it’s called SMES (superconducting magnetic energy storage). It’s biggest advantage is that it can dump all it’s power within seconds if needed, as it’s basically just electrical current in a zero-resistance wire going round and round for eternity. Once again, the coldness of space allows the use of cheap, light superconductors. To sweeten the deal further, the actual accelerator will likely be superconducting coils of wire, so you could create an elaborate grid which is both storage and accelerator at the same time to eliminate conversion costs. Although a superconducting loop has zero resistance, it does have a limit to how much it can store – the flowing current will set up an electromagnetic field that if strong enough, will break down the superconducting effect and create a “quench”. A quench basically means that all the stored power will suddenly encounter resistance, which will almost instantly turn it all into heat. Exajoules (billions of gigajoules) of power all released at once will result in an explosion ten thousand times more powerful than history’s biggest nuclear detonation, Russia’s stupidly overclocked “Tsar Bomb”. To prevent this, the likely design will be large, multiple loops of superconducting wire with a radius in the hundreds of metres. Since it’s space, aerodynamics don’t matter, so you can have large, bulky designs and get away with it as long as it’s assembled in orbit.
There are plenty of details being glossed over here; heat dissipation in space is tricky due to lack of atmospheric convection cooling, particle accelerators and SMES storage units are not 100% efficient, the thrust itself might degrade the engine itself over a period of decades, and plenty of other engineering challenges. Perhaps twenty-two years is far too ambitious, and the end result might be a scaled down version. Still, it is likely that any deepcraft humanity builds will have to use some form of relativistic thruster to bridge the chasms between solar systems. Rocket scientists are always trying to increase the velocity of their exhaust – this is merely the logical conclusion to that goal.
You would have to spend years building this contraption and charging up it’s crazy battery before you could launch it into the unforgiving oblivion of deep space, and in the end it would be worth it. Imagine humanity being able to reach the stars, not in some distant star-trek future, but using our primitive 21st century technology. Why procrastinate?