Notably, outer space has an annoying lack of power points (at least within any reasonably reachable distance). This, of course, means that powering your spacecraft becomes a more difficult exercise than turning on your TV or recharging your phone.
As the title to my post suggests, solar power is the obvious option. But before we delve into that, let's be whimsical and take a look at traditional methods of power generation and why they don't work in space.
Steam Power
Surprisingly, our mainstream method of generating power hasn't changed much since the industrial revolution:
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| I literally googled 'old timey steam train" |
- Boil water
- Water 'expands' into steam
- Steam pressure causes a turbine to spin
- Spinning turbine moves electrons - generates electricity
Why doesn't this work? Well for one, if you don't want your rocket to be the size of a small country, you'll want to design your system so that it can use a finite amount of water. The way that currently power plants do this is by having a fairly complex array of coolers and condensers and fancy tanks that regenerate the steam into water.
As it turns out, once you add up all the weight of the water + regenerating technology, you end up needing a rocket the size of a small country anyway.
Even then, the way we design all the high pressure equipment needed for a steam generator relies on being surrounded by one atmosphere of pressure. Throw all of this technology into the cold vacuum of space and suddenly having a steam generator becomes an incredibly complex engineering problem. No one to date could be really be bothered looking into the problem (though if you can find a paper I stand corrected).
One final problem - this process requires a heavy spinning shaft. Now on Earth, you can brace the spinning shaft against the ground. In Space? You won't be able to brace against anything. Newton's third law means that your satellite will start to spin in the opposite direction, which is bad if you're trying to take photos or point your antenna in a particular direction.
Combustion
Combustion engines are different. Instead of using expanding water pressure to spin a turbine, we instead explode petrol in cylinders that move pistons up and down. This spins a shaft in a dynamo and we get power. This is how diesel generators make electricity and how your car recharges its battery.
Why doesn't this work in space?
Well for one you still have a spinning shaft, so the problem of your satellite spinning out doesn't go away.
But let's say we solve that. Have you ever noticed how much a car engine vibrates? Have you ever noticed how much damping goes into engines, and yet they still vibrate and make noise? And you know how exhaust pipes on trucks judder and spit out gas? That's all excess kinetic energy. Again on Earth you can brace against the ground and add dampeners to make sure you don't go anywhere. In Space? That's a lot of force being braced by nothing. I wouldn't be so cruel as to deliver "the need to stabilise an engine block" on my Satellite motion control engineers.
Combustion also needs to operate at certain optimum temperatures. On the ground we have air coolers and cooling liquid to regulate temperature. In Space... we have no air and no non-wasteful way of keeping cooling liquid in a liquid state...
So how do you get stable electricity in space?
Radiation Poisoning
No I'm not joking, this is legitimately a solution. You get yourself some radioactive material. And you need stuff so radioactive that it heats up everything around it and you attach it to some magic electric elements, or thermocouples, that turn the excess heat into electricity. This is basically how you make a "Radioisotope Thermoelectric Generator" or "RTG".
This is actually a really popular method of generating power (as far as the term "popular" can go in modern space travel). The Cassini spacecraft (that you can thank for pretty flyby pictures of Jupiter) uses one, as does the New Horizons Spacecraft. In fact it turns out that most spacecraft that go beyond a certain distance of the Earth use one.
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| The same type of RTG used on New Horizons. Large, hot, radioactive column of death |
Note that I said beyond a certain distance of the Earth. Why beyond? For the obvious reason - if the Satellite decides to decay its orbit and burn up in our atmosphere (which most Earth orbiting satellites will do some day), it will rain fiery radioactive carcinogenic death through the atmosphere. And that's very bad.
For spacecraft closer to the Earth, we use...
The power of the Sun
Solar panel. Pretty simple. Get a tan, gather energy, use energy.
Except solar panels really suck at gathering energy.
A quick survey of papers on Google put the potential energy that we could capture from the sun at about 1000 watts per square meter. For reference - that's a surface area a little larger than a 42 inch TV, generating more than 10 times the amount of power required to power the TV. Satellites can be pretty smart with their power usage (more on this later) - so 1000 watts for a solar panel the size of a TV should be plenty for a satellite right?
Except that we can't capture all the damn energy. In fact not even close. Gallium Arsenide solar cells, the most efficient and widely used solar panels on space only capture on average 32% of this energy. Less than a third!
But what else do we have (short of putting a nuclear reactor in crashing distance of the Earth)? The answer is not much - which is why we end up having to use gigantic solar panels to generate enough energy, much like the monstrosities on the International Space Station
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That said, the inefficiencies of solar panels aren't actually a huge problem as far as the Earth's orbit is concerned. It only becomes a problem when you get into deep space - hence why New Horizons is using an RTG and Rosetta had to shut down to save power for most of its life.
We're trying. It's just unfortunate that the only alternative to solar power will probably give us cancer.
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