Last month I fielded a question on the Macrobusiness site about the timeframe in which asteroid mining would happen with a throwaway line of “Timeframe = never. Anyone who thinks otherwise hasn’t done the energy calculations,” to which, of course the response was “show me the numbers.”
So now I figure I’ll do an irregular posting to try and get some perspective around the ideas the cracpot optimists keep coming up with to stave off having to deal with declining resources and energy on this planet.
We’ll start with some ballpark estimates of moving an asteroid full of resources from the asteroid belt down to the same orbit as the earth’s around the sun.
The total orbital energy of a body is defined as:
E = GmM/(2r)
G is the gravitational constant
m is the mass of the body
M is the mass of the body around which it is orbiting (the sun, in this instance)
r is the radius of the orbit
I’ll assume a one million tonne asteroid, as I don’t see the point of anything smaller (we are trying to save life as the crackpot optimists would like it to be, after all, and it’ll need humungous quantities of resources to achieve this). I’ll also assume we can find the asteroid we want in the nearer section of the asteroid belt rather than having to go closer to Jupiter to find it.
in which case the orbital energy of our asteroid while it’s in its original orbit is:
We want to get it down to earth’s orbit, so we want the new orbital energy to be:
The energy difference, which we will need to provide, is 2.41×10^17 Joules, or a shade over 72 billion kilowatt hours…
This, of course, is the theoretical minimum energy and does not include such niceties as getting the thing into earth orbit without wiping out all life on the planet. It also assumes the fuel to make the journey is freely available out there, which it is not, and that the fuel is weightless which it also is not.
So, fuel. If we believe the idiot I heard on the radio a few weeks ago, who said “fuel is easy to find in the solar system, water is after all the same as the fuel used in the space shuttle, just slightly rearranged” we’ll use electrolysis to dissociate water into H2 and O2 for our fuel.
First up, where is this “fuel”, exactly? Possibly in the asteroid belt somewhere, but pretty bloody hard to find if it’s there in usable quantities at all. Ice’s habit of sublimating into water vapour under the right conditions suggests that it’s bloody unlikely that there are any ice asteroids, so the only certain source is on a couple of the moons of Jupiter, somewhat further away. OK, let’s say you go get it – how much do you need?
Based on data from the space shuttle’s main engines (the only source of hard data iI can find out there at the moment):
The space shuttle carries 735,000 kg of fuel (LOX and LH2)
The three main engines produce 6270kN of thrust combined, which roughs out to a shade under 2 million kW
They use all that fuel in 8.5 minutes
Which gives us 0.383kW/h and change per kilo of fuel
So, to move our 1,000,000 tonne asteroid, we need 188,516,153 tonnes of fuel
We need 3.66kW/h of electricity to dissociate 1kg of water into H2 and O2
We also need energy to compress and cool it to allow us to store it for later use.
So this works out to 1,004,162,710,943 kW/h of electricity to make our fuel out near Jupiter.
And where is that leccie going to come from? Solar? Out there, solar insolation is a tiny fraction of what we see here on earth, so we will need thousands of times more solar panels than we would need here to make the same power.
But, let’s assume we managed that somehow; now we need to get the fuel down to the asteroid, a roughly similar distance as the asteroid has to move to get to earth.
So if we need 188 times as much fuel as we have asteroid to move it where we want to go, we’ll need 188 times as much fuel to move the fuel to the asteroid (which works out to around 35,344,000,000 tonnes of fuel to move the asteroid’s fuel…)
And I haven’t calculated how much extra fuel will be needed to move the unburned fuel in each of these transfers, as before you’ve used all your fuel, you need to move the unburned fuel as well as the cargo…
And when you consider that neither Napoleon nor Hitler could manage supply lines from western Europe to Moscow in something as benign as a Russian winter (benign as compared to the cold and vacuum found once you get “off this rock”), it’s hard to see how even the most optimistic crackpot could consider such a venture possible, let alone having to make this work thousands of times every month…
So really, it’s just not going to happen.
And before you go criticising my choice of rocket motor, while we can look down our noses at 8088 powered IBM PCs as “80’s technology,” sadly the Rocketdyne RS-25, aka the space shuttle main engine, is pretty well the acme of LOX/LH2 engine design & is slated to continue in service as the core engine in NASA’s Space Launch System (SLS), should that ever get off the drawing board into reality.
I think that the blind faith that gets people fired up about this stuff comes from the inablity to see that although a CGI dude can stick a blue glow around the “exhaust” of picture of a “reactionless motor”, this is storytelling, not science and that wishing such things to be true will not automatically make them happen. Kind of Tony Robbins in Space. Sigh.
While it is certainly true that propulsion systems will be different in 100 years time , I’m more inclined to think they’ll involve a lot more muscle power and a lot less energy from other sources.
Really the only cure for the over reliance on non-existent technologies for imagining the next phase of human existence would be to ditch the television. Way too many people can’t tell the difference between CGI and real science/technology…
*photo of eros from the wikimedia commons