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Notes -
Contra sapce colonization
A couple arguments against space colonization, in order of how convincing they are to me. A lot of arguments in favor of space colonization like to make specious arguments based on the proposed similarity between the colonization of the Americas and Mars/Venus/Moons of Jupiter. While potentially highlighting psychologically similar explorer mindsets, I think these arguments completely miss the physical realities of space.
1. Ecology and Biology
The newest Tom Murphy post from DoTheMath has clarified what I believe to be a huge blindspot in the space colonization narrative that many on this forum: Ecology! Murphy's argument is that we've never successfully created a sealed, self-sustaining ecology that lasts for even anything close to a human lifespan. Biosphere 2 lasted for approximately 16 months, and the EcoSphere that Murphy uses as an example in this article lasts for about 10 years, but ultimately collapses because the shrimp fail to reproduce. Both of these "sealed" examples occur on Earth, shielded from radiation, and in moderate ambient temperatures. This will not be the case on Mars, nor on the 9 month journey to the Red Planet.
Even outside of sealed environments, island ecologies on Earth are notoriously unstable because of population bottlenecks that eliminate genetic diversity and make key species vulnerable to freak viruses or environmental disruption.
Of course a Mars colony won't be an ecological island, at least at first, because of constant shipments from Earth of supplies and genetic material (humans, bacteria, crops, etc.). But unless the colony can eventually become self-sustaining, I'm not sure what the point of "colonization" actually is. It's not clear that mammals can even reproduce in low gravity environments, and barring a large scale terraforming effort that would likely take millennia, any Mars colony will be a extraterrestrial version of Biosphere 2 without the built in radiation shielding and pleasant ambient temperature.
Constant immigration and resupply missions will also be incredibly challenging. 9 months in radiation-rich deep space in cramped, near solitary confinement is not something that is necessarily possible to endure for many humans. Every simulated Mars mission has ended with the participants at each others throats before arrival to the planet. Astronauts on the ISS, who receive relatively small doses of radiation compared to deep space, experience cancers at much higher rates, and probably damage their reproductive genetics significantly.
Contrast this to the colonization of the Americas. The initial colonists of both Massachusetts and Virginia were terribly unprepared for what was, at least compared to space, a relatively benign ecological context. There was clean air, water, shielding from radiation, and relatively plentiful food. Yet these colonies nearly died out in their first winter because of poor planning, and were only saved by the help of Native Americans. There are not Native Americans on Mars, no deer or wild berries to hunt in the woods if farming fails, or a supply ship is missed. Mars colonists won't be rugged frontiersmen, but extremely fragile dependents of techno-industrial society.
I'm not saying it's impossible to overcome these challenges, but it does seem irresponsible to waste trillions of dollars and thousands of lives on something we are pretty sure won't work.
2. Motivation
The primary initial motivation for New World colonization was $$$. The voyages of discovery were looking for trade routes to India to undercut the Muslim stranglehold on the spice trade. Initial Spanish colonization was focused on exploiting the mineral wealth of Mexico and Peru, French colonization on the fur trade, and English colonization on cash crops like tobacco.
In space, there is almost 0 monetary incentive for colonization. Satellites and telecommunications operate fine without any human astronauts, and even asteroid mining, which is a dubious economic proposition in the first place, doesn't really benefit from humans being in space. Everything kind of resource extraction that we might want to do in space is just better accomplished by robots for orders of magnitude less money.
What about Lebensraum? If that's really the issue, why don't we see the development of seasteds or self-sufficient cities in otherwise inhospitable regions of earth (the top of Everest for example).
3. Cost
Keeping an astronaut on the ISS costs about $1M/astronaut per day. And this is a space station that is relatively close to earth. Of course low earth orbit (LEO) where the ISS is, is halfway to most places in the inner solar system in terms of Delta V, so we're probably not talking about more than $10M/day per person for a Mars mission. For a colony on Mars with 100 people, that's close to a billion dollars a day. There is no national government, or corporation on earth that could support that.
Even if technology development by industry leaders such as SpaceX lowers launch costs by 1,000x, which I find to be an absurd proposition, that's still $1 million/day with no return on investment.
Even though SpaceX has improved the economics of launching to LEO and other near Earth orbits, our space capabilities seem to be degrading in most other areas. The promised Artemis moon missions are continually delayed by frankly embarrassing engineering oversights, and companies like Boeing, Lockheed Martin, and Northrup Grumman that were essential in the first space race can't seem to produce components without running over cost and under quality.
4. Narrative
This one is a little bit more speculative. The West, and much of the West of the world is entering a demographic spiral, with birth rates falling ever lower below replacement. This relieves a lot of the "population pressure" to colonize space, but also indicates a collapse in the narrative of progress that underpins the whole rationale that would lead us to even want to do such an absurd thing. If our leadership and population doesn't want to build the physical infrastructure and human capital necessary to embark on this kind of megaproject, doesn't this suggest that this dream is no longer appealing to the collective psyche? My read on the ground is that the general population is sick of the narrative of progress: we were promised flying cars and backyard nuclear power plants, but we instead got new financial instruments, addictive technology, and insurance.
China of course is held up as a positive example where the dream of the "engineering state" is kept alive, but I think this is misleading. China has potentially even worse of a demographic crisis than we do, and most of its smartest people (at least those I see in American academia) are desperate to leave.
Without a compelling narrative, the challenges facing potential space colonization become even more stark and difficult to overcome.
I think the idea of biological humans colonizing Mars is silly. It's very likely easier to make a strong AI than colonize Mars, certainly more profitable. Send robots and develop Mars. Or move inwards, there is lots of solar power closer to the Sun.
Likewise I've always been suspicious of chemical rockets. If it's not nuclear, why bother leaving Earth's gravity well? Chemical rockets are just too wimpy for serious space travel. Develop fusion first, then move out.
Now, obviously I love my LV-N as much as anyone. But fundamentally, the tyranny of the rocket equation can not be escaped by providing amazing energy density, because it is mostly not about energy density.
Fundamentally, rocket engines are characterized by two quantities: their thrust -- how much force they can deliver -- and their effective exhaust velocity (or specific impulse), which basically states how efficient they are at converting reaction mass into thrust. v_e is one of the factors which goes into the rocket equation for the delta v budget, the other being the logarithm of the initial and final mass quotient. If you want more delta v, trying to get to a higher v_e seems obvious.
For vanilla chemical rockets, the energy for accelerating the exhaust gas comes from having a fuel and an oxidizer react thermally. This puts some limitations on the exhaust velocity because chemical reactions will only yield so much energy per unit mass (especially as more exotic reagents like FOOF would have their own problems).
The nuclear thermal rocket avoids this energy bottleneck. The problem is that with hot gasses, the next bottleneck is right around the corner: you need materials to withstand the temperature. Energetically, you could just heat your reactor to 10000K (if your reactor can use fast neutrons, at least) and get an amazing exhaust velocity. [^1] Too bad your tungsten pipes will melt at 3700K, though.
The other thing you might do is just to forsake using thermal exhaust. You simply build an ion accelerator open at the downstream end (at least vaccum will not be a problem) and point it into the direction you do not want to go. If you have energy, there is no limit on how high your v_e can go.
Of course, the downside of ion drives is that their thrust is very tiny. This is a problem if you want to get somewhere before you die of old age. [^2] And using nuclear power over solar is not going to fix that -- even particle accelerators which we build on Earth, where we can just take power from a socket have abyssal thrust to weight ratios.
In conclusion, the use of nuclear energy (either fusion or fission) might help a bit, but it will not help you to escape from the tyranny of the rocket equation.
[^1]: Of course, what you get is the exhaust velocity, but what you pay for in energy is proportional to the exhaust velocity squared. This unfair business practice lead to a class-action suit against physics which was settled in 1905. Now as your exhaust velocities approach c, you the energy costs of marginal momentum get constant. Consumer advocates claim that this is a bad compromise because most households have exhaust velocities much lower than c and do not benefit from this at all. Attempts to lower c have been met by resistance of both high school students (who prefer Newtonian physics for real world problems) and gamers (who fear their ping times will increase).
[^2]: Additionally, in KSP, you can not speed up the simulation while you are accelerating. This makes missions which rely on ion drives rather cumbersome. (Though they would be even more cumbersome if the devs had not increased the thrust of the Dawn engine by a factor of 2000 over their Earthly counterparts.)
You have made several serious errors that invalidate your point.
a) The facts that you need high temperature to achieve high exhaust velocity, and that there are limits to the temperature of solids, do not imply that high exhaust velocity is impossible. All you need to do is ensure that your rocket motor is not in thermal equilibrium with your propellant or your fuel. The obvious way to do this is to have low thrust, as that allows your cooling system to keep up. Making your fuel also your propellant, and limiting its ability to thermalise before leaving the ship, also help (the limit is still proportional to F*Ve, but you can raise the proportionality constant). It helps a lot that plasma can be contained magnetically.
b) Thrust doesn't matter all that much except for takeoff. The time taken on a brachistochrone trajectory goes roughly as the inverse square root of your thrust (1), so a millionth the thrust is only a thousand times the travel time. 1 mG could get you the distance to Pluto in about a year and a half. What really matters for transit time is the delta-V needed to achieve brachistochrone at all, and that means nuclear.
c) Speaking of which, yes, nuclear has staggeringly-higher Isp than chemical, to the point that the rocket equation is generally in the linear rather than exponential regime for interplanetary flight and, hence, its "tyranny" is indeed "escaped" (interstellar's a different beast; if you want to go relativistic you're generally looking at antimatter fuel or external drives like light sails). "800 seconds" is a fucking joke compared to what nuclear's capable of - in the highest-Isp version of fission, the fission-fragment rocket, it's capable of millions.
I made the point about exhaust velocities mostly wrt fission engines using thermal gas as a propellant. If you have a fission reactor as an energy source, getting a heating your propellant to a much higher temperature than your fuel elements seems challenging.
The length of your brachistochrone would depend on your available acceleration. If you have unlimited thrust, the fastest path is a straight line (relativity aside). If your thrust is very limited, I would expect that you will spend a lot of time orbiting the Earth while prograding until you escape it eventually, and then you will spend a lot of time circling the sun until your intercept.
However, I agree that 10mm/s^2 is still a usable amount of thrust within the solar system. The Dawn spacecraft got around with much weaker engines, but it definitely increased the transition time.
I am more skeptical about fission fragment engines. Sure, the exhaust velocity -- a few percent of c -- is amazing. But for every fragment which escapes and generates thrust, another one (or three) will hit your spacecraft. Because energy scales with v^2, if you want a decent thrust, that will mean an ungodly amount of energy. 1kN times 0.01c is something like a few Gigawatts of thermal power, similar to what a large commercial nuclear reactor might have. Cooling this away in space would be challenging. And if you add a reaction gas to get more momentum per energy (at lower exhaust velocities), you still have to confine that gas magnetically, which also adds overhead.
Right. So you run open-cycle. You mix your fuel with your propellant, stick it into your nozzle, and burn it at plasma temperatures (remember, while terrestrial nuclear reactors burn up their fuel over the course of years, this is not actually required; a nuclear bomb burns its fuel to reasonable completion inside a microsecond). You will need cooling systems for the nozzle, of course, but the temperature gradients are all in the right direction.
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