Comment by ericd
17 hours ago
This seems sort of interesting as a sort of emergency backup, but it seems like the real solution for any sort of long distance/long duration in space is just making mass to orbit dramatically cheaper and shielding the spacecraft.
There more ways than just brute forcing massive shielding. If you know where the radiation is coming from & is directional (Sun, nuclear reactor) you can put the shielding some way from you & put the hab module in the shadow created by the shield - AKA a "shadow shield".
Some types of radiation can be deflected by strong magnetic fields - based on the technology and power source of your craft, this might end up lighter than pure mass based shielding.
Or you can just have your crewed ships go as fast as possible, reducing exposure & avoiding some radiation sources all together - zipping through radiation belts & doing fast transits in the quiet part of the solar cycle.
Mass requires reaction (generally: fuel) both to accelerate and decellerate. Absent alternative delta-V mechanisms (usually: aeroraking, as lithobraking is perceived as generally too extreme), this rapidly runs into the tyranny of the rocket equation.
<https://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation>
Even without earth-to-orbit costs, that mass has real costs, and reduces available payload.
If that shielding mass can be dual-use (e.g., water), reactive (e.g., electromagnets), or reduced to a very small amount (emergency shelters, wearable garments), it becomes more practicable. That still doesn't make it easy.
There's also a discontinuity in radiation exposure. Passing through radiation belts (e.g., Earth's van Allen belts, or those around Jupiter), and solar storms, are both predictable and special precautions can be taken. Cosmic ray radiation is unpredictable, high-energy, and is far harder to guard against. Some risk is inevitable.
Right, I don't think it makes sense to have our long distance transit vehicle be the same one that needs to decelerate/aerobrake - I'm assuming we'd do something like an Aldrin Cycler for transit between Earth/Mars, where we have taxis that go to/from the cyclers.
In KSR's Mars Trilogy, they had an emergency shelter, seems reasonable if the bulk of the radiation exposure is predictable. But I'd personally feel better being shielded the entire time, given cosmic radiation.
That long-distance transit vehicle has to change trajectory somehow. Orbital transfers may be cheap but they're not free, unless you're talking about a free-return trajectory in which case your specific transits are limited, and often fairly slow.
Though yes, that does make the option of providing a radiation-hardened shelter available with less concern as to total mass.
I thought i read somewhere that the soil on mars is probably pretty toxic to humans, so maybe getting there safely isn't as concerning as what you'd do once you are there...
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> Cosmic ray radiation is unpredictable, high-energy, and is ...
IIR, the big problem is that the cosmic ray background radiation is far too predictable - there is a concerning amount of it 24x7x365. So your baseline choices are gritting your teeth and bearing it, or spending most of your time in a seriously hard shelter.
Definitely outside my realm of expertise, but I'd expect that particles have a range of energies, and that the most energetic, or most damaging (not necessarily the same) occur somewhat rarely.
But yes, generally, it's at the very least a constant background flux (with occasional peaks), and all of it high-energy enough to make lightweight shielding of limited use.
Cheaper mass to orbit is undoubtably good, but not all mass, shielding or otherwise, needs to be lifted from Earth.
Yeah, even unprocessed Lunar or Martian regolith will help, provided the added mass does not cause issues - fine for space stations, more difficult for ships that have to drive the shielding around.
A spacecraft with a 4m diameter spherical living space and 4m of water shielding around it will weigh about 1,000 tons. The propellant tanks needed to move it around the solar system will be similarly titanic.
How many orders of magnitude were you figuring for your "dramatically cheaper"?
Also you should be able to ISRU[0] water from Moon/Mars/asteroids.
[0] https://en.wikipedia.org/wiki/In_situ_resource_utilization
That's 5 Starship block 4 launches, seems fine? Propellant, presumably for long duration voyages we're not relying entirely on chemical combustion. If we're using cyclers, once they're up to speed, you need barely any fuel for corrections, just to taxi to/from the cycler.
Maybe when its actually demonstrated that it can actually launch with that payload. The largest payload they'd lifted so far is ~45 tons, and the claims I see on Wikipedia say its estimated 200 tons for LEO, not even for geosynchronous orbit much less a lunar trajectory.
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And just for comparison on that number, the entire Apollo spacecraft (command+service+lunar modules) was about 50 tons to lunar orbit. And that took a gigantic Saturn V to launch.
SpaceX is planning on launching the significantly more gigantic Starship daily. It's not there yet, but they're steadily grinding toward regular launches, and they supposedly are retiring Falcon 9 from commercial launches within the next couple of years.
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