Mars Mission Radiation
The jury is still out on the topic of space radiation during an astronaut mission to Mars.
This has little to do with any lack of science or engineering. Plenty is known and plenty of clear avenues remain open to further investigation. Rather, space radiation is seen as such a huge show-stopping problem that almost no one making a living in spacecraft design wants to be the one saying why a mission to Mars is nearly impossible.
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Radiation takes two basic forms — electromagnetic — or photons (light of varying wavelengths) with increasing energy starting with long-wave radio, then ordinary radio, microwaves, infrared light, visible light, ultraviolet light, x-rays, ordinary gamma rays, and finally highly-energetic gamma rays. Damaging sorts of photons are found beginning at wavelengths just beyond the visible range in the UV spectrum. Truly nasty electromagnetic radiation, like x-rays, penetrates shielding materials — and people. Gamma rays cook cellular DNA on the way through.
Although a poor analogy, as wavelengths shorten I envision the energy of photons fired from a super cannon at increasingly faster velocities. UV bounces off most materials. X-rays cooks like a microwave oven. Gamma rays penetrate through just about anything to great depth.
Most electromagnetic radiation traveling between the Earth and Mars arrives from the sun. The worst of these photons are fairly ordinary x-rays. Even still, continuous exposure to0 radiation of this sort is not a great idea.
Except in exceeding rare instances (on the order of a billions of years) very little electromagnetic radiation arrives in dangerous quantities from deep space.
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The sun seen at x-ray wavelengths
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The other type of space radiation is high-speed particles — atomic nuclei and electrons accelerated by solar and cosmic events. Most of these are hydrogen nuclei — a.k.a, protons. When the sun tosses a big chunk of itself into space moving at several million MPH we call this a Coronal Mass Ejection (CME) — or more commonly — a solar storm.
The rest arrives from beyond the solar system — stars sloughing off pieces of themselves via their own massive CMEs, or blowing themselves to bits in various types of supernova and star-merging events. We call particles arriving from beyond the solar system Galactic Cosmic Rays (GCR). The name is somewhat misleading since the damaging part of this are atomic particles, not rays.
Supernova in particular have a nasty habit of blasting heavy atomic nuclei (especially iron) racing our way at nearly the speed of light. The energy of these particles is like a super cannon firing solid iron balls blowing huge holes in a castle wall — not just penetrating with a narrow beam. Anything solid flying close to the speed of light packs an enormous wallop, and at typical velocities like this, GCRs tear through human cell structure like tiny speeding bullets from a relentless machine gun.
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Supernova Simulation
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It would handy if shielding were the answer. Unfortunately shielding is like standing behind a concrete wall battered by artillery. The wall may not crumble right away, yet much of the energy makes it through in the form of shock waves and pieces of the inner wall shooting off the back side. When radiation hits shielding material, it sprays a bunch of secondary x-rays and gamma rays deeper into the shield — and usually out the other side. So a shield needs to block the initial collisions and the products of those events downstream.
Without adequate shielding, a single CME event would kill interplanetary astronauts in a matters of hours. To fully protect astronauts from the worst effects of solar storms the shield might need to be several feet of lead or many meters of concrete — hardly practical in a spacecraft design given the added mass. Fortunately, solar storm radiation arrives from just one direction and the heavy equipment portion of an interplanetary spacecraft can act as a substantial shield (engines, water, fuel tanks, batteries, etc). Keeping the bulk of the spacecraft facing the sun with the crew cabin on the opposite side might just do the job.
Arriving from all directions GCR are a more annoying problem. They come from all directions in a constant drizzle that won’t kill anyone outright except after many months of exposure and there are no tricks to defeating this threat beyond placing as much material between the astronauts and space as possible in all directions.
From a pure dosage number, it doesn’t sound too bad — maybe 100 rem for the whole mission — hardly much to worry about if this were from x-rays or some other wavelength of electromagnetic radiation. Yet this is GCR radiation, and a hundred rem of this stuff will kill astronauts during the expected eight month transit to Mars, or soon thereafter.
The best shielding might only induce a greater chance of cancer later in life, and in some people might not harm them noticeably at all. Yet no one can say how much shielding to reach even this level of safety is enough other than more shielding than we could ever possibly put aboard any reasonable spacecraft.
At present, with respect to the stew of space radiation actually out there — other than admitting how a lot is too much, we still don’t have an official limit for deep space flight exposure, and considering the cover-your-butt culture of space politics, it’s likely that no acceptable level will ever be decided with anyone’s signature attached.
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Nuclear Propulsion Concept
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Oddly, the solution to all of this might come down to nuclear propulsion.
With nuclear-powered engines we’d have the power to haul added shielding mass. (Some suggest mining these shielding materials from the moon — which is hardly cheaper than extra Earth-based launches). Nuclear-powered propulsion might also help solve the problem simply by flying to Mars a lot faster. Less time equals less radiation
Overall, the only viable solution is extra shielding and a faster round trip — requiring an even bigger engine. Yet there is a limit in this logic as well. A really big nuclear-powered engine would require it’s own massive shielding, and there’s considerable doubt that we could ever assemble such a fast and heavily-armored spacecraft in less than a hundred (or even a thousand) heavy-lift launches from Earth.
Nuclear propulsion is the only viable idea to date. Other engine types are in the works. Nothing usable is close to flight-ready or even ‘on the drawing board.’
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Living on Mars
Unlike the Earth, Mars has only tiny pockets of weak magnetism — nothing like the Earth’s powerful magnetic shield able to collect and redirect particle radiation to the poles. Mars also does not have much of an atmosphere to block x-rays and UV light from the sun.
This can be handled somewhat with crew quarters covered in martian soil thick enough to block just about everything — including secondary x-rays and gamma rays. Small side-facing windows would also make use of a thicker view through the atmosphere. Except for these windows and the bulk of the planet itself, there is no natural radiation shielding on Mars, and as long as astronauts spend the lion’s share of their time indoors, radiation dosage should be okay — not great — but okay.
Outdoors, a well-designed space suit might help a bit. Sorties would be limited to a total radiation dosage for the whole mission. After that point, astronauts would remain mostly below ground for the remainder of their visit. During solar storms, the crew would need to stay below ground — no if ands or buts.
Although seen as handy for exploration — from a radiation standpoint living aboard rovers would be a really bad idea.
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Underground Facilities at Mars (apparently using grow lights)
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Is radiation really a show stopper?
A mission to Mars that leaves radiation out of the design mix ignores one of its most basic design parameters. Maintaining habitability in a hostile radiation environment has a huge impact of spacecraft mass, systems configuration and mission operations and any design leaving this to be “solved later” is pure fantasy pissing dollars or Euros or Rubles down the drain while maintaining job security for space agency bureaucrats.
In a mission to Mars, space radiation is the central issue and the only place to begin. Once we begin to see serious discussions on the topic placed front and center, that is when we will begin to see a genuine mission to Mars in the works.
For my top ten mission hurdles, see Why Not Mars?
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