SwRi-led Team Calculates the Radiation Exposure with a trip to Mars
Southwest Research Institute Team Calculates the Radiation Exposure with a trip to Mars, SwRI
“Energetic protons constitute about 85 percent of the primary galactic cosmic ray flux and easily traverse even the most shielded paths (reds) inside the MSL spacecraft. Heavy ions tend to break up into lighter ions in thick shielding, but can survive traversal of thin shielding (blues) intact.”
Depressing news, but not unsurprising. There’s no realistic spacecraft in the near future that could carry enough shielding to fully protect against cosmic rays, so astronauts on longer missions would likely just have to take the dose and run the higher risk of cancer after they get back home.
Active magnetic shielding has been shown to be substantially lighter than passive absorbers and the use of superconducting magnets appears mandatory. http://adsabs.harvard.edu/a…
Full protection may not be necessary, so trades on trip time, adv. propulsion technology including chemical kick start stages, and combined active and passive shielding mass are required. Oops, DOA in NASA’s abundant chemical architecture dictated by Congress, adverse to technology development.
Since little funding has been directed at NASA’s Technology Challenges to Explore for decades, much misinformation exists regarding the design concepts.
For example, even though superconducting magnets populate MRIs, CERN’s collider, and many other devices for decades, advanced exploration systems managers today refer to superconducting magnets as a very immature technology, state that persistent mode magnets require lots of power, and state they do not understand the effects of magnetic fields on the human body assuming incorrectly that the deflecting field would encompass the crew as well, rather than surround the crew.
With superconducting material and refrigerator advances, another order of magnitude reduction mass is possible with active systems, with no such expectation possible with passive shielding.
Why would one want to subject the crew to higher doses without even investigating new technologies?
While I’m not going to argue against active shielding, wouldn’t the fact that it’s a mature technology tend to make it unlikely that anyone is going to get another order of magnitude reduction in mass for helium-cooled superconducters? And the warmer nitrogen-cooled superconducters tend to be ceramics and brittle materials.
I’m a little wary of the power requirements for this, too. That in of itself will greatly increase complexity and mass for the ship.
The key of course is to use higher temperature superconductors to reduce the power and mass of the cryocoolers, rather than rely on the low temperature superconductors.
Superconductors offer zero resistance, so once the current is flowing, it will require little additional power, and this is often referred to as a persistent mode magnet.
The power of course is needed to refrigerate the magnets below their critical temperature, and the higher T the better to reduce the power and mass. The good news is that deep space is quite cold, so this will be a significant benefit. Solar arrays would also block the sun to provide a lower effective sink temperature (and power for EP to reduce the trip time), with the initial dV provided by a chemical kick start with drop tanks. Perhaps nuclear is required also. As you and many others point out, trip time is key–not going to do with chemical alone.
We know that the earth’s magnetic field deflects GCR, so this is a good indicator that the concept is feasible. But no systems have been deployed TMK, so hence the immature statement on the overall concept, but not the magnets.
Magnets on earth, both LTS and HTS, are mature, but the superconducting wire continues to be improved, at costs substantially less than LV development. Following the path of 1st and 2nd Generation HTS tape, researchers now refer to a 2nd generation MgB2 wire, and different materials show promise of even higher critical temperatures and/or fields. So perhaps its the iron superconductors or a yet to be discovered SC material or a very lightweight refrigeration scheme that provides the breakthrough, but the chances of finding something better than hydrogen for the passive absorber seem unlikely, no? 😉
Per the brittle materials, they are supported by other materials and therefore would be considered in the overall mass trade.
Per system mass, for the “same” level of protection, the active systems are likely an order of magnitude less mass than passive. A subscale demonstrator at L2 could show that the concept is feasible. Since its subscale, the mass of course would be much lighter than the system sent to Mars, and could be be left untended by the crew to gather long term data on its effectiveness and reliability. The “flight” system would likely take advantage of likely technology improvements over the next decade or so prior to a 2030s mission. Further, one could offer only “a few thousand lbs” of active systems offering partial protection in trades yet to be done in any detail.
It is difficult to predict the future, but i see the risk being increased way more than the 3-5% projected without additional mitigation, because everything above the cutoff energy makes it way through.
Perhaps ESA would provide this shield to NASA as part of an IP contribution? This approach would make sense since the NASA does not seem to be interested in any technology development, except of course rebuilding engines from the 1970s.
Nicely said. If I can believe what I’ve read from various sources over the last couple of years, I get the impression that we’re almost at the point where, testing aside, the only major objection to attempting active radiation systems in spacecraft is the fact that it’s all or nothing — there are no redundancy or backup systems that are anywhere near as effective or as small, or use as little power. To me that suggests only two possible reversion schemes, either 1) take the chance and hope, or 2) fly every mission with two spacecraft large enough and equipped for the entire crew. Neither of these alternatives is great.
So the logical response is to start investing more heavily in spacecraft technology to mitigate this risk sufficiently to make a Mars mission possible. Faster propulsion – not using rockets – is a must, as is new ways of shielding spacecraft using both physical shielding, and potentially adapting M2P2 type technology to generate an artificial magnetosphere around the vehicle. Spacecraft design becomes a factor, and here, an Orion type capsule simply won’t cut it. What is needed is something that builds in a ‘storm shelter’, and perhaps has a design based around water tanks in the hull to absorb radiation. Cosmic rays are more of a challenge, given they are highly energetic. The best solution to beat cosmic rays is to make the vehicle go faster than what would be the case for rockets. The notional 180 day transit mission is way too long. This is an important issue, and something that NASA should be working on with much more emphasis, time and investment now that the risks are known.
Malcolm,
Good post. I would add that there is another, somewhat overlooked, testing and mitigation situation that needs to be pursued — instrumentation survival. It’s not just primary and secondary interplanetary radiation that pose a danger to the operation and lifetimes of spacecraft and scientific equipment, Also any shielding or propulsion systems that employ EM, plasma, or even just adjacent high voltages or magnetic fields are potentially going to affect various systems and their control electronics. To the best of my knowledge, nobody has a test facility geared to this sort of testing under space conditions. Everything would have to be tested/proven under atmospheric conditions as well, of course. People are not the only sensitive items that have to survive the trip.
NASA should have been investing in a high density, high power space nuclear reactor, able to power something like a VASIMR drive. Also, nuclear thermal propulsion should be resurrected. Chemical propulsion is not going to get us there (and back) fast enough…although I am sure there still will be people banging down the doors, ready to assume the risk of a long duration cruise phase.
This is the conclusion that Von Braun and Stuhlinger came to in 1972. Why is it that this is just coming back to light now?
I couldn’t agree with you more. We should all go vacation a Jackass Flats, pull out NERVA, brush the dust off and get to work. You know why the politicians killed the project back in 1972? Because like in “Field of Dreams” if they built it, they would have to go. Hold commissions…good; do studies…fine; hire constituents…great; but hike up you skirts and actually go…that gives the green eye shade boys gas.
NERVA was killed when the politicians pulled the money plug — at a point where it could almost be called finished and actually used. Webb even quietly “borrowed” money from other programs to try and let them finish it.
If they resurrected it now, they’d probably spend another 10 years “modernizing” it and we’d be no further ahead.
I was a huge NERVA fan, but if we had it available right now, we’d still have two problems that have always existed — getting it into orbit (you can’t use it as a launch vehicle), and not being stopped by all of the well-intentioned ecology people, who would insist it was dangerous, even if it was only used from orbit outward.
.66 Sv for a full round-trip to and from Mars is actually pretty low. Per the article, it’s less than a 5% increase in the risk of fatal cancer.
It will be decades before that’s the biggest risk to space travel. Astronauts are far more likely to die from all the other, harder-to-quantify, dangers out there until space travel becomes so routine that we iron out most of the bugs. Then, we should start worrying about radiation. Right now, it’s foolish to worry about that.
I think we obsess about radiation because we can measure it. The things that will kill the most astronauts in the next 50 years will be unforseen equipment failures, human error, etc.
One has to be very careful combining the effect of GCR into an effective dose rate.
The issue of course is that while the Earth’s magnetic field deflects much of the GCR at ISS, MSL RAD shows that energetic protons constitute about 85 percent of the primary galactic cosmic ray flux and easily traverse even the most shielded paths (reds) inside the MSL spacecraft. The sun also can deflect GCR, so flying at solar max rather than min also helps.
Another way to state this is that passive shielding (the MSL hardware) only blocks GCR particles below a certain energy level, likley around 150 MeV. The rest will pass through or the passive absorber may degrade the energy of the particles above the cutoff, but the dose inside the shelter can be substantially increased, making things worse.
What this means is that perhaps 1/3 of the crews DNA would be hit by ions for each year in deep space, and mutations of DNA can result in cancer, especially if the DNA cannot repair itself. It is really quite complicated, and to use quality factors to convert to effective dose rate appears to need much more R&D, IMHO.
Note the title of article SWRI “calculates” the radiation exposure to the crew, but the article does not mention the conversion in detail from the measured data.
In 1975, it was expected that about 1 in 4 humans would develop some type of cancer, and that has risen to about 1 in 3 today.
When one adds an additional 3% risk to the crew, they still cannot fly using the “effective dose rate” calculations, so it also not clear what increase risk of cancer was assumed to reach the 0.66 Sv, along with the quality factors. Perhaps the methodology is in the paper.
The primary reason GCR is not addressed is the large mass and hence mission cost to launch this mass. It is not an obsession, its just dealing with the physics of space exploration and crew health.
Chris,
I agree with muomega0 on this one. Everything being tossed around comes down to statistics, and unless your statistical universe is many thousands of people, at least, then it doesn’t really mean anything. We’ve all seen that statistical “analysis” involving small samples can be shown to “prove” anything, even exact opposite cases.
You may be right, in a sense, that it’s other things that’ll kill you, because it’s not just people who are subject to being diseased or killed by the space environment; every system on the spacecraft is likewise at risk to radiation and all of the other dangers of space.
It will be interesting to see what the dose is from a Mars stay, particularly a conjunction period. If this is significant, it further emphasizes the need for faster propulsion. Conjunction missions are often assumed to save propellant, but if the Mars dose is significant, that could change.
It’s also interesting that solar shielding is a no brainer – no need for magnets, etc.; the walls are up to the task, with a shelter for SPEs (provided you have enough warning). M2P2, etc., which at best shield protons, aren’t even needed.
The magnetic fields to shield GCR are huge – most studies show that most of the shielding arises from the sheer mass of the magnets, structure, coolants, etc., rather than from the magnetic fields.
The misconception is that the magnetic fields required are huge.
http://adsabs.harvard.edu/a…
but alas this was once stated in Scientific America incorrectly.
Would you provide references to any of these studies that state the the sheer mass of the magnets provide all of the shielding? Recall that passive shielding estimates are in the 100,000s KG range while active systems are in the 10,000s kg range, so are you stating that the passive guys are using the wrong material! Now that would be a breakthrough!
http://ntrs.nasa.gov/archiv…
http://ntrs.nasa.gov/archiv…
It’s important to specify the particles one is shielding. Your reference, from what I can see, talks about “solar cosmic particles” and “galactic protons” The GCR in question for shielding is GeV high Z ions such as iron, and it’s isotropic – not tied to solar field lines. I can’t quite tell from the figures what your reference is really calculating.
If you keep reading, the paper states “constructed for protrons, however, in a first approximation can be used for all GCRs, with the kinetic energy given in MeV/nucleon.”
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Per your linked 2005 workshop paper, the estimates of passive shielding have not changed over time, just a few of the input assumptions. So you clearly have no references stating that the “magnets themselves offer more protection than the field.” In addition, the paper mentions the use of “weak fields” rather than “huge magnetic fields”, a direct contradiction of your post.
The 2005 workshop by MSFC emphasizes again how much NASA has ignored active shielding.
Take for example, the ONE concept from 1991 considered for active shielding, which is not a magnet cooled by a refrigerator, but rather is “to insulate the sunward side of the coil and cool it by radiating heat to deep space from the shadowed side. This is technically feasible using well-understood techniques of multi-layer insulation and radiative cooling.The assessment of the Workshop is that this idea is impractical.”
…not really serious about anything but HLVs it would appear. Perhaps the problem is MSFC lead studies?
It also says that all of the mag shielding concepts contain energy comparable to high explosives . The structure to contain this energy was found to be effective shielding. What might of changed is that the structure is lighter and therefore poorer shielding.
The weak field concepts are coils 10’s of km in scale. The cooling by radiating to space fails near planets – the effective sink temperature is much higher. And the shade still gets hot.
NASA didn’t IGNORE active shielding; they examined it and found it wanting. If they’d ignored it, it wouldn’t be mentioned.
It’s only intuative, but my gut is telling me that Marshall “decided” against active shielding for unstated reasons of their own, or perhaps those of a senior person. The dismissal just seems too abrupt. Perhaps it’s as simple as no one there knew what the next steps to take with active shielding were, and they had no in-house data to draw from.
For the record, it’s “What might have changed”, not “What might of changed”.
Not all of the authors were from Marshall.
So lets keep reading the ESA paper provided in the link
http://adsabs.harvard.edu/a…
In 2004, the NIAC supported the study by former astronaut J. Hoffman and MIT “Use of Superconducting Magnet Technology for Astronaut Radiation Protection”, presented at the MIT-NASA Radiation Shielding Workshop in June 2005! They adopted the toroidal configuration without passive absorbers, unlike Spillanti, and was 15 m in diameter, with the mass of the coils ~9.4 ton, and the total mass around 30.1 ton, the equivalent mass of 1000 ton equivalent Aluminum absorber.
So here is a NASA paid study presented at a 2005 Workshop that was not included in the workshop paper that showed that 30 tons could replace 1000 tons? why was this not mentioned in the paper? Different assumptions but quite similar mass results.
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So how does the energy store in the magnet compare to the energy stored in the chemical tanks and how does this relate to the energy of high explosives? BTW what and why would the magnets explode?
I will have to read that again. My recollection from their NIAC report is that the authors ultimately did not endorse the magnetic shielding option. The data I saw was:
“In summary, the magnetic shield will:
`reduce the total radiation dose by a factor of at least three in a 200 m3 habitable
volume
-have a mass of between 400 and 1600 t
-require 52-75 tons of liquid helium for a three year mission
-need 117-169 kW of power for cryocoolers
-store 16 GJ of energy, requiring a dump mass of 5000 tons for a 30 C temperature increase.
Not sure where the 30 t number comes from.
The energy in the magnetic fields represents a total pressure that must be contained and supported, or the magnet coils expand and break.
The total effective dose for a round-trip Mars mission was estimated at about 1 Sv at the first Case for Mars conference back in 1981, and has changed very little since. The conclusion at that time, that one mission could be flown without substantially exceeding career limits, remains true except insofar as career limits have been reduced somewhat since the Apollo era.
Despite the high relative biological effectiveness of heavy ions like 56Fe in galactic cosmic rays, the risk of cancer from GCR is relatively low in comparison to solar protons, because a hit on DNA by a heavy, high-energy ion causes so many DNA breaks that it usually kills the cell. Dead cells don’t cause cancer and in most tissues are readily replaced. Neurons are generally not replaceable, however, and the cumulative loss of brain cells (as happens when we age, as well) will eventually be a limiting factor.
So radiation does not prevent a small number of individual missions to Mars, but until better shielding methods are developed, it does limit large-scale and permanent colonization of space.