Space Radiation + counter measures

Antius · 2013-05-31 03:27:02

New measurements suggest that a 6-month Mars trip would result in an accumulated dose of 330mSv.

http://news.cnet.com/8301-11386_3-57586 … radiation/

Once on the surface, an astronaut would then face an additional dose of 200-300mSv per year, depending upon location.

http://www.space.com/21353-space-radiat … hreat.html

2 five month journeys + a 2 year surface stay = ~1Sv = 5% increased risk of fatal cancer.

There are in fact counter measures that could substantially reduce the future risk of cancer. If astronauts fast for two days per week, insulin production is reduced and the human body's DNA repair mechanisms are greatly enhanced. On Earth, people that practice this sort of diet persistantly have dramatically reduced risk of cancer. Would it be enough to counteract the effect?

RobS · 2013-05-31 12:13:21

I just checked Zubrin's Case for Mars. He is estimating that a 30-month trip to Mars (6 months out, 6 months back, 18 months there) will result in a radiation dosage of 52 rems, which is 0.52 Sieverts, so he was off by a factor of two. Of course, the risk of cancer from cigarette smoking is a lot higher than it would be for astronauts traveling to Mars.

I have no idea what the scientific community thinks of fasting.

louis · 2013-05-31 11:55:45

That will be an average figure. Astronauts are by definition individuals with very strong physical constitutions (read The Right Stuff to get an idea of just how strong). It's likely they are much better than the average population at dealing with cancer threats, certainly to the extent of 5% increased risk. I'd say that was a "Green Means Go" message.

bobunf · 2013-05-31 17:19:32

Per NASA’s press release dated May 30, 2013 at https://webmail.west.cox.net/do/mail/me … DELIM29072 “The RAD [Radiation Assessment Detector] data showed the Curiosity rover was exposed to an average of 1.8 milliSieverts [mSv] of GCR [Galactic Cosmic Rays] per day on its journey to Mars.” Almost all of which was GCR. If the Sun had been more active, while the dangers of solar flares and coronal mass ejections would have been greater, the GCR would have been less - probably a net positive.

Note: NASA had previously defined all of the acronyms.

That works out to about 330 mSv in a six month voyage to Mars or about a 1.7% increase in the risk of developing a fatal cancer. In the United States the average background radiation is around 3 mSv, and average radiation from man-made sources is around 4 mSv – almost all medical – for a total of less than 4 mSv per six months. 330, an increase of two magnitudes; a very significant increase.

In addition, cancer is not the sole risk factor of increased exposure to radiation which also adversely affects reproductive tissues and the central nervous system.

And, of course, there is the trip back.

The simplest mitigation strategy would be to get the humans there faster, pre-positioning supplies and equipment that do not need to worry about cancer, reproduction or Alzheimer’s. Still others:

 It is very likely that better preventives and cures for cancer will be continuously developed over the next few decades.
 Shielding with hydrogen rich materials will make some difference depending on how much shielding is possible.
 Shielding with magnetic and/or electrostatic devices is another, more remote and difficult, but appealing possibility

On the planet, I suspect that RAD will find an incidence of GCR substantially less than 50% of 1.8 mSv per day; perhaps 0.7 mSv or lower.

 The planet will block out half the particles
 The atmosphere will have some protective effect
 Being within a crater will raise the average horizon so that GCR will enter from less than 180 degrees of sky.

In addition, a habitat would be organized with radiation protection as a prime concern addressed in numerous possible ways:

 Fossil magnetic fields may have a substantial mitigating effect.
 Living quarters can be located in valleys, in hollows, near crater walls, in the shadows of mountains and underground with substantial hydrogen rich shielding atop.
 Once substantial electrical power is available, it may be possible to protect a large area using magnetic and electrostatic shielding.

A two year stay on Mars may result in less than a 100 mSv exposure; quite possibly far less. That could bring the total exposure of voyaging to Mars, staying for two, perhaps more, years, and returning to Earth at less than 300 mSv – a 1½% increase in fatal cancer risk, or less.

I think that, like weightlessness, radiation is a very significant problem – not to be discounted - but one that can be managed, and does not fatally affect the long term possibilities of humans on Mars. People are very clever.

Last edited by bobunf (2013-05-31 19:38:31)

GW Johnson · 2013-06-02 07:56:50

The natural background in the US is not a "good figure" for estimating medical effects because it varies so much from place to place, even discounting radon gas in basements. There are lots of places with a little less than average, and a few places with a lot more, by factor 10 !!!, such as the mountains in Colorado. Even in those areas of far higher background, there is no discernible difference in cancer rates that I ever heard of. Very little is understood with certainty about acceptable doses of very low radiation, except that very little effective harm seems to occur with present lifespans 70+ to 90-something years.

As for shielding while on Mars, think a meter or two of regolith piled on top of whatever habitation structures you build there. Wet it down and let that freeze as you pile it, and you have a hydrogen-rich shield of all-indigenous materials (permafrost), with very little processing and no refinement. Subsurface ice seems quite common on Mars. Put it in a tank, melt it with solar heat (self-pressurizing as the vapor equilibriates inside the closed tank), and let the solid contaminates settle out. Easy source of water for a variety of uses.

GCR threats vary by roughly a factor of 3 through the solar activity cycle, a solar wind effect inside the heliopause. I don't know about the variation radially away from the sun. At min activity GCR around these parts (Earth's distance from the sun), is around 60 REM annually, compared to NASA's astronaut limit of 50 REM annually. At peak solar activity, GCR is somewhere around 22 to 24 REM annual.

20 cm water cuts those exposure levels in half without incurring secondary particle shower dangers, and that same 20 cm water will protect against the largest solar flare events. So, wrap the water and wastewater tanks, that you already know you must have, around a designated shelter on your ship. Easy shielding making water do double duty in your design.

The smart ship designer would make that designated shelter the flight control deck. That way, critical maneuvers can be made regardless of the solar weather.

As for peak GCR at solar min being slightly above the limit, I'd bet the proposed vehicle structures will cut the 60 REM in open space to pretty near the 50 REM limit, generally speaking. No water involved.

On Mars, I wouldn't count on any natural atmospheric or magnetic shielding effects as regards GCR exposures. But even without being down in a crater, the presence of the planet cuts the exposures in half to around 11 or 12 REM annually min, max around 30 REM annual. And, one is not outside one's habitations 24-7, either. If your buildings are shielded the way I suggest, your exposure is near 0 for all those portions of the day you are inside.

I don't remember the conversion from REM to Sieverts. It's all supposedly some kind of metric stuff, though.

GW

Last edited by GW Johnson (2013-06-02 08:00:45)

RobertDyck · 2013-06-03 02:59:46

I would count on Mars atmosphere for protection from GCR. There were 4 papers from the team for the MARIE instrument on Mars Odyssey. It included a chart of estimated radiation exposure to astronauts on the surface of Mars. The showed about 90% of heavy ion GCR blocked at a high altitude location 2km above the datum, and about 99% blocked at a low location 2km or more below the datum. Meridiani Planum is 2km above. Gale Crater or Mawrth Vallis (an alternate location for Curiosity) are 2km below. Utopia Planetia is 3km below.

Heavy Ion GCR is the most nasty. Light shielding causes a shower of secondary particles. So thin radiation shielding is worse than no shielding at all. But thin shielding can help vs. Solar Energetic Paricles (SEP), which is radiation spewing from the Sun. So shielding for SEP makes radiation from heavy ion GCR worse. It takes thick tanks of liquid hydrogen or water to block it without secondary radiation.

Proton SEP or proton GCR or light ion GCR does get through Mars atmosphere, but those types of radiation can be blocked by sand bags filled with Mars regolith. Without causing secondary radiation. Ideal radiation shielding is 2 metres of soil. That means burrying your Mars habitat.

RobertDyck · 2013-06-03 03:21:32

I posted this on the Mars DC email list.

Most of us see space radiation as an annoyance, not a show stopper. But here is one remediation: Mini-magnetosphere

There has been serious research into Mini-Magnetosphere Plasma Propulsion (M2P2). I don't think it's feasible for propulsion, but it is radiation shielding. And the M2P2 guys have looked at that, including integrating plasma sail forces with conventional propulsion for navigation.

This website describes it in simple terms, with cartoon-like pictures:
http://www.minimagnetosphere.org

And here is a formal academic paper:
http://www.ess.washington.edu/Space/M2P … elding.pdf

Abstract:
"The deployment of a mini-magnetosphere (or magnetic bubble) around a spacecraft has recently been proposed as a means to couple energy from the solar wind to provide in-space propulsion. In so doing mini-magnetospheric plasma propulsion (M2P2) would have both high specific propulsion and high thrust while requiring only modest (kW/unit) power levels to sustain the mini-magnetosphere. In order to obtained sufficient thrust for a manned mission, the mini-magnetosphere is anticipated to extend out to several 100 km to a few thousand km in radius, possibly supported by several units using a total of ~ 100 kW. At this size, the mini-magnetosphere has the potential for not only deflecting solar wind particles, but also the energetic particles that comprise galactic cosmic rays (GCRs) and solar energetic particle (SEP) events. These energetic particles provide a significant radiation hazard for any extended manned mission in space. This paper presents initial design characteristics for using an M2P2 system as a radiation shield. Each unit consists of a magnet with a radius of 10 – 20 cm, with strength of a few kilogauss to possibly a Tesla. Embedded in each magnet is a plasma source that is used to expand the magnetic field. It is shown that the magnetic field fall off approaches 1/r as the plasma energy density approaches the magnetic energy density. The effectiveness in shielding as determined by the integral of B×dr can be more than an order of magnitude larger than the magnet by itself. Pulsed operation of the system is used to prevent modification of spacecraft trajectory and prevent build up of radiation belts within the mini-magnetosphere."

The paper from University of Washington talks about 1 GeV shielding. That should block most GCR as well. About 99% of GCR is proton or alpha, only 1% is heavier or antiproton / positron. Even antiprotons arriving at Earth have a maximum of 2 GeV. Blocking (deflecting) ~90% of particles with energy <= 1 GeV would reduce radiation to equal ISS. This level of protection would equal protection by Earth's magnetosphere.

What I didn't say in the email: Equipment for the mini-magnetosphere is estimated to weigh 50kg and require 100kW of power. The SP100 nuclear reactor was part of the Earth Return Vehicle for Mars Direct. Design of Mars Direct started in 1989. SP100 was developed by the US military for space, it was already public domain by 1989. Since then (~2001) NASA has designed SAFE-400: 400kW thermal, or 100kW electric. 512kg

A spacecraft using a mini-magnetosphere would have to be designed for it. The plasma is thin, but hot. Thermal blankets used on ISS are sufficient to protect against that, so a spacecraft designed the same as an ISS module would work. But an aluminum antenna sticking out would get scorched.

JoshNH4H · 2013-06-03 19:08:30

I think that there's something important to note about the use of artificial magnetic or electrostatic fields as radiation shielding: These technologies can't even be considered vaporware. As fascinating a technology as it is, M2P2 hasn't demonstrated magnetic bubble expansion beyond the size of a small test chamber. Meanwhile electrostatic systems are power and mass intensive, and hard to set up, not to mention dangerous to crew doing EVAs and disruptive to electronics.

What we need to focus on, IMO, is better physical shielding once on Mars, and intelligent placing of mass both for in-space purposes and on-planet purposes so that the crew spends as much time as possible in well-shielded areas. Sleeping areas are a logical place to start. For the colony/base/hab itself, one could easily imagine a machine that packs sandbags full of regolith and piles them on top of the structure. This will help with pressure retention in the vertical direction as well. According to this image the pressure in Hellas (~12 mb) is enough to halve the radiation. 12 mb at 3.7 m/s^2 corresponds to a layer of water 32 cm thick. From this I conclude that a meter or two of regolith (much denser than water) should provide enough shielding to reduce radiation while on-planet to acceptable levels.

RobertDyck · 2013-06-05 06:37:50

What we need to do is test a mini-magnetosphere in Earth orbit. High enough that it it won't hit the Earth's atmosphere. The paper from University of Washington said it would expand to 100km radius. Tests in a small steel chamber won't go anywhere beyond what they've done.

Meanwhile, research has already shown that physical shielding won't work. Period. Lead or dense materials cause heavy ion GCR to shatter into multiple smaller particles, causing more damage than no shielding at all. That's called secondary radiation. Using liquid hydrogen fuel tanks as shielding doesn't work because the fuel is expended. Fuel tanks work when you're parked in Earth orbit, but as soon as you go anywhere the tanks are no longer full. Liquid hydrogen requires very large and heavy tanks, and constant refrigeration. Water is almost as good, and polymers of exclusively light elements such as polyethylene or polypropylene, but not nylon or polycarbonate. Light material slows radiation without causing secondary radiation, but it has to be very thick. At least a couple metres thick. Yup, I said surround the spacecraft in a tank of 2 metre depth of water on all sides. Or 2 metres of solid plastic. A centemetre or two just won't cut it. That means heavy. Any radiation shielding sufficient to be effective is prohibitively heavy.

I could argue the radiaiton issue is not as bad as the nay-sayers claim. We could have gone to Mars in the 1990s. But it isn't happening. We could sit on our hands doing nothing, or actually do something. Moving mini-magnetosphere development into space is doing something.

Last edited by RobertDyck (2013-06-05 14:02:50)

JoshNH4H · 2013-06-06 18:20:27

RobertDyck, I absolutely agree that further experimentation with and development of M2P2* units is a good idea. In addition to its potential as a radiation shield, it is also quite good as far as electric propulsion goes, insofar as the Sun provides most of the energy input to the system.

That said, I would like to see more development before M2P2s are settled upon as the prime method of radiation shielding. First off, I haven't seen any studies (be they physical experimentation or computer simulations) that say that an M2P2 unit has a strong enough or large enough magnetic field to have a significant effect on cosmic radiation. These particles are, after all, much higher in energy than the Solar Wind, and the simulations that I've seen suggest that even for the Solar Wind deflections won't be too large.

Beyond that, I have to call BS on the "Secondary Radiation" argument. Not because the physics is wrong, of course, because it's a fairly well-documented effect. Rather, if the regolith you're using as a shield generates secondary radiation, there is a simple answer: More regolith to shield from the secondary radiation. It's also worth noting that the most common element in regolith (a plurality by mass and a majority by moles) is Oxygen, which I would consider to be pretty light. Not Hydrogen, perhaps, but light. Once a true colony has been established, ice-based shielding is entirely reasonable, but I would expect a couple meters of regolith to do in the meantime, especially for an initial outpost.

I would like to read up more on the Secondary Radiation threat. This is obviously important, but I haven't found any good resources. Can you point me to a good overview of the matter?

I am actually of the opinion that a 5% increase in the probability of death from cancer is an acceptable risk for a one time mission followed by a life spent on planet Earth. For longer duration missions and colonies more effort should be put into reducing radiation but I think that plain old physical shielding while on-planet is a viable solution. I would like to add that it is in the nature of a frontier to be less safe than the homeland. I don't consider Jamestown to be an admirable model (in any respect) but the death rate there in the first winter was 56%. The winter two years later was even worse, with a mortality rate of 88%. In the first 17 years of the Jamestown colony, about 6000 people immigrated to Virginia. On the 17th year, the population was 1200, indicating that about 80% (Or more, given the tendency of humans to procreate) had died (source). I mention these statistics not because I think that these are enviable, or even reasonable mortality rates (Though one can't help but be impressed at the chutzpah of the early modern Europeans in going off into the unknown with a near total lack of preparation or prior knowledge). Rather, I do it to show that a frontier is not safe and it is the job of the frontiersperson to build society where none existed before. "You live until you die" is perhaps a relevant attitude-- There is such a thing as acceptable risk and at some point you have to say that things are "safe enough".

*I contend that this constitutes a proper name and thus the acronym does not need expansion or explanation.

RobertDyck · 2013-06-07 03:40:04

Robert Zubrin pointed out the increased chance of cancer is less than smoking. So radiation is there, but not so bad.

Re secondary radiation. The issue in space is quite different than the surface of Mars. The papers from the team for the MARIE instrument on the Mars Odyssey orbiter estimated surface radiation. Their surface map showed heavy ion GCR was blocked 90% at 2km above the datum (eg Maridiani Planum), and 98-99% blocked at 2km below the datum (eg Marwth Vallis or Gale Crater). So the best shielding against heavy ions is the atmosphere of Mars itself. And that means you don't have to worry about secondary radiation on Mars. Secondary is an issue for interplanetary space.

JoshNH4H · 2013-06-07 13:48:00

Just because heavy ion Cosmic Rays have been blocked doesn't mean that all of the high energy particle radiation has. Specifically, protons, which according to Wikipedia compose 90% of Cosmic Radiation (With a further 9% Helium, putting metals in the cosmological sense at just 1% of the radiation), and thus blocking of heavy ions is not that important in the context of radiation shielding, be it from primary or secondary radiation. I don't know exactly what is considered "heavy" in the context of radiation shielding but I would have to imagine that it would be the same as "heavier than Helium." If you can point me to the paper in which the claim was made I would love to look at it. In any case, this image (reproduced below) suggests that the annual radiation dose decreases from about 22 rem/year at the top of Olympus (where the pressure is nearly zero) to about 16 rem at the datum, where the pressure averages 610.5 Pa, by definition. At martian gravity of 3.7 m/s^2, this is 165 kg/m^2 (16.5 g/cm^2). These 165 kg of CO2 serve to reduce the radiation from Cosmic Rays by about 27%. Assuming exponential decrease, one would need 363 kg/m^2 (3.63 g/cm^2) of Carbon Dioxide to halve the cosmic radiation. Water ice is presumably better and regolith perhaps worse.

Hyperphysics suggests that this secondary radiation has three primary components about which we care: Energetic Protons, energetic Neutrons, and more exotic particles known as Pions and Kaons, each of which comes as a positively charged particle, a negatively charged antiparticle, or a neutrally charged particle. The charged Pions and kaons have half-lives of 2.6e-8 s for the Pion and 1.2e-8 s for the Kaon, and negligibly small (~1e-16 s) for the neutral particle. These decay into various combinations of electrons, positrons, and gamma rays (we don't care about neutrinos).

Unfortunately, I have no idea of the branching ratios for the formation of various particles. However, I would like to forward the following points for each: Protons will have lower energy than the particle that produced them, probably by a pretty significant amount. They should be stopped in short order by whatever shielding is in the way. Neutrons will also be stopped in a reasonable distance, depending on the material. Pions and Kaons will presumably have higher energies than protons or neutrons, since they are the result of subnuclear reactions as opposed to spallation. Between decay and impacts with shielding material, I would expect that they would have mostly decayed by the time they pass through the shielding. This will release gamma rays, which are also difficult to shield against. Note that this is the case for both heavy and light materials.

GW Johnson · 2013-06-08 02:43:52

Hi guys:

Interesting argument. The radiation map Josh provided is particularly nice. Atmosphere does help, even if it's thin.

Part of the "shielding" from secondary showers is simple distance: the shower decays before it hits you, because of the distance. That's partly why atmospheres and big solid bodies (dimensions in 10's+ of km) are such effective shields.

You don't get that benefit in man-made shields unless they are very thick. Piled-up dirt is the most practical. You put it on the roof about 2 meters or more deep, and it works pretty good. That's inherently rock particles separated by interstices. If you fill those interstices with ice, you get more shielding mass, and you get it from "light" elements that add little to the secondary shower problem. So a thick roof made of permafrost is a really good idea.

Both dirt and ice seem to be plentiful on Mars. Now, if there were just a good concrete that would set up strong in that cold, and that near-vacuum of an atmosphere....

GW

JoshNH4H · 2013-06-08 06:17:13

Interestingly, if in Martian Gravity you want to create a pressure of 50 kPa with regolith (assumed density: 2500 kg/m^3) one would need to use 5.4 m of regolith. For 40 kPa, 4.3 m of regolith*. When I think of martian habitats I tend to think of cylinders with regolith piled on top and sides rather than spheres or capped cylinders. The regolith provides the dual function of making structures simpler (uniaxial instead of bi-or tri-axial stresses) and shielding the structure's inhabitants from most of not all radiation.l

As an additional useful factoid, the characteristic distance (by which I mean the distance traveled by light in one half life of a particle) for Pions and Anti-Pions is 7.8 m, and for Kaons and Anti-Kaons is 3.6 m. This is the "half-distance" so to speak, because these particles may well be traveling with velocities only marginally slower than c. My particle physics is not what it could be, so I could be wrong in that respect, and if anyone is more well-informed on this matter corrections are welcome.

By the way-- If the argument is (as it appears to be) that an impact with a nucleus creates secondary radiation by spallation, then even Helium would be subject to this. Hydrogen would really be the only element that could not be turned into another element by spallation, especially given the high energies of cosmic rays. That said I can't really comment on which situations result in the production of Pions and Kaons, and nor can I comment on whether this is something we really need to worry about. I do know that there will be a lot of gamma rays produced in this situation and Gamma rays require heavy elements for shielding.

*I'm thinking of an atmosphere composed of about 20 kPa Oxygen, 3 kPa Carbon Dioxide, and the rest Nitrogen and Argon, in the 2.7:1.6 ratio found in the Martian atmosphere (e.g., why separate when both are perfectly good breathing gases?). For a 50 kPa atmosphere this results in 17 kPa of Nitrogen and 10 kPa of Argon, and for a 40 kPa atmosphere this results in 11 kPa of Nitrogen and 6 kPa of Argon. Midoshi has shown to my satisfaction in the "Minimal Martian Terraformed Atmospheres" thread that given sufficient levels of humidity these atmospheres are breathable, fire-safe, and conducive to plant life if any is desired.

Edit: By the way, GW, do you have any plans to attend the Mars Society convention this year?

RobertDyck · 2013-06-08 20:10:13

Interesting. The source I read said GCR consists of 85% protons, 14% alpha, and 1% others. Antiprotons and positrons are in "others". But since protons are so light, they don't produce much secondary radiation. Atmosphere of Mars hardly blocks them at all, but regolith does nicely. That's the great advantage: Mars atmosphere blocks the stuff that generates secondary radiation. So you don't have to use thick layers of water or plastic or liquid hydrogen, you can use dense shielding: lead, concrete, or the most practical - Mars regolith.

Proton GCR is much higher energy than proton SEP. But due to the low mass of protons, I wouldn't call them "high energy particle radiation". Much lower momentum, and most importantly a proton can't split.

Ideally you want 2 metres of regolith over your head. Soaking it with water and allowing to freeze to form permafrost would help. To conserve water, you would probably want layers: a thin layer of soaked soil, covered by a thick layer of dry soil. Repeat. The top of this "layer cake" would be a thick layer of soaked soil. This uses permafrost to hold the soil in place. Structural. If you build at a location with lots of water, such as beside a Mars glacier (orbiters have found a few), then you could soak it all. So we actually agree about what to build on Mars.

This addresses another concern. One you touched on. To prevent spallation (new word for me), you want shielding consisting of light elements. But those light elements don't stop X-rays or gamma. You want heavy elements to stop those. Again a conflict. But the fact Mars atmosphere blocks heavy ions solves this. Again, don't worry about spallation, just use regolith. Heavy elements such as iron in Mars regolith will block X-rays, and a significant portion of gamma.

As for neutron: very little leaves the Sun, and what does decays to proton and electron by the time it reaches Mercury. And there's none in GCR. Same reason: it decays to proton and beta radiation (high speed electrons) long before it reaches our solar system. So the only neutron radiation is secondary. Not much of that on Mars. One person at a Mars Society conference raised the issue of neutron radiation. She pointed out the neutron spectrometers on Mars Odyssey measure neutrons coming up from Mars surface. So there must be some neutron radation there. I asked one student at MIT to estimate the dose. She came up with 0.5 REM per year, just from neutron. That was assuming a Mars Direct habitat with aluminum walls, and restricting astronaut time in a spacesuit on the surface to 40 hours per week. I think 40 hours per week is hardly restrictive.

That 40 hours per week also came from radiation calculations. Assuming a permanent settlement has enough regolith piled on the roof to make radiation inside negligable, how much time in a spacesuit would result in the same exposure as a nuclear reactor worker in the US? That's 5 REM per year total. The result was 40 hours per week. But that would have to include time in a greenhouse, assuming ambient light greenhouse. A plastic film greenhouse would be metallized: spectrally selective. That blocks UV, controls IR, and the metal blocks what little X-rays get to Mars. Beta radiation is so weak it can be blocked by a single sheet of paper or plastic film. Alpha can be blocked by a single sheet of aluminum foil; the metal coating would do the job. The result would be about the same as a spacesuit. A glass greenhouse would provide a little more radiation protection, but negligable difference. Plastic film is light for transport from Earth, but glass is easier to make in-situ on Mars.

Last edited by RobertDyck (2013-06-09 22:02:14)

idiom · 2013-06-08 21:32:07

Quick question, its probably addressed somewhere and I missed it.

How so surface radiation affect greenhouses? What plants can tolerate radiation that high? Will we need to take breeds that have been cross bred with wild samples from Chernobyl and the like?

GW Johnson · 2013-06-09 03:15:48

Greenhouse-the-concept as applied here on Earth has a glass roof. Maybe that is the wrong concept for Mars. Think solid roof and terrain-bounced light through clear side walls. If terrain bounce is not enough, put mirrors outside. Then you can put a regolith radiation shield on the roof, same as the living quarters. Two radiation-related restrictions then go away: (1) the need for radiation-resistant plants, and (2) radiation exposure to persons working in the greenhouse.

It might need a thicker shield, but the same concept should work on the moon or any other airless world.

Josh: I plan to attend the convention and present a paper. My paper on a unique and simple low-density ceramic composite material-as-a-heat-shield was accepted. This was stuff I made a quarter century ago and used as a ram combustor liner. Turns out, it would be a dandy material at Mars, and would work for low ballistic coefficients here from LEO.

40 hr work week on Mars (or in vacuum anywhere) is an overtime stretch-of-the-imagination if we continue using the kinds of spacesuits we have used since the late 50's. Maybe not such a stretch if we get mechanical counterpressure suits working. That idea first worked successfully with a human subject in vacuum ca. 1969. Languished ever since.

Lots of imaginable outside activities on Mars involve vehicles or the adapted equivalents of Earthly construction equipment. So, pile a regolith shield on top of the crew cab. Pressurized, unpressurized, makes no difference. If you're under the dirt pile, you're shielded at one level or another. Just watch the film badge.

GW

RobertDyck · 2013-06-09 11:19:44

All plants can handle much more radiation than humans. Just grow normal crops in a glass greenhouse. Radiation on Mars is far below the levels dangerous to plants.

The description GW Johnson describes is what many have thought necessary. But now that we have measurements of Mars radiation, and researchers have done actual research, we don't need any shielding for plants. We do for the habitat, for people.

Also, Guelph University has done research in growing plants in low pressure. They can handle much lower pressure than humans. Pressure down to 10kPa (100 millibar) does not even slow plant growth. The catch is lower pressure causes faster transpiration of water through leaves. That means the plant requires more water as pressure goes down. But that water doesn't go anywhere in a sealed greenhouse, it just condenses on the cold walls and drips down back into soil. So it's a big circle: water is taken up by roots, transpires through leaves to become humidity, then condenses on cold windows/walls, drips down back into the soil. Below 10kPa plants will wilt and stop growing, but at 10kPa or higher they're fine. Furthermore, they tried another experiment. Decompressed spinach to Mars ambient pressure and held it there for 1 hour. It wilted, but when they restored pressure the plant perked back up and continued growing. That demonstrates plants can withstand complete pressure loss.

The only special consideration necessary is to keep a seed bank in the habitat, protected against radiation. If a solar flare or coronal mass ejection spews intense radiation at Mars during the day, then that will be enough to kill plants. That doesn't happen often, so just maintain a protected seed bank. Replant.

Also realize another very important fact. The reason NASA hasn't sent humans to Mars is nothing technical, it's political. You need to convince the political critters on Capital Hill. All politicians have Attention Deficit Disorder, and all have a very small intelligence. I could give a lot of example how they're screwing up to prove it. Every time you talk about the danger of radiation, what the politicians hear is it's too dangerous to send humans. When you blather on about technical details, they have already zoned out, they don't listen. When talking to a politician, keep it short and keep it simple. What they hear is if radiation is so bad that you need metres (yards) of radiation shielding on the roof just to grow food, then it's far too dangerous to send humans. So they cancel the entire humans to Mars project. Congress will never authorize NASA to send humans to Mars as long as Mars fans themselves repeat the radiation argument.

Last edited by RobertDyck (2013-06-09 22:10:01)

Decimator · 2013-06-09 15:58:08

RobertDyck wrote:

The only special consideration necessary is to keep a seed bank in the habitat, protected against radiation. If a solar flare or coronal mass ejection spews intense radiation at Mars during the day, then that will be enough to kill plants. That doesn't happen often, so just maintain a protected seed bank. Replant.

Do you have to let the plants die? It looks to me like the colony would have, at worst, an 18 hour warning that the CME is coming, with the majority giving 4+ days warning.

Link: Statistical Distributions of Speeds of Coronal Mass Ejections

JoshNH4H · 2013-06-09 19:44:53

Robertdyck-

I would suggest that the numbers you gave are basically the same as the numbers I gave, to within the margin of error and accounting for the potential variation in different parts of the solar cycle.

I think you're under a false impression regarding the source of the particles from spallation. Spallation can happen even when the impactor is a proton, so long as the nucleus that it is impacting consists of more than a proton. Subcritical, spallation-based nuclear reactors have been proposed in which protons are accelerated to 1 GeV to impact with Uranium atoms. They fracture the nucleus and the particle fragments are energetic enough to cause fissions in other nuclei. As you said, light element shielding would do a lot to alleviate this-- but not just light elements. It has to be Hydrogen.

Secondly, regarding "High energy radiation" protons can be every bit as energetic as other particles, be they heavier or lighter. Momentum is pretty irrelevant in this context, it's energy that matters. For our purposes, Cosmic Ray protons are just as energetic as other kinds of particles to be found in cosmic rays. By the way, some cosmic ray particles are moving so quickly that, due to relativistic time dilation, they experience time at rates that near zero. It's not inconceivable that we could get neutrons from other stars but I would think, and evidence seems to bear me out, that most of them are not moving quite that rapidly.

Most kinds of radiation are not particularly penetrating. Solar protons and x-rays, while a huge threat to a totally unprotected individual, are not nearly as hard to shield from as Cosmic Rays. I would imagine that any shielding that is sufficient to deal with cosmic rays would also be enough to deal with X-rays, even if it is composed of light elements.

The problem, again, is cosmic rays. I would simply use regolith. If you're using it in the dual function of pressurization and shielding, it will be more than enough to reduce radiation risk to acceptable levels.

GW and Robertdyck:

This really feeds into what GW was saying in his post: Given that simple and common-sensical measures can reduce radiation to acceptable levels, our response to anyone outside the scientific community with regards to radiation is (and ought to be) simple: Not a problem. No issue at all. Not dangerous. Perhaps our colonists will be subject to statistically higher rates of cancer or other diseases later in life, and this is unfortunate; It may sound callous but that's not unreasonable for a group of people who will be carving out a new society. If someone believes that people should be allowed to smoke then someone should certainly be allowed to colonize another planet (for much greater gain, to oneself and society) at comparable risk to theirself.

GW-

Regarding TMS Convention, when will your presentation be? I'd like to make sure that I attend. I'll be there that weekend, as will the user Bobunf and Midoshi.

The issue with that kind of shielding for greenhouses is that Mirrors aren't very good at capturing natural light like that at all hours of the day. In fact I'd suspect they're pretty bad at it. You'd need a good amount of land area and large greenhouses (good for thermal efficiency reasons) aren't really practical. Please recall that in a general sense agriculture is more productive when plants have more light to work with.

I've always been a fan of aeroponics to grow plants-- given that we'll want to economize on space and energy it seems like it is by far the most efficient way to do so. The basic method is that you suspend a plant with its roots in darkness and continuously spray them with a nutrient solution. Because the plants aren't in dirt it's relatively easy to move them, and the harvesting and planting (presumably the most labor-intensive parts of farming, given automation where possible) can be done by transferring racks of plants to a shielded location. Actual human presence in the greenhouse is unnecessary, and indeed providing for human access to the plants while they're in there could be seen as a waste of space.

MCP suits need to be revived, and I'd imagine that this could happen at low cost. I know it's been brought up before, but how different are they really from a full-body bathing suit? These "supersuits" as they were called prior to being banned certainly did not have price tags in the tens of millions of dollars.

For ease of human access in fixing machinery, etc. I favor a greenhouse atmosphere that is the same as the habitat environment as a whole. I think that this also makes it easier to use the greenhouse to regulate the atmosphere of the habitat, specifically with regards to CO2 levels and humidity.

Assuming that 18 hour warning can be given, I would imagine that a sheet of lead/iron/whatever could be pulled over the greenhouse to shield from X-rays.

RobertDyck · 2013-06-09 22:26:37

The head of the Mars Society Spacesuit Taskforce lead a project one year to invite researchers and developers of actual spacesuits to the Mars Society convention. Dr. Paul Webb was the inventor of the Mechanical Counter Pressuire spacesuit. We noticed his 80th birthday was that year, so we wanted him to pass on his knowledge; while he still could. We wanted to meet Dr. Webb, but primarilly we wanted him to pass on his knowledge to others currently working in the field.

One interesting fact was the first paper published by Dr. Webb was written a few months before it was published. It was published in the Journal of Aerospace Medicine, April 1968. But he wrote it in December 1967. That that's even a little earlier than 1969!

I had a grainy photocopy from another Mars Society member, who copied it out of his copy of the 1968 Journal. But when Dr. Webb himself presented, he had the originals. That included not just high resolution images from that paper, but he had transfered his film to a video file on his laptop. So we got to see video of his test subject in 1967. It worked, his test subject wore the suit in a vacuum chamber. That first test was equivalent to 50,000 feet, not hard vacuum of the Moon. But his later contractor report to NASA in 1972 was. Amazing.

There have been many attempts to revive this. But when Richard Nixon gutted NASA to transfer funding to the Vietnam War, all sorts of things got canceled. The MCP suit was one. Only workers for balloon suits got to keep their jobs, and they're still there now. So why won't NASA accept anything but a balloon suit? Hmmm. Am I implying that it's people protecting their jobs? Yup.

Decimator · 2013-06-10 11:20:26

JoshNH4H wrote:

Assuming that 18 hour warning can be given, I would imagine that a sheet of lead/iron/whatever could be pulled over the greenhouse to shield from X-rays.

I was going by the speed of the particles in a CME. The highest we've measured is around 3200 km/s, with most speeds closer to 450 km/s(you can view the distribution in the pdf I linked). If the problem is actually electromagnetic radiation, then there won't be any warning. Speed of light and such.

Last edited by Decimator (2013-06-10 11:23:58)

RobertDyck · 2013-06-10 14:27:40

The problem isn't X-rays, it's particle radiation. So your speed calculations are valid.

I just don't think there will be enough room for plants in the habitat. I envision a large greenhouse. I was part of the Mars Homestead Project phase 1: Hillside Settlement. The purpose was to design the first permanent human settlement. Since the project was building, we didn't want to waste a lot of time on how to get there. So I proposed (and they accepted) the premise that we would get to Mars via Mars Direct habitats. But greenhouses to supply a permanent settlement were quite large. I also argued for ambient light greenhouse, a couple others wanted buried. One of my arguments was for life support backup. Every life support system requires power, which means the power supply is a single point of failure. The only exception is an ambient light greenhouse. Plants recycle CO2 into O2 using sunlight. Biosphere 2 found difficulty getting the balance right, but they found it worked for a year without need for external oxygen. That gives plenty of time to repair the power plant. So one reason for ambient light greenhouse is life support backup. The project manager decided to compromise: some ambient, some buried. I would still like greenhouses to be all ambient light. With artificial light as a backup in case of dust storm. Those can last months. But with artificial light you again are dependant on power. Just hope the power plant doesn't fail during a dust storm.

The point is there would be far too many plants to fit in a habitat. And I don't expect them to be in flower pots that can be easily moved.

Here are a couple images of the Hillside Settlement.

Cross section. This shows 2 Mars Direct habitats on the left, then ambient light greenhouse with 1 tier of plants, then buried greenhouse with 2 tiers, then the habitat buried in the hill side. Notice the sun-tracking mirrors in a transparent sphere on top of the hill. Sunlight would be directed down via light pipes. That is a tube with mirrors on all interior surfaces. You don't need fibre optics, simple mirrors are good enough. A light diffuser at the bottom of the light pipe, which is on the ceiling of the habitat.

An artist's overview. The blue long things are greenhouses. The 4 cylinders with red rings are Mars Direct habitats. Three used to transport crew, 4 astronauts each. The fourth is a backup, and used to deliver cargo. The small greenhouses to the left of Mars Direct habitats are inflatable greenhouses, brought from Earth in the habs. The two large greenhouses are built using Mars resources. The two large shapes behind are greenhouses with regolith piled on top; what I call buried greenhouses.

Artists concept of a greenhouse. Again with regolith and artificial light. Notice the artist depicts a single tier, while the architect and engineers wanted 2 tiers.

And the central atrium. Notice the sunlight diffusers in the ceiling, at the apex of each groin arch. Not bad for in a hill.

And our artist's concept of an apartment. Common areas were deep within the hill, but apartments all have a window. The floor looks like hardwood. I don't know if that's realistic. There will be non-food plants grown in the greenhouse, such as bamboo for construction. But hardwood? You could apply vinyl flooring over the fibreglass or stainless steel pressure shell, but would want something nice. I'm thinking more along the lines of terracotta, ceramic tile, or even fancy stone. I don't think there will be any travertine: no hot springs. But there is calcite and dolomite in Mars soil, so would there be a deposit of limestone? And could limestone be metamorphed into marble? And there's basalt, so could there be granite? And there's clay, could we find slate?

JoshNH4H · 2013-06-10 20:44:06

In terms of radiation/solar storms, I would say that the problem is X-rays-- 2300 km/s is actually a pretty low energy, about 27 keV for a proton. That will easily be blocked by any matter that happens to be in the way. The x-rays emitted by the Sun, on the other hand, are rather more penetrating. Any shielding designed to significantly attenuate Cosmic Rays will take care of it easily, but humans outside in space suits or in the greenhouse are another story entirely.

The Mars Homestead project is truly wonderful and has come to a number of amazing results. I too favor greenhouses based purely on ambient light. I would like to point out that, given sufficient temperature, a dust storm will not kill the plants but simply reduce the rate of their growth. In dust storms there is a lot of indirect radiation, which plants can deal with just fine. I've heard that, including indirect illumination, insolation is reduced to about 30-50% what it would otherwise be, which is of course a huge loss but is not devastating. Artificial light will help things but depending on your power source (I don't want to get into solar-nuclear for imported power, but once you're building power supplies In-situ there's really no way to go other than Concentrated Solar Power).

That said, I disagree regarding bringing the plants to another indoor area. A greenhouse and a shielded area are approximately equal, in terms of per-volume cost for a structure of similar size. If we go the aeroponic route (which is the most efficient use of pressurized volume) the plants will be on racks and the racks can simply be moved. I would make it so that the machinery maintaining the plants (valves, hoses, and motors to transfer the plants into and out of the greenhouse) are accessible to people (so that they can be fixed if they break) but the plants themselves would have to be mechanically transferred to a shielded area to be tended to. Again, there's no implicit reason why shielded space should be much more costly than unshielded greenhouse space, and while it is an expense to have the space to transfer the plants to, a few at a time, it is more than made up for by the space savings achieved from not having people tend to the plants within the actual greenhouse (Because this space would only have to accomodate perhaps 5% or less of the plants at any given time). If I do say so myself, it's an intelligent application of simple technology to obtain real savings.

JoshNH4H · 2013-06-10 20:45:34

Oh, one objection: That apartment is too roomy! It's got more room than where I live

Edit: Oh, and no limestone. Limestone on Earth is the remnant of long-extinct crustaceans, and all signs point to there being none on Mars.

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