Scientific American and Space Travel

[Sawhet]

Forgive me if this has already been discussed, but I would like to hear the board's take on it.

In their last issue, Scientific American published an article by Eugine Parker on the dangers to space travel from cosmic rays. The author says that radiation makes for an irreducible threat that limits the time an astronaut could spend outside the earth's atmosphere. He went on to argue that there was no obvious way to shield a spacecraft from this damage.

The implications are disturbing. It seems to be saying that, baring a dramatic improvement in shielding technology, even the rest of the solar system is closed to human exploration.

Now, I've had as much fun as anybody following the exploits of all the robots, but this bugs me. Comments?

[Cugel]

I have read the same article, and it surprised me as well. It claims that it is actually the atmosphere (the column of air over your head) that offers the most protection against radiation. I always thought protection came (at least partly) from the Earth magnetic field. So the question is: how bad is it in low Earth orbit, where you don't have an atmosphere but you still are inside the magnetic field? So far, ISS visitors seem to be pretty safe. The article does not discus this.

For the Moon and Mars this is not relevant as they lack both protection mechanisms. According to the article it takes a column of 5 meters of water to offer roughly half the needed protection. Zubrin proposes to shelter astronauts inside the water tank of the spaceship during Solar storms but (as usual) didn't take any of this water (other than for consumption) into his mass calculations. Where one must shelter when the water is gone is probably discussed in his next book. Zubrin also makes his astronautes fill a couple of sandbags to put them on top of the tunacan, their living quarters. The article shows that this is useless. On the other hand, and Zubrin is certainly right in this, if you are willing to sit on top of a rocket that has a 2% failure rate (and knowing that 50% of Marslandings fail), you shouldn't be complaining about the 0.1% chance you will be killed by cancer.

[mugaliens]

Earth's atmosphere, all 14.7 pounds per square inch of it, provides some protection against radiation, more protection against some stuff than others.

But a simple and very thin layer of gold foil, so thin you can see through it, provides about as much.

[Van Rijn]

I have read the same article, and it surprised me as well. It claims that it is actually the atmosphere (the column of air over your head) that offers the most protection against radiation. I always thought protection came (at least partly) from the Earth magnetic field. So the question is: how bad is it in low Earth orbit, where you don't have an atmosphere but you still are inside the magnetic field? So far, ISS visitors seem to be pretty safe. The article does not discus this.


Haven't read the article, but: There are two sources of radiation, cosmic rays (which tend to be very high energy particles) and the sun (which produces lower energy particles). With the sun, you can get a solar storm with a huge number of charged particles heading your way and, without protection, you could potentially receive a lethal radiation dose in a short period of time. Cosmic rays are a long term radiation hazard. The earth's magnetic field deflects some cosmic rays and reduces the intensity of solar radiation substantially. Astronauts in ISS do receive a higher radiation dose than a person on the ground. It's a risk. For real long term life in space, we will have to think more about shielding, though we will probably always accept higher dosage levels than we receive on the ground.

One of the issues with shielding is that you can get a shower of secondary particles from high energy primary particles, so proper shielding design is important. Something with a lot of hydrogen is a good idea. Here's a good article on some of the thoughts about shielding (this is a pdf):

http://library-dspace.larc.nasa.gov/dspace/jsp/bitstream/2002/13284/1/NASA-97-tp3682.pdf

Anyway, you have to be careful about shielding or you can make things worse in the long term. One idea for Mars spacecraft would be to have a small "storm cellar" with additional shielding only to be used during a solar storm.


For the Moon and Mars this is not relevant as they lack both protection mechanisms.


Yes and no. You might get more secondaries off the regolith, but the world intercepts many of the particles before they reach you. Building next to a hill reduces this more. The Martian atmosphere helps some. If you put loose regolith on top of your shelter, you can reduce the dose levels further. It isn't ideal shielding material, but there is plenty of it!


According to the article it takes a column of 5 meters of water to offer roughly half the needed protection.


What is "needed protection"? I suspect the author has a very high (bordering on ludicrous) standard for what he thinks is "needed."


Zubrin also makes his astronautes fill a couple of sandbags to put them on top of the tunacan, their living quarters. The article shows that this is useless.


I don't understand that. Mass is mass. Granted, you need enough to handle secondaries that may be produced, so you may need a few feet for really good protection, but the principle is sound.


On the other hand, and Zubrin is certainly right in this, if you are willing to sit on top of a rocket that has a 2% failure rate (and knowing that 50% of Marslandings fail), you shouldn't be complaining about the 0.1% chance you will be killed by cancer.

I thought my chances of dying by cancer was already higher than that. The chances of getting cancer on the ground is far higher than that. The key question is: What is the increase in health risk due to this additional radiation?

[Cugel]

What is "needed protection"? I suspect the author has a very high (bordering on ludicrous) standard for what he thinks is "needed."

I'm just quoting from the article from my bare brains here (couldn't find the magazine). I believe 'needed protection' is roughly half of what you have on Earth surface.

I don't understand that. Mass is mass. Granted, you need enough to handle secondaries that may be produced, so you may need a few feet for really good protection, but the principle is sound.

The article described into some depth that water is by far a superior protection than any other stuff because of the hydrogen atoms it contains.


I thought my chances of dying by cancer was already higher than that. The chances of getting cancer on the ground is far higher than that. The key question is: What is the increase in health risk due to this additional radiation?

The risk increase from radiation is so small compared to all other risks of landing on Mars that it is not something to worry about. Where possible you should take counter measures of course. And for any 'would-be space invaders', you should accept that Mars/Space will probably kill you. Either spectacular or slowly in the long run.

[mugaliens]

Also, earlier talk in the 70s about colonies in space mentioned using up to 12 feet of rock to shield the colonies, but I think that included from micrometeorites, as well.

[Van Rijn]

I'm just quoting from the article from my bare brains here (couldn't find the magazine). I believe 'needed protection' is roughly half of what you have on Earth surface.


Ok. Until I see some specifics, I can't say too much, but assuming that means sea level that is being extremely conservative. You get double the radiation dose many places, and you don't see a noticable increase in cancer. There are places, like near a former natural nuclear reactor in Africa, where radiation dose is several times as much. Again, no obvious increase in cancer. Now, we assume there is, and as always, finding additional cancers in the "background noise" is very difficult unless the dosage is high, but the increased chance for cancer is tiny unless you really bump up the radiation level.


The article described into some depth that water is by far a superior protection than any other stuff because of the hydrogen atoms it contains.


Yes and no. More hydrogen means fewer secondaries, but if you have enough material to deal with the secondaries (like regolith) it doesn't matter. And the higher density can help you.


The risk increase from radiation is so small compared to all other risks of landing on Mars that it is not something to worry about. Where possible you should take counter measures of course. And for any 'would-be space invaders', you should accept that Mars/Space will probably kill you. Either spectacular or slowly in the long run.

Yep, same as earth. This is somewhat of an issue for space travel. We have to look at shielding both passive and active, and travel time. Still, if you gave me a ticket to Mars today, as long as semi-reasonable precautions were taken, I'd grab it.

[Relmuis]

Radiation would not be a problem in a Mars colony, provided that living quarters and most workplaces were underground. Agriculture, though, would have to be aboveground, in transparent, airfilled domes. All work inside these domes would, as much as possible, take place during the night, as would all other surface activities. Workers would carry dosimeters, and everone, except children and pregnant women, would have to take a fair share of the risk involved.

Travel to Mars might not be a problem if it did not take more than a few months for a one-way, never to be repeated trip, or a few days for a trip which people might expect to take a dozen times over their lifetime.

However, all interplanetary travel should cease, and all surface activities on Mars should stop during a solar flare.

[Sawhet]

Perhaps I should restate what the article as about. The author is not talking about solar raditation, he is talking about cosmic rays, which are very hard to shield against. He says that, once outside the earth's atmosphere, the level of radiation is so high that long term exposure is prohibitive. He believes it is a hard limit on the amount of time humans can spend in space or on planetary surfaces. He is arguing that, for all intents and purproses, it will make human flight into deep space impossible.

[Cugel]

Here is a doucument with additional information about the dangers of cosmic radiation related to manned missions to Mars.

http://www.thespacereview.com/article/602/1
(At the end of page 1)

It's written by Donald Rapp, somebody who knows what he is talking about. http://isdc.xisp.net/~kmiller/isdc_archive/isdc.php?link=personSelect&person_id=86)

I believe Rapp is taking a slightly more relaxed attitude towards the risk of radiation. But maybe that is because he is claiming that manned Mars missions are pretty much beyond our current state of (propulsion) technology anyway.

And as there is nothing beyond Mars for manned exploration, it's sort of a theoretical discussion from there.

[Van Rijn]

Perhaps I should restate what the article as about. The author is not talking about solar raditation, he is talking about cosmic rays, which are very hard to shield against. He says that, once outside the earth's atmosphere, the level of radiation is so high that long term exposure is prohibitive. He believes it is a hard limit on the amount of time humans can spend in space or on planetary surfaces. He is arguing that, for all intents and purproses, it will make human flight into deep space impossible.

I got that it was about cosmic rays. I don't get his conclusion. Does the author quantify what he means by a "hard limit" on time spent?

The link (note that it is a pdf) to the radiation article that Cugel mentioned is:

http://www.mars-lunar.net/Arch.Elements/Rad.Effects.Report.2.pdf

This is quite detailed and seems to cover the subject pretty well. I think it will have more to do with the question: How much risk are you willing to accept? If you go to space, you have to accept a higher cancer risk. Wouldn't be the first time people accepted risks to go to the frontier.

[Aireal]

Sawhet

I am very familier with the article in Scientific American as I have been doing research on this problem for 5-6 years now. Somewhere on this forum and others you will find posts on my concepts for efficent radiation shielding. After that article was published I even sent the editor a copy of my radiation shield concept. Over the years I have sent letters and e-mails to NASA and others about my work also, all to no avail. It seems that there is little to no work being done in this important area.

[loglo]

It seems that there is little to no work being done in this important area.

Human factors research for the VSE seems to have been put on the back burner while the launchers are developed. I think this is going to turn around and bite NASA when they least expect it. While I agree that radiation will be a big problem I don't think it is insurmountable. It needs to be looked at now however before they start designing the CEV, not after.

[JonClarke]

Earth's atmosphere provides ~1000 g/cm2 of radiation protection, equivalent to 10 m of water or 3.3 m of basalt.

Mean atmospheric pressure on Mars provides 7 g/cm2 of radiation protection, equivalent to 7 cm of water or 2.33 cm of basalt. Some places on Mars offer 12 g/cm2. This is OK for solar radiation but not much good for cosmic rays.

People also need to remember that these figures are a problem for Mars settlement but do not rule out either missions of research stations with crew rotation.

Jon

[publiusr]

Human factors research for the VSE seems to have been put on the back burner while the launchers are developed. I think this is going to turn around and bite NASA when they least expect it. While I agree that radiation will be a big problem I don't think it is insurmountable. It needs to be looked at now however before they start designing the CEV, not after.

Launchers have been neglected by NASA for far too long. Radiation is like the treat you face when storm chasing. Long duration missions can have LH2 tanks surround the central cabin as shielding. SCI-AM has its own slant on things, and no doubt the enemies of human spaceflight who want to limit all NASA flights to Delta II launched toys are behind stories like this.

[loglo]

Launchers have been neglected by NASA for far too long. Radiation is like the treat you face when storm chasing. Long duration missions can have LH2 tanks surround the central cabin as shielding. SCI-AM has its own slant on things, and no doubt the enemies of human spaceflight who want to limit all NASA flights to Delta II launched toys are behind stories like this.


I don't think LV design for the VSE can be completely uncoupled from the requirements of the CEV. The affects of radiation aren't just an abstract increased risk of cancer but can be mission killers. There is no point sending people to Mars who are going to die from radiation exposure en route. Shielding is an expensive mass requirement that needs to be accounted for adequately.

And frankly having an oxygen rich capsule surrounded by LH2 is not the safest option either. I want us to get to Mars too but stewed astronauts could be a programme killer, not just a mission killer.

[JonClarke]

Cosmic rays aren't going to kill people on route. The risk is long term, not immediate. Solar flares might, if there was not enough shielding, but this has been a well studied problem and the risk can be minimised by good design and strategic shielding. It's not a mission stopper.

Jon

[Van Rijn]

Right, GCR is a long term cancer risk. And, I take this argument with a very big block of salt. It sounds like the author is taking some fuzzy numbers and calling them hard limits. Yet, it is hard to show increases in cancer rates at low to moderate radiation levels. Also, it may well be possible to reduce cancer risk medically with antioxidants and related treatments.

Nobody is going to die on a mission due to GCR. Astronauts will choose to accept the possible cancer risk or won't go. I expect there are many people that would choose to go. It would take a lot to stop me, if I was given the option for a flight to Mars.


And frankly having an oxygen rich capsule surrounded by LH2 is not the safest option either. I want us to get to Mars too but stewed astronauts could be a programme killer, not just a mission killer.

But we drive every day in a car with gasoline next to us, sleep in houses with methane running through pipes in the walls and under the ground, and so on. It is a very minor issue with proper design.

[JonClarke]

I have gone through this article by Eugene Parker in some detail and it really is a mishmash of unsupported assumptions, poor logic, and outright factual errors. I am actually rather suprised at the editorial control at Scientific American in letting this through.

Several things must be pointed out.

First, the article is entirely about cosmic ray risk, not total radiation exposures. So solar flares, induced surface radiation, etc. are not covered.

Second, the article appears largely based on a study by Wallace Friedberg of the FAA, this study was reported by New Scientist, but credited to Keran O'Brien http://www.newscientist.com/article.ns?id=dn7753 . Both these authors have published extensively on cosmic ray risks to airline passengers and crew. According to New Scientist, the study was supposed to have been published in the journal Radioactivity in the Environment (vol 7, p 894), but I have not found any independent citation.

Third, the article does not address short term missions or acceptable exposure levels, but assumes that cosmic radiation exposure must be reduced to that encountered at 5,000' on earth. This is probably relevant to space settlements but is not relevant for Mars missions, or for Mars and Luna stations.

Errors

On page 24 Parker quotes Friedberg as saying that "Mars astronauts would receive a dose of more than 80 rems a year". This conflicts with the figure on page 25, which shows that in interplanetary space astronauts are exposed to 12-25 rem per year, those on the lunar surface 7-12 rem, and those in LEO 10 rem. LEO and lunar surface are basically half those of interplanetary space, as you would expect, with a planetary mass blocking half the cosmic rays. These numbers indicate that on average Mars astronauts on a 900 day mission would get a total mission dose of 35 rem.

This figure also conflicts with the IPS summary of space radiation http://www.ips.gov.au/Category/Educational/Space%20Weather/Space%20Weather%20Effects/guide-to-space-radiation.pdf which gives an in-spacecraft cosmic ray dose of 0.02 rem per hour (17.5 rem). this is in agreement with the page 25 figure but totally conflicts with the quote from Friedberg.

Unsupported assumptions

The punch line for Parker's article is on page 22: "They [cosmic rays] could be the show stoper for visiting Mars."

However the article does not address the actual acceptable exposure limits for human space flight. It assumes that cosmic ray exposure must be reduced to that encountered at 5,000' on earth. This is probably relevant to space settlements but is not relevant for Mars missions, or for Mars and Luna stations. The assumption that these short term scenarios (6-12 month rotations for a lunar station, 2.5 year mars missions, 3 year rotations to a Mars station) must abide by the standards of lifetime exposure is nowhere justified.

Poor logic

The article shifts the goal posts. On page 22 it is asserted that: "They [cosmic rays] could be the show stoper for visiting Mars." In other words even short expeditions to Mars may be ruled out. However, on page 29, the figure caption says: "The prospect for permanant settlement hinges on whether biomedical researchers can develop antradiation medicine." The goal posts have switcged from short visits being impossible to permanant settlements will require antiradiation medication.

Some facts

Finally, it is worth noting that people have already stend significant amounts of the in space. Krikalyov has spent 803.4 days in LEO over 6 flights, Avdeyev 747.6 days over 3 flights, and Polyakov 678.7 days in 2 flights. Using the 10 rem per year exposure to cosmic days in LEO given on page 25, Krikalyov has been exposed to 22 rem, Avdeyev to 20 rem, and Polyakov to 19 rem.

Given an average annual exposure rates in interplanetary space of 20 rem and on the Mars surface of 10 rem we can estimate the following exposures:

1. Sprint mission (4 months to and from, 4 months on the surface) 17 rem.

2. Standard mission (6 months to and from, 18 months on the surface) 35 rem.

3. Mars station (4 months to and from, 22 months on the surface in a well shielded - 2 m of basaltic regolith - base & assuming 1 day in 3 spent on the surface) 20 rem.

4. Lunar station (12 month rotations) 10 rem.

in other words options 1, 3, and 4 are below or equal to historical exposures to the most experienced space travelers. Option 2, a typical Mars mission scenario does have a higher exposure rate, but still only 60% above that of Kirkalyov.

Jon

Edited by JC

[Relmuis]

What is the official lethal dose?

[Van Rijn]

What is the official lethal dose?

A quick exposure (minutes, hour) of 600 REM will almost certainly kill a person. Fast exposure to a few hundred REM is very bad. However, as previously discussed in this thread, we're talking about GCR, which is a long term cancer risk, but will not cause radiation sickness - the exposure rate is too low. A bad solar flare could cause a massive exposure, but, again, those are lower energy particles and can be dealt with more effectively than GCR.

[Van Rijn]

Third, the article does not address short term missions or acceptable exposure levels, but assumes that cosmic radiation exposure must be reduced to that encountered at 5,000' on earth. This is probably relevant to space settlements but is not relevant for Mars missions, or for Mars and Luna stations.


Thanks for the information. It was pretty clear that the author was assuming a hard limit where none existed. Even for long term settlements, that's questionable - there are a number of examples of towns with people living long term with much higher radiation levels without obvious increases in cancer rates. Granted, it is a risk, but people choose risks all the time. Look at how many choose to smoke, for example. Space will be dangerous. With a bit of care, I don't see this being a significant additional risk.

I'm a bit surprised too, because Scientific American usually avoids radiation hysteria.

[JonClarke]

I think it is an example of journalistic perception of "balance". They have had articles in the past that strongly favour human missions (by Zubrin for example) so they feel in this case they have provide one that doubts the possibility. of course which issues get this sort of "balanced" treatment is editorial whim.

What is frustrating is that the article could have been good, with a little more editorial control. Had it addressed the shielding requirements for life-time space settlements on the Moon Mars, or in O'Neill colonies, it could have been very valuable. A discussion of the actual radiation issues for the Mars and Moon stations and Mars missions likely this century this too could have been valuable.

As to the actual consequences of the radiation exposure, the Friedberg article quoted said that an 80 rem dose in a year would lead to a 10% male and 16% female morbidity cancer morbidity. In actual fact a Mars mission would be exposed to 35 rem over 2.5 years, so the morbidity would be much less. The quote is unclear whether this is an increase in morbidity or total morbidity. from memory, 20% of the population dies from cancer anyway. So a 10% total morbidity is less than what you would get anyway. If it is a 10% increase, then that is more serious. A lot depends on the age of the Astronauts assumed for the study. A 10% increase in cancer morbidity if the study population was assumed to be in their 30's is a serious issue, as most of those deaths will be premature, given that most radiation related cancers take 20 years to show uo. On the other hand, if the study population was in their 50's (as is quite possible for a Mars mission crew) then many of the astronauts will be in their 70's before the cancers show up. Since most people die in their 70's and 80's, an increase in the cancer proportion from 20 to 30% is less than an issue.

On another issue, I think it is a bit unfair to have got Parker to have written this article in the first place. He is one of the GOM of space plasma physics, explaining the solar wind in 1958. Radiation medicine is not his field. It would have been better to have had Friedberg and O'Brien write the article, as they are the experts on cosmic rays and health and the source of the claim.

Jon

[mike alexander]

What is the cancer rate in people who live essentially their entire lives in high places, like the Tibetan plateau? That's a lot more than 5000' above sae level.

[Van Rijn]

What is the cancer rate in people who live essentially their entire lives in high places, like the Tibetan plateau? That's a lot more than 5000' above sae level.

There are people that are exposed to even higher radiation levels than that, but it is still often difficult to detect higher cancer rates. Papageno found this abstract:

http://www.inderscience.com/search/index.php?action=record&rec_id=7892&prevQuery=&ps=10&m=or

which suggests very high radiation levels aren't obviously increasing cancer rates. This is by no means the only article along these lines. We currently go with the "linear no threshold hypothesis" which assumes there is no threshold for "safe" radiation and that we can directly extrapolate incidence rates from higher radiation exposure levels, but I suspect it is, at least, not linear. In any event, I suspect we will learn a lot more about cancer and the details of the effects of radiation on cancer rates before we are ready to build long term habitats on Mars, the moon or in deep space.

[publiusr]

I have gone through this article by Eugene Parker in some detail and it really is a mishmash of unsupported assumptions, poor logic, and outright factual errors. I am actually rather suprised at the editorial control at Scientific American in letting this through.


I'm not--this is clearly an anti-VSE hit-piece aimed squarely at human spaceflight--typical of the folks at SCI-AM and their agenda---and written by folks who want nothing in space but itty-bitty Delta II payloads operated by petty academics who want to keep the rest of us Earthbound.

More thinkspeak from the "robot's only" crowd. This much should be obvious.