Surviving Radiation in Space

Apollo 10 image of Earth taken from 100,000 miles.  [Credit: NASA]
Apollo 10 image of Earth taken from 100,000 miles away.
[Credit: NASA]
For those who are interested in the reality of radiation exposure on Earth, in space, on the Moon, and what this exposure means for our prospects of manned exploration of the Solar System, read on!

The Myths and Truths of Death by Space Radiation

There are persistent groves of misinformation taking root about the lethality of radiation doses for astronauts, particularly for those who are bound for the Moon and/or Near-Earth-Objects, (such as asteroids for research or mining).

Unfortunately, these claims have been given the capacity for widespread proliferation in the fertile cyber-soil of the Internet, and worse, they usually sprout symbiotically with claims that the Moon landings were hoaxed, e.g.:

“We could never have landed on the Moon because the astronauts could never have survived the radiation from cosmic sources/the Van Allen Belts/solar wind.  Therefore, at a sound stage in the Nevada desert…”

Well, since most of these authors capitalize on the preexisting, prevalent fear of radiation to sugar-coat their misinformation pill, most people are unprepared to distinguish technically-compelling pseudoscientific fluff from interpretations of actual data.   So, the below is an effort to arm you, fellow readers, with a guide to help navigate these murky radiation/Moon hoax waters.

NOTE: NASA has produced a factsheet on space radiation as well, which covers the basics of radiation and its effects and measurement.

By reviewing some of this information, you’ll ideally emerge with an enhanced ability to see for yourselves if these radiation-lethality claims hold water.

(SPOILER ALERT: They don’t.)

So, to begin, let’s review what we know about radiation exposure right here on planet Earth.

Current Regulation Levels and Common Radiation Doses

After nearly a half-century of dedicated research, it has been found that there is no detectable increase in the incidence of cancer (the primary threat of penetrating gamma-ray radiation exposure) for people who receive an annual radiation dose of 5,000 millirem (5 rem) or less.

Consequently, the U.S. Nuclear Regulatory Commission’s (NRC) federal regulations currently limit nuclear workers to an annual dose of that amount.  Further, the U.S. Department of Energy’s (DOE) federal regulations, to be on the safe side, currently limit radiological workers’ annual doses to one tenth of the NRC’s limit (500 millirem) unless there is some sort of extreme circumstance or emergency.

But what do these numbers mean?  To help visualize this data, please see the below graph, which places these numbers in simple context with radiation doses we receive naturally from things we all can more easily comprehend, like a chest x-ray:

Current radiation exposure limits and common doses.  [Chart credit: Ben W. McGee]
Current radiation exposure limits and common doses. [Chart credit: Ben W. McGee]
As you can see, there is a certain amount of radiation exposure that we all receive just from standing on planet Earth (see the far right-hand side of the graph).  This natural radiation is unavoidable – cosmic rays can penetrate just about any shield that is not located deep within the Earth, which is itself radioactive and contributing gamma rays from below.  In fact, you will note that the DOE administrative limit mentioned above is actually less than the amount of radiation we all already receive from Earth, plants*, rocks, air, and even ourselves* in a given year.  (*Roughly 1-2% of all potassium on earth is the radioactive isotope, K-40.)

The take-home here is that none of the numbers in the above graph indicate any sort of imminent danger.  In fact, all doses depicted above are evidenced to be “safe” levels, in that they are either natural or below any exposure that the data indicates increases the incidence of cancer in a population (see: ICRP, NCRP).

NOTE:  There are actually two separate dangers that get confused during conversations about the health effects of radiation.  The first kind of danger is for lower-level exposures, which is the danger of increasing your risk of developing cancer later in life.  -This is exactly like the common knowledge that more time spent sunning or tanning during youth equates to an increased risk of skin cancer later on in life.  (It won’t harm you now, but it could harm you later.  It’s a roll of the dice based on your own health, habits, luck, and genetics.) 

The second kind of danger is immediate – the damage and destruction of cells due to a brief, intense exposure to radiation.  Following the sun-tanning analogy, this is akin to a sunburn but spread throughout your body – damage directly caused by the radiation due to its intensity.  While this may also increase your risk of cancer, the threat here is direct injury and your body’s ability to cope.

How do these natural and regulated levels of radiation exposure compare to the radiation dose levels we really know to be definitely unsafe?  For that, see the following expanded graph, which has been color-coded to relate it to the previous one:

Dangerous radiation levels in context.  [Chart credit: Ben W. McGee]
Dangerous radiation levels in context. [Chart credit: Ben W. McGee]
So, as you can see, this graph allows you to immediately identify relationships between ordinary and dangerous radiation exposures to help you understand the concept of radiation exposure and recognize how intense radiation has to be in order to be considered truly dangerous.

  • For instance, you have to be exposed to an intensity of radiation ten million times that of Earth’s normal background levels before worrying about developing radiation sickness.  That’s ten thousand times more powerful than a chest x-ray.
  • You would also need to receive 1,000 chest x-ray scans before worrying about definitely having increased your risk of developing cancer later in life by a single percentage point.

Now, with a little context, we can start to evaluate how bad the space radiation environment really is.

Explorer-1, that discovered the Van Allen Radiation Belts in 1958.  [Credit: NASA/MSFC]
Explorer-1, launched in 1958. [Credit: NASA/MSFC]
Debunking Lethal Radiation from the Van Allen Belts

The United States’ first spacecraft, Explorer-1, detected the presence of so-called “belts” of radiation around the Earth.  These were named after the scientist who designed the instrument that discovered them, Dr. James Van Allen of the University of Iowa.  However, we have learned much since then, including measurements from the radiation instrument RADOM aboard the much more recent Chandrayaan-1 spacecraft (launched in 2008).

Results from RADOM showed that the inner Van Allen belt, which extends from roughly 1,000 miles above the Earth to a little more than 6,000 miles up, appears to be composed of highly energetic particles, such as solar protons, (meaning they pack a higher radiation “kick”).  The outer belt, on the other hand, extends from a little more than 9,000 miles up to a full 33,000 miles up, and it appears to be a little gentler – it is composed primarily of electrons (beta particles).

So, just how bad was the radiation measured there?  Well, it wasn’t something to dismiss (and was academically quite interesting), but it also wasn’t something that would strike fear into the hearts of mission planners:

Peak radiation exposure while traveling through the inner, more powerful belt reached 13,000 millirem per hour, (or 13 rem per hour).  So, if an astronaut were to park in worst part of the inner Van Allen belt for an hour with no shielding, he or she would receive a radiation dose nearly three times the annual “safe” dose for DOE workers and may have bumped up their lifetime risk of a fatal cancer by a percentage point.

Fortunately, however, the time the Apollo astronauts spent traveling through the highest radiation zone of the inner Van Allen belt (at a screaming 11,000+ miles per hr) was fractional – their doses averaged 120 millirem per day.

Go ahead and compare this to the above graphs.

So, it is clear that the Apollo astronauts’ radiation doses in this case were much less than a common CT scan and far less than what a modern astronaut on the International Space Station receives during a 6-month tour (~7,000 millirem).

Hence, simply passing through the Van Allen Belts is anything but lethal.

Astronaut exposed to the raw space radiation environment on Apollo 8.  [Credit: NASA]
Astronaut exposed to the raw space radiation environment on Apollo 8. [Credit: NASA]
Debunking Lethal Radiation Doses from the Earth to the Moon

Like our own sun, all of the other stars in the night sky are nuclear reactors.  Consequently, a constant “rain” of high-energy particles and gamma rays comes at us from the rest of the galaxy, which we call Galactic Cosmic Radiation, or GCR.

Many claim that in “deep space,” e.g., the space between the Earth and the Moon or between Earth and Mars, GCR would prove lethal for a human being.  Yet, the data indicates otherwise.  (Actually, GCR is the primary source of radiation an astronaut normally experiences in all cases, whether in Earth orbit or beyond.)

Let’s have a look.

The data we have about radiation doses during travel from the Earth to the Moon, like with the Van Allen Belts, are not limited to the old Apollo mission data.  For example, the same Chandrayaan-1 spacecraft mentioned above also traveled from the Earth to the Moon and showed a dose during the five day trip (a.k.a. during “translunar injection“) of 1.2 millirem per hour.

Granted, while this is a level nearly a hundred times the average gamma-ray background radiation intensity on Earth, it is still low enough to not present an immediate concern.  Why?  See the above graphs for a comparison – An astronaut would have to spent more than 170 days in this radiation field before even reaching the NRC’s limit for nuclear workers, which equates to no statistical increase in developing cancer.

This sort of radiation exposure becomes an issue when planning long-term missions to the Moon or Mars, which involve several months to years of exposure time, but it certainly bore no immediate threat to Apollo astronauts traveling to-and-from the Moon in a matter of days.

View of the Taurus-Littrow Apollo 17 landing site, 7-19 Dec. 1972.  (Credit: NASA)
View of the Taurus-Littrow Apollo 17 landing site. [Credit: NASA]
Debunking Lethal Radiation on the Moon

Like with the trip from the Earth to the moon, radiation doses on the lunar surface did not even approach immediate danger levels, and while they fluctuated strongly with changes in the Sun’s output, the Moon itself was observed to act as a shield from galactic cosmic radiation.

Consequently, doses received by astronauts on the lunar surface were actually less than that received in lunar orbit, and again, averaged 120 millirem per day.

This value is completely consistent with measurements from the RADOM instrument in 2008 that showed radiation dose rates in lunar orbit of approximately 1-2 millirem per hour.

And again, these are far from doses that would pose an imminent threat to an astronaut’s ability to function.  An astronaut would, quite simply, need to stand in a radiation field of an intensity one hundred thousand times greater for a full hour before suffering the effects of radiation sickness.

The final space radiation threat data in context, plotted in green, can be seen in the following chart:

Space radiation doses in context.  [Chart credit: Ben W. McGee]
Space radiation doses in context. [Chart credit: Ben W. McGee]
Looking Ahead to Planetary Exploration

What does this all mean for the future of manned space exploration?  While all of this does show that claims of radiation lethality in space are plainly false, it also indicates that radiation mitigation will have to be a central planning issue in order for future astronauts to remain within the current bounds of acceptable risk.

Prevailing wisdom accepts that spaceflight and planetary exploration is inherently dangerous and limits what is considered to be acceptable risk to a 3% increase in fatal consequences as a result of radiation exposure – regulations for radiation exposure that are more lenient for astronauts than for other radiation workers.  (Surprisingly, however, this level of risk acceptance is actually more conservative than what is currently accepted for workers in other, much more prosaic terrestrial jobs in many industrial and natural resource fields… but that’s another story.)

There is some research to suggest that chronic, lower-intensity radiation exposure to some of the soft tissues of the eye may lead to secondary negative health effects, such as cataracts, but we’ve only just begun to learn what effects the many alien factors of the space environment have on human physiology, including gravity-induced modifications of bone, muscle, and organ function.  -And again, these effects are not imminently prohibitive and are certainly not immediately lethal.

The Take-Home

Radiation exposure is one of space’s primary threats – but it is not the primary threat.

A lack of atmospheric pressure, the presence of boiling and/or freezing temperature extremes, an intrinsic lack of breathable air and water, and the necessity of shielding against (or avoidance of) micrometeoroids are all arguably more pressing threats.

Radiation at any exposure rate measured in cislunar space certainly wouldn’t prevent an astronaut from visiting the moon, and only if trapped in the most unlikely and unfortunate of orbits would an astronaut ever need be concerned about the possibility of developing a radiation-induced depression of the immune system and – at the extreme – acute radiation sickness.

Take alarmists with a grain of salt and look to the data for the truth.  In fact, it can be seriously argued that conquering our fear of the atom may actually be the means by which the rest of the solar system is opened to humanity.

In my view, that’s where the real conversation is.


For more information on space radiation doses to astronauts, link (PDF) to the following landmark document, “Space Radiation Organ Doses for Astronauts on Past and Future Missions” by F.A. Cucinotta.

21 thoughts on “Surviving Radiation in Space

  1. Excellent overview of the radiation issues for spaceflight. One topic you might expand on is shielding. Our atmosphere is the primary shield from high energy GCRs and at ground level the atmospheric density is roughly the equivalent of about two meters of rock. So on the Moon, Mars, or an asteroid, piling up local materials around habitats can easily provide very good rad protection. Space stations can also gradually build up shielding as they accumulate material delivered to them over time. For example, a NASA team recently demonstrated how ISS waste materials could be compressed into blocks suitable for packing around the habitats for rad shielding rather than being dumped into the atmosphere.

    Space transports are obviously more difficult to shield due to the need to limit their mass but even there the design of the vehicles could be optimized for shielding. E.g. arrange water, propellant, and waste tanks as well as food and other stored materials around the living quarters to maximize their shielding effect.

    1. Clark,

      Thank you so much for reading and for taking the time to offer your positive feedback and thoughts! I always find myself in a tug-of-war between depth and readability when it comes to technical articles like this, and I completely agree that the subject of radiation shielding is one that deserves greater attention. As a radiological engineer, I am deeply troubled by the rampant misinformation (disinformation?) out there regarding the actual state of knowledge regarding radiation exposure, particularly in the space context. It is primarily to address these misconceptions that I put this article together. However, your (and others) specific mention of shielding definitely makes it appear that there is an audience for a deeper discussion of the space radiation issue, so with that in mind I think my next technical article will be on radiation shielding in space! Thanks once again for your readership!


  2. An excellent article and quite factual. However, as an astrophysicist, I can state that there is one particular radiation threat that does have catastrophic potential and that is a badly timed and unfortunately directed CME (Coronal Mass Ejection) from the Sun. Fortunately Apollo took place during a time of relative solar quiescence. Thus, if we are to ever go back to the Moon on a continual basis or if we are to go on extensive interplanetary flights, robust radiation protection will be a must. This issue has been addressed in the design of NASA’s proposed NAUTILUS-X deep spacecraft by using the crew’s water supply as radiation shielding in a special refuge the crew would go to when such events occur.

    1. Rick,

      Thanks so much for reading and taking the time to offer your thoughts. As I mentioned in an earlier comment, I’m always fighting to decide where to trim these articles for readability (they always start long). I originally had a section on CMEs and shielding (hinted at by the statement about dose on the lunar surface varying with solar output), which I ultimately cut because the article was simply getting too long. (I know I have only a finite number of technical words and concepts before number of readers drops off precipitously…) However, I completely agree with your points and as a result am now putting together a follow-on article where we really get into shielding – w.g., regolith, NAUTILUS-X-style approaches, electrostatic deflection, etc. Thanks so much once again for taking the time to weigh in, and stay tuned for more on space radiation!


  3. Thanks for a discussion that (among other human factors issues in spaceflight) does not receive the attention that it should. However, I differ a bit in your assessment that radiation risk is not as high as we think. In fact, it may be greater. To that point, Cuccinotta et al recently updated their radiation risk assessments and have suggested that older, non-smoker astronauts (>40 yrs) should have their dose limits lowered. Here’s the citation: Cucinotta FA,Chappell LJ. Updates to astronaut radiation limits: radiation risks for never-smokers. Radiat Res 2011; 176: 102-114.
    I also concur with Rick Boozer above regarding CMEs. The radiation that is of greatest concern is ionizing radiation/high energy particles (such as that coming from CMEs I believe)… but let’s face it– any radiation of a sufficient dose, rapid onset and duration is of concern. And, to Clark Linsay’s point, we have much work to do in order to address the shielding issue.
    Finally– the moon is not without its hazards. As I understand it, astronauts on the surface of the moon are actually double-dosed with GCR from space plus a secondary spray of ionizing radiation coming from GCR hitting the lunar regolith. Thus, whlle you are correct that the short sortis the Apollo astronauts made were probably not a huge safety hazard for them… it begs the question whether longer sortis associated with more permanent colonization will be possible. As for me, I’d definitely want my hab buried in the regolith under a bunch of water balloons!

    1. Eleanor,

      First, thank you very sincerely both for reading as well as for your in-depth comments! I would also like to point out that, despite your mentioning to the contrary, I think our assessments regarding the risk of radiation exposure actually agree with respect to the claims the article was specifically intended to address. Keeping in mind that there are many out there who are (loudly) claiming that simple passage through the Van Allen belts or a few-day sortie to the Moon is not survivable (deterministic effects), I hoped to with this article offer a (relatively) concise presentation of what we know about radiation exposure and risk (comparing hard terrestrial with ET numbers) to highlight those claims as simply untrue.
      –So, yes, I agree with you wholeheartedly that space dosimetry is woefully coarse and that as we collect more data the risk values will almost certainly shift around. However, my point is that I’m sure we both agree that none of this refinement of epidemiology and health effects numbers will suddenly justify the claim that no astronaut could expect to travel to the moon and live to tell the tale (thereby justifying the claim that the moon landings had to be hoaxed). =)

      Secondly, you’ve now made me regret an editorial decision! I was actually aware of the more recent Cucinotta article and had included it in what was originally a much longer section on the current “prevailing wisdom” of the 3% risk acceptance. As I mentioned in an earlier comment, however, in the interests of brevity/readability I ended up cutting the detail on Cucinotta’s more recent research (since it didn’t affect either the 3% statement or alter the plotted information about lunar dose) – the article was simply getting too long for there to be any expectation of a general reader hanging on until the end. And again, I agree that the dose/risk relationship may be higher in some cases than we currently accept – but it will never suddenly swell beyond the error bars of the data we already have.

      As for CMEs, again, these were originally mentioned but were cut for brevity. (I think all of the very-related comments on shielding will justify a follow-up article!) Since they’re episodic events, I decided this sort of detail (along with mentioning our having conducted the Apollo missions during a lull in solar activity) was non-essential in a broad-strokes article where to many readers the concept of GCR may have been encountered for the first time. I think in addition to CMEs as very real radiation threats, one could make the same argument for discussing protection against sudden exposure from a nearby incidental GRB – something for which evidence of past interactions with the terrestrial environment is mounting even in the archaeological record.

      Regarding secondary lunar backscatter/albedo neutrons, this is again extra detail that I think I’m now engaged to cover in a follow-up article, (e.g. Apollo 17 LNPE, etc). Absolutely these surficial effects contributed to the total dose experienced by astronauts while on the lunar surface, but I think I would disagree with your characterization (or at least clarify) that this equates to a “double dose.” Yes, there is more bang for the buck via GCR due to these scatter effects, so in this sense there is a “double dose” from incident GCR, but to be clear for readers, the dose estimates for Apollo astronauts included these contributions – so the dose is not, I would argue, double the recorded dose. And the moon does simultaneously act as a shield for the rest of the GCR coming from (pseudo-2pi geometry) underfoot. (This can be validated by comparing average lunar surface dose rates with dose rates measured more recently in lunar orbit. The higher the orbit, the greater the dose due to the effective decrease in size of the lunar “shield” with respect to 3D incidence of GCR.)

      So – at the end of the day, I think we absolutely agree: Radiation exposure is a serious mission planning issue for longer endeavors, such as a Mars mission or (as you suggest) lunar habitation. In this sense I do not mean to downplay the risks, which can be severe. However, considering that in the cultural mindset the risks from radiation exposure are already far overplayed (compared with the accepted mortality risks of, say, exposure to gasoline fumes at a gas station or from using a ladder at heights greater than 4 feet, etc., etc.), I believe what we need are more outreach endeavors that illustrate the risks of space travel in context, i.e.:

      An astronaut could survive:
      ~30 seconds without a pressurized atmosphere;
      ~30 minutes without thermal protection;
      ~3 days without water;
      ~3 weeks without food;
      ~3 years in LEO without radiation shielding
      (@ 0.005 mrem/minute, as measured on STS-39)

      Thanks so much for contributing – I hope to continue the conversation in the future! (…and don’t you mean burying your lab under lunar regolith and ice balloons? 😉


  4. Thanks for writing this, Ive been studying the Apollo programme for many years and, as a simple mechanic, this was always the hardest of the subject matter for me to understand, there aren’t actually that many hardened Apollo hoax theorists out there but there are many who listen to them and these are the people who i love to talk to, this will help me greatly in that endeavour. of course, i will have to convert from millisiverts to rad before i do because that’s what hoax theorists almost always use

    1. Yonkis — The issue is that of acceptable risk. NASA has set a risk ‘ceiling’ of a 3% increase in the chance of an astronaut dying from cancer as a result of their space radiation exposure(s). This is far, far below the radiation intensity level necessary to cause acute effects (radiation sickness). So, the issue is serious, but the risk needs to be placed in a proper context. Ultimately, the take-home I was after is that (in all cases except for a solar storm) the level of radiation exposure that gets NASA excited is nowhere near the level where I think people would expect NASA to get excited (i.e., no one’s faces are melting off, etc.).

  5. In a microwave radiation heats up solids. In a tin can like Apollo used the can would explode. On Earth heat from an irradiated can can be released through contact with air. In space or on the moon there is no way to disperse the buildup of heat and life forms would soon cook— to death.

    1. Mike – Not just solids, but solids containing water. Also, microwaves cannot penetrate a faraday shield, like a microwave oven (trapping microwaves in) or a spacecraft fuselage/spacesuit (trapping microwaves out). No show-stopper here.

    1. Not impossible, but you’re right that radiators and thermal control systems are a big deal. (Did you note the ammonia/heat exchanger problems on the ISS?)

  6. With the weight reduction of computers since 1969, and the immense improvement in rockets there are reasons why NASA has never explored 1% of the surface of the moon. Could it be related to why the space station stays under 400 miles above the earth’s oceans?

    If we somehow got through that radiation ( nothing like a microwave oven) in 1969 it should be a breeze today to go to the moon. Also note the Chinese said on July 23, 1969, that we faked the whole mission, and they have not tried to go to the moon.

    1. Mike,

      While you’re totally correct about the weight reduction in computers (but not much of the other hardware), there hasn’t been a huge improvement in rocket *power*. In fact, the world doesn’t have a rocket engine today as powerful as the Saturn V’s F-1s. (NASA is trying w/ the development of the SLS rocket.). See:

      So, after Nixon pulled the plug on the Apollo program (and the Saturn Vs) and directed all of the dwindling NASA dollars into the Shuttle and then ISS, the world simply hasn’t had a rocket fast enough to catch up to the Moon. Remember that higher orbital altitude is all about speed. The station is at it’s altitude because that’s the highest the Shuttle and int’l partners could loft and rendezvous with the components. The Shuttle could *barely* make it to Hubble for servicing.

      Note that the radiation field produced by the Sun and other stars (while substantial!) is no secret. All GPS satellites orbit above 22,000 miles (over ISS’s paltry 250), while the Sun-observing STEREO satellites sit way out in front and behind the Earth acting exclusively as radiation monitors for solar storms. The radiation out there will cook circuits as much as it’ll cook DNA, so communication/navigation satellite managers are awfully interested in the radiation out there (and it being accurate — our satellites aren’t falling out of the sky). But you absolutely shouldn’t take my word for it — you can see for yourself here at the Space Weather Prediction Center’s stream of the STEREO and GOES radiation monitoring page — you can calculate the doses for yourself if you’d like: .

      So, given the wrong time in the solar cycle or too much time spent out there, it causes a problem for astronauts’ career dose limits, but no ones’ faces are melting off unless they catch a direct hit from a Coronal Mass Ejection or get marooned right in the thick of the Van Allan proton belt.

      1. I worked in NAS Headquarers during the Apollo Program. I worked on which missions to come next. Given the importance of the moon on human activities further ecploration of the other 99% was obvious. Once designed and flown building a few more Saturn V rockets is cheap. The DOD marching bands cost dozens of times more each year.

        It was not money that precluded more lunar visits. No one thought endless circling the earth at 350 miles was going to prove much. We knew we could double the Saturn V payload with the next generation. We knew how to use much lighter metals, etc.

        The decision under the Nixon Administration reflected a reality different than economics. Obviously our military never gives up capabilities, so larger rockets were built, but not publicized.

  7. This article is great. I really don’t understand why people insist it is fake, just like people insist the earth is flat. Great article!

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