What Were You Saying About That Space Radiation? Here Are The Facts.

A recent report has been pummeling the internet over the past week proclaiming that Apollo astronauts who flew to the Moon and back have been dieing of cardiovascular disease at a higher rate than average. The authors of this report conclude that exposure to cosmic and solar radiation may be the cause of this uptake in deaths by cardiovascular disease.

The problem with this report is that it only samples seven deceased Apollo astronauts, so drawing any conclusions with regards to this limited data-set is nonsensical. And yet, that's exactly what science journalists have been doing.

The Apollo 11 Lunar Lander, piloted by Neil Armstrong.
Armstrong later died from cardiovascular disease.

Image Credit: NASA

Getting The Facts Straight

Of the 24 human beings that have flown to the Moon and back, eight have died in the decades following their 1968-1972 missions. Only seven of those eight were included in this study, because Apollo 14's Edgar Mitchell died relatively recently, presumably of old age (he was 85). 

Of those seven, three died of cardiovascular disease. The authors of this report make the claim that an anomalous 43% of deceased Apollo astronauts have died of cardiovascular disease, compared to 11% of astronauts who only flew on Low-Earth Orbit (LEO) missions and never ventured out into deep space. They then go on to say that Astronauts who flew to the Moon die of cardiovascular disease at a rate four times higher than Astronauts who went into space but never left LEO. And that claim is outright ridiculous.

For one, the sample size in question is minuscule. A sample of 1,000 astronaut deaths in which 430 of were related to cardiovascular disease would be a significant finding. But three out of seven just isn't significant at all. 

What the authors don't mention is that the average age of those who died of cardiovascular disease was 66.3 years old—and that in the 65-74 age group in the United States, 35% of deaths are a result of cardiovascular disease. That's only slightly lower than this reported occurrence in these seven Apollo astronauts, and yet it's been made to look like a substantial finding. Spoiler alert: it's not.

There's a selective fallacy at play here. Only 11% of deceased LEO astronauts died of cardiovascular disease. But that's because 14 LEO astronauts were killed in two separate Space Shuttle accidents alone, and that accounts for a giant percentage of astronaut deaths in modern times (remember, human spaceflight is only 55 years old, and most astronauts are selected by NASA while in their 30's). The national average of death by cardiovascular disease is just under 30%, and yet the authors don't attempt to make the claim that being an astronaut that only flies in LEO diminishes your risk of dieing of cardiovascular disease by two thirds.

Space Shuttle Challenger explodes after liftoff.
This alone was responsible for the deaths of seven astronauts.

Image Credit: Bruce Weaver/AP

They also don't mention anything about mitigated mortality. No Apollo astronauts have died from diabetes, so does that mean radiation exposure from going to the Moon prevents diabetes? Logically, that's no more ridiculous of a claim than saying that Apollo astronauts are four times more likely to die of cardiovascular disease than LEO astronauts. 

Realistically, we can't draw any conclusions based on such a limited sample size.

What About The Cancer?

Something that's strikingly absent in these statistics is any increased risk of developing cancer among Apollo astronauts—or astronauts in general. In fact, death by cancer is significantly lower in all astronaut groups than it is in the general population. Which is oddly not talked about (perhaps it doesn't fit in with the doom & gloom motif), and even runs contrary to conventional wisdom.

Exposure to cosmic and solar radiation is generally understood to result in an increased risk of developing various types of cancer. NASA often cites this fact as one of the major obstacles in its 'Journey to Mars'. After all, if Apollo astronauts have an increased risk of cardiovascular disease as a result of radiation exposure, shouldn't we also expect to see an increased incidence of cancer among the same group? Probably, but we don't.

Apollo 16 astronaut Charles Duke, currently the youngest living Apollo astronaut.
Image Credit: NASA

Of the seven Apollo astronaut deaths, only two were cancer-related. These occurred at an average age of 62.5 years old. In the 55-64 age group in the United States, 38% of deaths were a result of cancer. But, for Apollo astronauts, only 28.5% of deaths were from cancer. Does that mean radiation exposure actually prevents cancer and causes cardiovascular disease instead? Well, no. It's simply a symptom of an inadequate sample size. And in such cases, conclusions should not be made.

Robert Zubrin, aerospace engineer and founder of the Mars Society, has frequently stated that radiation exposure isn't as big of a deal as many people think. According to measurements taken by NASA's Curiosity rover while journeying to Mars, a human crew on a 6-month transit to Mars could reasonably expect to receive a dose of 30 rem each way (meaning 60 rem for a two-way, 12 month journey). Also, according to further measurements by Curiosity on the surface of Mars, an 18 month stay on the red planet would result in another 33 rem of radiation exposure. 

That's a total of 93 rem for a two and a half year expedition to Mars. Keep in mind that none of these figures include any sort of radiation shielding or mitigation, just straight exposure to a cocktail of cosmic and solar radiation. And that's not really as bad as it sounds. 

93 rem is around twice as much as what a human being on Earth would be exposed to during their entire lifetimes from just everyday background radiation. So, for Mars explorers, their lifetime radiation dose would be approximately triple that of the general population after one journey to Mars (again, without any sort of shielding/mitigation). And yet, that would not result in three times as much cancer. 

Radiation doses for various outer space activities.
Image Credit: NASA

Radiation exposure is a very small factor in the development of cancer. Most cancers originate from other sources—either genetic, diet-related, environmental, or chemical. Studies have shown that 20 rem accumulated over time is associated with an approximately 1% increased risk of developing fatal cancer. At 93 rem, that equates to a 4.65% increased lifetime risk of developing fatal cancer for future astronauts traveling to Mars and back. 

In 2014, 23% of all deaths in the United States were caused by cancer. So, assuming all else is equal, astronauts traveling to Mars would instead have a 27.65% chance of dieing from cancer. But isn't that a very small price to pay for the opportunity to go to another planet?

For comparison, commercial airline pilots may receive upwards of 10 rem over the course of their careers and have no statistically significant increased risk of cancer (or cardiovascular disease, for that matter). Some studies have even suggested much lower risks of cancer per rem than what I've cited above. 

And how much radiation did Apollo astronauts actually receive? Calculations have shown that the short-duration Apollo excursions actually received a rather inconsequential amount of rem during their trip to the Moon—comparable to a single CT scan. This small dose should not have manifested in any long-term health consequences.

Apollo 17 astronaut/geologist Harrison Schmitt on the Moon.
Image Credit: NASA

Critics of NASA (of which Zubrin is one) have pointed out that the space agency has often blown the radiation exposure problem out of proportion. Subsequently, news outlets have spun a tale of doom & gloom, writing about this excessively fatal and cancer-causing deep-space radiation as a barrier to deep space exploration.

Even Mark Watney, the fictional main character in The Martian, states that without proper radiation shielding of his habitat, 'even his cancer would have cancer'. That turns out to be incredibly false, because 18 months on Mars—if left totally exposed on the surface—would only provide about 33 rem, or a 1.65% increased chance of developing fatal cancer within his lifetime. 

NASA has an internal threshold of 3% increased risk of fatal cancer for it's astronauts. That means, after a certain amount of exposure, astronauts are grounded. Six months on the ISS is estimated to expose astronauts to 10 rem; Scott Kelly recently completed a year-long mission in which he would have been exposed to 20 rem, a 1% increased risk of cancer all on its own. 

The Real problem Is Interpretation

So what's the big deal here? These numbers seem to show that a short jaunt to the Moon couldn't possibly have caused a significant spike in cardiovascular disease, especially considering how little radiation Apollo astronauts were actually exposed to.

Apollo 17: Ridin' dirty.
Image Credit: NASA

There's an issue going on here with statistical analysis, which even the authors of this report acknowledge. And all of this can all be explained by the simple function of natural statistical variance.

Flipping a coin is known to be a 50/50 chance of getting either heads of tails, right? Well, let's assume that you hit a natural variance on two instances of coin flipping—that is, two of those flips land opposite of what is to be statistically expected.

  • Flip the coin ten times, and seven of those land on heads, three land on tails. That results in a 70% probability of landing on heads, according to statistics.
  • Flip the coin 100 times, and 52 land on heads, 48 on tails. Still only two flips went wrong, but now it's only a 52% probability of landing on heads—close to what would be expected if the odds really were 50/50.
  • Now, flip the coin 1,000 times. 502 land on heads, 498 land on tails. That equates to a 50.2% probability of landing on heads—even closer to the 50/50 mark.

And so on, ad infinitum. The more instances there are to measure, the more accurate your measurements become. Seven deaths are not significant enough to draw conclusions from, so drawing conclusions is not what any of us should be doing at this point.

And yet, science journalists have written in their own conclusion: that venturing into deep space leads to increased mortality from cardiovascular disease, and that human spaceflight is dangerous. Maybe next time you should try looking at the facts.


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