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Why is it believed that astronauts live longer than earthlings?

So, what I would like to know is that "If the belief is correct". If the belief is correct, how does that happen (what are the principles behind it).

And also from the INTERSTELLAR movie, which shows that the protagonist is still lesser in age than his daughter.

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    $\begingroup$ You need to explain what you're after better @Amar, the question is very unclear. $\endgroup$ – GdD Jul 19 '18 at 10:28
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    $\begingroup$ @Amar: You should, at the very minimum, include the text you're asking about and the source. It would be better to explain in your own words the assertion you believe the source is making and what it is that seems to be missing from their argument. $\endgroup$ – Nathan Tuggy Jul 19 '18 at 10:28
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    $\begingroup$ @Amar I removed the word "cosmonaut" from your post and replaced most uses with "astronaut". We decided as a community to use "astronaut" as the default term and only use "cosmonaut" in reference to astronauts from space programs that use the term "cosmonaut" (such as Roscosmos). See this meta discussion. $\endgroup$ – called2voyage Jul 19 '18 at 12:57
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    $\begingroup$ answers.com/Q/What_was_the_first_living_thing_sent_in_to_space - actually rather bad site... At list two answers there are total crap - about astronauts living longer, and about the first woman in space. Please don't recommend the site to anybody. $\endgroup$ – Heopps Jul 19 '18 at 13:29
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    $\begingroup$ Don't be discouraged if you get closed. It will be reopened this is a good question score. Some questions get closed a reopen many times $\endgroup$ – Muze Jul 19 '18 at 17:13
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Correlation is not causation.

The time dilation effects are indeed present, but they account for making the astronauts "younger" by a couple milliseconds over their lifetime. Not something detectable in the grand scheme of things.

But astronauts aren't just Joe Averages taken off the street. They are chosen from, amongst other features, the healthiest, most fit, most durable candidates. They get optimal nutrition, they are required to remain in perfect condition, they get absolutely best medical care - and they end up rather rich when retiring, meaning they can afford to continue receiving great health care and living fit, healthy lives.

And obviously, people matching such health and lifestyle profile tend to live longer than the average, regardless of being astronauts or not - but people not matching such health profile don't become astronauts.

Note - as others in comments mentioned: radiation takes its toll, especially increasing mortality in astronauts who crossed the Van Allen belts source. Interestingly, LEO missions don't seem to have this effect despite radiation dose being similar (though more spread over time) and 0g effects on health being far more pronounced. Still, an average astronaut (space-faring or just "reserve") lives considerably longer than their country average.


I couldn't find actual papers on life expectancy of astronauts, so I performed some general calculations to give the number a 'bottom bar'. Using the list of space travelers I averaged lifespan and birth date of all space travelers deceased to date. Average birth year: 1937; average lifespan: 61.4 years. Only two of the 79 entries were women, so the result predominantly favors men; US life expectancy for 1937 was 58.0 years; I couldn't find the number for Soviet Union, only information that it overtook the US expectancy much later. Regardless, my figure is a heavy underestimation of the life expectancy - as it doesn't include any of the astronauts who were born before 1952 (when male expectancy was 65.8, meaning average man born then would die about now) and are still alive, bringing the average life expectancy of the astronauts way up.

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    $\begingroup$ I would suspect that astronauts live shorter lives compared to a comparable group of ground dwellers. The detrimental effects of higher doses of radiation are multiple orders magnitude larger than the few microseconds of time dilation. $\endgroup$ – David Hammen Jul 19 '18 at 13:53
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    $\begingroup$ @DavidHammen I rather suspect that the long term effects of the radiation astronauts are exposed to is negligible compared to their fitness and nutrition advantages they have over the average person (although they might be at a disadvantage compared to fit earth-exclusives). That being said, their average life span might still be a bit worse than average simply because improvements aside, being an astronaut is still a somewhat dangerous occupation with a fairly high accident rate. $\endgroup$ – Cubic Jul 19 '18 at 14:32
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    $\begingroup$ Probably statistics exist about longevity of astronauts vs astronaut candidates, and the space twin study probably deserves a mention. $\endgroup$ – user17699 Jul 19 '18 at 14:53
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    $\begingroup$ I'm leaving this comment on your answer since yours is the one that addresses astronaut health: Hammen is right, there is a fair body of evidence that astronauts live shorter than they otherwise would have for their fitness group. See for example arstechnica.com/science/2016/07/… $\endgroup$ – Bear Jul 19 '18 at 15:35
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    $\begingroup$ @PeterCordes: On the flip side, following my point, people who die as infants aren't viable astronaut candidates. ;-) But you have a point, although I should then really include live astronauts in my calculation, and my social sciences schooling is very superficial, so I don't really know how to go about calculating the life expectancy for population that is mostly alive. Nor how to compare it to general (I think naive averaging of birth year makes sense when the spread of birth years is somewhat small.) $\endgroup$ – SF. Jul 20 '18 at 6:20
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So the article mentioned by OP did refer to time dilation. An astronaut in orbit does indeed experience time slowing down compared to humans on the surface on earth. However, this difference is incredible tiny for current astronauts. Quoting the Wikipedia article:

[A]fter 6 months on the International Space Station (ISS) (which orbits Earth at a speed of about 7,700 m/s) an astronaut would have aged about 0.005 seconds less than those on Earth.

You already experience a time dilation if you climb up a mountain or even a ladder. However, we're talking about incredibly small amounts of time here which are hard to measure.

See also: Have we attempted to experimentally confirm gravitational time dilation?

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    $\begingroup$ Probably worth noting that what occurred in Interstellar was an extreme case that currently is only possible in fiction, though it is based in GR theory. $\endgroup$ – called2voyage Jul 19 '18 at 12:58
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    $\begingroup$ The question is ambiguous and so this answer could be somewhat misleading. Time dilation won't make the astronauts live longer. If they were to die at 80, they'll still die at 80 even if a thousand years have passed on Earth. They won't live longer from their perspective, but observers will disagree with them on how much time has passed. $\endgroup$ – isanae Jul 19 '18 at 15:34
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    $\begingroup$ Actually the astronaut doesn't experience time slowing down. From his perspective, time passes as usual. It's just that indeed he can measure time dilation by comparison with the earthlings' time. $\endgroup$ – Ruslan Jul 19 '18 at 15:49
  • $\begingroup$ Note: going up a mountain causes FASTER time due to reduced gravity. The slower time of LEO astronauts is due to velocity, partly counteracted by reduced gravity. See en.wikipedia.org/wiki/Gravitational_time_dilation#/media/… $\endgroup$ – Foo Bar Jul 20 '18 at 10:39
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Your question is difficult to answer because it is ambiguous and conflates several ideas. I'll try to take them one by one.

Does being in space change anything regarding time?

No. There is nothing special about space itself that changes the perception of time. Even the word space is ill-defined, but it usually means "outside of Earth's atmosphere", that is, if you go up far enough, you'll end up there. An altitude of 100km is commonly accepted.

Therefore, if we define an astronaut simply as someone who goes into space, then obviously that won't change anything to their perception of time.

Can time pass faster or slower for yourself?

No. From your perspective, the rate at which time passes never changes. If you have a working clock with you, nothing you can do will alter the speed of the second hand.

Can time pass faster or slower for yourself compared to somebody else?

Yes. It is possible for two people with identical clocks to meet again at a later time and see a difference in what the clocks indicate. This can happen even though neither individual ever saw a change in how fast the second hand moves.

This phenomenon is called time dilation. It happens when there is a difference in either velocity ("speed") or gravity. If I go faster than you, or if I am on a planet that has greater gravity than yours, then when we meet again, my clock will show a time earlier than yours.

This will happen even though both of us felt the time pass at the same rate. It's only when we compare our clocks that we'll notice a difference.

Does this affect astronauts at all then?

Sure. It will affect anything that moves at a different speed or experiences a different gravitational field. In the the case of an astronaut, they're probably in orbit, and so are going very fast. Therefore, their clock would show a time earlier than yours when they come back down on Earth.

But they're also at a higher altitude, and so will experience less gravity than you who is standing on Earth (about 90% on the ISS). Less gravity means that their clock would show a time later than yours when they come back.

The effects are opposite, but not equal. The net result is that after 6 months on the ISS, the astronaut's clock would be about 0.005 seconds earlier than a clock on earth.

Does that mean they age less than people on Earth?

Indeed. If we're both 30 years old and I hop on a very fast ship, I might come back after 50 years have passed on Earth, but only one year has passed for me. I will have aged less than you.

Does that mean they live longer?

This is the ambiguous part and it is more of a language issue.

No, in the sense that if we were born at the same time and we both died at 80, we will both have lived the exact same time. If we had counted every second since our birth, we would end up with the same number. We both experienced the same amount of time even though we didn't die at the same time.

Yes, in the sense that after we reunited on Earth, I would die 50 years after you did. I would have been able to see things you couldn't. But once I die at 80, my second count will be the same as yours.

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