# Do astronauts age while traveling in space?

Yes - they will experience aging in what they perceive as a normal way. An Earth-based observer will observe an astronaut aging slower or faster depending on various factors discussed below.

Let us investigate whether astronauts age during space travel.

The first thing to remember is that, from the astronaut's perspective, time will pass normally; that is, a 24-hour period will be experienced as a 24-hour period (and not as something shorter or longer).

Things get complicated when we start comparing the aging rate to someone who is in a different place, such as an observer on Earth.

Time can dilate in two ways, meaning that different observers may perceive it to be passing more slowly:

High speed: According to Einstein's Theory of Special Relativity, time dilates (slows down) for an individual experiencing high speed. Astronauts experience a velocity that is significantly higher than that of an Earthly observer; therefore, the Earthly observer will see that the astronaut ages more slowly.

High gravity: Astronauts experience lower gravity than on Earth, so an observer on Earth will notice that the astronaut ages more quickly. Einstein's Theory of General Relativity states that time will dilate for someone in a high gravitational field.

I should point out that the effects for both of these effects are quite small, measurable, and do affect everyday life because we are not dealing with extreme gravity (like around a black hole) or extreme velocity (a significant fraction of the speed of light).

Now let's get specific.

Unique Relativity

Now let's examine the equation pertaining to time contraction at high velocities:

For the sake of curiosity, let us pose the following question: What would it take for an astronaut to be immortal, or, alternatively, what would it take for an Earthly observer to look upon an astronaut and realize that he is not aging? The astronaut would need to be traveling at the speed of light.

However, it's possible that an astronaut will age more slowly in space than on Earth. Let's investigate that; first, let's reapply the equation:

All Things Considered

As we can see from the above, the effects are very slight. Einstein also created General Relativity, which states that an individual within a gravitational field will age more slowly than an individual outside of it. Therefore, since our astronaut will be in a lower gravitational field than the observer on Earth, the astronaut will be observed to be aging more quickly.

Assembling it

We experience time in our "normal" Earth-based way on Earth; an observer would see us aging faster as we get higher and the amount of gravity we experience decreases; similarly, an Earth-based observer would see us aging slower as we orbit the Earth (or go to the Moon or somewhere else) and our velocity increases; and it turns out there is a point in space where the two effects cancel (thanks to the amazing work of @phillip-e!). He writes:

The calculation looks like this: the time dilation resulting from orbital speed for a satellite in orbit is:

Newtonian gravity in use:

Next:

This results in:

Thus, time moves more slowly when a satellite, like the ISS, is orbiting below the height of a half-Earth radius, or roughly 3,000 km, and more quickly when the satellite is orbiting higher, like GPS.

satellites with GPS

According to https://tutor.hix.ai "For GPS satellites, GR predicts that the atomic clocks at GPS orbital altitudes will tick faster by about 45,900 ns/day because they are in a weaker gravitational field than atomic clocks on Earth's surface. Special Relativity (SR) predicts that atomic clocks moving at GPS orbital speeds will tick slower by about 7,200 ns/day than stationary ground clocks."

(The metaresearch link seems to be broken for some reason, but the Ohio State University Department of Astronomy offers the same analysis and goes into further detail about how it affects the GPS network at https://tutor.hix.ai)

Ultimately, this means that GPS satellite clocks will appear to run 38,700 nanoseconds faster per day, or.00000387 seconds, or roughly 122 seconds annually, or roughly 2 minutes.

ISS

Because the International Space Station orbits at a lower altitude than 3000 km, time appears to pass more slowly for the astronauts there than it does on Earth.

Space Expeditions

However, Special Relativity causes more time dilation as astronauts travel faster toward the Moon and other destinations. Time also accelerates as gravity decreases, changing the calculations for which prevails.

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Indeed, due to time dilation effects predicted by Einstein's theory of relativity, astronauts age while in space, albeit somewhat more slowly than the general population. That being said, for the majority of space missions, the difference in aging would be negligible and would not be noticeable during the astronauts' lifetimes.

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When evaluating a one-sided limit, you need to be careful when a quantity is approaching zero since its sign is different depending on which way it is approaching zero from. Let us look at some examples.

When evaluating a one-sided limit, you need to be careful when a quantity is approaching zero since its sign is different depending on which way it is approaching zero from. Let us look at some examples.

When evaluating a one-sided limit, you need to be careful when a quantity is approaching zero since its sign is different depending on which way it is approaching zero from. Let us look at some examples.

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