Can a variable of physical traits like height, weight, complexion be changed in outer space? Maybe by gravity or by something else that determines these traits (like the UV rays of the sun that determines the production of melanin in the body)? As UV ray presence is not constant in space, would we expect a person with dark complexion to get fairer after living for a sufficient amount of time in outer space where UV rays vary compared to Earth?
$\begingroup$ The answer to could height or mass be changed in space, the answer is no except in the normal ways. (Like if you eat too much your mass increases.) Gravity won't change skin color, but prolonged lack of sunlight inside a space ship might change it a bit. Unlikely to be much change. All astronauts will need vitamin D supplementation if they are up there for very long. $\endgroup$– zeta-bandDec 17, 2018 at 18:56
$\begingroup$ Well a source says that astronauts in space can grow up to 3 percent taller during the time spent living in microgravity. That means that a 6-foot-tall (1.8 meters) person could gain as many as 2 inches (5 centimeters) while in orbit. Then gravity (may be not directly) influences the height of a person right? $\endgroup$– ShalumiaDec 17, 2018 at 19:07
$\begingroup$ @zeta-band Shalumia is correct that height is impacted by microgravity in the long run. $\endgroup$– called2voyage ♦Dec 17, 2018 at 19:35
$\begingroup$ Related: Does space flight have any positive effects on the human body? $\endgroup$– MuzeDec 17, 2018 at 20:31
$\begingroup$ There are many previous question about specific aspects of the effects of microgravity on humans, but I can't see a previous question asking for an overall summary, so I'm voting this up. Google finds plenty of answers, eg space.com/23017-weightlessness.html, but it would be valuable if someone could take the time to synthesize a thorough answer. $\endgroup$– Steve LintonDec 17, 2018 at 21:43
There is a Wikipedia article dedicated to the effects of spaceflight on the human body. Some of this is about launch and other aspects outside "prolonged space flight". Still, you might want to see that, but I'll mention a few things below.
For some time, it has been known that long duration space flight leads to spinal extension, making the person taller. However, this effect is reversed after the space traveler returns to Earth. For more, see this space.com article.
Spending an extended time in microgravity leads to deteriorated bones and muscles. This reduces body mass (weight).
There are many other effects, including: redistribution of bodily fluids, changes in eye sight, atrophying of the heart, reduction in aerobic capacity, brain reposutioning within the skull, sleep problems, etc.
I am unaware of any effect on skin complexion (except for the potential of sunburn, which I would expect to be temporary).
$\begingroup$ The space.com reference doesn't link to a space.com article. $\endgroup$ Oct 30, 2019 at 18:59
$\begingroup$ @WaterMolecule: Oops! Fixed. Thanks for the heads up. $\endgroup$ Oct 30, 2019 at 20:02