What space does to the body

Humans are preparing to venture deeper into space. Scientists aren’t sure if our bodies are up to the challenge.

The moon.
NASA is planning to build houses on the moon by 2040, but researchers have concerns
(Image credit: Master / Getty Images)

Humans are preparing to venture deeper into space. Scientists aren’t sure if our bodies are up to the challenge. Here's everything you need to know: 

What missions are in the works?

SpaceX and Tesla boss Elon Musk has said he wants to put a million people on Mars by 2050, while Amazon and Blue Origin founder Jeff Bezos dreams of trillions living permanently on floating space colonies in coming centuries. Perhaps more realistically, NASA is planning to build houses on the moon by 2040, inhabited by astronauts and ordinary civilians, and then to construct colonies on Mars. But beyond the engineering challenges of rocketing humans to those far-flung locales — a one-way trip to the red planet would take at least nine months — researchers are also grappling with the huge health problems posed by living in space or on other planets for long periods of time. Our bodies evolved on a world where there is constant and plentiful gravity and relatively low background radiation. But in space, those conditions are flipped. Without massive scientific advances, humans may never be able to colonize these ultra-hostile environments. "Space is just not very hospitable to the human body," said Emmanuel Urquieta, chief medical officer at the Translational Research Institute for Space Health in Houston.

How does low gravity affect humans?

First, it makes you want to puke. Much like motion sickness on Earth, space adaptation syndrome stems from a mismatch between what the eyes see and what the inner ear tells your brain. But unlike on Earth, that mismatch results from the organs and vestibular system operating without their usual gravitational signals. Those feelings of queasiness eventually dissipate, but other bodily problems remain. Without Earth’s gravitational pull, urine doesn’t pool at the base of the bladder but rather around the organ’s walls. Because spacegoers don’t necessarily feel the pressure associated with a full bladder, one academic paper notes, "urination may happen suddenly and spontaneously." Some astronauts fit themselves with catheters because their bladders won’t empty at all. Low gravity also takes a toll on astronauts’ bones and muscles.

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What happens?

Floating around in low gravity "is a lot like lying around doing nothing," says NASA scientist Michael Stenger. Without the need to support the body’s weight, muscles atrophy and bones lose density. Research shows that, for otherwise healthy astronauts, bone density in the hips and spine decreases by 1 to 2% a month in space; among elderly people on Earth, it can drop 0.5 to 1% a year. By exercising for more than two hours a day, space travelers can limit muscle and bone loss, but not shrinkage of the heart, which doesn’t have to pump as hard in space. When astronaut Scott Kelly returned to terra firma in 2016 after nearly a year on the International Space Station, his heart had diminished in size by nearly a third and become rounder. (His eyeballs, meanwhile, had become less spherical.) That shrinkage didn’t result in negative health effects for Scott. But researchers wonder whether weaker hearts might cause explorers to become lightheaded when they walk around Mars after a months-long space journey. With a spinning space station or spacecraft, it might be possible to create artificial gravity, easing some of those health worries.

What about cosmic radiation?

Earth’s magnetic field shields the relatively close ISS and other near-orbit missions from the most damaging solar and galactic radiation. But missions to the moon and Mars won’t have that protection. A 2016 study found that the 24 Apollo astronauts — who flew beyond the reach of our planet’s magnetosphere for no more than a week at a time — were nearly five times more likely to die from cardiovascular disease than astronauts of the same era who never left Earth’s orbit or who never flew at all. While the sample size is tiny, researchers suspect exposure to cosmic radiation damaged the Apollo astronauts’ DNA, arteries and veins, leaving them vulnerable to heart disease. NASA researchers say that a shield of water around a spaceship or Mars colony about 8 inches thick would absorb most space radiation, but water is dense and expensive to ferry from Earth. Some scientists have suggested tweaking space fliers’ genomes with CRISPR technology to better protect them against radiation-related health issues. "You can’t dismiss it as an idea, but right now but we don’t know the mechanisms," NASA radiation health officer Edward Semones said of that radical plan.

Will crew members face psychological challenges?

Humans are social creatures, and so the isolation, homesickness and interpersonal drama experienced during a months or years-long mission will have an impact on astronauts’ minds. A joint Russian, European and Chinese study from 2007 to 2011 simulated a 520 day mission to Mars with an all-male crew of six. It found that as the subjects experienced less contact with other humans — because of the distances, there is an average communication delay of 13 minutes between Earth and any Mars mission — and were denied images from home they suffered "detachment phenomena." The crew increasingly saw mission control as redundant and started to resist instruction. To see how humans might cope on Mars, four NASA crew members in June began a year-long stay at a 3D-printed habitat in Houston. "If we get to the end of the year and the crew is complete," said mission commander Kelly Haston, "that would be, for me, a huge thing." Despite mounting evidence of the massive health and psychological risks of deep-space exploration, there is no shortage of candidates willing to sign up for such missions. "[I’d go] in a heartbeat," said space health expert Urquieta. "It’s so inspiring. It’s space."

The science of sex in space

If people are going to spend years or perhaps their entire lives in space, scientists want to know if it will be possible and safe for them to engage in that most innately human activity: sex. The limited information available from previous ISS visitors suggests a reduction in libido, possibly because when you’re traveling around Earth "every 90 minutes, your circadian rhythms are altered and that alters everything, including your sex hormones," said former NASA medical adviser Saralyn Mark. Hormonal changes and gravitational challenges aside, researchers believe sex in space is feasible. But cosmic radiation could pose other problems to reproduction, rendering deep-space explorers infertile or causing potentially fatal defects in embryos and fetuses. For psychologist Simon Dubé, the most pressing concern is how any hookup would affect the crew dynamic in a "remote, isolated, very small space, with limited partners who are people you work with and depend on." That worries him more, he says, "than, are people going to be able to have sex in the space station?"

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