What if you lived on a space station?

Imagine living way up in space. You float to breakfast. You float to bed!

What would happen to your body if you lived there for months?

High above Earth, astronauts live and work on the International Space Station on missions that NASA says last six to 12 months. They float from room to room, and so does everything they let go of.

So what if you moved in? Floating sounds amazing. But your body grew up with gravity pulling on it every moment of every day. What happens when that pull stops being felt? Let's follow the clues NASA has gathered from years of astronauts living in orbit.

The International Space Station hosts crews for missions that NASA describes as typically six to 12 months long. Its environment is microgravity: everything on board is in free fall together, so objects and people float relative to the station.

This thought experiment asks what months in that environment would do to a human body, and how NASA counteracts it. It follows three threads: why things float, what happens to bones and muscles, and what astronauts do about it. The measured effects come from NASA; anything beyond them is labeled as an idea.

On Earth, gravity pulls on you all the time.

When you walk, run and jump, your bones and muscles work against that pull. That keeps them strong!

On Earth, gravity pulls you down every moment, even while you sit. Every time you stand, walk, climb stairs or carry a backpack, your bones hold you up and your muscles work against that pull.

That everyday work keeps bones and muscles strong. NASA explains that in space, muscles and bones can become weaker because gravity no longer makes them work as hard. In other words, the pull you hardly notice on Earth is quietly giving your body a workout all day long.

On Earth the skeleton carries the body's weight continuously, and muscles contract against gravity to hold posture and move. That constant mechanical load is part of what keeps bone dense and muscle strong.

NASA's education materials note that in microgravity, muscles and bones can become weaker without gravity making them work as hard. The losses NASA measures are in the weight-bearing bones, the ones that hold up the body's weight on Earth.

On the space station, you float. Your legs don't have to hold you up.

So your bones and muscles don't work as hard. Slowly, they get weaker.

In orbit, you float, so your legs and back no longer hold up your weight. NASA has found that in spaceflight, astronauts' weight-bearing bones lose on average about 1 to 1.5 percent of their mineral density every month. Muscles weaken too, because they don't need to work as hard. Scientists call this loss atrophy.

Other things change as well. Body fluids shift upward toward the head. And after months in orbit, astronauts have to get used to gravity again: their balance and coordination need time to readjust when they come home.

NASA reports that weight-bearing bones lose on average 1 to 1.5 percent of mineral density per month in spaceflight; another NASA page gives roughly 1 percent a month for crews who take no precautions. Bone-building cells slow down while the cells that break down bone keep their normal pace, so breakdown outpaces growth. Muscle mass also falls faster than it would on Earth without proper diet and exercise.

Body fluids shift toward the head, which may press on the eyes and affect vision, and returning crews must readapt to gravity, with effects on spatial orientation, balance and coordination. NASA adds that bone loss might not be completely corrected after return, although the astronauts' risk of fracture is not higher.

Gravity is still there in space! The station and everyone in it are falling around Earth together.

Falling together makes everything float.

Surprisingly, gravity is still strong up at the space station: about 90 percent as strong as on the ground, NASA says. Astronauts float because they and the station are falling toward and around Earth together. Scientists call this free fall.

Because everything falls together, nothing presses down on anything else. Your feet don't push on the floor, and your bones don't carry your weight. That's the real reason bones and muscles get a much easier ride in orbit.

The station orbits between about 200 and 250 miles up, where gravity is still about 90 percent of its surface value (NASA). Crew and station are in free fall together, accelerating toward Earth at the same rate while moving sideways fast enough to stay in orbit, so nothing presses on anything else: that is microgravity.

The physiological consequence follows directly. With no contact forces from standing or walking, the skeleton is unloaded and muscles that resist gravity are barely used. On Earth, NASA creates the same free fall for 20 to 30 seconds at a time in airplanes flying up-and-down parabolas.

Astronauts exercise for about two hours every day!

They ride a bike, run on a treadmill and use a special machine that works like lifting weights.

To stay strong, NASA says, space station crews exercise for an average of two hours a day. They ride an exercise bike called CEVIS, run on a treadmill, and use ARED, a machine that lets them do exercises like weightlifting even though nothing has weight.

NASA keeps experimenting too. In one study, some astronauts skip the treadmill to see whether other exercises work as well. Another project builds virtual-reality scenery to make bike rides more fun, because two hours a day is a lot of exercise!

NASA reports that crews exercise for an average of two hours a day, using the CEVIS cycle ergometer, a treadmill and the Advanced Resistive Exercise Device (ARED), which mimics weightlifting; TVIS, one of the station's first treadmills, used a harness to tether the user and add a gravity-like force. NASA notes aerobic and resistive exercise keep the heart healthy and bones and muscles strong, and medications called bisphosphonates have been effective against bone loss in NASA studies.

Research continues: the Zero T2 study compares crews who skip the treadmill with those who use it, and other projects test virtual-reality environments to help motivation. NASA is developing protective measures with longer Mars missions in mind.