It is probable that even today many people would instantly recognize famous astronauts like John Glenn, Neil Armstrong, or even Sally Ride. Even though they have aged since their famous flights into space, their face is still recognized when they appear in public. There was a time when their faces (and bodies) changed in an unusual way. This happened when they were in space.

On earth, you can easily change the appearance of your face. You can wrinkle your forehead or hold your breath and puff out your cheeks. You can even have cosmetic surgery. Of course, children don't need to worry about such surgery to make them look younger!

In space, astronauts' faces also change and there is little they can do to stop the change. Their faces will look "puffy" when they are in weightlessness.

When you are in space, you are not pulled down to the floor of your spacecraft. You don't feel the pull of gravity as you do on earth. You can float! This also means every part of your body is also floating - including everything inside your body.

Fluids in your body (such as water and blood) are no longer pulled down into the middle of your body in your chest and stomach areas. These fluids begin to move outward into such areas as your head.

Astronauts can feel the fluid moving into their heads. They often talk of having a "stuffy nose", "sinus pressure", or even "headaches" as their heads start holding more fluid than before. The extra fluid creates more pressure inside the head. As the pressure builds, it pushes outward on the skin. This makes the skin on their face "puff out" and they can look different.

Studies have shown that astronauts can lose 1-2% of their bone density every month they are in space. This seems to happen because your bones aren't needed in space to support your weight like they do on Earth. This can be a problem if they lose too much of the bone material and then return to earth. Their bones could become weak and possibly even break!

Muscles also do not work as hard in space. It doesn't take much work when you normally just float rather than walk around the shuttle or space station. Even the heart muscles normally have less work to do in space. NASA scientists and doctors want the muscles to stay healthy so the astronauts can easily adjust when they return to Earth.

Space shuttle flights are much shorter than a month. These short flights would not cause much change in astronaut's bones and muscles. Astronauts on the space station are usually in space for at least three months. That means astronauts must do things in orbit to stay as healthy and physically fit, as possible.

Special exercise equipment is used in space that produces pressure against the muscles and makes them still do work. Newly designed treadmills and bicycles (actually called "ergometers") are two things they can use in space. Astronauts are held against the equipment with cords. If they were not held against the machine by these cords, they would just float away. Astronauts usually exercise at least one hour every day while in space to keep their bodies in shape.

Weightlessness also causes other effects on the astronauts' bodies. Some become dizzy for a while since there is no "up or down" in space and their inner ears become "confused" from the different feelings.

Astronauts also grow 1-2 inches in space. Gravity no longer pulls on the bones of the backbone. This allows those bones to move apart a bit which lets you literally "stretch out" while in space. Of course, you will shrink back to your normal height when you return to earth!

When astronauts return to earth, they need to be careful to take time to allow their blood to return to "normal" as it is pulled out of their head again into their chest and legs. Astronauts risk fainting if they stand or move too quickly.

Perhaps, you will be on the first manned spaceflight to Mars. That trip will take you at least six months in space to get there. NASA scientists need to conduct more research to help astronauts know how to stay healthy in space for that long time period.

There is no doubt NASA scientists will find ways to work against any changes in space that endanger the health of astronauts. Originally, many scientists did not know if astronauts could safely swallow food in space. Some thought gravity must be present to pull food down your throat to your stomach and that you could choke in space without gravity to pull on the food. We now know it is possible to eat normally in space - as well as survive and even have fun in space.