The term "zero gravity" is not completely accurate in describing conditions in an orbiting spacecraft. You're not totally weightless in space, you just feel that way because you and the spaceship are both falling. There is still plenty of gravity in the orbiting spacecraft; you just don't feel the effects of gravity in freefall because there is nothing to resist its fall. If there was truly zero gravity, the spacecraft would not have gravity keeping it in orbit; it would simply float away into space. Therefore, gravity keeps the spacecraft in orbit while the astronauts float around inside. In training, astronauts simulate weightlessness by riding in an airplane that flies steeply upward and then arcs down into a dive. Conditions during that arc are similar to those in orbit. When the airplane flies up, the astronauts are held by gravity to the floor of the airplane. When the airplane arcs down into the dive, the astronauts are falling, so they look like and feel like they are floating even though the plane is still being held by gravity to the Earth. Three problems caused by weightlessness are eating, drinking, and sleeping. Astronauts have to eat dehydrated food. In space, food could just float away if not handled correctly. If you weren't careful, your dinner could go floating away. In space, there is not much gravity. Your water could go drifting away too, and do serious damage to something. If you don’t strap yourself down while you sleep, then you will float around and you might bump into someone or something, and cause a lot of trouble. You get taller when you are in space. Your waistline gets thinner as your body’s fluids redistributed. Gravity no longer compresses your spine in space; so you’ll be about 2.5 cm (1 in) taller. You do not get shorter in space. The heart grows weaker in space. Your heart has a lot less work to do in free fall. When you are on Earth, gravity is fighting the force of the heart so it can pump b