Weightlessness is the sensation of having no weight, which happens when nothing is pushing against your body. Astronauts experience weightlessness when they orbit Earth in a spacecraft. They float through the cabin, and if they let go of a pencil, it hangs in the air instead of falling to the floor.

This happens because the spacecraft and everything inside it are constantly falling toward Earth at exactly the same rate. They're moving so fast sideways that they keep missing the planet, falling in a circle around it instead. Since the astronauts fall at the same speed as their spacecraft, nothing pushes against them, and they feel weightless. Gravity is still there, pulling on them just fine, but they can't feel it.

You experience a tiny taste of weightlessness at the top of a roller coaster's big drop, or when you jump on a trampoline at the highest point before you start coming back down. For that brief moment, you're in free fall, and your stomach does that funny flip because you're temporarily weightless.

Scientists sometimes call weightlessness microgravity because tiny gravitational forces still exist in orbit. Astronauts must exercise constantly during long missions because weightlessness causes their muscles and bones to weaken without the normal resistance of standing and moving against gravity's pull.