Why Do Objects Float in Orbit?
A spacecraft could go so far from Earth that a person would feel very little gravity. But this is not why things float on the International Space Station. The space station orbits Earth at about 200 to 250 miles high. At that height, Earth’s gravity is still very strong. In fact, a person who weighs 100 pounds on the ground would weigh 90 pounds there.
So why do astronauts float in space? The answer is that they are in free fall. Gravity pulls all objects the same way, even if they are different sizes. If you drop a hammer and a feather on Earth, the hammer will fall faster. But that is not because gravity pulls them differently. Air makes the feather fall more slowly. If there were no air, they would fall together at the same speed. Some amusement parks have free-fall rides. On those rides, a cabin falls along a tall tower. If you let go of a ball at the start of the fall, you and the ball would fall together. The ball would appear to float in front of you! That is what happens in a spacecraft. The spacecraft, its crew and everything aboard are all falling around Earth. Since they are all falling together, the crew and objects appear to float.
How Can Spacecraft Fall Around Earth?
What does it mean to “fall around Earth”? Earth’s gravity pulls objects toward the surface. Gravity pulls on the space station, too. As a result, it is falling toward Earth’s surface. The station also is moving very fast. It moves so fast it matches the way Earth’s surface curves. If you throw a baseball, gravity will cause it to curve down. It will hit the ground soon.
A spacecraft in orbit moves at the right speed so that the curve of its fall matches the curve of Earth. For the space station, that speed is 17,500 miles per hour. The spacecraft keeps falling toward the ground but never hits it. Instead, it falls around the planet. The moon stays in orbit around Earth for this same reason. The moon also is falling around Earth.
from Nasa: What is Microgravity ?, first picture via physics.uiowa, second one Nasa