Microgravity is a state of freefall that occurs when orbiting spacecraft and satellites fall around the Earth at the same force that gravity pulls down on the craft. This works because everything is in a vacuum in which gravity acts upon every object with the exact same force. Microgravity in Earth orbit is 1x10^-6 g, where g equals the force of gravity on the Earth's surface.
Gravity pulls on orbiting objects, which travel at 17,500 miles per hour. Because they travel at such high speeds, these objects fall around the Earth, while falling toward it at the same time. The rate of falling toward the Earth is the same as the rate at which gravity pulls on everything in the spacecraft. For instance, everything floats inside the International Space Station because the force of gravity is counteracted by the speed of craft.
Microgravity is achieved in five ways. Earth orbit is the most complicated way. Payloads can be dropped from the top of a tower to achieve two seconds of microgravity. A special airplane from NASA flies in a parabolic flight plan that creates 25 seconds of microgravity several times per flight. Sounding rockets rise to the upper reaches of Earth's atmosphere and then fall back to the surface for up to nine minutes of microgravity conditions. Slowly spinning an object simulates microgravity for certain biological processes.