The term micro-g environment (also μg, often referred to by the term microgravity) is more or less synonymous with the terms weightlessness and zero-g, but with an emphasis on the fact that g-forces are never exactly zero—just very small (on the ISS, for example, the small g-forces come from tidal effects, gravity from objects other than the Earth, such as astronauts, the spacecraft, and the Sun, air resistance, and astronaut movements that impart momentum to the space station). The symbol for microgravity, μg, was used on the insignias of Space Shuttle flights STS-87 and STS-107, because these flights were devoted to microgravity research in low Earth orbit.
The most commonly known microgravity environment can be found aboard the International Space Station (ISS) which is located in low-earth orbit at an altitude of around 400km, orbiting Earth approximately 15 times per day in what is considered free fall.
The effects of free fall also enable the creation of short-duration microgravity environments on Earth. This is accomplished by using droptube, parabolic flights and Random-positioning machines (RPMs).