On Earth, "What goes up must come down." We know gravity makes that statement true. But in space, what goes up may just stay there. Is it because there's no gravity in space? No. Where there are two objects together, there is always gravity.
When astronauts on the space station drop an object, it floats. The floating is not because of zero gravity but because of microgravity.
Gravity is the force of attraction between two objects. Gravity keeps planets in orbit around the sun and moons in orbit around the planets. The gravity that we experience on Earth is called normal gravity, one gravity or 1 g. An object falling toward Earth (without interference of air resistance) will fall at a rate of 9.8 meters per second squared, or 9.8 m/s² (32 ft/s²). The rate is called the acceleration due to gravity.
The word microgravity begins with the prefix micro-, which can mean "very small." The symbol for micro- is the Greek letter µ, which is pronounced "mu." Microgravity, or µg, is less than 1 g but greater than 0 g.
Earth's gravity pulls on the International Space Station as it orbits. To stay in orbit and not fall back to Earth, the station and other Earth satellites must travel at a speed of 28,000 kph (17,500 mph). This speed is the orbital velocity for objects orbiting Earth 304-528 km (190-330 miles) above sea level.
As the space station orbits, it is in a state of free fall, meaning only gravity is pulling on it. If the station is falling around Earth, then so is everything inside the station. The free falling is what produces microgravity. Since everything inside the station is falling, dropped objects tend to float. If astronauts do not want an object to float, they must secure it to the walls, floor or ceiling. Sometimes, they stick food packages to the wall with Velcro®.
Microgravity on Earth To prepare astronauts for space, NASA simulates microgravity. The Neutral Buoyancy Laboratory in Houston, Texas, is a huge pool that holds 22.7 million liters (6.2 million gallons) of water. Full-sized models or "mock-ups" of the space station or other equipment are in the pool. Astronauts wear spacesuits in the NBL to practice spacewalks. The NBL simulates microgravity because astronauts float from one place to another. But they're not really in microgravity.
Astronauts also train on the reduced-gravity aircraft. The airplane flies a parabolic (up-and-down) path like a roller coaster. This type of flight path produces forces from microgravity to hypergravity (up to two times normal gravity) for short periods of time.
For experiments, NASA uses drop towers to produce microgravity. The 2.2-second drop tower is about 24 meters (79 feet) tall. Experiments dropped inside this chamber experience 2.2 seconds of microgravity. A taller drop tower called the Zero Gravity Research Facility allows an experiment to be dropped in microgravity for a little over five seconds. NASA also uses sounding rockets that allow six to seven minutes of microgravity. These small rockets soar high into Earth's atmosphere and free fall back to produce microgravity.
NASA performs experiments in microgravity for different reasons. NASA has learned that some objects, including the human body, react differently in space than on Earth. Because no weight or load is on the body in space, bones and muscles weaken. Astronauts must exercise to stay healthy in space. NASA studies how microgravity affects humans because the agency must prepare explorers to live in space for long periods of time, whether on the space station, missions to asteroids, or trips to Mars.
Liquids and flames behave differently in microgravity too. Crystals grow larger in microgravity. The structure of these crystals is better than those that grow on Earth.
Scientists are able to apply what NASA learns in microgravity to experiments on Earth.
In This Module This module has video and audio clips from:
A NASA microgravity expert on Earth.
An expert who has spent more than a year total time in microgravity on the International Space Station.
Two astronauts on the station who demonstrate the effects of microgravity.
Plus, fun video shows astronauts and other things floating on the station.
Preview video clips, audio clips and images under Micro-g Resources on your right. Download the ones you want to include in your podcast.