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# What is Microgravity?

Gravity is a force that governs motion throughout the universe. It holds us to the ground, keeps the Moon in orbit around the Earth, and the Earth in orbit around the Sun.

Many people mistakenly think that there is no gravity above the Earth's atmosphere, i.e., in "space," and this is why there appears to be no gravity aboard orbiting spacecraft. Typical orbital altitudes for human spaceflight vary between 120 - 360 miles (192 to 576 km) above the surface of the Earth. The gravitational field is still quite strong in these regions, since this is only about 1.8% the distance to the Moon. The Earth's gravitational field at about 250 miles (400 km) above the surface maintains 88.8% of its strength at the surface. Therefore, orbiting spacecraft, like the Space Shuttle or Space Station, are kept in orbit around the Earth by gravity.

The nature of gravity was first described by Sir Isaac Newton, more than 300 years ago. Gravity is the attraction between any two masses, most apparent when one mass is very large (like the Earth). The acceleration of an object toward the ground caused by gravity alone, near the surface of the Earth, is called normal gravity, or 1g. This acceleration is equal to 32.2 ft/sec2 (9.8 m/sec2).

If you drop an apple on Earth, it falls at 1g. If an astronaut on the Space Station drops an apple, it falls too; it just doesn't look like it's falling. That's because they're all falling together; the apple, the astronaut, and the Station. But they're not falling towards the Earth, they're falling around it. But since they're all falling at the same rate, objects inside of the Station appear to float in a state we call zero gravity (0g), or more accurately microgravity (1x10-6 g.)

## Creating Microgravity

The condition of microgravity comes about whenever an object is in "free fall": that is, it falls faster and faster, accelerating with exactly the acceleration due to gravity (1g). As soon as you drop something (like an apple) it is in a state of "free fall". The same is true if you throw something: it immediately starts falling towards the Earth. But how does something fall around the Earth?

Newton developed a "thought experiment" to demonstrate this concept. Imagine placing a cannon at the top of a very tall mountain.

Once fired, a cannonball falls to the Earth. The greater the speed, the farther it will travel before landing. If fired with the proper speed, the cannonball would achieve a state of continuous free-fall which we call orbit. The same principle applies to the Space Shuttle or Space Station. While objects inside them appear to be floating and motionless, they are actually traveling at the same orbital speed as their spacecraft: 17,500 miles per hour (28,000 km per hour)!

Objects in a state of free-fall or orbit are said to be "weightless." The object's mass is the same, but it would register "0" on a scale. Weight varies depending on if you are on the Earth, the Moon, or in orbit. But your mass stays the same, unless you go on a diet!

NASA uses a variety of facilities to create microgravity conditions. The most "famous" way is by aircraft flying in parabolic arcs to create microgravity for tests and simulations that last 20-25 seconds. NASA's Johnson Space Center, for example, operates a C-9 Low-G Flight Research aircraft also known as the "Vomit Comet." It make several trips each year to NASA Glenn in support of ground-based microgravity research. It's predecessor, a KC-135 aircraft, was used to shoot the weightless scenes in the movie "Apollo 13."

The facilities most-likely to be misconstrued as "anti-gravity chambers," are NASA's drop towers. Specifically, NASA Glenn has the "Zero Gravity Research Facility." It is a large shaft some 500 feet deep that allows test packages to free fall in a vacuum for just over 5 seconds. In this state of free fall, weightlessness (at or near microgravity) can be obtained. NASA Glenn also has a 2.2 second drop tower.

You may have experienced weightlessness yourself and not realized. Many amusement park rides create brief periods of free fall. Some rides that operate vertically without any applied forces are actually classified as "free fall rides." Most roller coasters have a set of parabolic (rolling) hills that also create brief periods of weightlessness. For less adventurous people, a car ride on the rolling hills of a country road or jumping on a trampoline also create brief experiences of weightlessness.

For more information on microgravity research at Glenn in support of for space exploration, visit the Human System Research and Technology page.