A condition in which the apparent effects of gravity are very small.

Microgravity is sometimes used interchangeably with terms such as weightlessness, free fall, or zero gravity. However, some of these terms can be misleading. For example, the term microgravity implies a condition in which very little gravity present. This condition would be possible to achieve at very large distances from any planet or star, but it is a condition that is currently not easily obtained.

From a practical standpoint, microgravity environments occur in relatively strong gravitational fields. To obtain the effects of microgravity, an object is dropped or put into a state of free fall. For example, astronauts orbiting the earth in the space shuttle can be described as being in a microgravity environment. However, in this orbit, gravity is still about 90 percent as strong as it is on the surface of the earth. The difference is that the astronauts and the shuttle are in free fall around the earth. They are all falling, but they have enough horizontal velocity so that the earth’s surface curves away from them about as fast as they drop and therefore, they do not hit the ground.

Microgravity environments can be used for many different applications that are impossible to achieve in a normal-gravity environment. One such application involves the creation of certain alloys. On Earth, when two materials are mixed, the new compound contains elements that have different densities. In this new compound, the elements that are denser will settle to the bottom of the mixture in a process called sedimentation. In a microgravity environment, sedimentation does not occur, and the material can be cured in a state where the elements with different densities are equally distributed. Many high-quality materials can be manufactured this way.

Microgravity is also used to develop and study many topics associated with biology and life sciences. By studying things such as crystals, plants, animals, and medicines in a microgravity environment, scientists can learn more about how each of these works, both in microgravity and in normal gravity. The result is a better scientific understanding and the potential to create new and better medicines.

Along with the benefits of microgravity also come problems. One of the big problems of microgravity relates to human space travel. Although the human body functions well in a one-gravity environment, when it is subject to a microgravity environment, as in orbit, the body experiences a decreased hydrostatic gradient, a shift of fluid from the lower body to the upper body. The body eventually rids itself of this extra fluid in the upper body, but upon return to Earth, the problem is reversed. When it leaves the free fall environment, the fluid pools in the lower body and can causes light-headedness or blackouts. Vestibular functions that sense a body’s orientation are confused by microgravity and can cause space sickness. Fortunately, the body usually adapts within the first few days of orbit. While in microgravity, the muscles in the body begin to atrophy due to the lack of use and bones lose calcium, which causes them to weaken. Vigorous exercise in orbit can help alleviate the muscle atrophy and some of the calcium loss, but no good long-term solution to these problems has yet been developed.

There are several methods that have been used to simulate microgravity via free fall. For centuries drop facilities have been used to create microgravity conditions. In the mid-sixteenth century, artillerists discovered that lead shot for muskets could be made almost spherical by dropping molten lead from a tall tower. During free fall, the lead would cool into a spherical shape and then land in a container of water. There are several different drop facilities around the world and in order to get relatively long periods of free fall, these facilities must be very tall. For example, one of the longest drop times comes from a facility in Japan that has been built in a vertical mine shaft that is 490 meters deep. This drop facility can provide free fall for up to 10 seconds.

Aircraft such as the National Aeronautics and Space Administration (NASA) KC-135 are used for free-fall environments by flying parabolic curves. During these parabolic trajectories, the occupants feel alternating 15- to 30-second forces of near-free fall and twice normal gravity.

Sounding rockets launched to high altitudes are yet another way to simulate microgravity. When their engines shut off, the coasting rockets can experience several minutes of free fall. To obtain longer periods of free fall, orbiting spacecraft can be used. These orbiting spacecraft, such as shuttles and space stations, are in constant states of free fall, achieving very long periods of microgravity. One problem that may develop in these situations is that as astronauts move around, they must push off the walls of the vehicle. The pushing causes small accelerations in the vehicle and this can disturb experiments that require an almost perfect free fall environment.

• Logsdon, Tom. Orbital Mechanics: Theory and Applications. New York: John Wiley & Sons, 1998. A generally readable introduction to the theory of satellite motion, with technically challenging mathematical points.
• Rogers, Melissa J. B., Gregory Vogt, and Michael Wargo. Microgravity. Washington, D.C.: National Aeronautics and Space Administration, 1997. A publication dedicated to the topic of microgravity, with many diagrams and examples that help explain the concept of microgravity and how it is used.
• Sellers, Jerry Jon. Understanding Space: An Introduction to Astronautics. 2d ed. New York: McGraw-Hill, 2000. A great book about many different aspects of space with technical details in the appendices for those wanting more information.

Astronauts and cosmonauts

Crewed spaceflight

Forces of flight

Gravity

Orbiting

Rockets

Spaceflight

“Vomit Comet”