Gravity is an omnipresent force in our lives. Without it, water from a drinking fountain would simply shoot up from the spout without arcing into the fountain again. Chocolate syrup on a sundae would stay put without dripping down a scoop of ice cream. In fact, gravity, the force of attraction that draws one object to another, is so powerful on Earth that scientists sometimes have to get away from its influence—if only for a short while—to better understand other forces at work in the universe. To do this, they must be in a microgravity environment.
Microgravity, where the effects of gravity are minimized (approximating one millionth that of Earth's normal gravity), is achieved during freefall. At first glance, astronauts working on the International Space Station may appear to be floating. In fact, they are in freefall inside the spacecraft, which is also in freefall. To understand this phenomenon, it may help to think through a mental experiment by English physicist and mathematician Isaac Newton (1642-1727). He understood that the force that causes apples and other objects to fall to the ground is the same force that holds celestial bodies such as the Moon in orbit. If a cannon is fired from atop a high hill, the cannonball will fall to Earth, landing some distance away. If more force is used, the cannonball travels farther before hitting the ground. If the cannonball is propelled with enough force, it will fall all the way around Earth, orbiting the planet, just as the Space Station or any space shuttle does.
Scientists who have conducted experiments in microgravity have discovered countless phenomenon that they would not see in normal gravity. For example, during space shuttle flight STS-95, which carried Senator John Glenn back into orbit in 1998, scientists saw ordered crystals of two different sizes of particles form together in one solution. On Earth, where metals (such as copper and zinc) are melted together to form alloys (such as brass), materials scientists contend with buoyant convection, which is fluid flow that causes denser particles to sink and less dense particles to rise. Convection makes it more difficult to blend uniform alloys and other materials.
Convection also affects how a flame burns. On Earth, gravity pulls cooler, denser air closer to the planet, causing soot and hot, less dense flame gases to rise. This can lead to an unsteady, flickering flame. In microgravity, a candle flame produces minimal soot for a brief time then appears spherical and blue. American combustion researchers found on the Russian space station Mir in 1998 that while a flame in microgravity does need airflow to burn, as it does on Earth, that flow is only a fraction of a centimeter per second, so small one would not feel it. The findings confirmed that materials considered to be flame-resistant on Earth might burn in low-gravity conditions in space.
As astronauts learn how physical phenomena are affected in microgravity, they are also finding out how the microgravity environment affects their own bodies. For example, during long-duration flights, such as on the International Space Station, human muscles begin to atrophy and bones can become more porous as they do in someone with osteoporosis . Scientists are researching methods of exercise and bone-replacement therapy that will help astronauts stay in top condition as they continue their discoveries of how forces behave with—or without—gravity.
see also Gravity (volume 2); Newton, Isaac (volume 2); Living in Space (volume 3); Living on Other Worlds (volume 4); Long-Duration Spaceflight (volume 3); Zero Gravity (volume 3).
Julie A. Moberly
National Aeronautics and Space Administration, Microgravity Research Division. Combustion Science. Huntsville, AL: Author, 1995.
——. Fluid Physics. Huntsville, AL: Author, 1995.
Ducheyne, Paul. "Surface Transformation of Reactive Glass in a Microgravity Environment." Microgravity Research Division's Online Task Book. 1999. <http://peer1.idi.usra.edu/cfpro/peer_review/mtb1_99.cfn?id=269>. Microgravity Research Program Page. National Aeronautics and Space Administration. <http://microgravity.nasa.gov/>.