While it may appear that these astronauts are floating, they are actually in a state known as "freefall."

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...