Resource: Microgravity

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Nearly all of the physical challenges we face on an average day are magnified, either directly or indirectly, by the force of gravity. Climbing stairs, running to catch a bus, and even brushing your teeth all require effort—and much of the energy we expend in this effort is used in the struggle against gravity. It seems that life might be easier if we could just escape gravity, or at least lessen its strength for a little while. However, weightlessness presents a range of challenges along with its advantages. Just ask an astronaut.

We've all seen images of astronauts floating around inside their spacecrafts, weightless, as if the force of gravity didn't exist. Yet even 300 km (186 mi) or so above Earth's surface, where most spacecrafts travel, the force of gravity is still 90 percent as strong as it is on land. Thus, the weightlessness that astronauts experience does not occur because space travel automatically takes them out of gravity’s reach. Instead, the sensation, also called microgravity, occurs because astronauts and their spacecraft are in a constant state of free-fall. By traveling at a high enough speed—about 7.5 km (4.7 mi) per second—perpendicular to the force of gravity, a spacecraft's path of free-fall can remain parallel to Earth's curvature. In other words, the spacecraft falls all the way around the planet, and continues to do so as long as it maintains its horizontal velocity.

The first astronauts could probably only guess at what it would feel like to be weightless in space. They may have had the idea that tasks in orbit would be easier than they are on Earth. However, their first attempts to do work in space quickly proved that life could actually be a great deal more difficult without Earth's familiar gravitational pull holding one's feet firmly to the ground. Each time an astronaut attempted to turn a valve or tighten a bolt, he was sent hurtling in the opposite direction. Although this presented a surprising challenge, it probably should have been expected. After all, it was nearly 300 years ago that Isaac Newton presented what came to be known as his third law of motion, which states that for every action there is an equal and opposite reaction. In this case, whenever the astronauts applied pressure to a bolt, the bolt pushed back with equal force. In response to this challenge, engineers began developing footholds and other devices to help astronauts on later missions anchor themselves to the spacecraft while performing tasks.

Despite the difficulties that weightlessness poses, space travel has provided scientists with an exciting opportunity to observe the effects of microgravity conditions over long periods of time on such things as biological systems, the development of crystals, and the flow of fluids. Orbiting spacecraft have become important experimental laboratories for many scientific investigations, and have contributed to our understanding of the influence of gravity on organisms and a wide variety of structures and substances. Scientists hope these studies will one day lead to the development of substances and materials that have never before existed—at least not on Earth.

To learn more about space travel, microgravity, and weightlessness, check out Gravity on Earth and in Space, Galileo on the Moon, and What Is "Weightlessness"?.

To learn more about the challenges of doing work in space, check out How Would You Turn a Bolt in Space?, Astronauts in Hard Hats, and Astronauts Speak: Gene Cernan.

Print Questions for Discussion

• What does the term "microgravity" mean?
• How would you explain the apparent lack of gravity in videos of astronauts on the space shuttle or space station?
• When these students dropped various objects in motion from their "drop tower," what changes in motion did they observe? How did they explain these changes? What explanations or questions do you have?
• Do you agree with Tiana's explanation for the behavior of the pendulum in microgravity? Why or why not?
• Have you ever experienced weightlessness for a short time? What were the conditions? What did it feel like?