About Microgravity

What does it mean to be “weightless”?

We have all seen images, such as the one at the right, of astronauts floating inside a spacecraft. If these astronauts used a spring scale to weigh themselves, they would detect no weight at all. Does no weight mean no gravity?

There certainly is gravity in the space around Earth, and that's what holds satellites in their orbits. Spacecraft with astronauts typically orbit at an altitude of only about 400 km (250 mi), and the acceleration of gravity at this distance is about 90% of its value on the surface of Earth. With all this gravity, how can astronauts be weightless?

The answer is that both the astronauts and the spacecraft are in free fall. Gravity accelerates BOTH equally. It's like being in an elevator with the cable cut. Since you and the elevator accelerate towards the center of Earth equally, you are not pressed down against the floor of the elevator. If you stood on a scale, it would read zero as long as you are falling freely.

The environment inside an orbiting space station is frequently called "microgravity," due to small residual accelerations of the spacecraft, including aerodynamic drag, vehicle rotation, and venting of gases.

This drawing shows how Newton imagined that a cannon could fire a projectile horizontally at various muzzle velocities. If the velocity were high enough, the projectile would follow a circular orbit. In each path shown in the drawing, the cannonball would be in free fall. (drawing courtesy of NASA)

In the SkyLab orbiter, one astronaut balances another on his index finger. (photo courtesy of NASA)

Astronauts-in-training affectionately call this aircraft the "Vomit Comet." To simulate microgravity, the Comet flies a parabolic arc for about 25 seconds, as shown in the inset. A typical training flight consists of flying about 40 such arcs.  (photo courtesy of NASA).