Exposing PseudoAstronomy

December 6, 2008

What is “Weightlessness?”

Filed under: misconceptions,terminology — Stuart Robbins @ 8:00 am


The term “weightlessness” is tossed around a lot in the media, every-day speech, and in science fiction. There are many different ways to experience the same feeling of weightlessness, but there really is only one way to truly experience it … and this is not the way that’s often used.


Mass Defined Every object that is made of normal, baryonic matter (like protons and neutrons being made of quarks) has mass. What this means is that it has the ability of gravitationally affecting the space around it, bending it to create the effect of pulling on other objects.

Weight Defined Many of us have probably been to a museum where they have different scales showing you what you weigh on different planets in the solar system. My favorite was always Pluto (which is no longer classified as a planet) because I weighed fairly little. However, even though my weight changed, my mass didn’t.

The concept of “weight” is based on a gravitational pull on an object that has mass. You “weigh” something on Earth because Earth is pulling down on you due to its mass. If you were to jump up in the air, you would fall back down to Earth with an acceleration of 9.8 meters per second per second (which we’re going to round to 10). That means that if you started falling from 100 meters in the air, with an initial velocity of 0 meters per second, after the first second you would travel at 10 meters per second. After the second second, you would travel at 20 meters per second. After the third second, you would travel at 30 meters per second. This would continue until you lithobraked (hit the ground with a splat).

This acceleration is what gives you weight. The rate of acceleration is directly linked to the mass of the accelerator and the distance-squared you are from it. If you were twice as far away from Earth, its force on you would be 4 times less and you would weigh only 1/4 of what you do on the ground. If Earth’s mass were cut in half, you would weigh only 1/2 of what you do now. If your mass were cut in half, you would weigh 1/2 of what you do now, but your mass would also be 1/2 of what it is now.

And that’s the difference between weight and mass.

Experiencing Weightlessness by Falling

This is the “fake” way to experience weightlessness. We “feel” heavy because we are pushing against something: Earth’s surface. When I got out of bed this morning, I had to exert an effort to sit up and climb out from under my blankets. I exerted no effort when I rolled onto the floor. Earth did. I only felt my weight when I landed on the carpet.

This apocryphal tale is the idea behind experiencing weightlessness by falling. When you fall, there is nothing* pushing up against your body, and hence you do not experience your weight. You only experience it when you land.

This is part of the fun of roller coasters. And sky diving. And riding in NASA’s “vomit comet” that simulates weightlessness or low-gravity environments.

This is also what astronauts experience. They are falling. When the space shuttle or the International Space Station are orbiting Earth, they are moving sideways at the exact same velocity that they are falling towards Earth. They are constantly falling, but they stay in orbit because Earth’s curved sphere is falling away from them at the same rate. They are not nearly far enough away from Earth (a few hundred km vs. the 6438 km radius of Earth is miniscule) to experience weightlessness in its true form. And to be truthful, this is the real misconception that I wanted to address in this post … but I will continue to finish out the story.

* Air is actually pushing up against you, but it is not nearly as dense as the ground. When skydiving, you will actually reach a terminal velocity where you can no longer accelerate due to the force of the air pushing against you

True Weightlessness

The only way to experience true weightlessness is to be far enough away from all other massive objects that there is no net acceleration in any direction. Even being in free orbit around the Sun, you would experience some weight because the Sun would be pulling you towards it. Consequently, there is no practical way to experience true weightlessness, only the illusion by falling.


It is often said that the astronauts in the shuttle are far enough away from Earth to experience weightlessness. They actually explicitly state that it’s the distance from Earth that causes this. This is the same thing as earlier this year when a skydiver was going to travel higher than anyone else had for his fall — the news sources all said he would be high enough to experience weightlessness.

However, as I have now explained, this is not the case. If the station remained fixed in space relative to the stars but still around Earth, the astronauts would still experience their weight, though it may be slightly less than when on the ground.


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