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Ask A Scientist

Why is there no gravity in space? 

Asked by: Avrey Stallman
School: Johnson City Intermediate School
Grade: 4
Teacher: Michele Zakrajsek
Hobbies/Interests: Dance, cheerleading, baseball 
Career Interest: College professor, dance instructor 

 

Answer from Stephen Levy

Assistant Professor of Physics

Research area: Biophysics, microfluidics Interests/hobbies: Science fiction, Mixels 

There is gravity in space! More precisely, there is gravity in space as long as space is not empty. Gravity keeps the moon moving around the earth and vice versa. Gravity pulls objects together throughout the universe. Isaac Newton discovered that gravity is a force between two objects that depends on the mass (amount of stuff) of each of the objects and the distance between the objects. If you double the distance between the objects, the force of gravity between them decreases by four times; if you halve the distance, the force of gravity is four times stronger.

So maybe you are wondering whether the force of gravity gets very weak in space. Let’s compare the force of gravity for a person standing on the surface of the earth to the force of gravity for an astronaut orbiting in the International Space Station about 200 miles above the surface. Using Newton’s distance relationship and the known size of the earth, we find that the force of gravity is less by one part out of ten for the astronaut on the space station. The astronaut would have to get about halfway to the moon before the force of gravity (between the astronaut and the earth) dropped to one part out of one thousand compared to when the astronaut stands on the earth’s surface.

But something definitely looks different to us about the orbiting astronaut — the astronaut is floating. So does floating, or weightlessness, mean there is no gravity? Clearly not, based on our calculation from Newton. The situation for the astronaut is the same as when you jump off a diving board into a swimming pool — you are weightless, still acted on by the force of gravity, until you hit the pool and the water pushes back on you. The astronaut and the space station are freely falling due to earth’s gravity but they are falling around instead of into the earth. Imagine if you could throw a baseball so far that the earth started to curve away underneath the baseball as it was falling back down.

One interesting thing about the orbiting astronaut is that everything on the space station is falling at the same rate, including the floor and walls. It is not obvious why this has to be the case, but it turns out that if we drop a bowling bowl and a feather on the surface of the earth in a chamber where all the air has been sucked out, they will both hit the floor at the same time. Gravity makes all objects fall at the same rate. 

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Last Updated: 9/25/14