When astronauts are in orbit whether in the International Space Station (ISS) or in some other space vehicle they seem to float and be beyond gravity’s effect. Why is this? At about 200 miles above the earth the ISS experiences the gravitational pull of the earth only about 10% less than we do on the surface of our planet. What gives? Why do they “float?”
The common statement “zero gravity in space” is not accurate. Of course there is gravity in space. Gravitational pull is a function of the mass of the objects and distance, not whether there is an atmosphere. The moon revolves around the earth and the earth revolves around the Sun due to gravity.
Why astronauts experience “zero gravity” in space is not because there is no gravity. Rather it is because they are falling! They are in so-called “free-fall.” The key to being in free-fall without crashing into the earth (or the Sun or whatever) is that the sideways velocity around the earth is balanced with the gravitational pull so the “falling” occurs as much horizontally as vertically.
Here’s a great explanation from the book Storm in a Teacup discussing an object in orbit and how it can be in free-fall:
“[An object in orbit free-falling has] such a huge sideways speed, by the time it falls a little way down toward the Earth, it goes so far forward that the Earth curves away beneath it. And as it keeps falling, so the Earth’s surface keeps curving away. This is the beautiful balance of being in orbit. You’re going sideways so quickly that you fall toward the ground and miss. And because there’s almost no air resistance, you can just keep falling and missing, as you go around and around.*
While you’re in free fall, you can’t feel gravity because there’s nothing pushing back. Since the astronauts can’t feel anything pushing back, they can’t tell that gravity is there.”
A key point about gravity is that it exists all around us whether we are on the surface of the earth or not. As mentioned in the quote above, in order to feel the effects of gravity we must have something to push against that is resisting the gravity. When standing on the ground, the gravity of the earth pulling us towards it’s center at 9.8m/sec/sec is resisted by the hard rocky surface. In a plane, we feel gravity as the plane’s lift counteracts gravity to keep us aloft (see IFOD: The Four Forces of Flight). Jumping from an airplane results in free-fall, but the effects of gravity are felt as you can feel moving through the atmosphere and, of course, seeing the ground getting closer and closer. A bit about solid objects, water and gravity: How Do Big Steel Ships Float?
Astronauts do not feel the effects of gravity because there is no resistance – nothing pushing back. No atmosphere, no solid surface.
Here’s a short video from NASA astronomer and astronaut, Michelle Thaller that does a good job of explaining “floating in space”:
Now here’s the 3-ish minute start of the 1981 classic cult movie Heavy Metal where the astronaut rides a 1959 Corvette from the Space Shuttle down to earth. Why? Just because:
*I changed the tense in this quote in a few spots for consistency.
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