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Q & A: When do you feel weightless in space?

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Most recent answer: 03/27/2018
Many people believe there is no gravity in space because astronauts on the Shuttles or ISS appear to float. In reality, gravity is everywhere, and the astronauts are actually falling AROUND our planet. But what were to happen if we launched a manned vehicle straight out into space away from Earth at a steady velocity? (If I'm correct, human travelers have never intentionally left an Earth orbit for long, yes?...even our trip to the moon was still an Earth orbit since we were chasing down the moon also orbiting Earth.) On this imaginary "straight out" space trip, would these astronauts still feel weightless? Or would they be able to walk around inside their vehicle until eventually the effects of Earth's gravity seem to disappear? Would that feeling also be affected by the velocity and acceleration of the vehicle? Also, what would happen as we pass other gravitational bodies (like Mars or Jupiter)? Thanks! This has been bugging me!
- Craig (age 18)

Great questions! A good way to understand this is to think of all motion under the influence of gravity alone as "falling." Whether you're in low earth orbit like the ISS, coasting to the moon like an Apollo astronaut, or drifting through interstellar space, you're in free fall whenever you're moving only under the influence of gravity. In free fall, gravity affects you and the ship in the same way—if you accelerate, the ship accelerates too—so you won't be pulled down to the floor no matter how close you are to a planet. If you move through space on any trajectory without firing your spaceship's engines, you'll feel weightless because your ship won't exert any force on you.

If you wanted to travel away from the earth in a straight line at constant velocity, you couldn't do it in free fall. You'd have to fire your engines constantly to keep the pull of the earth's gravity from slowing you down. With the engines on, the ship would exert a force on you and you'd feel pulled to the "floor." As you got farther away, the force of gravity from the earth would decrease, and you'd need less force from the engines to keep moving at a constant speed. The ship would exert less force on you and your apparent weight would decrease. At about 9,000 km from Earth, you'd feel about half your weight on the earth's surface.

If you cut off the engines at any time, you'd feel weightless again. As long as the engines were off, you could pass by other planets without noticing anything (unless you entered an atmosphere and the ship started to decelerate, or you passed so close to a black hole that you started to experience general relativistic effects).

Rebecca H.

PS: If you're interested in orbital mechanics, you might enjoying playing . You can design your own spaceships and use realistic orbits to get to the moon and other planets. 

(published on 08/10/2015)

Follow-Up #1: weightless in space

Why is it incorrect to say the Astronauts are weight-less in space while orbiting Earth in space in a space shuttle?
- Lul $tephanie (age 14)

It is correct to say they are weightless. A gravitational field doesn't by itself make any of the effects of weight. Those effects come from something (e.g. the Earth) getting in the way and preventing free-fall.

Mike W.


The astronauts are not massless though.  LeeH

(published on 03/27/2018)

Follow-up on this answer.