Why Are Astronauts Weightless in Space?

by Nancy Atkinson on May 21, 2012

Most of our regular readers understand why astronauts and objects appear to float around on the International Space Station, but there are some misconceptions and preconceived notions out there on this topic that aren’t true and which don’t represent a very good understanding of physics! This video provides an entertaining look at some of the ideas people have about the zero-gravity environment on board an orbiting spacecraft, and shows why the astronauts actually appear weightless.

But let’s discuss it, too:

When asked why objects and astronauts in spacecraft appear weightless, many people give these answers:

1. There is no gravity in space and they do not weigh anything.

2. Space is a vacuum and there is no gravity in a vacuum.

3. The astronauts are too far away from Earth’s surface to be subject to its gravitational pull.

These answers are all wrong!

The main thing to understand here is that there IS gravity in space. This is a very common misconception. What keeps the Moon in its orbit around the Earth? Gravity. What keeps the Earth in orbit about the Sun? Gravity. What holds galaxies together? Gravity.

Gravity is everywhere in space!

If you built a tower on the Earth 370 km (230 miles) high, about as high as the Space Station’s orbit, the gravity on top of the tower would be almost as strong as if you were on the ground. If you stepped off the top of the tower, you would drop to the Earth just as Felix Baumgartner will do later this year when he attempts to jump from the edge of space. (Of course, this does not account for the freezing temperatures that would ultimately cause your demise, or how no air or air pressure would kill you, or how dropping through the atmosphere would seriously do a number on your body parts. And then that sudden stop would be bad, too.)

So, why doesn’t the Space Station or satellites in orbit fall to the Earth, and why do the astronauts and objects inside the ISS or other spacecraft appear to be floating?

Because of speed!

The astronauts, the ISS itself and other objects in Earth orbit aren’t floating, they are actually falling. But they don’t fall to the Earth because of their huge orbital velocity. Instead, they fall around Earth. Objects in Earth orbit have to travel at least 28,160 km/h (17,500 mph). So, as they accelerate towards the Earth, the Earth curves away beneath them and they never get any closer. Since the astronauts have the same acceleration as the space station, they feel weightless.

There are times when we can be weightless — briefly — on Earth, when you are falling. Have you ever been on a roller coaster and just past the peak of a hill as the car starts to go down, your body lifts from the seat? If you were in elevator a hundred stories high, and the cable broke, as the elevator fell, you would float inside the elevator car. Of course, in that case the ending would be rather disastrous.

And also you’ve probably heard about the “Vomit Comet” — the KC 135 airplane that NASA uses to create short periods of weightlessness for astronaut training and to test out experiments or equipment in zero-G, as well as the commercial Zero-G flights where the plane flies in a parabola, and like a roller coaster (but at greater speeds and higher altitudes) when the plane goes over the top of the parabola and heads downward, a zero gravity environment is created as the plane falls. Luckily, the plane pulls out of the fall and levels off.

Let’s go back to the tower. If instead of just stepping off the tower, you took a running leap, your forward energy would carry you away from the tower at the same time that gravity pulled you down. Instead of hitting the ground at the base of the tower, you would land a distance away. If you ran faster, you could jump further from the tower before you hit the ground. If you could run as fast as the space shuttle and ISS orbits the Earth, at 28,160 km/h (17,500 mph), the arc of your jump would make a circle around the Earth. You would be in orbit and weightless. You would be falling without hitting the ground. Spacesuit and ample breathable air needed, however.

And if you could run at about 40,555 km/h (25,200 mph) you would jump right past Earth and start orbiting the Sun.

The International Space Station, the space shuttle, and satellites are designed to stay in orbit, neither falling to the ground nor shooting off into space. They orbit the Earth about every 90 minutes.

So, when you are in orbit, you are in free fall, and are weightless.


Nancy Atkinson is Universe Today's Senior Editor. She also works with Astronomy Cast, and is a NASA/JPL Solar System Ambassador.

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