≡ Menu

Why Are Astronauts Weightless in Space?

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.

About 

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

Comments on this entry are closed.

  • octopusmagnificens May 21, 2012, 10:46 PM

    There would not be instantly freeze.

  • Deniz Yalcin May 22, 2012, 12:32 AM

    What’s the situation when you’re on the way to the moon ? For example the movie Apollo 13 did show us that the astronauts in the capsule were weightless on the way to moon. On this route they don’t have any kind of orbital velocity. What’s the explanation for their weightlessness in this situation ? (Or was the movie false ?)

    • wjwbudro May 22, 2012, 1:26 AM
    • wjwbudro May 22, 2012, 1:26 AM
    • andrew g May 22, 2012, 5:14 PM

      It’s still a kind of orbit, even though it’s a somewhat extreme one, very eccentric, and you get captured by the Moon’s gravity before you can complete it. Then, you are in another eccentric orbit around the Moon, until you ‘fire your retros’ and enter a more conventional, less eccentric lunar orbit.

      Of course, Apollo fired its engines once and then coasted to the Moon, allowing the astronauts to be weightless on the way, but if you were in some kind of craft that travelled under constant rocket power then you would feel this acceleration as weight.

  • fanISS May 22, 2012, 5:07 AM

    Well… ISS might be really falling to the Earth, even with her speed, since they have to put her higher by engine… Since the perigee is 391 km (243 mi) and her apogee is 406 km (252 mi).

    I suppose that it’s better to get to a speed just sufficiant not to fall too fast to the ground, so they can adjust the altitude, than too much velocity that could put the ISS far away and eventually quit the Earth’s environment on a trip far away into space.

    Am I right ? Or not !

    • squidgeny May 22, 2012, 11:54 AM

      Not quite – objects in orbit around a body won’t spiral in toward that body, nor will they spiral out (except in the case where they exceed the escape velocity) without an external force acting upon them (i.e. atmospheric drag, or thrust)

      Instead, they’ll orbit in an ellipse. So as you said, the perigee is less than the apogee (by definition) but this is the nature of an elliptical orbit – no thrust has to be applied to maintain this orbital path.

      So a “precise” velocity isn’t necessary – though obviously we wouldn’t want the ellipse to become too distorted, else it might intersect with a denser part of the atmosphere…

  • Geoff Clare May 22, 2012, 7:23 AM

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

    You are in free fall but you only feel weightless, you are not actually weightless. Your weight provides the centripetal force that makes you travel in a circular (or elliptical) path.

    • Bill Farty May 22, 2012, 4:15 PM

      ACTUALLY YOU ARE WEIGHTLESS. YOU STILL HAVE MASS THOUGH. MASS PROVIDES NO FORCES EITHER. THE FORCE OF GRAVITY BETWEEN THE MASS OF THE EARTH AND YOUR BODY, AS WELL AS THE FORCE PROVIDED BY THE SHUTTLE PROVIDES THE FORCE.

    • lcrowell May 22, 2012, 5:05 PM

      You are confusing mass with weight. Weight is mass subjected to the acceleration of gravity. Newton’s second law, force equals mass times acceleration (F = ma) , tells us that the force of gravity g = 9.8m/s^2 on a mass m gives a force F we call weight. So objects in orbit fall as the same rate of acceleration due to gravity have no mutual acceleration between themselves. This is the source of weightlessness.

      LC

      • Geoff Clare May 23, 2012, 8:20 AM

        You are the one who’s confused. As you rightly state, F = ma, F is the weight, a is 9.8 m/s^2. Given those facts, a person with mass 70 kg weighs (70 * 9.8) Newtons, or a little less than 700 Newtons.

        If there was no force (i.e. you are weightless) Newton’s first law says you would travel in a straight line. To travel in a circular or elliptical path there must be a force and that force is your weight.

        • lcrowell May 23, 2012, 1:31 PM

          In a Newtonian frame everything on the spacecraft including you are falling at the same rate of acceleration. Therefore the absence of differential accelerations means everything is weightless. This means that being on a reference frame that is falling or in an orbit is indistinguishable from being on a frame far removed from gravity fields, say in interstellar space or way out between galaxies. In both cases if you are in a capsule and can’t look out you can’t perform an experiment to distinguish between the two. This is the basis for the equivalence principle Einstein invoked to work general relativity. It is then for this reason the force of gravity in Newtonian mechanics is really a pseudo-force. Gravity is not really a force in a strict sense, but is due to the motion of paths in space or spacetime, where if that is curved the paths are extremal curves in the space.

          This works for the capsule or the spatial extent of the frame very small. Gravity in the Newtonian perspective has a radial dependency, and a capsule with some spatial extent will then have a tidal acceleration on it. This tidal force is physically real, while the standard Newtonian force of gravity can be “removed” as a pseudo-force. In general relativity the tidal force is due to a portion of the Riemann curvature, but the Newtonian force of gravity is due to the connection terms that can be removed by coordinate choice.

          LC

          • Geoff Clare May 23, 2012, 2:17 PM

            Looks like we are talking at cross-purposes. The statement I originally commented on was “So, when you are in orbit, you are in free fall, and are weightless.” This clearly establishes the reference frame as the body being orbited (i.e. Earth). You are talking about a different reference frame (the spacecraft).

            With Earth as the reference frame, the astronaut and the spacecraft are both accelerating at 9.8 m/s^2 and each has a weight (in Newtons) equal to 9.8 times its mass (in kg), and this weight provides the centripetal force that keeps them in orbit.

          • andrew g May 24, 2012, 2:34 PM

            In which case no matter where you are anywhere in the Universe you can never be weightless and the term becomes meaningless. The statement you are referring to explicitly equates being in free fall and being weightless, which therefore defines “weight” as the reaction force of something on the Earth’s surface to your downward acceleration. This is intuitive as it allows your weight to change if, for example, you are in an accelerating lift (elevator) or aeroplane (airplane).

            Nothing to do with frames of reference.

          • Geoff Clare May 24, 2012, 5:10 PM

            You can redefine the word “weight” if you want, but successful communication relies on the people who are communicating each using the same definitions for the words they use.

            If you won’t take my word for it, since we are discussing astronauts in orbit perhaps you’ll take NASA’s:

            http://www.grc.nasa.gov/WWW/k-12/airplane/wteq.html

            “Let’s do another problem and compute the weight of the Space Shuttle in low earth orbit. On the ground, the orbiter weighs about 250,000 pounds. In orbit, the shuttle is about 200 miles above the surface of the earth. As before, the gravitational constant ratio is the square of (4000/4200) which equals .9523*.9523 = .907. On orbit, the shuttle weighs 250,000 * .907 = 226,757 pounds. Notice: the weight is not zero. The shuttle is not weightless in orbit.”

            They even put those last two sentences in bold.

  • Jeff Boerst May 22, 2012, 6:29 PM

    So if one was on a craft, say 1LY from any gravity well and the craft’s F.O.R wasn’t accelerating or decelerating on it’s own, what would be causing the floating then?

    • squidgeny May 22, 2012, 10:02 PM

      The cause of the floating would be the lack of anything to cause it to not float. If that’s a bit of a confusing answer, it’s because it’s a pretty nonsensical question ;)

  • The Muss May 23, 2012, 6:20 AM

    My understanding was this: Mass = Weight multiplied by gravity. I know in earth the gravity is 9.81 meters per second. Weight is how much kilogramm you’re.

    I thought gravity was zero in vacuum space. So if you multiply your weight by zero you get ZERO. But I was wrong and you know why..

hide