• Question: Hi, enjoyed the lectures but didn't think you really answered why the astronauts experience weightlessness on the ISS. There was good video of the Vomit Comet, but I didn't think the question was answered. Could you explain further please? David

    Asked by David to Kevin, Katie, Julia, Jon F, Jon, Jean-François, Floris, Delma, Claudie, Beth on 2 Jan 2016.
    • Photo: Julia Attias

      Julia Attias answered on 2 Jan 2016:

      Hi David!

      The International Space Station is located in what we call “Low Earth Orbit”; that is, just to the edge of the Earth’s atmosphere (only 250 miles up from where you are sitting!). Therefore, it is not subjected to zero-gravity. In fact, it is still very much within the pull of Earth’s gravity. The ISS is actually continually being pulled towards the centre of the Earth based on the gravity it is exposed to. But what you can do is make something move fast enough, so that that object will constantly be being “pulled in” towards the Earth but will never happen because it’s moving too fast. So what you end up with is the ISS constantly “free-falling” around the Earth due to the speed its moving + the gravitational pull, rendering everything inside it to be “weightless” (the astronauts are free falling too with it).

      Imagine that you are on a vertical-drop roller coaster. As the roller coaster drops, you move upwards; essentially you are weightless for those moments. If you weren’t strapped in you would float – its the same for them! I hope that helps you to understand a little clearer.

    • Photo: Jon Farrow

      Jon Farrow answered on 5 Jan 2016:

      Julia answered this pretty well already, but I’ll try and give it a go as well.

      The reason astronauts feel weightless aboard the ISS is that they are in freefall. This brings up two really common and surprisingly tricky questions – 1) why is the ISS in freefall? and 2) why does freefall means weightlessness?

      1) Why is the ISS in freefall?
      The ISS is only 400km up. That’s really not very far. The strength of gravity is all about distance between masses and if you plug the radius of the Earth and the radius of the Earth+400km into the right equation (F=GMm/r^2), you’ll find that the answers aren’t really that different. There’s still lots of gravity on the ISS. And that gravity is pulling the ISS towards the centre of the Earth. If it weren’t moving sideways at an incredible rate, the ISS would fall down to the Earth in matter of minutes.

      The secret to the ISS’s orbit is its horizontal speed: 27 580 km/h! It travels so incredibly fast sideways that even though the Earth is pulling hard on it, the ISS just keeps missing. It doesn’t fly off into the solar system because the Earth continues to pull the ISS towards its centre. That means the ISS is constantly falling.

      2) Why does freefall mean weightlessness
      Weight is the feeling you get when you support yourself against forces. Force, like Newton told us in his second law, depends on mass and acceleration. Standing here on Earth, you experience the pull of gravity and have to support yourself against it. If you increase your mass by picking up something heavy, you’ll feel heavier. The other way to feel heavier is to accelerate.

      Standing in an elevator as it starts to move up, your body needs to counteract not just the force of gravity, but also the force of the elevator moving up in order to support you. This extra force makes you feel heavier. If you take the elevator back down, you’ll notice that you feel a bit lighter when it starts to move down because the elevator is already doing some of the work for you by moving away from you. If you cut the cord, both you and the elevator would accelerate at the same rate. You wouldn’t need to support yourself against the elevator anymore. That is, until it hit the bottom.

      The ISS is like an elevator with its cable cut but that never has to worry about hitting the ground.

      To link this back to the lectures, just like when Kevin was jumping on the trampoline, the Vomit Comet plane launches its passengers up and then catches them again on the other side. Between throw and catch (jumping and landing in the case of the trampoline), objects are all accelerating at the same rate and so don’t have to support themselves against each other.

    • Photo: Jean-François Clervoy

      Jean-François Clervoy answered on 18 Jan 2016:

      Bonjour David,

      ‘Weightlessness’ is the state of an object subject to only the force of gravitational attraction.

      In other words: when the only force applied to your body is your own weight, your natural motion follows a freefall trajectory, which is the same for all objects, regardless their weight, size and shape. The ISS and its astronauts are attracted by Earth and fall freely and continuously together towards it, but their horizontal velocities provided by the launcher makes them ‘miss’ Earth continuously. If you jump up in the air, and if we neglect air drag at low speed, you are in the same weightless state as the ISS astronauts until your freefall is stopped because your feet hit the floor.