• Question: I watched last nights lecture explaining that zero gravity on the ISS is actually because it is constantly falling. I want to know is the weightlessness of the astronauts affected when the ISS has to use thrusters to move it in orbit?

    Asked by Aiden Childs to Steve, Rochelle, Kevin, Katie, Julia, Jon F, Jon, Jean-François, Floris, Delma, Claudie, Beth, Adrian on 2 Jan 2016.
    • Photo: Steve Price

      Steve Price answered on 2 Jan 2016:

      In practice, the thrusters are fairly low power and so do not cause noticeable effect.
      If an astronaut was standing or sitting or… in the direction of motion they would feel the effect of the push from the thrusters. Temporary loss of zero g would occur, but the change would be small.

    • Photo: Kevin Fong

      Kevin Fong answered on 6 Jan 2016:

      Good question. The answer is yes. Weightlessness for crews in orbit is caused by freefall. The vehicle which you are in is essentially falling away from you as fast as you are falling towards it and so there is no reaction force for your to experience. If there were no vibrations and no other accelerations aboard space station then that weightlessness would be perfect. But the station does vibrate because of machinery and that, plus the small accelerations produced by thrusters, do produce (very) small accelerations. This is why scientists formally refer to the environment up there as “microgravity”. This takes into account the small disturbances that astronauts experience when these additional vibrations and accelerations are taken into account.

    • Photo: Jean-François Clervoy

      Jean-François Clervoy answered on 18 Jan 2016:

      Hi Aiden,

      Indeed, as soon as another force than gravity is applied to the ISS, its crew members feel attracted momentarily to the wall opposite to the thrust direction and all free-floating objects do the same. But it remains very small compared to normal gravity (except during the launch itself) because the orbit thrusters push with a 1000 or even 10000 times lower force than the gravity force.