imagine astronauts inside the ISS while everything are in free fall, the astronauts are floating around. Now if the moon are also in free fall then how come astronauts can perform moonwalk on the surface but not on ISS?

btw ISS stands for international space station.

  • 3
    $\begingroup$ Think about what it's in free fall relative to in each case and which bodies of significant gravitational force are involved. Here's a hint, Earth is in free fall around the sun. How do you walk? $\endgroup$ Apr 10, 2018 at 1:54
  • $\begingroup$ The mass of the moon is $7.3\times10^{23}$ kg. The mass of ISS is about $4.2\times10^6$ kg. Start doing some calculations with Newton's law of graviation and see where it leads you. $\endgroup$
    – The Photon
    Apr 10, 2018 at 1:55
  • $\begingroup$ Also realize that the earth is in free fall around the sun. $\endgroup$ Aug 5, 2021 at 11:47

1 Answer 1


The astronauts on ISS are attracted to the space station, just as they are attracted to the Moon and to the Earth which are also in free fall. However, the mass of ISS is so very much smaller than the mass of the Moon, by many orders of magnitude (as Photon says, $4.2 \times 10^6kg$ compared with $7.3 \times 10^{23} kg$), so the gravitational pull of ISS is many orders of magnitude less.

In addition, the astronauts are surrounded by the mass of ISS, instead of it being on one side of them. If the mass of ISS were distributed with spherical symmetry, then the astronauts inside it would be weightless even if ISS were as massive as the Sun, whatever position they occupied inside it. This is a consequence of the Shell Theorem. The strongest pull from the space station would be felt just outside of its surface.

  • $\begingroup$ so if the moon starts crash-landing towards Earth, can astronaut still moonwalk? im trying to figure what is free fall about ;D $\endgroup$
    – user6760
    Apr 10, 2018 at 2:55
  • $\begingroup$ Yes. Even if the trajectory of the Moon (or ISS) takes it on a collision course with Earth, the astronauts are still attracted to the Moon (or ISS) with the same force as when the Moon (or ISS) is in a circular orbit. So they can still moonwalk (or ISS-walk). Free fall means that the object is moving freely under the influence of gravity alone. The astronauts and the Moon are like projectiles in Earth's gravity : if they are launched together with the same speed at the same angle they follow the same trajectory together regardless of their mass. $\endgroup$ Apr 10, 2018 at 3:20

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