Astronauts on the International Space Station feel weightless because Astronauts on the International Space Station feel weightless because The gravity of the earth and the sun are directed opposite each other, so there is no net gravitational force. They are above the earth’s atmosphere. Their apparent weight is zero. They are far enough above the earth’s surface that the gravitational force is negligible.

Although gravity works on ISS, but it in an orbit. Orbit is nothing but free falling. ISS is continuously trying to fall on Earth but it misses it every time owing to its fast velocity. Let's understand this concept by an example.

Consider a Cannon Gun sitting on the Earth, firing parallel to the surface. If the cannonball's velocity is too slow it will travel forward and curve towards the Earth and hit the Earth. If the velocity is high enough it will fire parallel to the Earth and keep going until it escapes the Earth, in a slightly curved path. However, if the velocity is just right the cannonball will fall towards the Earth in such a way that it completes a full circle and returns to its starting point.

Similarly, ISS is moving with this "just right" velocity such that it misses the earth every time thus, experiencing a free fall . So the astronauts feel like they are in zero gravity. Hence their apparent weight is zero.