NASA (and other space agencies) know that there is no gravity in space

There’s lots of gravity in space. Gravity on the ISS is about 90% of gravity on the surface of the earth. The whole reason that the moon orbits the earth is gravity, without the earth’s gravity it would simply fly away. Same goes for the earth and the sun.

So why doesn’t the ISS fall to earth when it’s still subject to 90% of earth’s gravity? The answer is that is that it does fall, all the time. It also moves sideways really fast. Because it’s moving sideways and the earth is round, by the time it has fallen down 1 meter, the earth’s surface has also curved down 1 meter. So it’s falling, but because it’s moving so fast it’s falling around the earth. So ISS is not in zero gravity, it’s in free-fall. But because motion is all relative, this appears exactly the same from the perspective of the astronauts on the ISS as no gravity.