How does weightlessness occur?

Weightlessness doesn't arise from the absence of gravity, but from free fall. Weight is only perceived because a surface pushes back in the opposite direction: the floor, a chair, the sole of a shoe. If that surface is also falling—in a falling elevator, in the capsule of a drop tower, in a spacecraft on its trajectory—that resistance disappears. The body and its surroundings accelerate equally, nothing presses against anything anymore, and anything not secured floats away. In orbit, that fall never ends, because the sideways speed is so high that Earth's surface curves away at the same rate as the falling trajectory.

For research and training, this state can also be created on Earth. In the drop tower in Bremen, an experiment capsule falls inside a tube from which the air has been removed; it doesn't last long, but with very few disturbances. A parabolic aircraft climbs steeply and then follows the same trajectory a thrown stone would take; this produces about 20 seconds of weightlessness per parabola. That's enough for experiments with people on board, though with many more stray accelerations than in the drop tower. Sounding rockets offer longer phases, but without a crew.

None of these conditions is perfect. Air resistance, attitude control, pumps, fans, and any movement on board generate tiny residual accelerations. Sensitive experiments on crystal growth or the behavior of liquids and flames react measurably to this, which is why technical language speaks of microgravity. For everyday life on board, this changes nothing: anyone who doesn't hold on to something drifts away.