What does weightlessness feel like?

The sensation doesn't arise because gravity disappears: at the ISS's altitude it acts almost undiminished. The station, the crew and any loose pen fall together around the Earth, and orbital velocity carries them past the ground again and again without ever touching it. Anyone who knows the seconds of drop on a roller coaster knows the principle behind this sensation. Parabolic research flights offer about 20 seconds of that each time: enough for experiments, too little to get used to it.

Without weight, blood no longer pools in the legs. Fluid shifts upward: a swollen face, thin calves, head pressure like a cold, dulled senses of smell and taste. At the same time, the balance organ reports stillness while the eyes perceive motion. Space sickness arises from that contradiction, with nausea and headaches. Up and down cease to exist; which wall counts as the floor becomes a choice.

After a few days, the sensation becomes pleasant. You move with your fingertips instead of your legs and learn to dose force carefully, because any push that's too strong ends against the next wall. Tools get tied down, or they disappear. At rest, the body adopts a slightly hunched posture on its own. Muscles and bones lose load, which is why daily exercise is part of the onboard program. Upon returning, one's own arm suddenly feels surprisingly heavy.