What happens if there's no gravity in space?

Gravity doesn't end when you leave the atmosphere: it weakens with distance, and very slowly. At the International Space Station's altitude of 400 km, Earth's pull still retains close to 90% of its surface-level value, and it doesn't vanish even at the Moon's distance — that same gravity is what keeps our satellite in orbit. What's missing up there isn't gravity, but ground. The station and everything in it are continuously falling toward Earth while moving sideways fast enough to never actually land; inside, that shared fall is perceived as weightlessness, just like in an elevator whose cable had snapped.

The body interprets the absence of weight as a signal to economize. Bone that no longer bears load demineralizes, the antigravity muscles of the legs and back lose volume, the heart pumps against less resistance, and fluids shift up into the torso and face — hence the puffy look and nasal congestion of the first few weeks. Pressure on the eyeball also alters vision in many crew members, a syndrome NASA monitors closely. That's why daily exercise on resistance machines is mandatory and takes up roughly two hours of the day.

Returning is the hard part. After months in orbit, an astronaut needs help walking, feels dizzy on standing because the circulatory system has forgotten how to push blood upward, and spends weeks or months regaining bone density and balance. That rehabilitation is why extended microgravity is such a concern for Mars mission planning: when they arrive, no one will be there to carry them off on a stretcher.