Is there gravity in space?

The phrase "zero gravity" is one of the most durable errors in popular science. Gravity is a property of mass and its reach has no edge; it simply weakens with distance. The International Space Station orbits a few hundred kilometres up, and at that altitude the Earth still pulls on it with roughly 90 per cent of the strength felt at sea level. If gravity really vanished there, the station would fly off in a straight line instead of circling the planet.

What astronauts experience is free fall. The station is falling towards the Earth continuously, but it is also moving sideways fast enough that the ground curves away beneath it at the same rate. Crew, walls and water bottles all fall together, so nothing pushes against anything else, and there is no sensation of weight. The correct term is microgravity: not the absence of gravity but the absence of a supporting force.

The same effect can be produced without leaving the atmosphere. Aircraft flying parabolic arcs give twenty or thirty seconds of weightlessness at a time, and research agencies use them to test hardware and train crews. Nothing about Earth's gravitational field changes during those seconds.

Even in interstellar space, far from any star, gravity persists at a vanishingly small level, because every mass in the universe contributes something. The practical consequence for human spaceflight is unavoidable: astronauts on long missions lose bone density and muscle mass not because gravity is missing from the universe, but because their bodies no longer have to resist it. Countermeasures on the station revolve around daily resistance and cardiovascular exercise for precisely that reason.