The International Space Station has been continuously inhabited since November 2000. Somebody has been off the planet every day for a quarter of a century.
It is roughly the size of a football field, circles Earth about every ninety minutes, and travels at around 28,000 kilometres per hour. From the inside, none of that is what defines the experience. What defines it is that nothing stays where you put it.
Freefall, not zero gravity
The station is not beyond Earth’s gravity. At its altitude of roughly 400 kilometres, gravity is still about ninety percent as strong as at the surface.
What produces weightlessness is that the station and everything inside it are falling continuously, and moving sideways fast enough that the ground curves away beneath them at the same rate. Orbit is not the absence of falling. It is falling and missing.
The correct term is microgravity, since small residual accelerations remain from atmospheric drag, equipment vibration, and crew movement. For sensitive experiments, those tiny forces matter.
What the body does about it
Human physiology assumes a downward direction, and removing it triggers a cascade of adaptations.
Fluids that gravity normally pools in the legs shift upward, producing puffy faces and congestion for the first days. Bones lose density at a rate comparable to advanced osteoporosis, concentrated in the load-bearing structures that no longer bear load. Muscles atrophy, particularly in the legs and back.
The countermeasure is exercise, treated as a non-negotiable part of the schedule: around two hours a day on a treadmill with a harness to hold the runner down, a stationary cycle, and a resistance device that simulates weightlifting using vacuum cylinders.
Vision changes have emerged as one of the more concerning long-duration effects, apparently related to pressure changes from the fluid shift, and some cases have not fully reversed after return. Radiation exposure is elevated, since the station sits above most of the atmosphere though still within the protection of Earth’s magnetic field.
Astronauts return measurably taller, having lost the spinal compression that gravity imposes all day on the ground. It does not last.
Ordinary tasks, redesigned
Every routine activity had to be reconsidered.
- Sleeping happens in a padded booth roughly the size of a phone box, in a sleeping bag attached to the wall. Orientation is irrelevant; there is no down.
- Eating works because surface tension holds liquids in place. Most food is rehydrated or thermostabilised, eaten from packages, and seasoning is supplied as liquid so it does not drift into equipment.
- Washing is done with rinseless soap and towels. There is no shower, since water would not drain.
- The toilet uses airflow instead of gravity, with separate systems for solid and liquid waste. Urine is recovered and processed back into drinking water, which recycles a large fraction of the station’s supply.
- Sixteen sunrises and sunsets per day make natural light cues useless, so the crew works on coordinated universal time with a carefully managed schedule.
Where the time goes
A typical day runs roughly twelve hours, with science and maintenance dominating.
The station’s purpose is to be a laboratory, and the research spans protein crystal growth in the absence of convection, combustion physics without buoyancy, materials science, plant biology, and long-term studies of the human body itself. Much of it is work that cannot be done on the ground because gravity interferes.
Maintenance is a larger share than most people assume. The station is an ageing spacecraft with life support, thermal control, power, and communication systems that require constant attention. Filters get cleaned. Pumps get replaced. Occasionally something breaks that requires a spacewalk, which involves hours of preparation, a suit that functions as a personal spacecraft, and a task list rehearsed extensively in a pool on Earth.
The cooperative part
The station is assembled from modules built by multiple space agencies and operated jointly by the United States, Russia, Europe, Japan, and Canada. Segments are structurally and functionally interdependent — power, propulsion, and life support are shared across national boundaries by design.
It has continued operating through periods of considerable political friction on the ground, which is either a testament to the durability of technical cooperation or simply a consequence of the fact that the modules cannot be separated. Probably both.
The end of the programme
The station has a finite life. Structural fatigue accumulates, and hardware designed decades ago becomes progressively harder to support. Current planning points toward retirement around the end of this decade, with a controlled deorbit over the remote Pacific.
Several commercial stations are in development to succeed it. Whether the handover is seamless is an open question — but the goal is that the streak of continuous human presence in orbit, now well past twenty-five years, does not break.