A laboratory among the stars

What the space station was built for, what it cost, and what happens when it comes down

A The largest object ever assembled in orbit was not built as a scientific instrument. It was built as a diplomatic one. Two national programmes that had spent thirty years competing found themselves, at the end of the Cold War, with a partly designed American station that had no political constituency and a Russian programme with unmatched experience of long-duration flight and no money to use it. Combining them solved both problems, and the result has been continuously occupied since November 2000 by crews drawn from countries whose governments have frequently not been on speaking terms.

B The arrangement was not merely symbolic. Each side contributed what the other lacked: American funding and laboratory modules, Russian propulsion and life support derived from decades of operating earlier stations, and the two halves remain, technically, semi-independent segments that depend on one another for power, attitude control and reboosting. The engineering is easier to admire than to describe. The station masses over four hundred tonnes and was carried up in more than thirty separate launches over a decade, with modules built on three continents that had never been physically joined until they met in orbit. It circles the earth roughly every ninety minutes, which means its occupants see sixteen sunrises a day, and it must be periodically pushed to a higher orbit because even at four hundred kilometres the atmosphere is thick enough to slow it down.

C What it is for has always been the harder question. The official answer is research in conditions of continuous free fall, and the work that most obviously requires such conditions concerns the human body. Weightlessness removes the load that maintains bone and muscle, and astronauts lose both at rates comparable to accelerated ageing; fluid shifts towards the head and alters the shape of the eye; the immune system behaves oddly. None of this can be studied properly on the ground, and all of it must be understood before anyone spends years travelling to another planet. The countermeasures developed in response — resistance exercise on machines that simulate weight, drugs that slow bone loss, carefully specified daily schedules — have kept losses to a level from which crews recover after landing, which is itself among the station's clearer achievements.

D The other research has a more mixed record. Crystals of proteins grown without convection are more regularly formed than those grown on earth, which assists in working out their structure, and combustion behaves differently enough in free fall to have taught engineers things about flames that terrestrial experiments could not. Against this must be set the cost. The station has consumed something on the order of a hundred and fifty billion dollars, and a persistent criticism, made by scientists rather than by opponents of spaceflight, is that a fraction of that sum spent on uncrewed missions would have produced more knowledge by any measure one cares to apply.

E The defence of the expenditure rests on the parts that do not appear in a research budget. The station has kept two space programmes working together through the collapse of a superpower and several subsequent political crises; it has kept an industrial base assembled; and it has established, in practice rather than in theory, how to operate a complicated machine continuously in an environment that kills the unprepared. Everything now proposed for the moon or Mars assumes competence that was acquired here, mostly through the accumulation of unglamorous procedure.

F That competence includes a repertoire of improvisation that the official accounts underplay. Leaks have been traced with floating tea leaves, a cooling system has been repaired by a spacewalk devised over a weekend, and an air scrubber has been kept running well beyond its design life by crews cannibalising spares. The station is old, and old machinery is maintained rather than operated. A significant share of crew time goes not to the experiments the mission is justified by but to keeping the platform on which they are conducted alive.

G Its end is now scheduled. The structure is fatiguing, the leaks are increasing, and the plan is to bring it down over an empty stretch of the southern Pacific around the end of this decade, using a purpose-built vehicle to push it into the atmosphere in a controlled way. What follows is intended to be commercial: several companies propose smaller stations, with national agencies as customers rather than owners. Whether that arrangement will sustain the same continuity of human presence is not known, and the honest assessment of the station's legacy may turn out to depend on the answer. An unbroken occupancy of low orbit lasting a quarter of a century is either the beginning of something or an episode, and which it was will be decided by what is flying in the 2030s.