Crewed Spaceflight

Keeping people alive off Earth is a logistics problem disguised as an engineering one. Air has to be scrubbed of carbon dioxide continuously, water recovered to about 98% because resupply costs thousands a litre, and food, spares and propellant lifted from a gravity well at enormous expense. The human body cooperates poorly: bone density falls about 1% a month, fluid shifts flatten the back of the eyeball in ways that do not always reverse, and radiation exposure accumulates from the moment you leave the magnetosphere.

The countermeasures are unglamorous and mostly work. Two hours of resistance exercise a day against a machine that simulates weights holds most of the bone and muscle, which is why crews walk off the capsule rather than being carried. The eye problem — spaceflight-associated neuro-ocular syndrome — has no fix at all yet, and it is the one that worries flight surgeons, because it is a structural change to the back of the eyeball rather than something that rebounds. Radiation is the other unsolved one. Inside the magnetosphere a six-month increment costs roughly what a few years of ground-level background would; outside it, on the way to Mars, there is no field and no practical shielding thin enough to lift, and the dose is the mission's hardest single constraint.

The International Space Station has been continuously occupied since November 2000, and it is not primarily a laboratory — it is the entire basis of what is known about long-duration flight, accumulated one increment at a time on a few hundred people. That matters because it is due to be retired around 2030, deliberately deorbited over the Pacific, and the commercial stations meant to replace it are still drawings and funding rounds. If there is a gap, the dataset stops growing at exactly the point where it is being asked to underwrite a three-year mission. Everything about going to Mars is downstream of these numbers rather than of propulsion, and the numbers are thinner than the renderings suggest.