The medical results from the Gemini 5 eight-day mission are in and I’ve been reading the summaries. The human body changes in microgravity. Some of it is manageable. Some of it is alarming.

The cardiovascular system: without gravity, blood doesn’t pool in the legs. The heart has less work to do to circulate blood. The body, being efficient about things, responds by reducing blood volume — you excrete more fluid because your body thinks you have too much. When you return to gravity, you have less blood than you started with, and your heart has to work harder than before to circulate it, which manifests as orthostatic hypotension (lightheadedness when standing up) and general fatigue.

Cooper and Conrad had measurable cardiovascular deconditioning after eight days. They needed rest after landing. This means Apollo astronauts returning from the Moon will need recovery time before they can function normally. The medical team is working on exercise protocols to mitigate this — periodic bungee cord exercises, designed to load the cardiovascular system during flight.

Bones and muscles: eight days showed early signs of loss. The body doesn’t maintain bone density it doesn’t need. Without the constant loading of gravity, bone resorption exceeds bone formation. Eight days isn’t enough time to produce dramatic loss, but the trend was measurable. Longer missions — like the eventual Skylab station or a Mars mission — will face this seriously.

Kidneys and calcium: related to the bone issue. The calcium released from bone has to go somewhere, and it goes through the kidneys. High urinary calcium over long periods can cause kidney stones. Nobody wants a kidney stone on the way to the Moon.

Inner ear adaptation: the vestibular system that senses orientation and motion adapts to weightlessness within a few days. Most astronauts experience some nausea in the first day or two as the inner ear recalibrates. The danger point is re-entry, when gravity returns suddenly and a still-adapted vestibular system can cause severe disorientation.

What does all this mean for the Moon? The Apollo mission is about eight days round trip. Not fourteen days like the Gemini maximum duration, but not trivial either. The medical team’s assessment, as I understand it, is that the deconditioning at eight days is manageable — not negligible, but manageable. The crew will be somewhat weakened on return but functional.

I find myself thinking about the astronauts themselves. They train for the engineering and the piloting. They practice procedures until they can do them in the dark. They are exceptionally fit. And then they go into an environment where their body starts quietly dismantling itself, and they have to perform their professional best anyway.

The body is another system that has to work. Like the fuel cells and the guidance computer and the heat shield. The body is a system, and systems have to be engineered and maintained.

Remarkable people, doing an extraordinary thing, in a body that would prefer to be on the ground.