Gordon Cooper and Pete Conrad just spent eight days in space. Eight days.

The Moon and back takes about eight days. The Gemini 5 mission, which ended today, has now proven that two human beings can survive in a spacecraft for the duration of a lunar mission. That’s the simple bottom line.

It wasn’t a comfortable eight days. The fuel cells that were supposed to power the spacecraft developed a problem early on — the pressure in the oxygen reactant supply tank dropped, which threatened to cut the mission short. Cooper and Conrad powered down to minimum consumption and coasted for a day while engineers figured out if they could keep going. They could, and did. Eight days.

Both men landed with some orthostatic hypotension — when they stood up after landing, their blood pressure dropped significantly because their cardiovascular systems had adjusted to weightlessness. They were physically weakened. After eight days, the human body has adapted to the weightless environment in ways that make re-adaptation to gravity taxing. Their muscles had thinned somewhat. Their blood volume had decreased.

This is important information for the Moon program. It means recovery time is needed after a lunar mission. It means exercise protocols in space need to be developed and enforced. It means we can’t assume a crew will step off the lunar lander and perform at full capacity.

But they survived it. They were functional. They did their jobs for eight days, dealt with a fuel cell emergency, and came home. The biological case for lunar duration has been made.

Cooper flew Mercury’s last mission. Now he’s flown Gemini 5. He keeps adding miles to his record in a quiet, professional way. I’ve always liked Cooper — something laconic and capable about him. He doesn’t perform for the cameras the way some of them do.

The Gemini program is moving fast now. Gemini 6 and 7 are the rendezvous mission. That’s next. That’s the last big piece before we can say: we know we can go to the Moon.