Astronauts carry suitcases attached to their suits what are they for?

Air conditioners.? Expert Explain

Humans are mammals. We crank out a lot of heat. Even just sitting around, we generate about 100 watts equivalent to waste heat. We have to get rid of that heat, 24 hours a day, or we’ll die.

Spacesuits (and high altitude suits like those worn by X15 & SR-71 and U2 pilots) are (naturally enough) air-tight. Perspiration doesn’t cool you inside an air-tight suit, and the insulation of the bulky suits doesn’t help. Add to this the fact that astronauts often have a fairly long journey from where they suit up to where they board. Heatstroke is not a good way to begin a flight.

This is an SR-71 pilot wearing a high altitude suit made by David Clark:

That case is pumping in cool, fresh air so he can breathe. Even with the helmet removed, he would need the conditioner to keep from overheating while walking around in that sun. You do sometimes see photos of pilots in these suits without conditioners: