How long can an astronaut survive in space without a space suit?

Space is a vacuum with no air and no pressure, so an astronaut outside without a space suit is in danger from the very first moment. Suitless, you cannot breathe, and fluids in the body start to react almost at once.

Within 10 to 15 seconds, you lose consciousness due to lack of oxygen in your blood. That is about how long you could remain awake and useful outside a spacecraft or space station without a suit. You absolutely cannot hold your breath doing so could rupture your lungs due to the pressure difference.

After about 15 seconds, all the oxygen in your blood is used up. If you have not been recovered and repressurised, you will be unconscious. Some animal and rare human experiments show that the brain and body can survive up to 90 seconds without permanent harm if re-pressurisation is rapid.

Your body fluids begin to boil at such low pressure, a process known as ebullism. Skin and muscle keep the body together, but you swell up noticeably your tissues expand, but you do not explode. Body water, including saliva, boils quickly, so your mouth dries, and gas escapes from your lungs and stomach.

In the sun, body parts heat up rapidly to over 100°C; in the shade, they cool to -100°C but not instantly. Loss of heat is slower than you might think, because vacuum is a poor conductor. However, you get severe exposure to cosmic rays and solar radiation, which can burn skin within moments.

If you are repressurised within about two minutes, there is a good chance of surviving without permanent harm, though the risk of damage climbs each second. After 90-120 seconds, death is nearly certain; the brain and heart can no longer function.

This is why space suits are so critical. They keep in pressure, provide oxygen and shield the body from cold, heat, and radiation. An astronaut can only survive in space without a suit for up to 90 seconds before lasting damage sets in.