As an expert in the field of space medicine and human factors, I can provide a detailed explanation of what would happen if an astronaut were to remove their helmet in the vacuum of space. Firstly, it's important to understand that the human body is not adapted to survive in a vacuum. The vacuum of space is an environment where there is no air or pressure to support life as we know it. If an astronaut were to take off their helmet, they would be exposed to this harsh environment. Upon exposure to vacuum, several things would happen almost immediately: 1. Ebullism: The boiling of body fluids, particularly at the warmest parts of the body, such as the lungs and the mucous membranes. However, contrary to popular belief, the body's fluids would not boil instantly due to the low pressure rather than the temperature causing the fluids to vaporize. 2. Decompression sickness: Also known as the bends, this occurs because the reduced pressure would cause dissolved gases in the body to come out of solution and form bubbles, which could cause damage to the body's tissues. 3. Asphyxiation: The lack of oxygen in the vacuum would lead to asphyxiation. However, the body's oxygen supply in the bloodstream would last for about 15 seconds, giving the astronaut some time before losing consciousness. 4. Pressure differential: The pressure difference between the inside of the body and the vacuum outside would cause the body to expand. The air in the lungs would be forced out through the mouth and nose, as the vacuum would quickly equalize the pressure. 5. Radiation exposure: Without the protection of the Earth's atmosphere or a spacesuit, the astronaut would be exposed to high levels of radiation from the sun and cosmic rays. 6. Thermal issues: The skin and body would be vulnerable to temperature extremes. In direct sunlight, temperatures could rise rapidly, while in the shade, they could drop just as quickly. The effects would be severe and potentially fatal. However, it's worth noting that the exact outcome would depend on various factors, including the individual's health, the duration of exposure, and the specific conditions of the vacuum. read more >>

Even most astronomers don't know what would happen. NASA, however, does know. Occasionally during astronaut training, an astronaut's helmet accidentally comes loose in a vacuum chamber or something like that. ... When you're exposed to vacuum, the air in your lungs is forced out through your mouth.read more >>