Pressure drop (Image: NASA)

Sadly we know how long humans can survive if suddenly exposed to the vacuum of space. Three Soviet cosmonauts died in 1971 when a faulty valve caused their Soyuz 11 capsule to depressurise at an altitude of 168 kilometres, shortly before re-entry into the Earth’s atmosphere. Investigations revealed that the cabin pressure dropped to zero for 11 minutes and 40 seconds, until the capsule hit the atmosphere. The crew died within 30 to 40 seconds from hypoxia. “You need both oxygen and air pressure to deliver oxygen to the brain,” says Jonathan Clark, a former space shuttle crew surgeon.

It is possible to recover from shorter spells in a vacuum, however. In 1966 a NASA technician was testing a spacesuit in a vacuum chamber when the pressure dropped to the level you would experience at an altitude of 36,500 metres. He passed out after 12 to 15 seconds. The last thing he recalled was the saliva boiling off his tongue; that’s because water vaporises at low pressure. He regained consciousness within 27 seconds when the chamber was repressurised to the equivalent of an altitude of 4200 metres. Although he was pale, he suffered no adverse health effects.

When the external pressure reduces, gas bubbles form in the blood, leading to lung damage within minutes. And the nervous system can be damaged within hours due to nitrogen dissolving out of the blood. A sudden drop in pressure can be devastating: air trapped in the lungs explodes within seconds. However, if you approach near-vacuum in a more gentle fashion and with intensive medical care standing by, it …