This entry has been adapted from an answer I gave on Quora.com (see https://www.quora.com/What-is-the-scariest-thing-that-has-happened-to-someone-while-on-the-International-Space-Station/answer/Calum-Hervieu).
I can think of three events that are closely tired for the accolade of most terrifying event on the International Space Station.
1. Incoming space debris
Everybody knows that in recent years space debris has become a major problem.
NASA and other organisations continuously track around 22,000 pieces of space debris of considerable size (bigger than a few inches) and perform orbital boosts to move the space station out of the danger zone if a collision course is determined.
One day, during Scott Kelly’s ‘Year in Space’ mission (July 2015, Exp. 46) NASA became aware of a large piece of debris heading right at the space station just a mere 1.5 hrs before an expected collision. This meant there was no time to perform an orbital maneuver so the crew was instructed to hunker down in their Soyuz capsule ready for a quick return to Earth if a collision should take place. This is standard procedure and something they are trained for.
However, as Scott points out in his book, the ISS is travelling approximately 27,600 km/h. Due to the laws of physics, in order for a piece of debris to be travelling on the same orbit in the opposite direction it must also be going 27,600 km/h. That makes the combined speed of the collision 55,000km/h! In SI units this represents a kinetic energy of 48 MJ, or in perhaps more familiar terms it is the equivalent of setting of 12kg of TNT on the station.
Clearly, there would be no hope for survival or any chance to undock the Soyuz capsule successfully after this collision occurs.
So, imagine, sitting there in a tin can waiting and hoping that this piece of space metal doesn’t reduce your entire existence back into star dust. Pretty scary stuff.
Fortunately, the debris passed within about 1.5 miles of the station and they resumed normal activities immediately!
2. Going blind in space
In April 2001, while on STS-100 to the Canadian Space Agency Chris Hadfield was performing a space walk. NASA coats the inside of the helmets with an anti-fog solution. During his first spacewalk of the mission Hadfield experienced a severe irritation of his left eye when some of this anti-fog solution floating into it. This triggered the standard physiological reaction of watery eyes, in his bodies’ futile attempt to clear the irritating solution.
However, in space tears do not fall down the face as there is no gravity - their motion is dictated mainly by friction. Clearly, in his suit it was impossible to wipe the water from his eyes. So his tears gradually grew into a large ball of water & anti-fog solution that eventually bridged his nose and covered both of his eyes, rendering him almost completely blind.
Imagine orbiting the Earth, clinging to a chunk of metal, with nothing but a few thin layers of fabric between you and the vacuum of space, and then you go temporarily blind. You have no useful sense of sight or smell and severely reduced touch. Chris had to rely on his training, his crew mate and mission control to get the situation under control. That must’ve been pretty scary.
The tears grew so large that it effectively diluted the solution and he managed to shake off the tears and complete the space walk. NASA now uses Johnson no-more-tears to prevent this situation from arising again!
He gave a really great TED talk on the incident:
3. Drowning in space.
Last but certainly not least, during expedition 36/37, European Space Agency astronaut Luca Parmitano was performing a space walk when his water cooling garment developed a fault. It began to leak water into his helmet from behind his head.
This was much more dangerous than Chris Hadfield’s situation as the volume of water was copious. In a matter of mere minutes from realising the cold water was not sweat and was indeed a growing ball, Luca became almost completely emerged in water.
His eyes were covered and his ears were blocked meaning he could not hear or see mission control or his crew mate.
Fortunately, astronauts are trained for this scenario and spend hours on the ground practicing a quick return to the airlock via a 'buddy system' - whereby one crew member directs the other along a long metal tether which prevent them from slipping and floating off to burn up in the atmosphere.
By the time Luca returned to the airlock the crew estimated his helmet was half full of water!
I recommend watching the live video from the incident along with a press briefing below and note how relaxed he is!
You can also read Luca’s thoughts about the incident on his blog.
Pretty scary stuff!
If you know of any other scary incidents that I have missed then please comment below and don't forget to subscribe for more great posts!