A bullet shot from a modern rifle travels at about 1200 m/s. Most people would agree that in the scope of things that humans interact with every day, this is pretty fast.

The international space station is traveling over six times the speed of our modern rifle bullet — about 7660 m/s. This is faster than anything humans have direct experience with on the earth. To put this into perspective, if the ISS flew past you at sea level it would take a little over a second for it to get from horizon to horizon. You wouldn’t have much time to register that it was coming toward you (aside from the atmospheric havoc it would wreak, but we don’t have to deal with that sort of thing in our pleasant thought experiment).

On September 5th, 1977, the Voyager 1 spacecraft was launched. It has been traveling for over 40 years, and is now beyond what most people consider to be the boundary of our solar system, well into the interstellar medium. As of this writing, it is about three times the distance from the sun as Pluto (Voyager Mission Status). Voyager 1 is currently traveling at about 17000 m/s relative to the sun, well over twice the orbital speed of the ISS. This is faster than most of our intuitions can grasp, but we’re still on Earth’s doorstep on a galactic scale.

The closest star to our solar system is Proxima Centauri, about 4.2 light years away. At the current stupendous speed of Voyager 1, it would take about 74,000 years to reach Proxima Centauri (if that’s the way Voyager was headed, which it is not). 74,000 years encompasses just about all of the history of Homo Sapiens after we expanded out of Africa — a long time.

So now we’ve established that Proxima Centauri is really far away. The distance is 4.2 light years. The Milky Way galaxy, our own corner of the universe, is about 100,000 light years across. For Voyager to make that journey at its current speed would take about 1,763,500,000 years. That’s a significant percentage of the age of the earth itself. We’re now at distances that I cannot intuit at all.

The Milky Way is a part of a cluster of galaxies called the Local Group. We know that there are at least 54 galaxies in the Local Group, but the Milky Way partially occludes our view. Each of these galaxies contains millions to hundreds of billions of stars. Andromeda, about 2.5 million light years from Earth and one of the biggest galaxies in the local group, is estimated to contain a trillion stars. The entire Local Group of galaxies is about 10 million light-years across.

The Local Group is gravitationally bound to an even bigger structure, the Virgo Supercluster. Within the Virgo Supercluster are least 100 groups and clusters of galaxies, many of which are larger than the Local Group. The Virgo Supercluster is about 110 million light years across.

This ludicrous distance is dwarfed, however, by the size of the observable universe, which is estimated to be about 93 billion light years in diameter. It’s estimated to contain about 10 million superclusters. This is absurdly, incomprehensibly large.

Thanks for stretching with me!

The first big thing to understand is how heat is transferred in space. You may remember that there are four main ways that heat can be bled off: conduction, convection, radiation, and phase change transfer (e.g. ‘enthalpy of vaporization’). When you’re in space, conduction and convection are out, because nothing cold is touching you (conduction) and there are no fluids to transfer heat away from you (convection). That leaves only radiation and phase changes that can cool you down. The infrared radiation leaving the human body is only about as much as a lightbulb, which is not going to drop your temperature extremely rapidly. You’ll also be cooled when the water in your skin boils away, but that’s only going to affect your outer layer. Your internals will be fine for a while, until they completely run out of oxygen.

So now that we’re clear that insta-freeze won’t happen, what are the actual steps in our grisly space demise? Here they are:

• As soon as you get into space, the air will be violently forced out of your lungs. Your lungs can’t take a one atmosphere pressure differential. In an experimental dog decompression study, most dogs also underwent “a high degree of subcutaneous emphysema and expulsion of gas from the stomach and lower bowel, often with simultaneous projectile vomiting, defecation, and urination”.
• Your fluids will start boiling away instantly. As pressure decreases, boiling point temperature also decreases until you reach the triple point (the point at which water, ice, and vapor exist at equilibrium). The triple point temperature for water is 32 degrees fahrenheit, well below body temperature. There was a NASA volunteer test subject, Jim LeBlanc, who was accidentally exposed to near-vacuum on camera while testing a space suit in 1966. He reported feeling the saliva boil away from his tongue before passing out.
• Generally speaking, you’ll pass out after about 10-15 seconds due to lack of oxygen. Since you can’t hold your breath, there’s not much wiggle room there. Your organs will continue to work for a while longer, but you’ll really want to get back to a pressurized environment as soon as you possibly can, because while all this is happening, you’ll also start bloating.
• Severe bloating will begin immediately, as water in your body turns to water vapor and the gases dissolved in your bloodstream start coming out of solution. It’s a good thing you’re passed out, because this is likely to be extremely painful.
• If you don’t get back to safety within about 2-3 minutes, the oxygen deprivation combined with organ hemorrhaging from lack of pressure will kill you. Animal studies with dogs and chimpanzees were conducted at near-vacuum (high altitude) to obtain those numbers, which I’ll discuss shortly.
• After death, your fluids will continue to outgas until your body is mummified. You’ll last quite some time at this point, if you can avoid gravity wells!

So how do we know the approximate point of irreparable damage? As I mentioned above, in 1964 an interesting (and disturbing) paper was presented called “Experimental Animal Decompressions to a Near-Vacuum Environment”, which contains descriptions of the rapid decompression of 126 dogs to near-vacuum (2 mmHg). Basically they decompressed batches of 6 dogs each for periods of time varying from 5 seconds to 180 seconds. At two minutes the dogs suffered 15% fatalities, and at three minutes the dogs suffered over 80% fatalities. Denitrogenated dogs (dogs that breathed pure oxygen for a period of time to remove most of the nitrogen from their bloodstreams) survived for slightly longer than non-denitrogenated dogs. The bloating was so bad that the dogs appeared inflated like balloons – in the words of the paper, “The water vapor effect and gas expansion were of such magnitude that the animals became completely immobilized with the extremities, neck, and body in an extended position, similar in appearance to an inflated goat-skin bag.” Many of the dogs died from heart failure long before recompression.

The dogs rapidly deflated once recompression commenced, and the dogs that survived quickly regained functionality in the majority of cases. To quote the paper, “During recompression to higher pressures, the subcutaneous gases were, of course, also recompressed and the animals quickly and dramatically deflated to their normal appearance, still remaining, however, in an obvious state of flaccid paralysis, unconsciousness, and apnea. This deflation process appeared to begin rather gradually with the onset of recompression. At 25 to 30 mm. Hg absolute, deflation became more rapid ; at approximately 70 mm. Hg the animals appeared to have returned to their normal size. At 45 to 50 mm. Hg, however, a major portion of the deflation is complete, suggesting that water vapor is probably the predominant gas concerned with the excessive distention of the animals.” Even in those decompressed for 180 seconds, lung trauma appeared to clear up in 24 hours.

Similar studies were done on chimpanzees, which were interesting because the chimps could perform complex tasks before and after the decompression. In one study, all eight chimps survived decompression, one of which was decompressed twice (the last decompression lasting 150 seconds). All the chimpanzees were able to perform at or above their pre-decompression mental levels, and in subsequent check-ins they displayed no noticeable long-term side-effects. In a subsequent study, one chimpanzee survived a 210 second decompression (subject 254, one tough chimp). I’m happy to report that S-254 demonstrated complete behavioral recovery after 199.25 minutes.

As you can see, reality is much more dramatic than fiction when it comes to rapid decompression in a vacuum. Try to keep your spacesuit intact.