The SF writer Charlie Stross recently wrote on his blog about the absurdity of self-sufficient space colonies (“Insufficient Data”).  He noted that it takes an extraordinary number of people to maintain a technological civilization, because even the most common artifacts embody a vast range of skills.  There are tens of millions of lines of software in every modern car, and a huge variety of individual mechanisms.  He thought that maintaining even the current level of technology would probably need at least 100 million people.   It’s impossible to estimate such a thing of course, but worth noting that the 10 to 20 million people in North Korea and Cuba have great trouble supporting a modern economy.  100M is not a big percentage of the Earth’s population of 7 billion, but it’s hard to imagine plunking that many people down on the Moon or Mars.

But this made me wonder – what is the actual population of outer space today?   The quick answer is zero, since there are no permanent residents of space.  But what is it if you add up all the person-years of the transient visitors?  There have been continuously occupied space stations since the early 90s, so there’s been someone overhead for a quite a while now.  As far as my Google-fu can tell, no one seems to have added this up.

I did find a list of all manned space missions here, thanks to the efforts of Robert A. Braeunig.  I turned that into a list of all the people who had ever visited space, and calculated how much time they had each spent there, and where they had lived (E.g. on the Shuttle, on the Mir station, etc.). The results are shown below (please click for a larger view):

Average Population of Space, by Year and Habitat

The habitats are:

• Salyut 1-7: a series of small stations put up by the Soviets
• Skylab: a big station put up with one of the last Saturn 5s and occupied jointly by the US and USSR.
• Shuttle: Missions by one of the 5 US Space Shuttles.  In the 2000s the Shuttle has been almost entirely used for building the ISS, so I’ve counted  those flights in the ISS category
• Mir: A large station put up by the Soviets and occupied for 14 years
• ISS: The International Space Station, a gigantic station with contributions from the US, Russia, EU, Japan, and Canada.  It’s now the most expensive single structure ever built, with cost estimates ranging from 35 to 150 billion dollars.
• Other: all the other various launches.   These have all been done by the US or Russia, except for 2 by China in 2003 and 2005, and 3 suborbital flights by Scaled Composites Inc. in 2004.

While I’m at it, let me throw out some of the other statistics I learned:

• Total number of manned flights: 289
• Total number of visitors to space (counting repeats): 1262
• Total number of individuals: 520
• Max number of flights: 7 by Jerry Ross and Franklin Chang-Diaz
• Max total time in space: 2.2 years by Sergei Krikalyov
• Number of people killed in space: 17 (Soyuz-1, Soyuz-11, Challenger, Columbia)
• Number of space tourists (people who paid their own way): 7 (Charles Simonyi has been twice, thanks to Microsoft stock)

So the full answer to “What is the population of space?” is “About five”, at least as of 2009.  Future growth will come from the full occupation of the ISS, which can handle up to 6 people at a time, and the launch next year of a small Chinese station, Tiangong 1.

The population of space has bounced up and down a lot, but the  growth since 1980 is about 4% per year.  At that rate we’ll have 100 million people in space in about 470 years, about the era of Star Trek.  That’s way beyond the the disasters we see now!  Space colonization will not save humanity.