Looking up into the black void of space, humans have often wondered what is out there. As a species, we’ve been explorers from the start, spreading out into unoccupied territory in search of freedom or resources. Our curiosity drives us to look elsewhere, and when it comes to the stars, we’re no different. Recently, humans have been able to take our first steps into space, and some of us want to go farther than our doorstep. We have sent robots and probes out into the solar system, and hopefully soon, humanity will send people to another planet. Assuming that we keep exploring, eventually, we’ll look beyond the confines of our small star. Luckily, there exist numerous thought experiments of journey beyond our tiny blue dot in the genre of science fiction (SF). SF’s ideas are always huge, and as a genre, it ranges from extrapolation that adheres rigidly to the laws of physics as understood at the time to full on technological magic.

One of these huge ideas is a vehicle that obeys the laws of physics but is an ark sent out of the solar system on a voyage to find a new habitable planet. Since this ship cannot go faster than light and since distance in space is bigger than we can comprehend, this journey would take longer than one human lifetime. In fact, the voyage’s timeline would require at least three generations¹ for the shortest distance. There will be entire generations of humans that are born, live, and die inside this vehicle, not knowing the planet from which the ship launched, hence the name Generation Ship.

Let’s start with the basics. The generation ship is a giant, completely self-contained vehicle. It is larger than anything humans have ever created. Ever. The minimum estimate of passengers required to make the journey and be genetically diverse enough for the survival of the species ranges from 150 to 10,000 individuals.² That’s a minimum estimate. Based on engineering design principles, a factor of safety should be applied to that number to ensure success. For such a risky endeavor, that factor would be three to five times the minimum number. Assuming a net zero population growth on the journey – birth rate equal to number of deaths – the vehicle would have to be large enough to house 150 to 50,000 human beings for their entire lives. This number ranges between fifteen times the maximum astronauts on the STS-112 International Space Station³ to the average amount of people that visit the Mall of America in a twelve hour period.⁴

The International Space Station
Photo courtesy of NASA and used with Fair Use

Assuming the passenger estimates are correct and assuming a per person livable space of six foot by ten foot, which is slightly bigger than the average jail cell, the sleeping space would range in size from one and a half STS-122 International Space Stations⁵ to just over half the Mall of America.⁶ This space estimate doesn’t include working, eating, and recreational spaces. Assuming sixty square foot per person only for working space and assuming that eating and recreation can combine to sixty square feet, the size of the ship has just increased to five space stations all the way to two Malls of America, rounding up. Nor does that estimate consider the space needed for engines, fuel tanks, power plants, maintenance and transit pathways, armor, sensor arrays, various wiring and plumbing conduits, supply storage, weapons, and additional vehicles, like landing shuttles or exterior maintenance trams. Simply put, the generation ship needs to be huge.

Mall of America Interior
Photo courtesy of Explore Minnesota
Displayed under Fair Use

Since one hundred fifty people is the bare minimum with no safety factors, I think it’s safe to say more passengers are needed. Personally, the fifty thousand estimate seems a better choice to maximize success. This population is slightly more than the capacity of Busch Stadium, home of the St. Louis Cardinals. That doesn’t seem like much when you consider the size of cities. But remember that ships are enclosed spaces, i.e. buildings. A generation ship can be thought of as a giant apartment complex sailing through the void. It’s also a giant apartment complex that most of the passengers can/will never leave. While it doesn’t seem like much, imagine for a moment a life lived entirely within a baseball stadium, only there’s no sky above, just more metal.

This article went longer than I expected; so, it’s getting broken up into two parts. This first section deals with size requirements; the follow up deals with constraints placed upon the ship.

Click here for the list of articles for this series.

1. This is assuming an average lifespan of 75 to 80 years, i.e. no longevity technology and a relatively health journey. This is a decent estimate as in 2014, the average American lifespan was 78 years. It is safe to assume that by the time building a generation ship is possible, humans have found a way to expand the average lifespan across the planet to this range. Click here for the reference.

2. “How Many People Does It Take to Colonize Another Star System?”, Sarah Fecht, Popular Mechanics, 2 April 2014, http://www.popularmechanics.com/space/deep-space/a10369/how-many-people-does-it-take-to-colonize-another-star-system-16654747/

3. During a question and answer session, NASA said that as many as ten people can be on the station at one time. So for the minimum of 150 passengers, 15 STS-112’s would be needed. Now, this would be a very tight squeeze because the permanent crew on the space station is three members. Click for the reference.

4. According to the Mall of America fun facts, forty million people visit the mall every year. That means the mall hosts an average of one hundred thousand visitors per day. Malls are not known for large areas of personal space; so, the passengers on the high end of the estimate would still be very close and personal with the other passengers. Mall of America facts.

5. Per PBS, the livable volume of the space station is 43,000 cubic feet, which is roughly the average of three American homes. Also, the square footage is roughly 6,000 sq ft.

6. According to the Mall of America fun facts, the gross building area is five and a half million square feet