Science Fiction & Fantasy Stack Exchange is a question and answer site for science fiction and fantasy enthusiasts. It only takes a minute to sign up.
Sign up to join this community
In WALL•E, it's established that the 5-year spaceship has been keeping people alive for 700 years. Also, it's clear that (at least before they all left the Earth), the garbage problem was never solved. We even see the Axiom itself keeps throwing garbage to outer space with the help of the WALL•As.
So, if they keep constantly throwing and throwing garbage, how can they keep living? What's all their food made of?
The short answer is that the ship is quite literally cannibalising itself, along with the passengers.
We know from the "All Aboard the Axiom" special feature that the ship's population was originally around 600,000 however by the time of the Wall•E film, there appears to be dramatically fewer passengers on board the ship.
This is confirmed in a couple of graphics from the "History of BNL" featurette (from the Wall-E bluray) which show the before/after populations (in binary), starting at around 700,000 but declining to less than 10,000 in the second panel
This is also backed up by the 'Get to know the bots' featurette which refers to the ship's "thousands of passengers" (e.g. rather than "hundreds of thousands") and the image of the lido deck where all of the (approximately 8-10000) passengers have gathered when the alarm sounds.
As to what they're eating, the film's script mentions that one of the Captain's duties is to maintain the "regenerative food buffet". It seems likely that in the absence of the fresh food, Auto has tasked the ship with liquefying the dead and feeding them to each successive generation. This would also explain the fall in population since the nutritive value would decline each time a corpse is recycled.
The same applies to the machinery used to maintain the ship. With a heavily declining population, robots that are now surplus to requirement can be disassembled and dumped overboard. Note that all of the "non-recyclables" seen in the disposal bay are pieces of metal or pieces of broken robots rather than any organic or plastic waste
Richard's soylent green explanation covers everything really well -- I hadn't even noticed that little throwaway frame with the projected population decline, and I'm perfectly happy with this theory in its entirety. That puts the population decline at ~842 every year -- just another century would have dropped the population to nearly 2,000 passengers! (I'm sure the math is much more complicated than I'm making it out to be, but even simplified that's quite impressive.)
I did want to contribute a few additional thoughts, though.
I've been thinking for a while that it was strange for the AXIOM to be called a flagship of a fleet of starliners, yet throughout the course of the movie, the credits, and the additional DVD and Blu-Ray commentary we hardly see these ships, and only ever from footage taken on Earth before and during the mass exodus.
I've read around online that the other ships went to other locations in the Kuiper Belt and to other extra-solar sights; however, not all of these vessels were Executive Starliners, and considering the height of luxury advertised on BNL commercials I'm betting the other ships weren't as nice, and wouldn't have lasted as long in space without guidance from BNL's CEO.
So maybe the AXIOM 1) cannibalized the passengers and resources from other ships in the fleet over the course of 700 years, and/or 2) routinely traces the last known locations of offline ships to salvage bodies and resources to sustain its own crew and passengers? Traveling long distances isn't a problem, obviously, if it goes from the Kuiper Belt to Earth in fifteen seconds or so -- especially not if it's siphoning fuel from dead ships.
