A staple solution to these problems in science fiction is to simply put the void-travelers to bed for a while. In a sleep-like state akin to hibernation or torpor, metabolism drops, and the mind is spared the boredom of waiting out endless empty hours.
Unlike faster-than-light travel and wormholes, the premise of putting astronauts into a form of hibernation feels like it's within grasp. Enough so that even the European Space Agency is seriously looking into the science behind it.
Implications of a new study by a trio of researchers from Chile now reveal a mathematical hurdle to turning the potential of long-term human stasis into reality, one that might mean it's as forever beyond our reach.
Roberto F. Nespolo and Carlos Mejias from the Millennium Institute for Integrative Biology and Francisco Bozinovic from the Pontifical Catholic University of Chile set out to unravel the relationship between body mass and energy expenditure in animals that hibernate.
They discovered a minimum level of metabolism that allows cells to persist under cold, low-oxygen conditions. For relatively heavy animals like us, the energy savings we might expect from entering a deep, hibernation-like state would be negligible.
In fact, we'd probably be better off just napping our days away the old-fashioned way.
The word hibernation often invokes images of a bear tucked away in a den for a long winter's rest.
While bears do shut down for several long, cold months, their dormancy isn't quite like the true hibernation among smaller critters like ground squirrels and bats.
In these animals, body temperature plummets, metabolism shrinks, and heart rate and breathing slow. This process can reduce energy expenditure by as much as 98 percent in some cases, removing the need to waste effort hunting or foraging.
However, even in this state, the animal can still lose more than a quarter of its body weight as it burns through its fuel reserves.
If we applied the same basic mathematics to a hibernating adult human, a daily food intake of around 12,000 kilojoules would be replaced by a need for just a couple hundred kilojoules of body fat.
Keeping with this scenario, we might imagine our intrepid space tourist tucked up in their specially-kitted bed would lose just over six grams of fat a day. Over a year, this would add up to around two kilograms of weight.
This might be fine for a rapid journey to the Jovian moons, but if the average adult wants to survive decades floating through interstellar space to a nearby star, they'd need to pack on an additional few hundred kilograms of fat. That, or routinely wake to throw back a lard milkshake or three.
These back-of-the-envelope calculations rely on many assumptions, not least of which is how hibernation might scale. After all, there's probably a good reason behind the scarcity of massive hibernating mammals our size (or larger).
So the researchers carried out a statistical analysis across a variety of hibernating species, as detailed in previous studies.
From this, they concluded the daily energy expenditure of hibernating animals scales in a fairly balanced way, so a gram of tissue from a tiny mammal, like the 25-gram leaf-eared bat, consumes as much energy as a gram of tissue from an 820-gram hibernating ground squirrel.
We could assume that if we ever worked out how to hibernate as efficiently as a dormouse, every gram of our tissue would require the same energy as every gram of theirs.
It's a different story when mammals are active, however. The scaling of the relationship between active metabolism and mass produces a slightly different graph that reveals a point at which hibernating doesn't really save a great deal of energy for bigger beasts.
That point is near our own mass, implying our total energy needs while hibernating aren't going to be significantly different from those while we're merely at rest.
This could be why bears don't really hibernate in the same way smaller animals do. And it also means for us humans, going to all the risk and trouble of cooling our bodies, dropping our heart rate and breathing, and artificially depressing our metabolism just might not give us the results we'd hope for.
If we want to save our boredom and keep from munching through the ship's supply of freeze-dried ice cream, we might as well binge The Expanse, take a bunch of sedatives, and doze our way to Mars.
Forcing humans to hibernate just isn't going to be worth the hassle.
This research was published in Proceedings of the Royal Society B.