I was thinking of an organism that lives its entire life in interstellar space

  • It eats, defecates, moves, grows, ejects eggs, attracts mates, and has sex in interstellar space.
  • It has skin that pressurizes its insides so that its insides are pressurized even when the organism lives in the vacuum of space.
  • It has thrusters on its back that it can eject fuel out of to change its velocity.
  • It is capable of accelerating to 50% the speed of light relative to the rest of the galaxy in 3 months allowing it to cover astronomical distances in search of food and mates.
  • Its eyesight is so good that it can see an object the size of Saturn from as far as Earth with as much detail as a human seeing a baseball at a distance of 0.1m.
  • It has bioluminescence that allows it to be seen by potential mates.

What type of diet would be most efficient for this organism to have considering that it would have to get all its food from the vacuum of space?

Also: How might its digestive system evolve to process food that it finds and eats in interstellar space?

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    $\begingroup$ You know that "the vacuum of space" means "the absolute emptiness devoid of everything, including food, of space", right? Without making planet-fall every now and then, or living in extremely dense gas clusters, your creature is not viable at all if you expect it to be anything like you described it. It will expend its reserves of energy and matter fairly quickly and then have no way to replenish them. $\endgroup$
    – MichaelK
    May 13, 2016 at 5:29
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    $\begingroup$ You may underestimate how mindbogglingly hard it is to see things at the kinds of distances you are talking about. Something which can see a comet from a light year away, and track it, probably doesn't actually need to feed because its pure magic. $\endgroup$
    – Cort Ammon
    May 13, 2016 at 6:03
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    $\begingroup$ Obviously Astronaut Ice Cream, right... $\endgroup$
    – Aron
    May 13, 2016 at 10:22
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    $\begingroup$ Why not have it absorb sunlight every now and then? Similar to how a aquatic mammals surface for air once in a while, this creature could spend some time orbiting a star. This could be when it hibernates and gathers up reserves of energy. $\endgroup$ May 13, 2016 at 14:39
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    $\begingroup$ "It is capable of accelerating to 50% the speed of light relative to the rest of the galaxy in 3 months" holy cow, where does it get that kind of energy? There's nothing it could eat to have this kind of energy budget. $\endgroup$
    – JDługosz
    May 13, 2016 at 16:15

13 Answers 13


Such a creature would most likely dine on a diet of the finest pixie dust

Unfortunately, the creature you describe is so far from reality and the laws of physics that we are pretty much 100% in the realm of magic.

First off, let's talk eyesight. The metaphor of a baseball at 0.1 m isn't quite good enough. Let's take a look at the most likely food source, a comet. Comets themselves are not light sources. They get their light from a nearby star. A absolute-best-case-scenario for our creature is a comet or asteroid at 110 AU, which is right on the edge of interstellar space. Unfortunately for our poor creature, at that distance comets have no tail. They only get a tail when they get into the inner solar system, so they look like small asteroids at a distance.

There's a decent number of 100 km in diameter asteroids (larger than that, they're extremely rare). Assuming it was a sphere, that'd give it a frontal surface area of about 8×109 m2. At a distance of 110 AU, that covers 3×10−17 steradians. Doing the math with our own Sun's luminosity, that's about 9×108 watts of energy hitting the asteroid.

Now let's re-radiate that energy out, so that our creature can see it. You mention its ability to travel across the galaxy, which is thousands of light years wide. Let's give it an easy morsel: it's only 1 light year away (1016 m). We can do some more math here, to determine that the intensity of the light 1 light year away is 8×10−25 W/m2.

This is a tiny amount of light. Let's try to put a number on it. The most likely band to be looking at is the hydrogen gap, where hydrogen absorbs the least, 27 cm. At this frequency, a photon has about 10−24 J. This means, on average, a 1.2-square meter detector will see about 1 photon per second from our comet! This creature is going to have to be massive! So massive that eating comets is going to be a pretty paltry diet.

That's also assuming a very nice cool detector that can pick up those photons. It's going to have to be near a whopper of a thruster. A single hydrogen atom at 50% the speed of light had to be given 2.3×10−11 J of energy. Let's pretend the creature harnesses pure antimatter to store energy. That amount of energy would take the annihilation of 2.6×10−28 kg of matter and antimatter to produce. Coincidentally, a hydrogen atom is 1.6×10−27 kg, so basically for every 6 hydrogen atoms in your creature, you need a corresponding pair of hydrogen/anti-hydrogen atoms to be used as fuel. Given that you also want to slow down, this basically says the creature is physically impossible because its energy needs are too high, even given perfect hardware. (None of these "thrusters." You'd never make them efficient enough.)

The energy requirements of the bio luminescence is even more of a problem. And, of course, there's the issue that any light you see from a mate 10 light years away is now 10 years old.

Your best bet is to remove the insanely fast movement, ignore the eye sight, and do what Lostinfrance recommended: have it consume interstellar hydrogen. Of course, you'd still need to dip into planetary systems from time to time to get more materials besides hydrogen. There's not much else out there. I'd also make them live for a few billion years to solve the issue of finding a mate.

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    $\begingroup$ Let's not forget. If it had all these characteristics, it would STILL find it easier to land on a planet and eat that, one mouthful at a time. $\endgroup$
    – Aron
    May 13, 2016 at 10:27
  • $\begingroup$ There's a small problem with your math, which is that only a small amount of the Sun's luminosity is radiated as radio waves; the calculation would probably be more realistic if you used a visible-light photon (but, because they're higher-energy, the required detector surface goes up and reinforces your point). $\endgroup$ May 14, 2016 at 20:40
  • $\begingroup$ @2012rcampion True. I chose the best case scenario, using a band that minimized absorption, and assumed all solar output was absorbed as though it was in that band. $\endgroup$
    – Cort Ammon
    May 14, 2016 at 21:55

Give it the ability to generate a vast magnetic field, making it a living Bussard ramjet. Then it could scoop up and "eat" interstellar hydrogen. It wouldn't exactly have a digestive system, more a combustion chamber.

Unfortunately the poor thing might get hungry even so.

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    $\begingroup$ I think I've read about those somewhere. I think it's called a main sequence star. Unfortunately, wouldn't it's heliopause prevent it from eating? $\endgroup$
    – Aron
    May 13, 2016 at 10:40
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    $\begingroup$ @Aron, that shouldn't be a comment, it should be an answer. It has real star quality. $\endgroup$ May 13, 2016 at 10:42

Note: I am not saying this is realistic, just that this is how I could see it working. Well, this is as close to what your thinking about you can get IMO while not being totally impossible.

Perhaps your creature evolved to go into a dormant, VERY energy-conservative state known as hibernation while in interstellar space. It goes from star system to star system at some fraction of the speed of light by slingshotting itself off of multiple gas giants in each system (most systems have 1 or two of them it seems). Your creature would not be able to survive solely on junk available in interstellar space because it is to sparse.

Once it reaches the Kuiper belt of a system, though, it would have (depending on size) enough minerals to survive and would come out of hibernation.

Animal itself would likely just float around with minimal control as to the direction it is going (so as to conserve energy). This could happen by the animal evolving some sort of biological thruster system to propel it in the desired direction. I imagine that this animal would gain minerals from asteroids and comets and gain energy by absorbing radiation/sunlight in a process similar to photosynthesis. Otherwise it would absorb what was needed from passing comets and asteroids and dispose of the junk in the universal waste-disposing system: crapping.

You can look at a Baleen Whale to get an idea of its behavior patterns. Baleen whales essentially swim around with their mouths open, swallowing anything useful that come in (plankton, small fish) and getting rid of everything else.

  • $\begingroup$ Reminds me a little of Clarke's novel Rendezvous with Rama. $\endgroup$ May 15, 2016 at 13:03

tl;dr: You need an ocean in space.

As many other answers have stated, this isn't within the realm of known physics. Accelerating anything substantial to .5c takes a ridiculous amount of energy, let alone something that does so with the biological equivalent of rocket engines. Forget thrusters and go with something more exotic like pushing on the quantum virtual vacuum. Now on to your real question... What to eat.

Whatever this creature's method of locomotion, it's going to need massive amounts of energy. Unfortunately for our friendly neighborhood space whales, space is big[citation needed] and there's not much in it[dubious-discuss]. In order to harvest the amount of energy required, it's going to take an entire ecosystem.

This makes an intuitive sense when you think about it. It would be impossible for only one type of creature to have evolved on Earth, every part of the planet depends on every other part to produce its food in some way, shape or form. Plants absorb energy from the sun and concentrate it, herbivores eat those plants, carnivores eat those herbivores, then ultimately microorganisms eat the carnivores when they die and supply nutrients the plants need to keep harvesting sunlight (if you cite this explanation for any formal discussion of biology, you have only yourself to blame). So we need a similar ecosystem in space to support your space whales.

You could have swarms of plankton-like creatures that cluster around stars and soak up sunlight, and maybe others that feed on raw materials in planets (probably gas giants) and asteroids. Then perhaps a chain of other creatures eat those and concentrate their energy, and these whales are the apex predator of the system (as @oberron mentioned). In fact, it's probably better to think of these as space sharks instead of whales, because no whale on Earth is capable of accelerating its massive bulk very quickly (that would take more energy than they're capable of harvesting from plankton alone). I'd also save the high-end acceleration for migratory purposes, when food runs low in one solar system they need to boost on over to another, risking their energy reserves on the bet that there's more food there. Once accelerated they could go into a sort of hibernation where they turn off most body functions until they get where they're going or sense food somewhere, and just coast along in the meantime.

The moral of the story is there isn't enough energy just floating around in space as we know it to make this possible, so you're going to have to flesh out your ecosystem to make it work in some plausible way. Time to create more creatures!


Let me start by looking at the rockets.

If it's capable of reaching a speed of c/2 - and then slowing back down so as not to have a very 'impacting' arrival, it needs to have a delta-V of c, which requires its fuel to be expelled at light speed. (Where does that energy come from?) And even then the half lightspeed feat will require ejecting exactly 100% of its mass (at light speed) by the time it has slowed down.

If you tone it down to reasonable levels, the key point remains energy management. All non-extinct forms of life do their energy management well. This thing is going to require immense levels of energy just for propulsion. So after you tone it down sufficiently, you're going to be looking at something that eats stars. Maybe not whole (how big are they supposed to be?), but the delicious stuff would be well below the surface. (Do they sun-dive for their food? See also the huge velocities things would have as they impact the sun, just from the sun's gravity.)

This brings thermodynamics into the picture, as they will need to tame the high heat levels of not just the dive/eating but keeping and digesting their food. Who needs bioluminescence anyway?

Alternatively it would need to eat through huge amounts of matter to extract fissionables. Or have a magic matter-to-energy conversion thing that would be quite desired by all intelligent space-faring species, to the point of hunting them whenever they were seen, unless they learned to make their own.

Also, why does it need that pressurization? It'll need a membrane sturdy enough to survive high-velocity dust and keep things from falling off. But beyond that, the pressure fluid is probably useless extra mass that will eat into its energy budget.

  • $\begingroup$ Stars offer all the energy in the void of space. Any truly stellar creature or civilization will learn how to feed off them. My guess is that this thing will just charge up like a plant/solar-panel. $\endgroup$
    – LukeN
    May 13, 2016 at 21:21

What if it's made wholly of antimatter? Its food source of regular matter would therefore be quite tasty indeed (and extremely energetic) provide it had a means to properly ingest it (some sort of magnetics) so it doesn't simply collide and burn. Its food? (plot twist time) the unfortunate craft of all the space-faring species' who discover a strange, moving object in interstellar space and send a big rocket to investigate.


This is a tuffy, Let's ditch most of the science and hit specific problems.

it eats

It can't eat in the normal way. There is just nothing to eat. Photosynthesis gives us a decent model, but stars are really far a part, a space turnip isn't going to make it much further from the sun then earth is. So let's look at what is more "available" then light. Gamma Radiation, X-Ray Radiation, and the like. So basically "Radiation" is a food source. It could work, but it wouldn't give very much energy.

It defecates I don't see this happening. It would have to "use" almost 100% of the energy that it collects. See the section on thrusters though.

It moves Well, ok, so long as it doesn't move much, sure why not. If it moves too much you have an energy problem again. I would say it would be ok to "spasm" every once in a long while.

It grows Sure, why not. The problem is growing takes more energy then maintaining. With such a rare food intake, that precious energy probably can't go too much to growing.

It ejects eggs, attracts mates, and has sex in interstellar space Unless your writing space whale porn, let this one go. For all known lifeforms, sexual reproduction is a energy intensive task. Even plants, and they let someone else do all the work. Super abstract, all the energy of making an egg and a sperm, then finding a mate, sharing it, and hatching it, that's quite a bit of waste. Have them produce a-sexually. It's "easier". Once they reach a certain size, they split. In this case the "mother" split would need to die in 90% of the cases. Only if it was really lucky would the mother bit get to live.

It has skin that pressurizes its insides so that its insides are pressurized even when the organism lives in the vacuum of space. Why, There is no need for this. In this odd lifeforms, it's insides don't need air-pressure. It can evolve ways around that.

It has thrusters on its back that it can eject fuel out of to change its velocity. This is another odd one. I think you might be better off with "farts to change direction" There is not a lot in space, so where are you going to get this fuel. Assuming that this thing eats "radiation" it's gonna need a lot of it. You can get some propulsion by "emitting radiation" but not as much as your going to need. Look to satellites' magnetic engines for your best, currently known bet, and do something like that.

It is capable of accelerating to 50% the speed of light relative to the rest of the galaxy in 3 months allowing it to cover astronomical distances in search of food and mates. First, that's horribly slow. That's 8 years between star systems. The good news is, that as long as you don't care about slowing down, I suppose it's doable. It can speed up really really slowly, absorbing the radiation of a star as it gets closer, smashing into it, destroying it, and absorbing some matter in the process. Then fart propel it's self in a minor coarse correction, leaving behind a decimated ruin or a solar system.

Its eyesight is so good that it can see an object the size of Saturn from as far as Earth as a human seeing a baseball at a distance of 0.1m. Yeah, that's pretty crappy on interstellar terms. Using the normal analogy of Sol to Alpha century when both are the size of grapefruit, you would need to see that grapefruit from across the US. The good news is that because it eats "radiation" it doesn't have to "see" just has to point it's self at the most tasty radiation it can. And because it only has minor fart propulsion, it's not going to get a bunch of choices.

It has bioluminescence that allows it to be seen by potential mates. Unless it can light up brighter then a star, no it doesn't. It simply won't work. You can make it shiny though as a side effect of absorbing the "radiation".

What your left with:

You would have a MASSIVE lifeform, that basically, breaks wind in a single direction, hoping to find enough "Radiation" to sustain it's self. It wouldn't be very smart, it doesn't have the energy for that. Every 200 billion years or so it would clone it's self, without slowing down mind you, let out a massive fart (of radiation) and change direction by 0.0000001 degrees.

The creature would need to live near forever, it would be HUGE and would wipe out any solar system it came near.

I would not want to run into one of these.

  • $\begingroup$ Fart propulsion, I love it. What's the delta-v per bean? $\endgroup$
    – Jens
    May 15, 2016 at 19:46
  • $\begingroup$ Well remember, it's eating "radiation", so it would expel some byproduct of that process. Maybe a different type of radiation. And because of this things huge size, it would need a massive amount of thrust, sustained over a long period of time. A sustainable 1 m/s delta-v would be plenty over a long enough period of time. That's still a lot of thrust though. $\endgroup$
    – coteyr
    May 15, 2016 at 21:21
  • $\begingroup$ no, no, no, if it eats radiation, and SWALLOWS hydrogen... it can light it's farts. $\endgroup$
    – Seeds
    May 27, 2016 at 20:35

Beyond the 50% of light speed aspect, if any advanced life form was living in space it would be an apex predator for space life.

As mentioned in comments food in space would be scarce but if we consider panspermia theory there is some organic material in space which could be used by an equivalent space-plancton as food source which in turn would be the base food for other forms of space life.

Similar to whales in our oceans size would probably be key to maximise chances of capturing such space-plancton.


Your creature may consist of dark matter and obtain it's energy (food) from dark energy. IMHO it is more reasonable this way, since space is full of both dark matter and dark energy.

Estimated percentages for matter, dark matter and dark energy


The OP did not ask whether such creatures were feasible.

For purposes of this discussion I assume that "eating" provides two benefits: Eating provides raw materials used to further growth, procreation, and other activities, and eating also provides a source of energy to power those activities.

Given that such creatures exist in the ficton*(see below) and that they eat, there are a few internally consistent possibilities (not mutually exclusive).

  1. They eat other examples of the same "species" (cannibalism).
  2. They eat other creatures similar but not identical to themselves, which you must add to the ecosystem in your ficton.
  3. They eat other creatures that are much more primitive than themselves, which you must add to your ecosystem in your ficton.
  4. They eat non-creature organic material (e.g., plant life) which you must add to your ecosystem.
  5. They eat non-organic material and derive energy from subsequent chemical or other molecular or submolecular interactions.
  6. They gain energy from some source other than eating: electromagnetic radiation, gravitational fields, etc., and they take in raw materials via some other method that might be described as eating but would include such activities as absorption of (admittedly thin) interstellar gas.

EDIT: A "ficton" is "A fictional universe. A place or environment created for the purpose of fictional storytelling."



If you're ruling out eating planets, then what's left is asteroids. See this link for how dense the asteroid belt isn't. That's going to be one very hungry organism


It can 'eat' energy coming from stars (absorbing some spectrum of radiation). For this to work, you need to assume it has efficient energy/matter conversion. It can go close to suns when it needs to charge up, then use stored energy to travel between stars.


The creature can drink water and eat close to the usual food: there are various organic carbon molecules, as well as water, in the interstellar space. They are even called "seeds of life", as may be the origin of life in the Earth. Light that powers many organisms in Earth is also available in space, even interstellar (we see light coming from other stars).

The concentration, however, is extremely low, and some really mystical capabilities are required to concentrate the reasonable amounts. Maybe could dwell near black holes or the forming stars where the gravity of these bodies could concentrate the food.


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