I recently came across a game called dead space and a certain main antagonist species inside of the game. In this game you play as the main character who is trying to protect humanity from sentient race of beings called the the brethren moons.

These are creatures that can reach a massive size of 2000 miles. They live in the void of space feeding off of intelligent species that become over populated throughout the galaxy.

Ignoring the evolution aspect of the creatures and where they would of originated from. What I want to know is if such a creature that size could even survive and reproduce, and what would happen to the core of the creature if it was that size.

Almost certainly not.

But that's not much fun, so let's try and think of ways to make it even remotely plausible:

First, the creature could use an actual moon or large asteroid as a core; i.e. the biological portion is only a relatively thin (though still stupendously massive) layer on top. This would dramatically cut down on the energy required to reproduce, and give it a structural core that it didn't need to grow itself.

The host planetoid could be the source of most the raw materials for the actual creature. It could also serve as a giant heat sink/source to help stabilize its temperature. The creature could dump heat into its core when energy is plentiful (when near a star, or feeding,) and draw upon it when it's scarce (i.e. in deep space.)

How it moves is going to be a problem. Barring science fiction Warps and such, the only remotely plausible (still nuts, note) method I can think of is something like a planetary-scale ion drive, where it continuously accelerates and ejects a small amount of core mass at extremely high velocity. Very little thrust, but excellent efficiency. It would also take advantage of gravitational slingshots as much as it can (being intelligent would be very helpful here.)

All of this means that the sucker is going to be very slow. You can model it pretty well from the more realistic generational space ship ideas. Hundreds if not thousands of years between target systems, during which it will need to be in a basically-dead hibernation state ("In his house at R'lyeh dead Cthulhu waits dreaming...")

Then of course there's the issue of how & why it feeds on intelligent species... In any realistic scenario it would be much easier and filling to just feed on stellar radiation. You'd pretty much have to have a "psychic energy" angle in your world to make it plausible. Which given the Lovecraftian nature of the critter you may well have anyway.

Not in real life, no.

1. A 2000 mile diameter organism would be between the size of mercury and mars. This organism would crush itself under its own gravity and die.


2. Keeping that much organic material alive and warm in interstellar space is very costly from a caloric intake perspective. Devouring an entire civilization would not provide enough calories to keep it warm and alive.


3. Movement, moving something this size requires an enormous amount of energy, which it can only get from devouring an intelligent species, but it would have to use much more energy to get itself from solar system to solar system, and it may or may not find an overpopulated species in a system it visits.

There are surely other reasons why this would not be possible, but it mostly can't be reconciled due to the energy requirements of such an organism. This guy really needs some sort of ship with a handwavium power source in order to do what it wants.

Just to add.

• Energy

Your creature is close in size to Earth's moon and lets take the same for mass.

Just on fast calc for energy needed to move (hope I did not mess up), with perfect antimatter-matter conversion: To move in or out would take around 350 million tonnes of humans(???) And you need to go in and out and have food for your body, savings, extra movements. I would say, it would need minimum of 1 billion tonnes from each planet it visits. For note, 1 kg is ~43 megatons, not so far from Tsar Bomba.

All humans on Earth would be around 500 million tonnes so not even worth a trip fuel. But you can eat some trees for salad and go find something more fleshy.

• Pressure and heat

Pressure and heat inside such creature would be massive. The temperature in the core of Moon is probably about 1600–1700 K. There is no way any life, with no magic involved, that would live with that.

But there is solution! Creature is only some 50 km thick and center is close to hollow or has real moon inside, taken as home by creature.

Close to hollow structure would lower energy needs and make viable more real ways-to-get and manage energy.