Your Turn, Round 2-2: Life In Vacuum
For the second alien species in the Fermi Problems setting, I decided to go with another take on the “Where are they?” line. Fermi’s argument for a spacefaring civilization spreading across the galaxy in only a few million years depends on a long period of nearly exponential growth and on a travel speed that is a significant fraction of the speed of light. My friend Jacob Haqq-Misra has worked out some limitations on the rapid growth of civilizations, but I decided to design aliens who could not travel at more than 0.001 c. This requires two things: a species that cannot handle high acceleration, and a technology base that does not allow them to either engineer themselves or robotic emissaries to accelerate faster or to build a rocket that can continuously accelerate for a very long time.
The latter is a rather severe limitation. Accelerating at 0.03 m/s^2, a spacecraft would still reach 0.003 c after 1 year and cover ~8 lightyears in a century if the acceleration was sustained. The later is unlikely for something internally powered: fusion rockets are limited to maybe 0.1 c peak speed, given the requirement to stop at the destination. Even a Bussard ramjet is limited to 0.119 c by the exhaust velocity of the fusion rocket. But I have set a speed limit of 0.001 c or 300 km/s. Even a fission-reactor-powered ion engine could in theory reach or somewhat exceed that speed. So, I needed to design a species that cannot handle high acceleration and can travel interstellar distances with nothing fancier than a chemical rocket. Taking some liberties with biochemistry, I propose a biosphere that evolved on small asteroids in the nominal habitable zone of a binary made of two M-dwarf stars. I picked CM Draconis because there is a lot of data available on it, it doesn’t have any large planets in the habitable zone, and it isn’t too close to Earth. Once again, I gave the fictional version of it a name in addition to the catalog listing: Druk. It happens that CM Draconis forms a hierarchical triple system with a white dwarf, GJ 630.1B, several hundred AU away. Continuing the theme, call it Wyvern.
I fully confess that the following is far more improbable than the ursians are, because the reservoirs for biochemistry to develop in are far smaller if for no other reason. With that caveat, I ask for any reasons why it is impossible, rather than merely incredibly improbable (levels of likelihood comparable to large-scale quantum tunneling may be counted as impossible). The improbable makes for good science fiction; the impossible is not allowed.
Consider a water- and carbon-rich asteroid, between a few hundred meters and a few tens of kilometers across. The particular object we are discussing doesn’t exist anymore, but it was like billions of others: a rubble-pile of variously-sized grains. Fine-grained material accumulates in dust ponds on such objects, and on a carbonaceous-chondrite object those grains will be rich in a slew of various relatively complex carbon compounds. This much is well-established.
The fictional assumption that I invoked was for there to be enough chemistry going on in those dust ponds (using monolayers of water molecules on the grain surfaces?) to produce small closed membranes durable enough to keep their contents separated from the surrounding grains and protected from being shaken off a grain into the surrounding vacuum while still being permeable enough to allow new molecules in. From there, chemistry proceeds to something that is self-catalyzing and rather like a protocell – similar to the bubble model of abiogenesis.
Given this admittedly out-there assumption, I can invoke evolutionary processes to expand this strange biosphere. Cells with vacuum-tight walls, high radiation tolerance, and specialized molecule-sized valves survive better; on the surfaces of the ponds as well as beneath them and inside ejected meteoroids. Photosynthesis takes over from feeding on primordial and cosmic-ray-seeded high-energy compounds. Heterotrophs feed on the plants and on each other; larger and better-armored multicellular plants enjoy a survival advantage and larger animals evolve to feed on them. Eventually, three-dimensional forests grow out from small cores – collecting a hundred times as much light as compared to if they only coated the rock. I was inspired to this design by some speculation by Freeman Dyson and possibly by some of the pictures in Le Petit Prince. Conveniently, these forests will be incredibly hard to observe over interstellar distances: they are dark in the visible, and infrared telescopes don’t have the same resolution as optical or radio arrays. Even if they are seen, the forests will be confused for boring dead carbonaceous asteroids until and unless their orbits are known well enough to measure their masses from mutual perturbations – showing how low their densities are.
The first rock was destroyed in a collision within a few hundred million years of the first appearance of life, scattering microbes across the system in the debris. Objects within the entire habitable zone have things growing on them; going too far out from the star causes un-insulated cells to freeze. On a timescale of a hundred million years, each individual forest is destroyed. On a much much shorter timescale, the plants break off rocket-propelled seed pods and solar-sail leaves that search for other objects using chemical-imaging tracers (the sail leaf idea was used by Larry Niven in the form of the ‘sail seed’). The animals that don’t have sufficient hibernation and delta-v abilities of their own hitch rides.
And, because I said there would be one, an intelligent species of animal evolved in one of the larger forests and then spread. I called them the neari – the etymology is a bit strained, but I hope it sounds cool enough.
Anatomy and Psychology
There is gravity, even on the smallest asteroids, although it can be effectively canceled for fast-spinning objects. The neari can handle continuous loads of ~0.03 m/s^2 without having too many problems, and can do brief jerks up to several gees. But they don’t have a physiologically preferred up and down or left-and-right, since the gravitational pull is a very gradual change as they navigate through the forests. So I designed a body plan for them inspired by sea urchins, crabs, and T4-bacteriophage: an icosahedral central body with limbs (equally validly called ‘arms’ or ‘legs’) at the vertices. The facets have specialized functions, but the neari are equally capable from all directions:
Four facets contain mouths, with airlock throats leading into a central stomach (the mouths are also used to excrete waste). Four are optical eyes, based around fish-eye lenses. Four have lines of infrared-sensitive pits. Four are blank on the surface, and underneath them are the neari’s central brain lobes. And four have organs that work only in near-vacuum: nose-eyes. This is an idea from Hal Clement: in vacuum, molecules travel long distances without collisions. So a pinhole camera can make an image in molecules as easily as in light, as long as whatever it is looking at is putting out any detectable number of particles. Neari exoskeletons and plant leaves smell only faintly. A comet outgassing is detectable tens of thousands of kilometers away – far further along its tail. An simpler version of this organ is used by the rocket pods and sail leaves to home in on objects.
The neari grow by molting and shedding their exoskeleton, a process which also allows the regeneration of limbs. Molting grows less efficient with time, and eventually accumulated damage leads to death – although prompt injuries will kill a neari far more quickly. The other way neari die is by giving birth. To solve the problem of exchanging genetic material and maintaining a pressurized environment for the embryonic neari to grow in, two neari (of any two of the three different sexes) join along the blank faces and slowly fuse together. The embryonic neari grow inside the merged exoskeleton, fed by their parents’ bodies.
These biological differences led to some important psychological differences between neari and humans (or ursians). Neari never know their parents – they are raised by their aunts/uncles and cousins. The individual is generally seen as less important compared to the family than it is for us. And sex is not a recreational activity. I have not fully explored how such different psychology will be reflected in neari culture. There is one other relevant psychological trait, from the hazard of being stuck in space just out of reach of a claw-hold: neari get very anxious if they aren’t holding onto something.
Culture, Technology, and Starships
The neari culture at Druk is in many senses less fragmented than that of the ursians, even though they’re spread out over cubic astronomical units. As of 70,000 BCE, you might have been tempted to call them a bronze-age culture. But there is no such thing as technological levels – real technological developments are spurred by the environment, the available resources and knowledge, and random moments of inspiration. The neari may have not have any metal other than nickel-iron, but they formed it into knives and hooks and parabolic focusing mirrors. Reflecting telescopes and heliographs gave light-speed communication across all of Druk, at least at telegraph-equivalent bandwidth. Writing preserved information. The neari had chemical rocketry, using rocket pods, and solar sails, made by cultivating and then trimming the leaves of sail plants. Travel between the forests, while slow and mass-limited, was inexpensive, and the neari understood Newtonian mechanics, radiation pressure, and numerical integration.
That said, the forests were isolated enough from each other that they retained very different cultures. Wars were not unknown, although they were limited in scope by the very low population density. Enough cooperation between forests did happen to organize large scale projects, including starships.
Druk has two red dwarf stars very close to each other, with an orbital velocity of ~75 km/s. With a little extra help from the super-jovian gas giant (>5 AU from the stars) and Wyvern (~500 AU), a careful gravitational slingshot maneuver past both stars can give an escaping interstellar spacecraft a velocity of ~300 km/s. And the neari already have things that can be turned into starships: comets (I got this idea from Greg Benford and David Brin). Several cubic kilometers of comet ice/organics contain ~3e16 J of usable chemical energy, enough to support 100 neari and their life-support system for tens of thousands of years. Select a comet about to make a close flyby of the stars, trim its orbit with high-speed impactors so that it gets ejected at high speed, jump on board before the flyby, afterwards adjust your trajectory using the gas giant or the white dwarf, and then burrow down under the surface for the long ride.
The problem with these comet boats is that they can only be targeted onto a certain range of trajectories, only a small fraction of comets are suitable (e.g. the comet has to be big enough to survive the stellar flybys), and the accuracy of the navigation can only be so good. The comets jet out gas unpredictably at perihelion, and even if the comet boat’s trajectory was known perfectly that of the destination star isn’t. The neari can measure parallax and proper motion accurately, but radial velocity is only approximate. A 0.1 km/s error at launch would be 1000 AU at arrival, and the comet boat can’t change its velocity by too much more than that en route.
For purposes of the setting:
The neari did start to spread across interstellar space ~70000 years ago – launching comet boats every few thousand years. But only one of them got close enough to its target star for the neari then on board to ditch the comet and stop. The others are either confirmed to be dead, still flying through the void, or unknown in status because they stopped replying to heliograph calls centuries ago. So Druk and Wyvern (the latter has a small debris disc out to a few hundredths of an AU from the star that the neari have settled) are still the main place to find the neari and there they are hard to see from parsecs away.
Does all of this work for a species that can travel over interstellar distances without being readily detectable, while simultaneously not spreading so fast that we should expect them to be here already?