Interstellar travel: The smallest things matter the most

We are taking a look back at stories from Cosmos Magazine in print. This article appeared in December 2019.

Two
overarching questions confront would-be human spacefarers: where to go and how to
get there.

Much
attention has been given to the latter question. For interstellar travel to
become a reality, major engineering advances are required, probably involving
radically new propulsion systems. Many proposals are highly speculative, but we
know of no fundamental physical principles that forbid interstellar travel;
whether or not it becomes a reality boils down to technology, cost and
motivation.

I wish to
address the oft-neglected first question: the destination. Leaving aside
fantastical speculations about faster-than-light travel, it is clear that
journeying between the stars will take a very long time, even on the most
optimistic estimates of technological advance. Therefore, interstellar tourism,
or trade in physical substances (as opposed to information), is inconceivable.

Travel beyond the Solar System will be one-way only. Two possibilities then arise: that spacefarers will seek out and colonise other worlds, or that they will create permanent artificial habitats in space. Both scenarios have been popular in science fiction. I’m going to leave aside the vast engineering issues involved and dwell instead on a much trickier and more basic problem – the ecological requirements, especially those relating to microbiology.

Long-term
human survival means more than growing enough food to eat and making enough oxygen
to breathe. It demands creating a complete self-sustaining ecosystem. On Earth,
complex multicellular organisms (e.g. animals, plants) form merely the
conspicuous tip of a vast biological iceberg, the majority of which is
microbial.

Almost all
terrestrial species are microbes – bacteria, archaea and unicellular eukaryotes
– and to date microbiologists have scratched only the surface of the microbial
realm.

Microbes are everywhere – in the soil, in the air, in water, in the rocks beneath our feet, in the Earth’s crust to a depth of several kilometres. These busy little creatures are a vital part of the life-support system of our planet, both via their metabolic activity (such as recycling material) and through the exchange of genetic components. Even within your own body, microbes play a crucial role.

The
microbial inhabitants of your gut, lungs, etc – known as your microbiome –
outnumber your own cells. Without them you would die. So astronauts cannot be
sent to the stars without, at the very least, their own microbiomes.

It’s not just bacteria – a fraction of space worms came back with two heads.

But it
doesn’t stop there. Microbes do not live in isolation; they form a vast network
of biological interactions that remains very ill-understood. The basic
Darwinian process – replication with variation plus natural selection – is now
recognised as an incomplete account of evolution. Darwinism can be regarded as
the vertical transfer of information (from one generation to the next), but
there is also much horizontal information flow, via gene transfer, cell-cell signalling,
collective organisation of cells and much else.

Interwoven
into this network are the activities of viruses, which infect microbes just as
they do larger organisms. The subtle interplay of viruses, microbes and metazoa
constitutes an ecological web of such staggering complexity that scientists have
hardly begun to unveil it. The daunting nature of the problem may be glimpsed
from the work of my Arizona State University (ASU) colleagues Hyunju Kim and
Harrison Smith, who compiled data from over 28,000 genomes and 8658 biochemical
reactions to create a map of information flow taking place, not just in
localised ecosystems, but on a planetary scale. The biosphere, it seems, is the
original World Wide Web.

Given that we can’t send the entire biosphere to another world, a fundamental problem arises: what is the minimum complexity of an ecosystem necessary for long-term sustainability? At what point, as more and more microbial species are dropped from the inventory of interstellar passengers, does the remaining ecosystem become unstable and collapse? Which microbes are crucial and which would be irrelevant passengers, as far as humans (and their animal and plant food supply chain) are concerned?

This is a
Noah’s Ark conundrum with a vengeance: which species get chosen to go? Not only
have we no clue as to the answer, we have little idea of the solution to a much
simpler problem: identifying the smallest self-sustaining purely microbial
ecosystem.

Can we
pull the web of life to bits, extract a tiny subset of it, and expect such
mini-webs to function forever in isolation? Any plan to terraform a planet ahead
of human colonisation cannot proceed without a far deeper understanding of
microbial ecology.

Imagine
making a list of the minimum number of plants and animals needed to accompany
humans on a one-way mission, and leave aside the logistics of growing, feeding
and breeding all these organisms in space, many of which may require
environmental conditions (temperature, pH, oxygen levels, etc.) very different
from those congenial to humans.

We might think of cows, pigs, sheep, chickens, some fish, a few vegetables – that would do for a start. But how many, and which, microbial species do these animals and plants depend on? How many, and which, other microbes do those animal-and plant- servicing microbes depend on? Which of these might be pathogenic to humans, yet vital for some other part of the ecosystem? Without a full understanding of the principles of the networks involved, how can we be sure that we have done our ecological accounting exercise correctly?

It would
be no good getting half-way to Alpha Centauri only to find that a key bacterium
was overlooked and left back on Earth.

An added
complication is that the activities of microbes depend on which genes they
express (i.e. switch on). My ASU colleague Cheryl Nickerson found that bacteria
can change their gene expression in zero g, and is working with NASA to study changes
in astronaut microbiomes when they go into orbit. Of concern is whether a
relatively benign bacterium might turn into a toxin in space.

And it’s
not just bacteria that change under space conditions. Michel Levin’s lab at
Tufts University experimented with planaria worms that had flown on the space
station. Planaria can regenerate both head and tail if chopped up. Levin found
that a fraction of the space worms came back with two heads.

Some of these difficulties might be mitigated by biotechnology. The visionary physicist and futurist Freeman Dyson has articulated a hope that we may eventually map the genome of the entire biosphere, then use the colossal computing power envisaged to be available in future decades to make sense of it all. We might then be able to design an ecosystem customised to a target planet.

I am far
less confident, however, that the behaviour of an ecosystem can be captured in this
manner by mere number-crunching. Even if it was, the supercomputer may tell us
that there is no solution at all that matches the physical environment of our
intended destination. Or it may specify that tens of millions of species are
required. And that is not all. Because biological evolution involves a large
element of chance, a transplanted ecosystem will not endure indefinitely as
originally designed, but may evolve in ways incompatible with human habitation,
requiring complex “mid-course corrections” entailing planet-wide bioengineering.

In my
view, the best hope lies not with assembling an inventory of genes, but with
our discovering the underlying laws and principles that govern the flow and
organisation of information in living systems – what we might call “the software
of life”. I believe that there are universal informational patterns or motifs
in biology, which would be hallmarks of life whatever its chemical basis. If we
understand the properties of these patterns and how they change with time, or
how they can become disrupted, we might be able to create a transplantable
ecosystem small enough to be transported off Earth and robust enough to withstand
space conditions.

Rather
like software engineers can design a computer game without mapping a computer’s
circuitry, biological software engineers might be able to reprogram the
organisation and management of information in terrestrial ecosystems without unravelling
all the genetic details, and produce a system suitable for “playing” on another
world.

Suppose
the solutions for a sustainable miniecosystem are indeed one day worked out,
and a mighty one-way mission departs for the stars, destination: a planet many
light years away, where the spacefarers or, more likely, their very distant descendants,
will make a new home. Astronomers are now fairly certain that the Milky Way
contains millions, possibly billions, of Earthlike planets (depending a bit on
your definition of Earthlike), so there’s plenty of real estate to choose from.

In many science fiction stories, the heroic adventurers touch down and step out onto an equable and verdant planet, hosting a rich indigenous biosphere, though preferably not a hostile civilisation, and take up joyful residence. Unfortunately it’s not that simple. There is a vanishing chance that the neatly-excised and self-sufficient truncated terrestrial micro-ecology would peacefully co-exist alongside the (presumably more extensive) alien equivalent, and proceed to carry on business as usual.

But this
problem highlights a much deeper and more substantive obstacle to human
colonisation of other planets, which is the very existence or otherwise of
indigenous life.

Many
fictional scenarios envisage humans in search of a planet with abundant life to
take care of the colonists’ needs thereafter. An ideal world for human
colonisation is one with oxygen to breathe and edible indigenous plants and
animals. But this vision flies in the face of basic biology.

Organic
matter is edible only when its biochemistry closely matches that of the
consumer. Even on Earth, the vast majority of organisms are not suitable for
human consumption. There is no reason to suppose that terrestrial biochemistry,
which is highly specific to both the conditions on our planet and the accidents
of evolutionary history, is universal.

It is easy
to imagine carbon-based life on other worlds using different amino acids,
different informational molecules, different membrane molecules, and so forth.
It is also easy to imagine a mirror world in which familiar organic molecules are
replaced by their mirror images (e.g. righthanded amino acids instead of
left-handed). It is highly likely that this alien foodstuff would be unpalatable
and indigestible. (The same reasoning makes nonsense of the whimsical
suggestions that aliens coming to Earth might choose to eat humans.) Worse
still, the indigenous biota would serve as a barrier to the establishment of a
secondary transplanted terrestrial ecosystem.

There is,
however, a flip side to the biological incompatibility problem. An alien
biochemistry that offers little scope for consumption also poses little threat
for infection. Alien microbes and viruses (if they exist) would probably be
unable to invade terrestrial organisms, or to make much progress if they did.
And vice versa. Wells’ “happy ending” to the War of the Worlds, in which
the Martians succumb to terrestrial germs, is simply not credible.

The
foregoing issues would disappear if the host planet had no indigenous life;
that is, if it was habitable but uninhabited – terra nullius on a planetary
scale. Unfortunately, this scenario has its own difficulties, one of which is
crucial to human survival: oxygen.

Oxygen is
a very reactive element, and does not endure for long in planetary atmospheres
unless it’s replenished. A planet with breathable atmospheric oxygen implies
the presence of photosynthetic plants, or at least microbes. (An important
project in astrobiology is the construction of a space-based optical system
capable of detecting the spectral signature of oxygen in the atmospheres of
extra-solar planets as a surrogate for detecting life.) If there is no life on
the destination planet, then it very probably would not have breathable amounts
of free oxygen in the atmosphere. Nevertheless, setting up home on a previously
sterile planet, and breathing manufactured oxygen, would be far easier than
coping with an indigenous biosphere.

Quite
apart from the practicalities of colonising another planet, there are serious
ethical issues at stake. If a planet already hosts some form of life, the
question arises of whether human beings have the right to limit or threaten it
by transplanting Earthlife in its midst. Attitudes to this issue will depend on
how important human colonisation is deemed to be and how complex the alien life
forms are.

One
motivation for sending humans into space is as an insurance policy against a
megacatastrophe on Earth. Often cited is the impact of a large comet or
asteroid which might destroy our civilisation or even our entire species. More
likely in my view is a sudden pandemic, either naturally occurring or through
the accidental release of a virulent biowarfare pathogen.

In any
case, over a period of millennia, there is no lack of potentially
species-annihilating hazards. If all that stood in the way of human survival
were some indigenous microbes on another world, few people would have scruples
about ignoring their “rights”.

If a planet had complex plant and animal life, there should be strong ethical objections to contaminating it with terrestrial organisms. Even if the two forms of life were so biochemically different that direct infection was avoided, it may still be the case that the terrestrial invaders would plunder some vital resource and deplete the indigenous ecosystem. Earth organisms might spread like the rabbits in Australia, and elbow the indigenous life aside, driving it to extinction.

Earth organisms might spread like rabbits and elbow indigenous life aside.

That issue would be greatly sharpened if a target planet is found to host intelligent life. In the movie Avatar, resource-hungry humans muscle in on the planet Pandora to the extreme discomfort of its indigenous population, although in the interests of Hollywood-style justice, the pesky human invaders eventually receive their comeuppence. There is no guarantee that future generations of humans would exercise respect for the rights of alien beings, nor can we be sure that aliens would respect ours.

Even
aliens far in advance of us in technology and social development may not share
our ethical values. Because we cannot begin to guess the motives and attitudes
of truly alien beings, when it comes to the prospect of humans encountering an extraterrestrial
civilisation, all bets are off.

It seems to be generally accepted that interstellar travel should, and could, become part of our destiny. Why? A familiar answer is that humans have always had wanderlust, a sense of curiosity, a desire to explore the world about them and to push on to pastures new. That may be true, but people have always fought wars and oppressed minorities too; just because something is deeply ingrained in human nature does not make it a noble motivation.

Rather easier to justify is the argument that human society has produced much that is good, which it would therefore be good to preserve for posterity. Humans may choose to undertake interstellar colonisation to keep our species, and the flame of our culture, alive somewhere in the cosmos. By establishing a permanent settlement on another planet, human culture could continue even if disaster struck at home. It could be countered that this argument adopts an inflated view of human significance and human worth, and that it is life, as opposed to our specific species or culture, that should be perpetuated and perhaps disseminated around the cosmos.

We could
already begin sending microbes in tiny capsules out of the Solar System if we
were so minded, but it is hard to imagine much enthusiasm for the project.
Seeding a barren galaxy with DNA may one day fire people’s imagination
(assuming the galaxy is not already teeming with life), but today the appeal of
interstellar travel is deeply rooted in ideals of human adventure and
advancement.

When Neil
Armstrong took that first small step on the Moon, it was widely hailed as the
initial step on a stairway to the stars. Half a century on, with humans
seemingly stuck in low-Earth orbit, the prospects for interplanetary, let alone
interstellar, travel look bleak. These microbiology problems compound what is
already a formidable challenge in spacecraft design, propulsion systems and medical
technology. Yet if humans wish to secure a long-term future in an uncaring and
occasionally dangerous cosmos, some form of cosmic diaspora needs to be part of
our long-range plan.