I've just been listening to Elon Musk's much anticipated talk about his vision for colonization of our solar system. Many find this idea inspiring, that by technological means, we can become a multiplanetary species. 

Here is the talk, in case any of you haven't seen it yet.

For a summary of the technical details he revealed in the talk, see this article on Space.com

Note, Robert Zubrin has suggested an improvement on the approach suggested by Elon Musk. See also Jason Torchinsky's Here's How To Fix The Big Problems With Elon Musk's Mars Spaceship

It is great that he wants to use his wealth for the benefit of humanity. However, I'd like to challenge the idea that this approach will make Earth safer for all of us and prevent our eventual extinction. Is this the best use of this dazzling technology, able to send 300 tons into LEO? 

I'd argue for a more open ended approach. Yes, let's aim to send humans in spaceships throughout our solar system, as in his vision.

Let's also explore places we can't visit in person using telerobotics, set up bases throughout our solar system that rely on in situ resources, do manufacture in space, try all those exciting things we can do. Yes, let's do that! It was an inspiring vision that he presented there:

Departing Earth - screenshot from the talk

Humans exploring Saturn - from the SpaceX Flickr stream - many other images available there too. You might be surprised to see a photograph of a human piloted spaceship flying past Saturn - how would humans get there? It seems almost like something out of Star Trek. Well do listen to the talk if you haven't heard it yet. He plans refueling stations along the way. He envisions humans eventually exploring as far afield as Pluto, in person.

The Jupiter system is an obvious refueling stop for a mission to Saturn and beyond. He talked about humans landing on Europa but that doesn't make much sense for planetary protection, since Europa along with Enceladus and Mars are the three most vulnerable places in the solar system on present day knowledge. Also Europa is within the radiation belts of Jupiter, and the extreme radiation would kill humans on its surface within hours if unprotected.

However, another of Jupiter's moons, Callisto, would be an excellent place in the Jupiter system to build a refueling stop.

Callisto, outermost of the larger moons of Jupiter. Callisto consists mainly of ice, so has plenty of water for refueling - and though it is thought to have a subsurface ocean, it is deep below the surface with no communication with the surface. This needs more study with a robotic mission to Callisto first, to be sure, but at present it is thought to have no planetary protection issues.

It is also outside of Jupiter's intense radiation belts and so would be safe for astronauts with only standard cosmic radiation shielding. As the outermost of the large moons of Jupiter, it's also not so deep in the gravitational well of Jupiter, so you would need less delta v to get to it.

As anothe rmoon of Jupiter, it takes the same time to get there as for Europa. New Horizons took just over a year to get from Earth to Jupiter, though of course it didn't have to stop when it got there. It left Earth with a delta v of 16.26 kilometers per second and already had solar system escape velocity when it left Earth. If you do a type II Hohmann transfer, spanning less than 180 degrees around the sun, then you can get from Earth to Jupiter in under two years as Voyager 2 did, taking one and a half years to reach Jupiter from Earth. That's a timespan well within reach if we can achieve reliable long term closed system recycling

Let's also explore places we can't visit in person using telerobotics, set up bases throughout our solar system that rely on in situ resources, do manufacture in space, try all those exciting things we can do. Yes, let's do that! It was an inspiring vision that he presented there.

But what about this idea that we need to become a multi-planetary species to survive long term?

I'm going to say things here that some of you will find controversial, especially if you are really sold on this multiplanetary species vision for the future of humanity, as I know many of my readers are. What I say will be especially controversial for you if you have already decided that the best possible future for humans in space is to set out to colonize Mars as fast as possible right now, as our top priority. It's an opinion piece, a statement of strong views held by the author in the hope that it may interest the reader, and stimulate debate and discussion. 

In the Kardashev scale, which is one way to categorize possible future and extraterrestrial civilizations, a Type I civilization is one that has access to the equivalent of all the power available from the sun on its home planet. This map shows how far we are from becoming a type I civilization. The yellow squares show the area of solar panels needed to power the entire world with the electricity consumption projected for 2030 using solar power alone. The yellow box at bottom left shows the total area needed to the same scale as the map.

The three levels in this scale are:

Type I "A civilization with an energy capability equivalent to the solar insolation on Earth"
Type II "A civilization capable of utilizing and channeling the entire radiation output of its star. The energy utilization would then be comparable to the luminosity of our Sun"
Type III "A civilization with access to the power comparable to the luminosity of the entire Milky Way galaxy,"

Note that these levels are not defined in terms of territory at all, but in terms of the amount of energy available. You could be still living in a single planet, but if you have access to the equivalent of  all the energy incident on your planet you are level I. If you have access to the equivalent of all the energy from your sun, even in a solar system with just one planet, you are level II. If you have access to all the energy from your galaxy, even if it is all gathered using robotic spacecraft and none of your civilization ever leave your solar system, or if you have explorers throughout the galaxy, but nobody settles anywhere else, and you still have access to equivalent of the energy output of the galaxy you are level III.

There is nothing there about any need to go multiplanetary. Putting a few bases on the Moon, or Mars or the asteroids will not take us to a type II civilization, or even the beginnings of one. Indeed if it has the effect of diverting resources away from Earth it could delay us from getting to the type I civilization level. If rapid colonization leads to millions in space, and then to warfare in space at some later date, it could even be a major set back. 

So is it worthwhile to try to become a multiplanetary species, at present, when we are still at a very early stage in technology development, not yet type I? And in a situation where there are no other planets in our solar system anything like as habitable as Earth?

First, if we can't keep our Earth in order, we are nowhere near the level of understanding needed to turn another planet into a copy of Earth. Earth is by far the easiest planet for humans to live in, in our solar system. If we can't keep Earth terraformed, in a state suitable for humans to live in, what chance do we have on Mars? 

Luckily, Earth is robust. Even the worst of global warming is an issue mainly because of the speed of the change. Earth has been far hotter in the past indeed it is unusual for it to have ice at either pole, and we have ice at both poles. It's oceans have been acid before, so acid that corals can't form for millions of years, then new species evolve as the oceans become more alkali. What we are doing to Earth is nothing it hasn't seen before but the speed of change is unusual, and also we have settled in large numbers in many places that are vulnerable to climate changes and trees, plants, and species used to living in narrow temperature ranges can't move or adapt quickly. That's the main reason many species may face extinction if we don't do something about it.

To devastate Earth so much that it is as bad for humans as Mars, we would have to somehow remove all its oceans, all its water, and most of its ice, leaving just a tiny ice cap in Antarctica, much smaller than the one we have now. We'd have to remove all but 1% of its air, nearly all its oxygen and CO2. We'd have to make its land completely dry to depths of hundreds of meters in equatorial regions. We'd have to stop continental drift, get rid of its magnetic field, reduce its gravity to a third of what it is now (somehow), and move it further away from the sun so that it gets half the sunlight it does now. There is no way we are going to devastate Earth as much as this in the near future.

Our ecosystem is dependent on many particulars of the way Earth works to continue. It is balanced for a particular distance from the Sun, particular levels of sunlight, composition of the atmosphere with oxygen in it. Transfer it to Mars and it wouldn’t work; it would be far too cold, trees couldn’t survive even in the tropics, and there’s no continental drift so not enough volcanic activity to return CO2 to the atmosphere. 

Current best ideas for terraforming Mars involve planet sized mirrors (if the thin film mirror weighed, say, 1 gram per square meter, that would be a ton per square kilometer so with half the surface area of Mars needed as space mirrors, you are talking about of the order of 72 million tons of mirrors to double the sunlight levels on Mars). Alternatively it involves mining it for greenhouse gases, the "easy way to do it" (at a rate of 13 cubic kilometers of fluorite mined per century on Mars and 500 half gigawatt nuclear power stations running full time to make the gases). 

Then it involves  diverting numerous comets, to bring more water, nitrogen and carbon dioxide to the planet and other heroic feats of mega-engineering, to get to a Mars that a thousand years from now might support trees. And that's with humans still using full cycle air breathers because carbon dioxide is poisonous to humans above 1%. To sequester all the carbon from the atmosphere into organics, to make it breathable, would take 100,000 years of photosynthesis - and that's a huge speed up compared to the millions of years it took on Earth. Meanwhile we run into problems trying to maintain a complex closed ecosystem the size of Biosphere II. The Mars trilogy is science fiction, with the facts stretched a bit as artistic license to make a good story.

Even if we put huge efforts of mega engineering into terraforming Mars, the result would still be at most a pale copy of Earth, and the best place for humans to survive in our solar system would continue to be Earth.

So, no, I don't think Mars will be easy to terraform, or that it is the sensible thing to do at this stage, or that to do so will genuinely make us multiplanetary, or advance us even towards a type I civilization (if that is  a desirable goal), never mind to type II. 

I think we are nowhere near at the level of wisdom and understanding to make a start as planetary ecoengineers. I've written a fair few articles about this so see also my 

• Trouble With Terraforming Mars
• Imagined Colours Of Future Mars - What Happens If We Treat A Planet As A Giant Petri Dish?
• To Terraform Mars with Present Technology - Far into Realms of Magical Thinking - Opinion Piece 
• Why Nukes Can't Terraform Mars - Pack Less Punch Than A Comet Collision
• Our Ethical Responsibilities To Baby Terraformed Worlds - Like Parents 
• How Valuable is Pristine Mars for Humanity - Opinion Piece?

There have also been many failed colonizations such as the attempt of the Vikings to colonize America, and the attempt by the Scottish to colonize Panama, which was so disastrous it lead much of the lowland population of Scotland to bankruptcy, and resulted in an urgent need for unification with England to save them.

 
Flag of the Company of Scotland Trading to Africa and the Indies. Their "Darien Scheme"an attempt to colonize Panama, lead to the death of nearly all the colonists, and it also drained Scotland of an estimated a quarter of all its liquid assets. Scotland was saved from bankruptcy by England, in exchange for unification with England, higher taxes, and an agreement to service the English national debt.

If you focus just on the colonization attempts that succeed, you get only a partial picture, which may be over optimistic. I think it may well depend a lot on how we set about it.

Now imagine using all that megatechnology and all those resources to protect Earth and to preserve it as a habitable place for humans. It's not a comparison between an Earth where we do nothing to protect it and an Earth where we still do nothing here but attempt to set up another civilization on Mars. It's a comparison between Earth where we focus our main efforts in space for many decades into this colonization attempt, and an Earth where we use the same space technology to help and protect Earth itself, protect from asteroids, move heavy industry into space, explore other planets in ways that avoid introducing Earth microbes to them, so making new discoveries about origins of life possible or much easier, and so forth, and with scientific bases and explorers spreading through our solar system like the early Antarctic explorers, making many discoveries, but for now, focuses on what they can find and most likely not yet deciding to set up home in these desolate inhospitable places. Certainly not under any peer pressure or expectation to set up home outside Earth.

WE DON'T FACE NEAR TERM EXTINCTION FROM ASTEROIDS

I was glad to hear Elon Musk also say in his talk that we don't face near term extinction. There is too much doomsday talk and scaring people these days with non existent threats. There are plenty of real concerns to deal with such as global warming, or indeed the real threat of impacts by smaller asteroids.

The only asteroids that can hit Earth are too small to make humans extinct. As one of the most adaptable species, with minimal technology, there's not much that can make us extinct. Even if 99% of all species on Earth went extinct, then humans with the bare minimum of technology would be amongst the survivors.

We can be as sure as can be that we won't be hit by a 100 km diameter asteroid. There hasn't been an impact that large for over three billion years from the cratering record. The Aitken basin on the Moon, the Hellas basin on Mars, the Caloris basin on Mercury, the ancient impact on Earth by a 23 mile diameter asteroid over 3.26 billion years ago, they are all over 3 billion years ago and those bodies don't have any large craters like that younger than them. Most are from around 3.8 billion years ago. In case of Earth then some craters can be erased by continental drift but we have a good record on the Moon, Mars and Mercury to fill out the picture.

We risk a 10 km asteroid but not a 100 km asteroid. Jupiter protects us, it seems.

A ten kilometer diameter asteroid like that could devastate our civilization, but Earth would remain by far the most habitable planet even after and through it. And it couldn't make an adaptable species as humans with technology extinct.

There's a tendency to look at the Chicxulub impact and say "The big scary dinosaurs went extinct, along with 75% of species on Earth, so surely we'd have a 75% chance of extinction"? But humans are ominovores and we can survive on fruit, nuts, roots, grains, insects, animals, fish, birds, shellfish. We can survive anywhere from the arctic to the Kalahari desert with minimal technology, can build boats and cultivate crops and farm animals and fish. We just need something edible somewhere on Earth and we can go there, farm it if necessary, and we would survive, some of us. Even extinction of 99% of Earth species would be zero probability of human extinction.

And meanwhile, we have the capability to detect and deflect asteroids. The astronomers know what to do. Instead of colonizing another planet, if the focus is on protecting Earth, we can map out all the asteroids that have any chance of going close to Earth - and deflect or mine away any that are a significant risk in the next few thousand years.

With only 450 million dollars, even with present day technology, no heavy boost, we can send a space telescope into orbit between Earth and Venus to find nearly all the Near Earth Asteroids that pose a possible future threat to Earth within a decade. With the kind of technology levels Elon Musk envisages, we could map out every asteroid and comet in the entire solar system of any size, and would be able to divert them, easily, decades before impact. You only need centimeters per second delta v to divert an asteroid a decade before impact, and if it does a close flyby first, only microns per second. With just a tiny fraction of the effort needed to attempt to terraform Mars or even to send a million people there, we can ensure that Earth is never in the future hit by a giant asteroid again, for the next thousands or millions of years.

We don't need to be multi planetary to do this, we don't need to be a type I civilization, we can do it already. It just needs funding. For far far less cost than a colonization attempt for Mars.

Asteroid impacts are the easiest of all to prevent if you have the funding to do it. We can predict the impact exactly, and know what you need to do to prevent it - to apply delta v well in advance to the asteroid, with various methods for doing that. For more on this, Giant Asteroid Headed Your Way? - How We Can Detect And Deflect Them

90% OF SPECIES EXTINCT CONVERTS TO ZERO PROBABILITY OF HUMANS EXTINCT

This is an argument you often see which seems convincing at first. In some past extinction events, 90% of all species have gone extinct. It's easy to think "oh dear, that means we have a 90% chance of going extinct in such an event". But no, it's not like that. Some species are far more vulnerable to extinction than others and some can, and have, survived them all. For instance Horseshoe crabs have survived and continued to evolve through all the past extinction events for the last 450 million years.

We would be bound to be one of the survivors with even minimal technology as one of the most adaptable and robust creatures on our planet. Birds, river turtles, and small mammals, survived the dinosaur extinction. We are omnivores and can survive on fruit, on roots, on shellfish from the sea shore, on mammals, birds, grow crops, can travel in boats. We just need to find something edible and sustaining for humans, somewhere in the world that has survived, and if necessary, cultivate it. There's no way those events would make us extinct.

And if the Earth was devastated at all, it would still be by far the most habitable place for humans in the solar system. In an event like that the only place it makes sense to "terraform" is Earth.

OTHER FORMS OF EXTINCTION

The situation is the same for a global nuclear war. Even if all our nuclear weapons were exploded and the atmosphere filled with dusts from radioactive substances, and dense clouds plunged Earth into a nuclear winter, Earth would still be vastly more habitable than Mars, and some humans would certainly survive. It would still have a breathable atmosphere for starters and it is hard to top that.

Risks such as nanotechnology, artificial life etc caused by humans could as easily originate in a space colony as anywhere, as those colonies would need the most advanced cutting edge technology just to survive. So then, in an interconnected future space economy, with fast transit and hundreds of people traveling back and forth between the colonies, the technology would be able to spread civilization wide rapidly. That some of it is in space would not make any difference there. If quarantine is the reason for safety, e.g. for microbes / synthetic life, then you can set up quarantine on Earth, a base with a six month quarantine say. If the reason is to have a backup of our knowledge, and of species, of biology, then the Moon is an excellent place to locate an informational / biological seed bank repository not just for us but for all future civilizations as well, easy of access from Earth and very stable geologically. It could have caretakers - and it could be interrogated from Earth via radio or laser if for some reason we lose space capabilities but still have those - continually beam instructions on how to use it in a beacon to Earth.

Other disasters like super volcanoes can't be prevented, not with current or foreseeable future technology. All we can do is increase the warning time. However the motivation to go multiplanetary is to prevent extinction, and a super volcano would not make us extinct. Nor would a gamma ray burst, or any of the other possible near future natural disasters. I go through them all in my article: Could Anything Make Humans Extinct In The Near Future?

Our only significant risk in the "near future" is 500 million years in the future as the sun gets hotter, and who knows what species will be alive then and what their capabilities will be. If you are talking about long term distant future risks, then you have the problem that it is unlikely that we actually know what to do that would be of use to them in that distant future. For instance, attempts to terraform Mars now might well turn it into a planet that can't be terraformed 500 million years into the future when it is needed, with all its water and dry ice converted into carbonates or the water and atmosphere dissociated and lost to space.

Or even a century from now or a few centuries from now, maybe they have space colonies throughout the asteroid belt, they find it easy to build habitats to live in, they have eliminated all possibility of asteroid impacts on Earth, ad they don't need Mars as a place to live. It might be that the thing they would find most useful then would be able to have access to Europa or Mars or Enceladus as they were before the humans messed them up. Rather than being grateful to us, they might be exasperated by our short sightedness and wish they could somehow go back in time and change our actions.

If you look at projections of the situation in 2000 onwards from the 60s and 70s, it shows how difficult it is to plan for a future even a few decades in advance. Certainly, developing the capability to transform Mars in a global way might well be useful, doing the studies, getting ready to do it if it is ever needed. But actually using that capability means making decisions based on guesses about what we think our descendants will want several generations or even a thousand years into the future or 100,000 years into the future in the case of Mars terraforming plans. That is fraught with difficulty and I doubt if we have the understanding to set in motion things like that in a way that will actually benefit our descendants a thousand years from now - much as we might want to do so.

CLOSING OFF THE FUTURE ON MARS

Some of you may say - okay - maybe it is not going to make us multiplanetary in a genuine sense - but why not let them give it a go if they think they can do it?

So, first, I have no doubt about Elon Musk's intent to benefit humanity, what he says just rings so true. I admire him for this. But humans can make mistakes and nobody, however brilliant, is immune to mistakes. We have done so as a species many times, and often with the very best of intentions and on the basis of the best understanding of the experts of the day. We need to look at ideas carefully, and think about them independently, not just accept them because someone who is extremely clever, with a good heart, and with a lot of business acumen says it's what we should do. 

So the problem that I think we need to consider here is that you are closing off future possibilities by introducing Earth microbes to Mars. That wouldn't be the intent of Elon Musk at all, or the colonists of course, to make native Mars life extinct, to confuse scientific exploration of Mars, and so on, but it might be an unavoidable effect. Humans themselves are no problem, but the microbes that come with us are another matter. Sadly humans can't be sterilized of our microbes.

In this way colonists just by landing on Mars in their spaceships would be making decisions, not just for themselves, but for the whole of humanity. They would be saying by their actions "We know what is the best future for Mars for us all", based on limited understanding of Mars. 

INTERCONNECTED MARS

The problem is that Mars, unlike the Moon, is a strongly interconnected system through its global dust storms. If there are habitats suitable for Earth life on Mars,  the dust from the global dust storms can blow microbial spores into them. Then introducing Earth microbes to Mars is irreversible, like introducing rabbits to Australia.

Robert Zubrin says that this would be impossible as it would be like sharks competing with lions. However analogies can lead you far astray at times. It might instead be like non native rabbits competing with threatened wallabys, wombats, trees and plants in Australia. Researchers have found many microbes from Earth that can do just fine in habitats on Mars, able to cope with the UV light, the perchlorates, the thin atmosphere, the extreme cold at night and so on. See Candidate lifeforms for Mars

MICROBIAL DARK MATTER AND SHADOW BIOSPHERE APPLIED TO MARS

Another analogy Robert Zubrin often uses is of anthrax. He says we will be able to recognize Mars life as different from Earth life because we just need to examine the life we bring with us and since we can distinguish anthrax from other life, we'll be able to tell what is Earth life and what isn't.

But that doesn't work either. Yes, there are a few micro-organisms that we have studied in great detail. But most of the archaea, the microbes most likely to be on Mars, are so little known that 99% of them in a typical sample can't even be cultivated. This is the problem of the "microbial dark matter". They are only known from DNA fragments and we don't even know how many genuses of archaea there are, never mind have an idea of the range of species. We don't know their capabilities. When they do microbial assays of spacecraft cleaning facilities, it's just the same. 99% of what they find is DNA fragments of unknown species of archaea. Nowadays, they can sequence individual archaea cells.

So if you find a microbe on Mars, and it is DNA based, then yes, you could in principle sequence it. Then if you happen to have sequenced that same species on Earth, you could recognize that they are identical. However, even if you have a complete sequence, you can only compare Mars ones to the ones you have already sequenced on Earth. Most Earth microbes of course are not sequenced and most of the microbe species on the spacecraft would not be sequenced. It's only ones that are very important to us like anthrax are sequenced.

There are an estimated one trillion microbial species. Of those, 99.999% have not yet been identified. Only ten million species have been catalogued. Of those, only about 10,000 have ever been grown in a lab and only about 100,000 have classified sequences. So only 0.00001% of all microbial species on Earth have been sequenced to date. See Largest ever analysis of microbial data (May 2016).

So, no, if we find DNA on Mars, we won't be able to tell by some simple test if it is from Mars or from introduced Earth life. Not if it is related to us from any time since the very distant past, the first archaea, billions of years ago,.

GENE SWAPPING BETWEEN EARTH AND MARS MICROBES

Life can also exchange fragments through GTA (Gene Transfer Agents). This can happen even with higher organisms, the red colour of some aphids is the result of carotene which they produce due to a gene fragment that came originally from a fungus. It is something that archaea can do very readily, exchanging fragments even overnight in salty solutions.

This is an ancient mechanism so even if Mars life separated from Earth life three billion years ago, the archaea would still be able to swap gene fragments using horizontal gene transfer in that way. One of the most interesting questions would be how long ago the two separated evolutionarily and how closely connected they are, and you won't be able to answer this nearly so easily or maybe not at all.

Studying the archaea is tough. We don't want to make it tougher.

Then as well as that, our life detection experiments are searching for tiniest traces of life. They are exquisitely sensitive. Some of the experiments astrobiologists want to fly to Mars can detect a single amino acid in a sample. Such measurements will be meaningless if there is Earth life there introduced by us.

Here on Earth in attempts to study the organics in the meteorite ALH84001 and other similar ones in attempts to determine if they ever contained life, one of the biggest problems astrobiologists face is contamination by Earth life rendering most of their tests very hard to impossible. Introducing Earth life to Mars would contaminate present day habitats on Mars in this way making similar tests of those habitats ambiguous and as hard to interpret as our meteorite studies on Earth.

UNINHABITED HABITATS

Then also Mars may perhaps not have any life in some of the habitats, even though habitable. This is a very interesting situation in its own right. We want to understand how planets work and what happens when you have complex organics but no life. Does it build up to some form of proto life? Do you get anything resembling cells? You do in some experiments in laboratories, depending on the conditions. This is in some ways one of the most interesting cases of all, if perhaps not so exciting for astrobiologists. Again introducing Earth life would make it impossible to find out what was going on in these habitats before the life was introduced.

EXTINCTION OF MARS LIFE BEFORE IT IS DISCOVERED

And then, Earth life could make Mars life extinct, for instance if it is some early form of life, intermediate between uninhabited and inhabited. So in that case we don't even get to find it, it may be gone already before we are able to send robots to its habitat to sample it. More on that in a moment below.

SCIENCE IMPACT

Elon Musk does care about the science impact of introducing Earth microbes to Mars. Here he answers a question on this topic, in the 2015 AGU conference in San Francisco, 30 minutes into this video:

Q. "I am Jim Cole from Arizona State University. I was listening to Chris McKay, another advocate of humans to Mars, and he was talking about how if we do go to Mars and we find life either there or extinct, we should consider removing human presence so that we can allow the other life to thrive. I was wondering what your thoughts on that were. "

A. "Well it really doesn't seem that there is any life on Mars, on the surface at least, no sign of that. If we do find sign of it, for sure we need to understand what it is and try to make sure that we don't extinguish it, that's important. But I think the reality is that there isn't any life on the surface of Mars. There may be microbial life deep underground, where it is shielded from radiation and the cold. So that's a possibility but in that case I think anything we do on the surface is not going to have a big impact on the subterranean life.".

So, it's clear (as I'd expect actually), he does think it is important we don't extinguish any native Mars life. But he thinks there isn't any present day life on the surface. But is that right?

I did a survey of the scientific literature, to see what there is by way of proposed habitats and to investigate the range of views on the topic:

Are There Habitats For Life On Mars? - Salty Seeps, Clear Ice Greenhouses, Ice Fumaroles, Dune Bioreactors,... (long detailed survey article with many cites)

It's also available as a kindle booklet, and also online here with table of contents

As you see, there's an almost bewildering variety of suggestions for habitats on Mars for life. The main surface or near surface ones are (these links take you to the online booklet)

• Warm Seasonal flows (Recurrent Slope Lineae)
• Sun warmed dust grains embedded in ice
• Flow like features
• Life able to take up water from the 100% night time humidity of the Mars atmosphere
• Deliquescing salts taking up moisture from the Mars atmosphere
• Advancing sand dunes bioreactor
• Droplets of liquid water on salt / ice interfaces
• Shallow interfacial layers a few molecules thick

Most of those habitats are either above the permafrost layer or at most a few centimeters below it (the permafrost layer is typically 2 cms below the surface of Mars or less).

There's a wide variety of views also on the topic of whether any of these are habitable, and whether they actually have life in them, from almost impossible to very likely, see Views on the possibility of present day life on or near the surface, and for the idea that they may be inhabitable but uninhabited, see Uninhabited habitats.

If these habitats do exist and are habitable, there are many Earth microbes which have been shown to be able to survive in Mars conditions in Mars simulation conditions, and so could potentially survive in these habitats on Mars if they exist.

Researchers at DLR (German equivalent of NASA) testing lichens in Mars simulation experiments. They showed that some Earth life (Lichens and strains of chrooccocidiopsis, a green algae) can survive Mars surface conditions and photosynthesize and metabolize, slowly, in absence of any water at all. They could make use of the humidity of the Mars atmosphere.[46][47][48][49][50]

Though the absolute humidity is low, the relative humidity at night reaches 100% because of the large day / night swings in atmospheric pressure and temperature.

I did a survey of the literature a while back and compiled a list:

• Chroococcidiopsis - UV and radioresistant can form a single species ecosystem, and only requires CO2, sunlight and trace elements to survive.[50]
• Halobacteria - UV and radioresistant, photosynthetic (using a different mechanism), can form single species ecosystems, and highly salt tolerant. Some are tolerant of perchlorates and even use them as an energy source, examples include Haloferax mediterranei, Haloferax denitrificans, Haloferax gibbonsii, Haloarcula marismortui, and Haloarcula vallismortis [59]
• Some species of Carnobacterium extracted from permafrost layers on Earth which are able to grow in Mars simulation chambers in conditions of low atmospheric pressure, low temperature and CO2 dominated atmosphere as for Mars.[141][140]
• Geobacter metallireducens - it uses Fe(III) as the sole electron acceptor, and can use organic compounds, molecular hydrogen, or elemental sulfur as the electron donor.[200][203][204]
• Alkalilimnicola ehrlichii MLHE-1 (Euryarchaeota) - able to use CO in Mars simulation conditions, in salty brine with low water potentials (−19 MPa), in temperature within range for the RSL, oxygen free with nitrate, and unaffected by magnesium perchlorate and low atmospheric pressure (10 mbar). Another candidate, Halorubrum str. BV (Proteobacteria) could use the CO with a water potential of −39.6 MPa [202]
• black molds The microcolonial fungi, Cryomyces antarcticus (an extremophile fungi, one of several from Antarctic dry deserts) and Knufia perforans, adapted and recovered metabolic activity during exposure to a simulated Mars environment for 7 days using only night time humidity of the air; no chemical signs of stress.[55]
• black yeast Exophiala jeanselmei, also adapted and recovered metabolic activity during exposure to a simulated Mars environment for 7 days using only night time humidity of the air; no chemical signs of stress.[55]
• Methanogens such as Methanosarcina barkeri[200] - only require CO2, hydrogen and trace elements. The hydrogen could come from geothermal sources, volcanic action or action of water on basalt.
• Lichens such as Xanthoria elegans, Pleopsidium chlorophanum[53], and Circinaria gyrosa - some of these are able to metabolize and photosynthesize slowly in Mars simulation chambers using just the night time humidity, and have been shown to be able to survive Mars surface conditions such as the UV in Mars simulation experiments. [205][206][207][208][209]
• Microbial life from depths of kilometers below the surface on the Earth that rely on geochemical energy sources - relying on metabolic pathways that can't be traced back to the sun at all. Some of these are multi-cellular. Examples include the microbe Desulforudis audaxviator which metabolizes reduced sulfur as the electron acceptor, and hydrogen as the electron donor, can fix nitrogen and has every pathway needed to synthesize all the amino acids [210][211]
• Multicellular life from depths of kilometers below the surface such as Halicephalobus mephisto, a nematode feeding on bacteria, 0.5 mm long and up to 3.5 km deep, lives in water at 48°C, very low oxygen levels about a thousandth of the levels in oceans. Though it probably originates from the surface, carbon dating shows it has lived at those depths for between 3,000 and 10,000 years, and it's been suggested that this has implications for deep subsurface multi-cellular life on Mars.[212]

Most of these candidates are single cell microbes (or microbial films). The closest Mars analogue habitats on Earth such as the hyper arid core of the Atacama desert are inhabited by microbes, with no multicellular life. So even if multicellular life evolved on Mars, it seems that most life on Mars is likely to be microbial.

Because of the low levels of oxygen of 0.13% in the atmosphere, and (as far as we know) in any of the proposed habitats, all the candidate lifeforms are anaerobes or able to tolerate extremely low levels of oxygen. This also makes multicellular animal life unlikely, though not impossible as there are a few anaerobic multi-cellular creatures[213]. Some multicellular plant life such as lichens, however, may be well adapted to Martian conditions (this was a bit of a surprise to the researchers as lichens are symbionts of algae and fungi, and fungi need oxygen - however, it seems that the algae supply enough oxygen for the fungus even when there is hardly any oxygen in the atmosphere around them).

Also some multicellular life such as Halicephalobus mephisto can survive using very low levels of oxygen which may perhaps be present in some Mars habitats.

(This is a copy of my section Candidate lifeforms for Mars in my booklet Places on Mars to Look for Microbes, Lichens, ...)

Most of these can live in the postulated surface or near to surface habitats. The habitats deep below the surface would be less affected by microbes introduced to the surface, but as on Earth, Mars will have caves and crevices that link to the surface, as a result of meteorite impacts, water erosion and crustal movement (the meteorite impacts especially fracture the surface of Mars to considerable depth). Also the methane plume observations, if confirmed, suggest a connection between the deep subsurface and the atmosphere. So I included those as well, as I'm not sure that the subsurface can be treated as totally insulated from the surface, though the surface habitats of course are the ones of most immediate concern.

So when will we resolve this? Well not for some time. Most of these potential habitats would be hidden from view, a few millimeters or centimeters below the surface. Some of the habitats might be quite productive, for instance methanogens in warm humid locations deep below the surface heated by geothermal processes. There might be enough life there to cause obvious effects on the atmosphere, such as the methane plumes. But as Mars changed from a warmish wet planet to a cold dry planet, any surface life would probably become more and more sparse, and have less and less effect on the atmosphere.

As Mars slowly changed from the warmish humid planet on the left to the dry cold planet on the right, then any surface life may have become more and more sparse, and had less and less effect. Image from NASA (Goddard space center).

So, if the life from early Mars still lingers but is sparse, it might easily have almost no effect on the atmosphere by now. The most habitable areas of Mars such as the warm seasonal flows, if we are lucky, might be about as habitable as the Antarctic dry valleys or the high Atacama dry desert. If that's the way of it, life in those few square kilometers of the Martian surface would have almost no effect on the atmosphere. Mars already has small amounts of oxygen (0.145% as measured by Curiosity). The signal of oxygen from photosynthetic life on the surface, at such low levels, would just be hidden in the noise.

Indeed, even if the entire surface of Mars is as productive of oxygen as Antarctic ice covered lakes - and even if all that oxygen ends up in the atmosphere, the signal from all of that photosynthetic life would still be lost in the noise and not noticeable in the atmosphere (I made it about 0.0002%, in a very rough calculation, by just assuming a residence time of oxygen in the Mars atmosphere of 4500 years, the same as for Earth - at any rate it would be a tiny, surely undetectable, signal).

Why Mars Surface Life May Leave No Traces In Its Atmosphere: Our Rovers May Need To Go Up Close To See It

also my Our Spacecraft Could Look Straight At an Extraterrestrial Microbe - And Not See a Thing!

For more on this see Value of a non confrontational approach and following in the book.

It is an open question in exobiology, whether Mars life would be related to Earth life or not. The theory of panspermia is just a theory and even if there is some shared life, there could also be life that is not shared. A journey of a few thousand or hundreds of thousands of years in the cold and vacuum of space is not the same as a journey of a few meters from a spacesuit (they leak air and microbes all the time) to the ground. Even if some very hardy lifeforms from Earth have got to Mars, which is not yet known, most microbes would have no chance of making that journey on a meteorite.

The problem is that the Mars surface is interconnected, with the global dust storms. So, if these habitats exist, any life brought to Mars will gradually spread to the entire planet through hardy microbial spores, protected from the UV light imbedded n cracks in the dust. Once that is started, it can't be reversed. And as Elon Musk says, human missions to Mars will be dangerous with significant chance of loss of life. One of the most dangerous times in the mission would be the landing on Mars. After a human mission crashes on the Mars surface, that would be the end of any chance of protecting Mars from Earth life, with debris, tiny fragments of the astronauts bodies, air, water, food, scattered across the surface and the spores spread in the dust.

The introduced life from Earth could easily out compete Mars life. I think the easiest example here is the example of some earlier form of life that has been made extinct on Earth but still lingers on Mars.

One good example is RNA based life, which could exist without DNA or protein. These "RNA world" cells were hypothesized for a shadow biosphere here on Earth. So far we haven't found these cells on Earth, so if this form of life existed here in the past, perhaps DNA based life made it extinct. If so, what if it still exists on Mars? It might well be vulnerable to whatever made it extinct on Earth, from introduced Earth life. See One example of what we might find on Mars is some early form of life made extinct on Earth by DNA based life 

Or, it could be some completely unrelated form of life, in which case again, there is no reason particularly why it has to be invulnerable to Earth life, or indeed vice versa, maybe in the harsh Mars conditions it has evolved capabilities our life doesn't have, and Earth life could be vulnerable to it. A good example there - if it has a more efficient metabolism, or is marginally better at photosynthesis, then it could out compete Earth life, and vice versa, Earth life could out compete it. It wouldn't need much of an edge there, just a tiny edge due to Earth life's different biochemistry, and over many generations of microbial cells the Earth life would predominate. Just doubling once a month, if it has a significant advantage over Mars life, it wouldn't take long for every cell in a small habitat to be Earth life, and then if it spreads through spores in the dust, then it might not be many years before most of the habitats Mars wide are colonized by Earth life. And whether or not it happened as quickly as that, there would be no way to halt or reverse such a process once started, once you have spores in the dust and the life has spread significantly beyond its original first colonized habitat.

I'm not saying, don't go to Mars. But we can do a lot from Mars orbit. Meanwhile the Moon is the safest place to do early experiments in space settlement. It may have lunar caves up to 5 kilometers in diameter and over 100 kilometers long, according to the Grail data, already set up with radiation and regolith shielding. See Lunar caves and Lunar caves as a site for a lunar base We can also use materials, first from the Moon and NEOs, later from the asteroid belt to build colonies in space with up to a thousand times the population of Earth, see Asteroid Resources Could Create Space Habs For Trillions; Land Area Of A Thousand Earths. We have the potential of the Venus clouds as well, see my Will We Build Colonies That Float Over Venus Like Buckminster Fuller's "Cloud Nine"?

The three main places we need to hold off from landing humans on, if we care about planetary protection and the science value of the search for life in the solar system, are Mars, Europa, and Enceladus, and we also need to go cautiously with Ceres and the larger asteroids until we know if they have life or not (considered as a non negligible possibility by astrobiologists because of the water plume seen around Ceres by Hubble). All these are places we can explore by telerobotics using increasingly capable robots, also explore using robots controlled from Earth. 

There is no need to send humans to these places as quickly as possible. It won't help to make us multiplanetary, but it may mean we miss out on discoveries about the origins of life, and other lifeforms. Imagine if you could learn about life on a planet or in the ocean of an icy moon around another star? Even if it was just extraterrestrial microbes or lichens, imagine how exciting that discovery would be? Well Mars, Europa and Enceladus may be like exoplanets and exomoons in our own solar system, they may be as interesting as that. We don't know until we study them close up.

It's the aspect of our exploration of the solar system that gets most interest of all from the general public I think. And if we did find an early form of life, or something significantly different, it would be the greatest discovery in biology since the discovery of evolution, or perhaps the discovery of the helical nature of DNA, of that order of importance. Who knows what implications it would have, if you think of how much of modern biology comes from those two discoveries.

If we introduce Earth microbes to them, accidentally or intentionally, this may well be irreversible. It's the irreversibility that's the issue here. If it is biologically reversible, not so much of a problem. But if irreversible, that means it would change those places for all future time, not just for us, but for our descendants and all future civilizations that arise in our solar system, they won't be able to make the discoveries they could make by studying these places as they are now, without Earth microbes introduced to them. They also won't be able to transform them in other ways if they decide they wish to introduce a different mix of microbes from the ones we brought there.

I think we just know far too little at present to make such a decision for all those future generations and civilizations and indeed for ourselves.

TELEROBOTICS AS A FAST WAY FOR HUMANS TO EXPLORE MARS FROM ORBIT

Telerobotics lets us explore Mars much more quickly with humans in the loop. And you'd use an exciting and spectacular orbit for early stages of telerobotic exploration of Mars, following the HERRO plans. It comes in close to the poles of Mars, swings around over the sunny side in the equatorial regions and then out again close to the other pole, until Mars dwindles again into a small distant planet - and does this twice every day.

Imagine the view! From space Mars looks quite home-like, and the telerobotics will let you experience the Martian surface more directly than you could with spacecraft, actually touch and see things on the surface without the spacesuit in your way and with enhanced vision, blue sky also if you like. It's like being in the ISS, but orbiting another planet.

12th April 2011: International Space Station astronaut Cady Coleman takes pictures of the Earth from inside the cupola viewing window.- I've "photoshopped" in Hubble's photograph of Mars from 2003 to give an impression of the view of an astronaut exploring Mars from orbit.

This is a video I did which simulates the orbit they would use - in orbiter. I use a futuristic spacecraft as that was the easiest way to do it. Apart from that, it is the same as the orbit suggested for HERRO.

It would be a spectacular orbit and a tremendously humanly interesting and exciting mission to explore Mars this way. The study for HERRO found that a single mission to explore Mars by telepresence from orbit would achieve more science return than three missions by the same number of crew to the surface - which of course would cost vastly more. Here is a powerpoint presentation from the HERRO team, with details of the comparison.

Then, you'd also have broadband streaming from Mars. As well as being very safe, also comfortable for the crew, you'd also have wide-field 3D binocular vision. It's amazing what a difference this makes, I recently tried out the HT Vive 3D recreation of Apollo 11. We'd have similar 3D virtual reality experience of the Mars surface.

Also, it would actually be a much clearer vision than you'd have from the surface in spacesuits, digitally enhanced to make it easier to distinguish colours (without white balancing the Mars surface is an almost uniform reddish grayish brown to human eyes)|.

Here is the hololens vision, which though it's not telepresence, I think gives a good idea of what it might be like for those operating rovers on Mars in real time from orbit, some time in the future with this vision.

It's safer too. No need to suit up. No risk from solar storms - at worst you have to go to a storm shelter in your spaceship, not rush back to your habitat as fast as you can to get out of the storm in time. No risk of falling over and damaging your spacesuit. And when you need to take a break, have your lunch, or whatever, you can just take it up again where you left off, indeed leave the robot doing some task while you have your lunch or sleep.

As for how long it would take to do a biological survey of Mars, Carl Sagan took a figure of 60 landers, 57 of those successful, and 30 orbiters, all devoted to biological exploration like Viking, as a starting point. So that's as good an estimate as any. The ability to explore from orbit would help hugely. It's just a preliminary survey, there are a dozen different types of habitat to explore, and you have an area the size of Earth's land mass, so it is like landing eight rovers on each of the seven continents on Earth. You would get a first rough idea. But you wouldn't find some rare lifeform in some unexpected location. I don't think we should say in advance what counts as a completed survey - as we would find out things as we go that would help us understand how complete it is, which we can't know in advance. But exploration from orbit by telerobotics, and sending lots of small robots to Mars would speed it up a lot. For more on this, see How many years are needed to do a biological survey of Mars?

NO HURRY

Why the haste to land humans on the Mars surface? 

In the natural course of events, Earth will remain habitable for five hundred million years. It's hard to grasp what a vast timescale that is. That's long enough for humans to evolve again from the smallest microscopic primitive multicellular creatures.

So yes we need to find somewhere else to live if we still are here half a billion years from now - or rather - whatever intelligent species is still here by then - we need to protect Earth from the Sun, or move the Earth itself. If we survive half a billion years as a technological species, that should surely not be hard for us to do. But there is no hurry to go anywhere else right now. I think we need to focus on protecting Earth, and to go into space, yes, but do so to find ways to help Earth.Carl Sagan expressed a similar sentiment in Pale Blue Dot

"The Earth is the only world known, so far, to harbor life. There is nowhere else, at least in the near future, to which our species could migrate. Visit, yes. Settle, not yet. Like it or not, for the moment, the Earth is where we make our stand

MOON AS FIRST DESTINATION FOR HIS SPACECRAFT

When you think about it this way, the Moon becomes a natural destination for his spacecraft. Not as a refueling stop on the way to Mars, that probably doesn't make too much sense, but a place of major interest in its own right. It is actually resource rich for instance, you can make solar cells in situ using the high grade vacuum and silicon, with a paving robot that could drive over the surface turning it into solar panels. 

The 14 day lunar night is not the disadvantage you'd think when you realize it has sunlight 24/7 at the poles and no dust storms, also the high grade vacuum is an advantage for making electronics, it has volatiles at the poles, it is rich in many metals, it probably has caves kilometers in diameter and over 100 kilometers long, and it is always at the same distance from Earth. See The Moon is resource rich. There are solutions also for growing plants on the Moon with a 14 day night. See Earth length day on Mars versus advantages of close to 24/7 solar power at the lunar poles

It is also a place where we can also make first experiments in off planet settlement, close to Earth, and likely to be able to provide resources even for cities of millions of people. If the amount of water at the poles matches expectations, it's enough for a million people to have at least an entire olympic pool swimming lane's worth of water each.

It also might have a commercial value, because of its proximity and low escape velocity, while Mars is hard to justify economically in the near future - see my article in  Forbes magazine: Is there a Fortune to be made on Mars? And with the Hoyt cislunar tether system, the delta v to return materials to low Earth orbit, and to the Earth's surface, can be effectively almost zero.

His spaceship could also make a big difference for transport of goods and materials to and from the Moon as well as people. And it could help support a major tourist industry on the Moon in the near term. Surely many more people will be interested in spending a weekend or a week on the Moon than will want to go to Mars orbit for two years, never mind go there and stay.

We are likely to have a Lunar village at any rate, whatever SpaceX or NASA do, as the ESA is keen on the idea as is Russia, as well as other countries.

REALITY CHECK

I think we do also need a bit of a reality check here. So far SpaceX haven't yet sent a human being into space, not even a test pilot. They have had many unmanned flights using the same rocket they will use for humans, but that's not quite the same as actually sending a human into space.

They have also had two incidents with the Falcon nine, the second leading to it blowing up while refueling, stationary on the launch site, and before the test burn of the engine - such a rare form of accident it hasn't been seen for decades. Doug Messier has an interesting opinion piece here, questioning the wisdom of bringing Silicon Valley working practices of 60-80 hour weeks, multi-tasking, and frequent hardware and software upgrades to the rocket industry. See Are SpaceX’s 60 to 80 Hour Work Weeks Really Such a Good Idea?

There's no question, they have already achieved amazing things and have reached the reliability levels needed for unmanned launches. But are they ready for manned flights? This spaceship for a hundred people is a fair way down the road yet. And what happens if the rocket blows up on the launch pad? How do they ensure that those 100 people survive such an accident safely? I'm not saying they won't do it, not at all, just a reality check and a caution that it is early days yet.

MOON AS A SAFER FIRST DESTINATION

This idea of putting 100 people into one big spacecraft is bold - but if the spacecraft crashes - or its systems fail, it loses its atmosphere due to an explosion of an oxygen tank or some such -  that's a hundred people who will die. And if the issue happens soon after they leave Earth, the spaceship they are in continues on its path to Mars with no possibility of return. It's not like the old days of sail. Then, you could at least breathe the air, and you could get in a lifeboat with some chance of survival, and you might find an island that you can land on. Here, you are in a boat for 100 people, but with no lifeboats, committed to a six month journey. 

The essential difference here is that you can have a lifeboat on the Moon to get you back to Earth in 2 days. Many space accidents unfold over a period of days, like Apollo 13, and especially if your lifeboat is separate with its own self contained systems, and supplies to last you for a couple of days, this is plenty of time to get into the lifeboat and escape if necessary. The first thing the crew of the ISS do when there is a risk of impact with space debris is to get into the Soyuz TMAs which are always attached to the ISS ready to escape back to Earth if necessary.

This makes it far far safer than if you don't have a lifeboat. Spacecraft to Mars or other deep space missions currently can't be fitted with lifeboats with present day technology.

Examples of the sort of tragedy that could unfold, could be water contamination, or the food spoiled, or the oxygen regeneration failing slowly, or an explosion that breaches the hull, or even, amongst a hundred people, someone on board doing something stupid or going crazy and damaging the ship's systems.

You would need a lot of confidence in your rocket, the crew, and especially with the environment control system, as such have often failed. Including the 2005 oxygen generator failure and oxygen issues in 2011 on the ISS. We haven't had a major failure for over ten years, agreed. But many minor issues. Have we put all major failures behind us for good? If something like this happened, they might know already that there is nothing they can do when they leave Earth orbit, with too much delta v to get back again, and with an environment control system that can't supply them enough oxygen for the round trip or a slow build up of noxious gases (there are many such that can build up in a human occupied spacecraft and have to be scrubbed constantly), or whatever the problem is.

The problem here is that you don't know what is going to fail until it happens, and the environment control system and other spaceflight systems are immensely complex. Of course you'd have astronauts experts in repairing the systems, as was the case with Apollo 13, where Jack Swigert, on the flight as the result of a last minute change of crew, had literally "written the book" on command module malfunctions. But depending on what goes wrong, they still might not be able to fix it, especially with a fix to last for 500 days to over two years (for the first flyby missions of Mars).

I'm not saying that we have to stay close to Earth for all time, but we do need to test it closer to Earth, for similar mission durations before we send it as far as Mars orbit.

The Moon is a destination you can reach in two days. If you have an Apollo 13 type disaster, there is at least some chance of recovery from it, as for Apollo 13. Apollo started off with orbital missions around Earth, docking tests, flybys of the Moon, finally a test of all the systems with Apollo 10 going right down nearly to the surface but not landing, before they landed anyone there, and it was still a high risk mission.

In the same way we can do long duration missions on the Moon or at lunar L2 which gives you the opportunity for a mission without Earth in the sky and with no direct communication to Earth. There's plenty of motivation to do that - a mission that can survive two years without resupply would cost far less than one that needs to be resupplied on average every three months. Then, if you can send a mission there, that lasts for 2 years without resupply from Earth, and probably more than one such mission, then perhaps you are ready to set off on the first multi year deep space mission to Mars without lifeboats. It's still a major risk, but more on the lines of the risks run by the Apollo astronauts, an acceptable level of risk of the type that test pilots face.

OTHER PLACES WE CAN EXPLORE, AND POTENTIALLY SETTLE, IN THE SOLAR SYSTEM

Once we have longer duration missions,we can then go anywhere in the solar system, as explorers at least, Antarctic base type settlements, and perhaps eventually living there permanently. 

There are many possible futures here, such as exploring with humans traveling throughout our solar system but not colonizing, settling places other than Mars, such as the Moon, using materials from Near Earth Objects to construct habitats for humans, doing the same with the asteroid belt, settling on Mercury, Jupiter's Callisto, the moons of Mars, the upper atmosphere of Venus, and settling on Mars but at a later date, as not our top priority right now.

The key to settlement, other than for scientific research, tourism and such like, I think is reducing the maintenance cost. But it would make this article long to go into such things here, see my section on Maintenance costs in case for Moon first.

Suppose eventually we do have self sustaining, low maintenance colonies throughout our solar system, what next? That leads us to a larger philosophical question.

IS IT GOOD TO BECOME A GALACTIC CIVILIZATION?

Then what about the larger view. Should we set out with the aim of becoming a galactic civilization? This is a question that space colonization enthusiasts rarely ask. It is just assumed that spreading through the galaxy is good. The sooner we do it the better, according to most enthusiasts. 

But, let's stop and think. Is that true? What would the consequences be?

IS IT PRACTIALLY POSSIBLE

First we do need to ask if it is possible at all. And there, I would say that yes, practically, I think we could start this in the near future on timescales of centuries, maybe less. There are many habitable planets out there. These are just the best ones we know of to date:

There must be many more that are not discovered yet. And of course there's the newly discovered probable planet around Alpha Centauri.

THIS IS NOT STAR TREK THOUGH

There’s a tendency to think it would be like Star Trek. But that’s a very idealized picture and even then you have the likes of the Borg in their universe. What happens a million years into their future in their timeline? What happens to the various large scale conflicts that have already happened just a few thousand years into that new era of space faring civilizations? Even the rather idealist Star Trek universe has many seeds for potentially endless chaos a few thousand or million years down the road.

But Star Trek is just the products of talented script writers’ imagination, not future casting. It makes lots of implausible assumptions. For instance, it has as its basic premise that ETs throughout our galaxy have similar exobiology and often even compatible DNA, can all eat each other’s food nearly always, and most are basically humanoid, looking more like each other even than humans look like gorillas, never mind elephants or parrots or octopuses. And all arose independently around multiple star systems at the exact same moment of time to within a few thousand years. 

They have an “in house” explanation based on the ancient humanoids who billions of years ago seeded worlds throughout the galaxy and who were so advanced they could arrange for them all to evolve humanoids like themselves at the exact same moment more or less, billions of years into the future. 

Right. And borders between great empires of many stars, light years apart are measured and patrolled exactly to the kilometer. And if you are threatened by an exploding star, you get out of danger by backing away by a couple of hundred thousand kilometers (less than the distance from Earth to Moon). It’s fun but it is a product of script writers, and requires a lot of suspension of disbelief if you are scientifically minded. I greatly enjoy star trek actually, am in the middle of watching the entire series of Star Trek Voyager again just now :). But it's not future casting :).

Ancient humanoid who in the Star Trek universe seeded many planets with life which independently evolved to humanoid lifeforms which then evolve space faring technology within a few centuries of each other 4.6 billion years later

WHAT WOULD HAPPEN IN OUR REAL UNIVERSE?

Instead, imagine a galaxy that is filling with peaceful colonists, yes, maybe some pirates etc as in Star Trek - but let's make it so they are all originating from Earth (vastly more probable than that they arise simultaneously on multiple worlds at the same time).

Then amongst them, amongst those trillions of colonists on initially thousands and eventually tens of billions of habitable worlds of our galaxy, you will be sure to have the likes of ISIS, North Korea, Hitler, whatever present or past figures you imagine as the worst we could have. But now give them the technology we would have a million years into the future. They have the ability to make self replicating machines probably, and cyborgs, and super intelligent uplifted animals of all sorts made intelligent by DNA manipulation.

Now let them expand through the galaxy - so that they are beyond reach. Assuming we don’t get warp drive, which is just  sci. fi. at present - then if some of our descendants are a thousand light years away, only a hundredth of the diameter of our galaxy, we wouldn’t know what they have done more recently than a thousand years ago.

EXPANSION OF A SOCIALLY PRIMITIVE, TECHNOLOGICALLY ADVANCED CIVILIZATION THROUGHOUT A GALAXY

We are technologically advanced, yes, at least compared with previous generations here on Earth, if surely not compared to a million years old technological civilization - but that example shows that socially we have a lot of catching up to do to be ready to use this technology on a galaxy spanning scale. I can’t see any way that such a galaxy could be anything but total chaos. This expansion of our technological civilization into a galaxy would also quickly go exponential, and favour the most aggressive and most rapidly reproducing species, the ones with most children and that reach adult hood and have their own progeny as quickly as possible, or reproduce using cloning.

The most aggressive colonists wouldn’t need to set up stable civilizations on planets even, as they would surely by then have the ability to live in the Oort clouds, with fusion power. In that future, maybe just a few centuries ahead, small groups, even just family groupings or a few friends on a whim, could just hop from one comet to another throughout the galaxy. There would be no limit to this comet hopping, as those clouds of comets mingle and probably spread most of the way to the nearest stars, probably all the way to nearby Oort clouds around other stars, and then onwards and outwards from one star to another.

FILL GALAXY AND THEN UNIVERSE

They could then fill our galaxy in the not so distant future. A long time perhaps in human lifetimes, but compared to geological time, it would be just a blink of time. Probably it's already getting crowded throughout the galaxy in less than a million years. Crossing over to other galaxies is quite a challenge - but the space between galaxies is not devoid of material. There are stars in between and gas clouds also, and with the technology we’d have by then we’d probably be able to move planets with fusion mini suns, at close to the speed of light, a bit like the Puppeteers in the Larry Niven novels. There are parts of the universe disappearing beyond the speed of light horizon under its expansion, so without warp drive our expansion would have limits, but we could spread to millions of light years, eventually billions of light years in every direction. If we set it off here, without foresight and planning, that’s surely the inevitable outcome, with no possibility of stopping it once it has been underway for a few centuries.

So - in principle, yes we could fill the galaxy and much of the universe also, at least of the part closest to us.

But, if we did it as we are now we wouldn’t turn into a galaxy and universe spanning civilization.

We’d become a galaxy and universe spanning chaos of beserker robots and creatures just running amok. Not our selves necessarily, our creations, self replicating machines, uplifted lifeforms, synthetic lifeforms and so on.

NIGHTMARE FUTURE

I think a future like that would be the worst nightmare not just for us but for any beings in this galaxy and eventually the entire observable universe. If someone somewhere establishes a peaceful spot in the galaxy - they would never know when some horde of beings with strange ideas would suddenly appear having developed for a thousand years over a thousand light years away - and then arrive at close to the speed of light using unfamiliar technology. They would be our distant cousins, or the creations of our cousins, but that wouldn’t help. And once started, how can this ever stop?

Even if people learn to co-operate in one part of the universe - that’s only going to work in a small region, perhaps a few tens of light years in diameter. Beyond that, the chaos would just start up again or rather just continue without check.

IMPOSSIBILITY OF EXPONENTIAL GROWTH FOR MORE THAN A FEW MILLENNIA

Also you just can’t keep expanding exponentially. Surprisingly quickly, even doubling only once a century, within a eighteen thousand years, even with warp drive, you reach the limits of what is possible with the matter available in the universe. Even if your colonists become so small that each one masses less than an atom, through some unimaginable future technology, that just adds an extra ten thousand years to the length of time you can continue at that doubling rate before you run out of enough matter in the entire observable universe to make colonists.

If you don't have warp drive, then the crunch time comes much sooner, within a few millennia.

Populations don't have to expand exponentially. We have already reached peak child and the middle of the range predictions for the future see our population leveling off at about ten billion people. At the moment our population is continuing to grow only because the average life expectancy is increasing - people world wide are living longer.

So why not stop at a point where you have some reasonable control and reasonable prospects for your civilization for the future? For more on this including the calculations, see Why ET Populations Can't Continue To Expand For More Than A Few Millennia, and Why Only Very Young ET Civilizations Will Have Expanding Populations - Opinion Piece

SO NO SENSIBLE ET WOULD BE A GALACTIC IMPERIALIST

For these reasons I think that no sensible ET would set out to colonize a galaxy. Not in an imperialist conquering way at least. If they do explore the entire galaxy it will be in some way that involves restraint and has minimal impact on it, chances are.

There’s plenty of space. We could have countless trillions of us in a single star system. There’s enough material in just the asteroid belt of our solar system for a thousand times the land area of Earth - so that is already a population of trillions living in comfort if we develop that technology.

SETTING UP NEW COLONIES BEYOND THE HORIZON OF COMMUNICATION WITHOUT FORESIGHT - LIKE A NUCLEAR BOMB IN YOUR HOME CITY

So, I expect our galaxy to quite possibly have trillions of colonists from various ETIs, but they would keep them localized in a small region. Setting up new colonies “beyond the horizon” of your civilization's space travel capabilities without foresight about the possible effects on the galaxy would be seen like letting off a nuclear bomb in the middle of your home city. Something that no sane person, or being, would do.

I think they, and we also, should use robots instead. The robots, paradoxically, are safer for the galaxy, just because we can, ethically, make them completely safe by design. We can make self replicating machines, designed with many safety restrictions so that they can't be a nuisance to the galaxy (e.g. telomere style maximum number of generations, and a "keep alive" signal from Earth so that they stop functioning and self destruct if they no longer get it). 

We evolve because our replication system has inexactness built into it, with the possibility of error. Creatures with inexact replication systems would out compete the ones that replicate exactly every time.

Self replicating robots are designed rather than evolved however, and can be designed to replicate exactly with many failsafes and no chance of error. The technology could be be tested on a small scale locally first. There are many things we could do to keep them safe. And to start with, not use replicating robots either, just lots of low mass but highly capable robots, with identical copies sent to many destinations in the nearby region of our galaxy. For more on this, see Self Replicating Robots - Safer For Galaxy (and Earth) Than Human Colonists - Is This Why ETs Didn't Colonize Earth?

WE CAN BE ONE OF THE WISE ETS

I think, there is evidence that we may be wiser than the most reckless ETs possible. the ones which might destroy themselves in space wars pretty much as soon as they begin on spaceflight. Carl Sagan refers to this as "the intrinsic instability of societies devoted to an aggressive galactic imperialism".

Though we have stumbled a lot, we have made many good decisions, such as dealing with the problems of DDT and CFCs, human rights (a lot of progress though much still to do), preventing chemical and biological warfare (even in the almost all out conflicts of WWII neither side used the chemical weapons of WWI, I know there have been exceptions but most wars don’t use them).

We've developed nuclear weapons, and yet, for decades we haven't used them. Indeed Carl Sagan suggests that maybe nuclear weapons are the deciding factor here. After talking about our own efforts to deal with nuclear bombs he then goes on:

"If every civilization that invents weapons of mass destruction must deal with comparable problems, then we have an additional principle of universal applicability. Weapons of mass destruction force upon every emerging society a behavioural discontinuity: if they are not aggressive they probably would not have developed such weapons; if they do not quickly learn how to control that aggression they rapidly self destruct. Those civilizations devoted to territoriality and aggression and violent settlement of disputes do not long survive after the development of apocalyptic weapons. Long before they are able to make any significant colonization of the Milky Way, they are gone from the galactic stage. Civilizations that do not self-destruct are pre-adapted to live with other groups in mutual respect."

He goes on to say that because we have only just reached this stage then this future scenario of mutual respect may seem unlikely because of our short term perspective, and that the required changes may take a thousand years or more for us to reach maturity as a species. From Carl Sagan's "The Solipsit approach to Extraterrestrial Intelligence", 1983 .

We’ve prevented starvation with the often forgotten Green Revolution between the 1930s and the 1960s, stopped nearly all whale hunting, done lots of work to preserve species and environments etc.

If you compare our present world with what it could have been without all those initiatives - I think it gives room for optimism for the future too. And I think we’ve made an excellent start on peaceful use of space with the Outer Space Treaty.

Although it’s frustrating that we don’t have warp drives or even the Star Trek “Impulse drive”, and easy ways to build habitats in space, I actually think it helps, that space is so hostile. Hopefully by the time we figure out how to live sustainably in space habitats, we also have figured out how to do it peacefully, or reasonably so. With competition of course, but more like the Olympic Games than WWIII.

Hopefully we can become more forward looking as we continue to colonize space. Perhaps the increased resources from space can help us to become more peaceful if we can handle it right.

If so we might well eventually have a chance to explore even our entire galaxy peacefully, and without harmful consequences to ourselves and other intelligent species that may exist in our galaxy. And if we meet ETs then they also I think would be ones that have figured out how to explore the galaxy in a similarly peaceful way.

THIS IS A GREAT FILTER IN THE FUTURE - ANY EXPANSIONIST ET MUST ALSO HAVE MINIMAL IMPACT - OR ELSE - WE ARE THE FIRST SUCH, AT LEAST IN OUR GALAXY

I’m pretty sure there can’t be any aggressive exponentially expanding ET out there (such as we could become potentially) except by some amazing coincidence. That’s because unless they started on their expansion less than a million years ago, a tiny slice of the age of the galaxy, they would have occupied Earth and our solar system already, indeed, probably long before we evolved, and would have been fighting over the spoils ever since. 

Any ET that managed to expand to a few star systems or to the Oort clouds, also, I think can never stop expanding, short of warp drives. That's because if there are any of their species left anywhere in the galaxy that are expansionist, they will start up again, and take over from all the others that give up. How can that ever stop? Even if they started billions of years ago, they would still be at it, I think, even if most of them retreat into Dyson spheres or whatever it is they do, the few who don't would continue to expand through the galaxy., over and over

So any "great filters" ahead of us have to operate before any ETs out there start on any major push of galactic imperialism.

I think the most likely reason is either

1. We are the first, so then it's our responsibility to make sure we take good care of our galaxy, and leave it in a decent state for future generations and civilizations just as we would want earlier civilizations to leave it for us.
   
   If so, we must be exceptionally far sighted and careful as our decisions can reshape the entire galaxy for ourselves and other intelligent creatures here for all future time. If we get this wrong, we can turn it into a never ending chaos that no-one will ever be able to calm down, neither ourselves nor any other species in our galaxy, nor any of our creations or their creations (short of warp drive and incredible technology).
2. Or if there are other technologically advanced ETs, they have all found some way to explore the galaxy without major impacts on it, and without doing this uncontrolled galaxy cramming exponential expansion. Or else they just “stay at home”. And the reason for that would be because the far sighted ones show restraint, eventually, maybe only after centuries or millennia of social development, or maybe quite quickly. And the ones who can't do that simply never get that far.

WILL WE BE A WEED OR A FLOWER IN THE GALAXY

The anthropologist Mary Dora Russell says:

'Anthropologists used to say that Homo sapiens was a unique and special species because we were the only ones who used tools, or who were self-aware, or had language, or passed culture to our offspring… Then we started finding out that chimps and dolphins and crows and African grey parrots and snow monkeys were making a mockery of our pretensions to uniqueness, so we’ve kind of shut up about all that in recent years.

If you want a nice reductive definition of our species, I could defend this: “Human beings are bipedal tailless primates who tell stories.”

That’s probably just as stupid as earlier definitions, but it’s catchier than my other version, which is

“Human beings are a dangerous, invasive weed species that has invented central heating, air conditioning, and food that can be stored for up to ten years, so not even a direct hit by an asteroid would likely make us extinct.”'

When nothing else matters, by Mary Dora Russell

I think that’s rather how I see the future of us in the galaxy if we just expand into it without foresight. But far worse than a weed on Earth. We’ve unnaturally made ourselves almost impossible to go extinct already by our technology, and if we expand through the universe without evolving social breakthroughs of some sort, to catch up with our technological breakthroughs, I think we’ll become the ineradicable weed of the galaxy. But harmful to ourselves as much as to everyone else, and able to create even more dangerous replicators through our technology.

It's not just us. Our galaxy may well contain many non technological species, for instance intelligent fish-like or octopus-like creatures, living in the oceans of icy moons, or ocean planets, where they have no chance to develop control of fire. Or creatures that are just not very strong, and don't have good "hands" like us for manipulation, like parrots or crows. Even an elephant would have a lot of trouble building a fire and smelting metal. Ancient civilizations, perhaps advanced in mathematics, art, poetry, music, perhaps socially very advanced, yet without technology they would be especially vulnerable to a new technological species spreading out of control like an ineradicable weed through the galaxy. Out of all the intelligent creatures on Earth, I think only humans also (and the other great apes) had a decent chance of developing technology based on fire, even with intelligence. I can't see parrots, crows, dolphins, whales, octopuses, elephants, dogs, cats, developing technology like us, with tools, fire, metal working, however clever they became and however skilled at communication. So if that's a good basis for generalizing, then the non technological civilizations universe wide may well outnumber the technological ones many to one. So, even a billions of years old civilization could still be highly vulnerable to a few centuries old civilization of technological ETs such as ourselves,

I think any sensible ET will look at that possible future for themselves and the galaxy, and find a way to become a flower of the galaxy instead of a weed that will eventually choke all the species in the galaxy, including themselves. If they can’t see a way to a future like that, then if they have any sense, they just stay at home until they can. And if they haven’t the sense to do that, I think, perhaps, that they either make themselves extinct, or they keep destroying their own spaceflight capabilities, and get nowhere, until they develop some sense.

Let’s be one of the civilizations in our galaxy and universe that flowers like a beautiful flower.

Let’s be one of the civilizations in our galaxy and universe that flowers like a beautiful flower.
- Amazonian giant water lily.

Or a field of flowers

Or we might even give rise to many flowering human originated civilizations each unique in its way, and all sufficiently in harmony with our galaxy or universe not to be a nuisance to each other or other ancient civilizations in our galaxy.

See also the section in my Case For Moon First: Further into the future, what about habitable planets around other stars?

See also: Robert Walker's answer to Will humans become extinct before we can colonize other planets?

You may also be interested in my article published today in Forbes magazine: Is there a Fortune to be made on Mars?

This answer is partly based on extracts from my book

Where to find "Case for Moon First "

• Buy this article from Amazon as a booklet for kindle
  
  Case For Moon First: Gateway to Entire Solar System - Open Ended Exploration, Planetary Protection at its Heart - kindle edition
   
• Read it online on my website (free).
   
• Some of you might also find Pocket useful - I've been suggested this by a keen reader of my posts. It lets you read articles offline without any internet connection. It's free (with a premium version which few people need).

The kindle book may be useful if you want it formatted as a book, which you can read on your kindle, and also on most major smartphones, tablets and computers, using the free kindle reading app.

FACEBOOK GROUP

I've made a new facebook group which you can join to discuss this and other visions for human exploration with planetary protection and biological reversibility as core principles. Case for Moon for Humans - Open Ended with Planetary Protection at its Core

SEE ALSO

Robert Walker's posts - on Quora

And on Science20

Robert Walker's posts on Science20

KINDLE BOOKSHELF ON MY AUTHOR'S PAGE

And I have many other booklets on my kindle bookshelf My kindle books author's page on amazon