I think we should build our first offworld backup on the Moon. We can start by storing seeds there, similar to the Svalbard seed vault in Norway. Within a few years we should have easy access to the Moon, and then it will be easy to do. The lunar caves are naturally at the right temperature. Add a vacuum sealed packet of dried seeds to a rover that explores a suitable lunar cave, and leave it there at the end of the mission, inside the rover, and that's it.That's the start of a future seed vault. From small beginnings ... 

Elon Musk says there are two futures, to stay on Earth and eventually go extinct, or to become a "multi-planetary species". He says Mars is our "plan B". But there is a third possibility. 

We can protect and preserve Earth so that we don't need to go extinct here either. And go into space as well. Maybe eventually we have settlements that fill our solar system right out to Pluto and beyond. But we are doing this with a secure Earth behind us, a planet that we have preserved and kept habitable and in tip-top condition. Isn't that a better future to aim for?

He talks about a new colony that people can go to, if we need to abandon Earth. But if we can’t look after our Earth, which is so hospitable to us, with the temperature just right, fresh water often falling from the sky to water our crops, air we can breathe, trees, plants, our food can grow almost anywhere, protection from cosmic radiation, solar storms and UV all filtered out naturally - if we can't look after this beautiful and so habitable planet, ideal for humans, how can we possibly build a civilization on Mars, with conditions so unbelievably harsh and difficult for us?

Yes a backup is good. We already have them with the seed vaults. It just makes sense. The more backups the better.

However, if our aim is to look after Earth, as our first priority, the Moon is by far the best place for a backup, starting with an off-world seed vault. It is easy to reach from Earth (when we have the technology once more to get back there) and geologically stable, and the caves are at just the right temperature naturally. Some of the lunar caves probably have an internal steady temperature of around -20 °C (see page 5 of this paper). This is similar to the -18 °C for the Svalbard Global Seed Vault and should be perfect for a passively cooled off world seed bank. Many seeds can survive for centuries in conditions like that before they need to be revived.

Seeds can be preserved for even longer periods, up to thousands of years, in liquid nitrogen. For instance fresh lettuce seeds have a projected "shelf life" of 3400 years in liquid nitrogen. Temperatures in the lunar cold traps can get as low as 38 K - far lower than that of liquid nitrogen which freezes at 63 K and boils at 77 K).We don't have anywhere on Earth where passive storage permits temperatures so low, even in Norway.

It's a place to put a library too. A backup of all the knowledge of humankind, designed to last for thousands of years.

Of course, eventually, you would have more than one copy of all the seeds. the library, and other materials on the Moon. You would need that as precautions on the remote chance that one of your backups gets a direct hit from an asteroid hitting the Moon. That's about the only thing that could happen there, and would be exceedingly rare, especially if it is located in a cave protected beneath the surface at a depth of many meters. And the Moon is safer than Mars for this, as it gets only a tenth of the impacts of Mars, so close to the asteroid belt.

That is all you need. You don’t need to go to any other planet or celestial body as there is no conceivable near future disaster, in the next few hundred million years, that would destroy the Moon. Even a huge impact would only make a new crater there. A seed and information vault elsewhere on the planet, in a lunar cave, would be completely unaffected. Of course there is no harm in extra backups. There are many other places we can do it, e.g. if someone was going to set up a colony in L4 or L5, or you could include a backup in a satellite in geostationary orbit etc. But the Moon has the advantage that it is close by, that we are going there anyway in the near future with some robotic missions already planned for this year and the next, and that it has conditions there that are ideal for preservation of seeds and information with passive cooling.

William Burroughs explored this in his book "The Survival Imperative, using Space to Protect Earth". Then, with Jim Burke, who was a project manager for Apollo, he presented a Doomsday Ark on the Moon which has interested ESA scientists. See also this account in National Geographic.

"In case a larger part of human race is wiped out along with plants and animals, the survivors will need information about Earth that will help them in the future. The European Space Agency is thinking about a base on the moon that will serve as an encyclopedia that will preserve the information about our planet, including data on plants and animals."

"The moon base will include information that people will be able to easily access. The encyclopedia on the moon will allow people that survived on Earth to get a better understanding of things. It will help the survivors reacquire various technologies."

"Using the DNA data, people will be able to revive different species of plants and animals that might have disappeared as a result of nuclear holocaust or asteroid collision."

Many others have explored the idea. See for instance the section about it in The Moon: Resources, Future Development and Settlement

So, it's a library and a seed vault also. Then, yes, we could have human caretakers there too. You don't need a million colonists for a "backup". A hundred caretakers would be fine to look after it. We now know that it's possible to sustain a human being using greenhouses covering just 30 square meters per person.That's using crops that can be harvested a month after seeding (surprisingly many including a variety of dwarf wheat), aeroponics and aquaponics. That's enough to guarantee human survival already.

There are some "recalcitrant" seeds that can't be preserved by drying and freezing, including avocado, mango, lychee and some horticultural trees, so those would need to be revived and grown from time to time. This would become more feasible in the more distant future as the core caretaker colony expands and perhaps occupies one of the many huge lunar caves on the Moon. A larger settlement could also have "backup" rare insects, animals, birds etc too.

A small to medium sized lunar settlement like that would be more than enough insurance for any possible disaster on Earth, even the most extraordinarily unlikely Hale Bopp type impact. I should say for those who are nervous about such things - there hasn't been an impact this big for over three billion years. Hale Bopp can't hit us in its current orbit and long term it's got only a 0.1% chance of hitting Earth every billion years. See my What will be the effects if comet Hale-Bopp hits the earth?

Rather than try to escape from such impacts and making backups - we should be making sure Earth can never be hit by something like that. And indeed we are, we are searching the space around Earth with increasingly more sensitive telescopes for anything that could endanger Earth. If we do find something with decades to centuries of warning, it becomes far easier to deflect it. There is no risk at all this century from large short period asteroids and comets as we have found all the ones of 10 km or larger. And there is almost no risk from large comets also as they are very rare at present. The closest any comet has got for centuries is six times the distance to the Moon (Lexell's comet in June 1770)

For more on this see my answer to How vulnerable are we to a catastrophic asteroid impact? Are there people whose job is to watch for them?

However, even if we got something like that, our oceans boiling for a year and a hundred meters depth boiled away - as soon as the Earth's surface cooled down, the only sensible place to terraform in our solar system would be the Earth. Reintroduce photosynthetic life, plants, animals, ourselves. Some life would be there still in the ocean depths. Oxygen in our atmosphere has a half life of thousands of years so the atmosphere would remain oxygen rich and breathable.

But that wouldn't need to be its primary purpose, as a human extinction event is extraordinarily unlikely, almost impossible. The solar system is a lot quieter now than it was back then, over three billion years ago, the last time we had impacts that large. For more on this and other events like supernovae and gamma ray bursts (those also are not possible in the next few centuries), as well as manmade disasters, see my Could anything make us extinct in this century?

Right from the start, even if we have no caretaker colony there yet, it's a backup in case that some disaster happens on Earth that destroys the repositories here. That doesn't need to mean losing ability to go into space, and if we can then we can go to the Moon to recover the seeds that were lost. If you lose that ability then you can't go up there to get the seeds, but still, they are there safe until whenever you do get the ability again. Looking in the very long term, even a thousand years from now, they would be able to look up

Then as well as that, the ARC idea is to have a library that you can look up via radio from Earth so you don't need spacecraft, you can just use radio.That way, you can read it even if for some reason our civilization loses the ability to send spacecraft to the Moon.

Their website is here: Alliance to Rescue Civilization.

"In the event of a major catastrophe, for example worldwide plague, comet impact, nuclear war or social collapse, the staff of ARC will function in a rescue capacity rather than as librarians. They will be prepared to help the survivors reestablish a functioning technological society, or in the worst instance, to repopulate the Earth themselves, and re-introduce the additionally needed biological species here. The primary mission of ARC will be to secure our tenancy of this planet, although it is fully compatible with plans to extend human settlement beyond the Earth-Moon system. ARC will provide our manned space program with the central purpose which it has so sorely lacked, linking it firmly to human survival on our home planet and elsewhere. The ARC facility will stand as a visible and inspiring symbol of our aspirations, one which can overcome the negative connotations associated with disaster relief. With ARC in place, of course, other scientific and commercial uses of space will be facilitated. ARC can serve as an engine that pulls many freight cars. "

As long as you can communicate via radio you can look things up in the library on the Moon. And - if we lose the ability to go to the Moon - well the seeds are still there for us when we regain the ability. Of course you continue to have the seed collections on Earth. It's not the idea of doing it instead, but as well as them.

So we can start like that, and then later on develop a caretaker settlement of a hundred people or s, and then later expand into thousands of people there and a large agricultural area in a lunar cave which you can use for plants, animals and insects that can't be cryopreserved for centuries in liquid nitrogen and that need to be continually cultivated to survive.

A focused backup like that would be much more effective than a rather nebulous idea to start up a civilization on Mars in the hope that it might help Earth in some way in the case of some future disaster.

Mars may look like an Earth desert, in the colour corrected white balanced photos - like frontier territory where you just need a bit of care and attention to grow crops. But you can't even breathe there. Not only that, the air is so thin that you can't survive even with an aqualung. The moisture lining your lungs would boil at blood temperature and you can't get even a single breath. You die quickly without a two million dollars good for a couple of dozen spacewalks. The spacesuits are so complex and have so many tasks to perform that I think it's optimistic to expect even a ten fold reduction in price to $200,000. And I think it will be a long time before we have spacesuits that can be used for years without frequent repairs and regular replacements by new suits. See What is a Spacesuit? (NASA).

At night it gets so cold that for 100 days of the two Earth year long Mars year dry ice covers the sands with frost even at the equator. How can we hope to make that into a better place than Earth before we have learnt to look after our Earth properly, which is such an easy place for us to live?

The longer term aim of all this is to protect and preserve Earth, as the precious "pale blue dot" of Carl Sagan, the only place in our solar system that is so habitable for humans, and the planet we evolved on. As he put it in 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."

And yes, longer term we may get humans in controlled environments in space, and maybe eventually these can be so self sufficient and easy to repair that they are easier places to live than Earth itself. However Earth I think would remain precious to us, as a place to protect and cherish.

National Geographic quote Kilian Engels as calling this "Plan B"

"Plan A involves creating an international network of astronomers to scan space for asteroids and comets that might threaten Earth, a global task force to formulate a strategy to prevent impacts with the planet, and a new generation of spacecraft to carry out these missions,"


With this perspective, then there is no longer a desperate need to try to colonize Mars as quickly as possible. We can start with small scale backups on the Moon, as the easiest and most convenient place to do it.

To start with it's a knowledge bank and seed bank, completely automated with nobody on site, and you can interrogate it via radio from the ground. This would be very easy to set up. If we lose the ability to get to the Moon, still, the vital seeds are there preserved for centuries through to millennia in liquid nitrogen for whenever we regain the capability.

After that there can be small caretaker settlements of up to maybe 100 people with the aim to become as self sufficient as possible so that they can remain on the Moon even if we lose the ability to get there from Earth (with the ability of course to get back to Earth if needs be in an emergency). Then, depending how easy it is to live on the Moon it can expand to a larger settlement of a few thousand in a lunar cave, and at this point they can conserve crops and other seeds like mangoes etc that can't be cryopreserved and need to be continually cultivated, and some animals, birds, fish, insects etc. Then that can gradually expand. and continue from there.

If we need a backup, make it a proper focused backup. With this approach we can do our backups on the Moon, the logical place to start, and can afford the time to take a careful look at what effect our microbes would have on Mars before we send humans there.

The astronomer Lucianne Walkowicz of the Adler planetarium who works for the NASA Kepler mission and studies habitability of planets in conditions of high levels of radiation has a good way of putting it in her TED talk: Let's not use Mars as a backup planet

She says about Elon Musk’s plan:

“I totally disagree with this idea. There are many excellent reasons to go to Mars. But for anyone to say you need to go to Mars to backup humanity is like the captain of the Titanic telling you the real party is happening later on the lifeboat.”

So, to elaborate on her analogy - there is no point in a lifeboat if you have nowhere to go.

The natural place to go in our solar system is always Earth. It is far more habitable than anywhere else. So - if you have a lifeboat, you want one that you can use to return to Earth.


Colonies often do fail. The reason Scotland had to become part of the UK and merge with England originally was through failure of its attempt to set up a colony in Panama leading to an enormous debt. The Vikings failed to colonize North America even though we now know for sure they got there. Mars is far more inhospitable than any place that humans have tried to settle. Some people have even tried to settle the sea bed and this attempt failed, yet it is far more habitable than Mars.

Jacques Cousteau's underwater colonies from the 1960s

There have been many attempts at undersea habitats with the idea of eventually developing undersea colonies, since then. It has never taken off. Underwater habitat - Wikipedia

I see this attempt to set up colonies on Mars as like that. It’s far far easier to set up a colony on the sea bed than on Mars or the Moon. For a long time I think the only reason we will have habitats in space will be as research stations, as tourist destinations, as bases for explorers - much like the reasons we have underwater hotels and underwater research stations.

That's being realistic, not pessimistic. Where is your plan B if the colony on Mars fails, or comes to nothing after trillions of dollars spent on it?

Mars won’t be a lifeboat for Earth, any time soon. They are not the lifeboats for us. It's the other way around. We are like the land that the lifeboat can take them back to, the habitable shore if things go belly up on Mars. .


It would be like a paradise, to live in a city floating on the Pacific, or living in the harshest of our deserts, after Mars or the Moon. Breathable air. Protection from cosmic and solar radiation and even UV. You can go out of doors not only without a full pressure $2 million, or perhaps eventually $200,000 spacesuit, not only without an aqualung; you don’t even need an oxygen mask. You can open a window to ventilate your habitat instead of using expensive and elaborate machines to scrub the atmosphere and avoid “sick building syndrome” an ever present hazard in an enclosed space habitat.

If you use the advanced methods of agriculture developed for space colonies, with aeroponics and conveyer system gardening, and short growth cycles of only a month between seeding and cropping then

  • Four times the population of Earth would need about 0.5% of the surface area of the Pacific to grow all of its own food using space colony type technology, if available on Earth rather than in space. Here of course, it would be much simpler, lower cost and lower tech and easier to maintain.
  • Or - is it too much of an engineering challenge to design the seasteadings to cope with typhoons and rogue waves?

    We could feed the entire world from only 2.5% of the Sahara desert using salt water greenhouses and the space age technology for agriculture already developed for space colonies (especially with the Russian BIOS-3 experiments).

For details see my An Astronaut Gardener On The Moon - Summits Of Sunlight And Vast Lunar Caves In Low Gravity

If it ever does become possible to have self sustaining colonies in space - and yes, I do think that there is a chance of this some day - then our issues with living on Earth in a sustainable way will be over. Because our population is set to level off at around 11 billion.

We could feed many times that number from our deserts, or from just a tiny part of the Pacific using just the sea water, the salt and other chemicals extracted from the water and the air to breathe. Sustainably, with the rest of the Earth, if we want it, allowed to revert to totally natural conditions once again.

It would make sense to have a backup yes. But a backup normally means you use it to restore the original. A backup of seeds, of knowledge, a caretaker colony. All that is worth doing but not on Mars. The obvious place to do it is on the Moon. Close by, perfect conditions for a seed vault in the lunar caves.


And meanwhile Mars though it seems to have lost all of its life at first sight - on a closer look, scientists are not so sure. There are many potential habitats still to search. The RSLs (Recurrent Slope Lineae) are only the best known of them.

We have not yet even sent a rover to search any of these habitats. And there are a dozen or more radically different types of habitat to examine. These might be some of the top priorities for the search for present day life:

If we find life there, it would be tragic to lose it, in our rush to set up an unnecessary backup to our vibrant beautiful Earth. It might just be microbes, probably is, but microbes that are ET microbes. They may have a completely different biology inside,. Or they may be relics of an early form of life.

The only evidence we have of what might be life on Mars is from ALH84001 which has tiny structures that some still think could be life. We have alternative non biological explanations so can't prove either way if it is life or not. But if it is, the cells are far too small for modern Earth life.

The structures in this photo from the original ALH84001 press release are between 20 and 100 nm across, well below the resolution of a diffraction limited optical microscope of 200 nm.

This suggests that if this is life (of course we don’t know yet), it could be RNA world cells or some other very early form of life that hasn't yet developed the complex machinery of ribosomes and DNA and messenger RNA and proteins. It might consist inside almost entirely of RNA and a few enzymes made up of fragments of RNA stitched together to make ribozymes.

This is from over 3 billion years ago on Mars. But Mars has hardly changed over those three billion years. It’s possible that its life has evolved no further and that it still consists of those tiny RNA world cells.

This would be an astonishing discovery that would revolutionize biology. And it could be destroyed so easily, be so fragile. Whatever destroyed it here could make it extinct on Mars. We shouldn't risk this for a backup that we can do as easily on the Moon.

This is one of many possibilities, but others have their planetary protection issues too. Even if there is no life on Mars, what an opportunity that would be, to study a planet that is like Earth but has never had life there? How far did it get? Were there any life precursors that developed over billions of years on a planet with water, organics but no life? And if not, why not, and what did happen? If there is life similar to Earth, how similar is it, and how different? Are we connected and if so, how long ago did life on Mars part from our line of evolution. Does it have capabilities Earth life doesn't have? For instance there are three main types of photosynthesis on Earth. Did Mars life develop a fourth type we don't have?


It would be difficult for life to get to Mars from Earth, despite Robert Zubrin’s claims, because the only impacts that can do it are ones that send meteorites from the Earth’s surface so fast that they EXIT THE ATMOSPHERE AT THE ESCAPE VELOCITY OF ABOUT 11.2 KM / SEC. Think how fast they must be when they leave the ground. They would punch through the entire atmosphere as a blazing fireball. Any photosynthetic life particularly is likely to be burnt off completely as it would tend to be on the surface. And any life that remains would be deep within, and it then has to find a habitat on the almost completely dry Mars. It’s entirely possible that no life has ever been transferred from Earth to Mars, or that it has only done so in the early solar system when there were impacts so large they punched a hole in the atmosphere so that the meteorites would exit through a vacuum with the atmosphere already blown away.

When astrobiologists design experiments to search for life on Mars they say we have to be open to finding anything. Including life not based on DNA and evolved separately with a completely alien biology inside every cell.

You might think that anything that can live on Mars, so harsh compared to Earth, must be much tougher than any Earth life, and can't possibly be harmed by a few microbes that get there on our spaceships. But far from it. We have many microbes that could survive on Mars, chrooccocidiopsis for one, which can withstand the cold, vacuum, ionizing radiation and live on nothing but the sunlight, and a little night time humidity and the rocks of Mars. It's a "polyextremophiles" also perfectly at home in a tropical pool. Our habitats and spaceships are homes to trillions of microbes that can hitch a ride there in a spaceship. One of the microbes best adapted to the low pressures of Mars was found on a human tongue.

Humans aren't the issue, our microbes are. There is a reason why we sterilize our spacecraft that we send to Mars and why we keep Curiosity kilometers away from a mark on Mars that may be the site of a subsurface seep of water that just possibly might have conditions habitable to Earth microbes.

We can go into space yes - but to support and to protect Earth. And please, please, if we are sensible at all - let’s not rush humans to Mars.


The Moon is a great place for us to go. It’s closer, safer, far more interesting than we used to think. And also far more dangerous than many realize. 

The Apollo astronauts made it seem easy. But they were test pilots with a cool head and they saved the day many times, sometimes with quick decisions that would have lead to everyone dying if they had waited a moment longer before acting. Apollo 10 spinning out of control within seconds of hitting the lunar surface springs to mind as one of several such incidents. If they had been anything but cool headed test pilots able to make snap decisions in a situation where nearly everyone else would panic, they would all have died right then.

Going back to the Moon will be an adventure and a challenge and push us to our limits.

The retired Canadian astronaut Chris Hadfield, former commander of the ISS, interviewed by New Scientist, put it like this in their article "We should live on the moon before a trip to Mars"

"I think ultimately we’ll be living on the moon for a generation before we get to Mars. If the world and the moon were threatened and the only way to preserve our species was to launch from Earth, we could go to Mars with yesterday’s technology, but we would probably kill just about everybody on the way."

"It’s as if you and I were in Paris, paddling around in the Seine in little canoes saying, 'We’ve got boats, we’ve got paddles, let’s go to Australia!' Australia? We can barely cross the English Channel. We’re sort of in that boat in space exploration right now. A journey to Mars is conceivable but it’s still a lot further away than most people think."

Further into the future, once we do have the ability to go further afield, whether it is the next generation or the next decade, Mars is one of many places we can go. The solar system is wide open to us.


Just as for Mars, we need to learn to become reasonably self sustaining for long term missions to the Moon. Up to multi year missions then something like the ISS system is not much different from a more self sufficient mission. But once you get beyond three year missions it makes a huge difference to be self sustaining. There is nothing special about Mars here - it's the same for anywhere off planet.

So if we have bases on the Moon for more than a few years it is well worth sending sufficient payload to make it self sufficient. The same applies to the ISS - it has been in orbit more than long enough for them to have achieved considerable savings if they had focused on sustainability involving growing plants in orbit. Not just a few lettuces but entire modules devoted to it would have been well worth doing financially. If they had done this from the outset, it would have saved them billions of dollars over the lifetime of the ISS, and would have been psychologically beneficial too and helped develop our capabilities for future long term missions.

On the ISS it would probably need centrifuges, not a big deal for plants and there were plans to add a farming centrifuge module fully worked out but never built. This is a proposal from 2010 which explored the idea of a "Farm module" which combined artificial gravity with fish and plants in space.

From 2010, this module idea has a rotating cylinder in the center if I understand right. The fish tanks and tomatoes are suspended from it, and then further out you have lettuces shown below and dwarf wheat above.

On the Moon that's probably not necessary for plants.

For more about all this:

The Moon also has volatiles at the poles, it's hard to know so far how easy they are to extract but it could be a major thing. But also, the amount of overhead you need to send into space to enable self sufficient aeroponics along with some hydroponics is not that great either. If you are sending humans there anyway it's entirely feasible to just supply everything from Earth for a closed system self sustaining and expanding colony.

The carbon dioxide in the Mars atmosphere is not a benefit for agriculture as plants need only a tiny fraction of a percent, a few kilograms in the atmosphere, and humans are constantly breathing it out anyway and if they import any food at all from Earth then they will produce CO2 in excess that needs to be scrubbed in a closed system.

If you have closed system recycling then the atmosphere as a whole is only a small fraction of the total mass of a habitat. Even the amount of water needed is not great, with decent recycling, and hardly any of it is needed for agriculture, if you have aeroponics for most of the plants (a system of farming using a mist of water vapour suspended in air to water the roots and at the same time supply them with oxygen).

As for sunlight you have it 24/7 nearly year round at the lunar poles, at double the light intensity for Mars and without the dust storms that sometimes can block out sunlight on Mars for weeks on end. For the lunar caves, you can use LED lighting and with the BIOS-3 systems with 30 square meters per person efficient LED lighting optimized for agriculture amounts to a few kilowatts per square meter. This can be supplied by fuel cells during the lunar night with many other possible solutions (including molten salt) but it's also possible to beam power from the sunny side of the Moon using microwaves in a future more developed Moon.

It is easy to make solar panels in situ on the Moon covering large areas because of the hard vacuum permitting direct vacuum deposition of the panels onto regolith with the top few microns turned to glass with lunar solar panel paving robots. This technology is not possible for Mars where its lack of a hard vacuum actually becomes a disadvantage for this application, also the iron oxides - on the Moon there is nanophase pure iron pervading the lunar regolith which makes it easy to sinter it into glass using microwaves.

For more about this, see also my Astronaut Gardener on the Moon


In this way, Keeping as our top priority to protect Earth, and to protect the scientific and human interest of the solar system, there are many places we can explore. There are so many possible destinations for humans with no planetary protection issues at all.

Right now we can explore the probably vast lunar caves and go to the poles of the Moon and find out what the ice is like there.

We can even do astrobiology on the Moon. We can search for billions of years old meteorites that may have preserved organics, in a freezer just a few degrees above zero, from ancient Earth, and Venus and Mars. If there ever was life on Venus and Mars we may have almost perfectly preserved microbes and even fragments of larger lifeforms on the Moon from the entire history. And we almost certainly do have meteorites there with microbes and fragments of larger lifeforms preserved from all the major impacts on Earth, if there are indeed deposits of ice there that have stayed there unchanged from the early Moon. For more on this see my:

Also, to explore the probably vast lunar caves, to build telescopes, habitats, mine the craters for their platinum meteorites, maybe eventually build domed cities.

The Moon is also the place where we can make our first experiments in living long term in bases on another celestial body. We are close enough to Earth to get back in a couple of days in an emergency, we can rush supplies to the Moon any day of the year. But it is far enough away to be a major challenge to us as we learn to live safely and deal with the many issues that will arise in a space colony. We can find ways to stay there for years on end with almost no resupply from Earth, as will be essential for us if we are to explore safely further into our solar system. Once we can do that, we are ready for multi-year interplanetary voyages.

We can send humans to Callisto, outermost moon of Jupiter beyond its radiation belts, and a valuable source of ice. Europa, like Mars is one of the places in our solar system most vulnerable to Earth microbes. It is also right in the middle of the Jupiter radiation belt. Humans without radiation shielding would get lethal doses of radiation in minutes. Europa is the last place we should send humans if we have planetary protection as a priority. But Callisto has no issues at all and is more accessible as well.

Elon Musk in one of his presentations shared this as an artist's impression of his skyscraper sized BFR landing on Europa as a "refueling stop". But it would work much better as an artist's impression of Callisto. Europa though smooth from a distance, close up has turned out to be rough, covered in crevasses, with regions where there are ice blocks kilometers across moving and tilting and turning over, one of the most dynamic places in our solar system. It's also right in the middle of the Jupiter radiation belt, deadly to humans without protection. And it is a place where there is a deep subsurface ocean and there's some evidence there may be liquid water that's upwelled from it in a kind of "water magma plume" to within meters from the surface. This water as it freezes may be what causes the ice flows to move and turn over. This makes Europa one of the three places in the solar system most vulnerable to our Earth microbes, along with Enceladus and Mars.

Meanwhile Callisto, though rough from a distance, has a stable geology, billions of years unchanged, is outside of Jupiter's radiation belts and may well have smooth surfaces as seen close up just like this artist's impression. It is also easier to get to, not so deep in Jupiter's gravity well and also has a thick ice shell. It is perfect for Elon Musk's refueling stop. And has no planetary protection issues whatsoever, has the same planetary protection classification as the Moon, class II.

We can also go to the moons of Mars, to study it via telepresence from orbit, to the poles of Mercury (which also has ice like the Moon), to many of the asteroids, and eventually further afield, to explore the moons of Saturn, to Neptune's Triton and beyond. Other places probably have no planetary protection issues but this needs to be confirmed first - for instance to Venus and its upper atmosphere and to Saturn's largest moon Titan, with its Earth pressure atmosphere of methane and ethane. Bitterly cold, yes, but it's far easier to contain heat than to keep in an Earth pressure atmosphere, making it possibly one of the easiest places to set up habitats for humans in the entire solar system outside of Earth of course. There are many places to explore next after the Moon.

Meanwhile we don't ignore Mars. We have robotic spacecraft there exploring, but with broadband communications. Communication satellites around Mars would speed up our rovers hugely and is a small change we can do in the near future. NASA plan a satellite to increase the bandwidth to a massive 800 gigabytes of information a day. That's hundreds of superbly detailed multi-gigabyte images every day. Our rovers will get more autonomous, and we can eventually send humans to drive over the surface of Mars and explore it with humanoid telerobots from shirt sleeve environments on its moons or in orbit around Mars. We can master 100% sterile robots, a technology just on the horizon with the ability now to build electronics and other components, even cameras, able to work fine after sterilizing temperatures of 500 C. There already is a design for a cryobot like this to drill into the subsurface ocean of Europa, sterilized at 500 C throughout the journey out. Once we can do that, we can send sterile robots by their thousands, anywhere we want to search for life in the solar system.

Doing it this way keeps our future options open for Mars until we find out more about it. To rush humans there is closing off futures that may be wonderful, awesome, teach us amazing things about biology and about our galaxy and universe, and about what happens to a planet like Mars after 4.5 billion years of evolution. Let's look and see what is there first, before we rush in and turn it into something else with our inevitable microbe hitchhikers. Maybe we do eventually have colonies on Mars, who knows. But let's look at Mars first as it is now, and decide what to do based on understanding the present day planet first.

And if we want to settle the solar system we have plenty of places to go. There are so many places we could set up space settlements. As well as Callisto, Titan, the poles of Mercury, the moons of Mars, Venus cloud colonies, we can build habitats in the asteroid belt, with enough materials there for radiation shielding for a thousand times the surface of Mars, as slowly spinning the habitats for artificial gravity, or using two habitats tethered together lazily revolving around their center of gravity. They keep revolving due to angular momentum, and don't need any source of power to keep them going. It's like a bicycle wheel spinning endlessly on frictionless bearings.

Video fly through of a Stanford Torus style space habitat by Uzi Bento

Stanford Torus showing how the mirror works. The station is edge on to the sun, and the mirror is at 45 degrees to the sunlight and reflects the light to the habitat where it is reflected by more mirrors into the inside of the habitat. Image NASASunlight is usually brought to the habitat via mirrors in these designs e.g. here is how it's done for the Stanford Torus.

This is a modern update of the design, Habitat 2, with  a big 2 km across mirror (aluminized Mylar) - the animation leaves out the cosmic radiation shielding for artistic reasons: 
Video of Habitat 2 and mirror

Sunlight gets reflected around the cosmic radiation shielding into the habitat. UV light can be absorbed on the way. Cosmic radiation, as highly energetic particles, goes right through the mirror.

We can build habitats like that out as far afield as Pluto and beyond, using large thin film mirrors to collect the sunlight for our habitats. If we do that, then we will hardly notice even billions of years into the future when the sun goes red giant. Just move our colonies (or those of our very distant descendants millions of times removed) a little further out from the expanding sun.

Artist's concept of the surface of Sedna, a distant ice dwarf. All we know about it is that it's surface is red in colour (probably organics). This shows the milky way to the left and the sun is the bright object in the middle of that misty cloud (which on Earth we know as the "Zodiacal light" faintly visible on clear starry nights in places with superb seeing and no light pollution). The other bright star, second brightest after the Sun, is Spica.

With thin film mirrors to concentrate the sunlight, we can have habitats spinning for full gravity as far from the sun as this. The mass of the mirror would still be tiny compared to the mass of the settlement. Ice, meanwhile, is perfect for radiation protection and would replace the regolith used to protect colonists from cosmic radiation in habitats made from materials from asteroids and the Moon.

Another idea, proposed by Ken Roy, is to have settlements in underground toruses excavated (melted) below the surface of the ice with maglev used to keep the habitat spinning slowly for artificial gravity. Maybe Elon Musk's "Boring company" can diversify to help build sub-ice habitats on distant ice dwarfs?

This is an alternative positive vision. It's not an attempt to stop people from being adventurous and to explore the solar system. It's saying, let's be responsible and careful as we do so.

I cover this idea for a backup on the Moon in more detail in Earth best for a backup - maybe with a small knowledge and seeds library on the Moon with caretakers in my Touch Mars? book, and then in even more detail in Natural disasters - resilience of humans and in Backup on the Moon - seed banks, libraries, and a small colony, in my Case for Moon First book.

For some of the other topics touched on here, see also my

This post started as my answer to the question: Should we build a Doomsday Seed Vault on either Mars, Europa, and/or Titan instead of on Svalbard? on Quora.


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I cover the ideas for exploring the solar system in an open ended way with planetary protection at its heart in detail in my three new books on kindle and also available to read in their entirety for free online. They are designed to be read on a computer, with videos and links to click through, so are not available as printed books and I have no plans to make printed text versions of them.

You can read my Touch Mars? book free online here:

Touch Mars? Europa? Enceladus? Or a tale of Missteps? (equivalent to 1,938 printed pages in a single web page, takes a while to load) also available on Amazon kindle

The other ones are