Leaving the Cradle

By Owen Lewis | 8 July 2020

An artist’s conception of a human Mars base, with a cutaway revealing an interior horticultural area. (Credit: Nasa)

As long as we have existed, humanity has looked up at the night sky with a mix of wonder, fear, and curiosity. Our ancestors often worshiped what they saw; and along with the Tower of Babel incident described in the Biblical book of Genesis, one has to wonder how many other ancient pyramids and towers were built to try and get closer to the heavens. Humanity has always wondered what is over the next hill, around the bend, on the other side of the river, or across the sea; and now we are finally at a point where our technology has caught up to our natural inclination and desire to ‘go and see for ourselves.’

Since Galileo started observing stars and planets with the recently invented telescope, our knowledge of what exists beyond the bounds of Earth has increased exponentially. From Copernicus’s theory that we orbit the Sun instead of the other way around, to our realizing that the stars are other suns, to 1924 when Edwin Hubble made public that the Andromeda — previously thought to be just a nebula — was actually a galaxy like our own, to our discovery that the visible universe contains at least 176 billion galaxies and likely many more; our knowledge of the cosmos seems to be expanding faster than the universe itself is.

As astronomer Carl Sagan said, “Exploration is in our nature. We began as wanderers, and we are wanderers still. We have lingered long enough on the shores of the cosmic ocean. We are ready at last to set sail for the stars.” While the stars may be out of our reach for another century or two, our own Solar System is wide open and waiting for us to explore and expand into it.

. . .

We started off strong. A mere 66 years after the Wright brothers made their historic flight, Neil Armstrong and Buzz Aldrin set foot on the Moon. The 1969 landing on our nearest celestial neighbor marked a turning point in human history. No longer were we confined to the planet that gave us birth; we could step out and start to see for ourselves what wonders, treasures and challenges await us outside the borders of our first and still only home. Sadly, it was not to be. After the triumph of Apollo 11, only 6 more manned missions to the Moon took place; and in 1972 the last of the only 12 humans to have ever set foot on another world left and came home. We have yet to return.

About 3 centuries after Galileo and his telescope, Konstantin Tsiolkovsky — the father of modern rocketry — opined that “The Earth is the cradle of humanity, but mankind cannot stay in the cradle forever.” In some ways we are like a toddler who after having taken their first few steps, simply went back to crawling around on the ground; unwilling or unable to even repeat those first shaky steps. We went to the Moon — one giant leap for Mankind as Neil Armstrong so eloquently put it. But since then we have apparently been content to crawl around in LEO (low Earth orbit), building a few space stations and not much else.

By saying not much else, I mean in terms of human spaceflight. The science missions launched by NASA and other space agencies have often been wildly successful. Opening up the vistas of our Solar System to us, sights never before seen by human eyes. Our probes have been to every planet (even Pluto!), along with many moons plus a few asteroids and comets. The twin Voyager spacecraft have recently left the Solar System entirely and embarked on a fantastic, though slow by cosmic standards, voyage into interstellar space. Yet despite the triumphs and discoveries of our robotic emissaries, we have done little but cheer them on from the sidelines.

Now that finally seems to be changing. NASA has a new goal of returning the first woman and the next man to the Moon by 2024. Though manned spaceflight goals seem to change depending on who is in the White House, this one is backed up with a significant increase in NASA’s budget. It remains to be seen if the U.S. will return to the Moon in 4 years or 8, but one can hope that whoever wins the Presidency this November, the motivation to at least make it back this decade will remain. Unlike so many times in the past however, there are added spurs this time for NASA to set and achieve some definite goals in finally get humans back out beyond LEO.

One is private companies like SpaceX and Blue Origin, that are determined to get us into space no matter what governments do. They seem quite happy however to work with NASA in mutually beneficial relationships, the goal of which is everyone getting to space sooner. These sorts of partnership might revitalize NASA and help it move away from twiddling its thumbs in LEO and buckle down to moving humans farther out into space. Or it might be that a governmental organization is only necessary to do the basic research and take the first steps, at which point private industry is able to take over; similar to what happened with the internet. It remains to be seen which possibility plays out, but it might mean a faster and more efficient push into space if NASA and private companies continue working together instead of acting separately and each going it alone. The fruit of this approach can be seen in the recent success of SpaceX in launching two American astronauts to the International Space Station; a world-first for a private company.

The second spur is China. They have significant long term ambitions in space and their program has already had some big successes despite just starting to gear up. American pride and a desire to maintain pre-eminence in space will provide motivation arising from competition much as it did to the American space program when Sputnik was launched by the Russians in 1957. Of course, from a historical point of view a few centuries from now, whoever gets humanity back to the Moon, to Mars, and then even farther out will have gone down in the history books. It is of greater importance that we leave the cradle and don’t stay crawling around in LEO forever — who gets us there is secondary. The argument has been made that the cause of freedom and liberty would be better served by a country like the United States leading the way into space instead of an autocratic one like China; but if America refuses to step up and China does, our descendants will probably thank them.

Where Can’t We Go (yet)?

Regardless of who takes the lead, the most important question is: Where should we go?

To answer this, perhaps a helpful question to ask is where the easiest places in the Solar System to settle are located. Given the harshness of space, the easiest places will by default probably also be the best, and thus answer for us the question of where to go. To help us narrow down our options, let’s look first at where we likely cannot go, at least not in the immediate future.

. . .

Anywhere Past the Orbit of Saturn

Distances in our Solar System are vast. In fact, along with getting out of Earth’s gravity well, journeying to our planned destinations is probably one of the most difficult problems we face as we step out from the cradle. Earth sits at 1 AU from the Sun, a distance of about 150 million kilometres (93 million miles). Mars is at 1.5 AU, so about half as far out again as we are. From us to Jupiter is 4.2 AU, and Saturn sits at a distance of 8.5 AU, or just about 17x as far from us as Mars. Next out is Uranus, over twice the distance from us as Saturn. By this point, the distances simply become too large for conventional, ion, or even small nuclear rockets to get us (with large amounts of gear in tow) there in anything like a reasonable amount of time. With the development of more advanced fission or fusion rockets those distances will be far easier to traverse, but because we don’t want to wait further decades than we already have those bigger and better sources of propulsion will be ignored for the purposes of this article. Even getting to the moons of Saturn would be a long trip; but until adequately powerful nuclear rockets are developed, going farther will probably need to be left to robots.


Another place we probably do not want to go is Mercury. Closest to the Sun at less than half an AU, Mercury is just a little too close to our local flaming ball of fusing hydrogen for comfort. While it appears that there is some water ice protected in deep craters at the poles of the planet, the high levels of solar radiation, combined with scorching temperatures hot enough to melt lead during the day (which lasts almost 59 Earth days) fail to make the small planet very appealing. Though only 40% larger than our Moon, because of its density Mercury actually has a similar gravity to Mars. Therefore, if we find that Mars has enough gravity for humans to thrive, then by default Mercury does too. If it isn’t the lack of water or the gravity that are the problems, what is? The Sun. In addition to the problem of establishing a base anywhere but the eternally shadowed craters of the poles, Mercury is so close to the Sun any approaching spacecraft has to be careful not to end up pulled off course by our local star’s immense gravitation. Also, because you are deep within the Sun’s gravity well, it is more difficult to ‘climb’ back up to the Earth than it would be say from Venus, which is farther out. The only potential resource there which might be worth considering is Helium-3, which could be used in advanced nuclear fusion reactors. However, there are easier sources to be found closer to home on our Moon; so probably not a great reason to go and build a city.

Venus (probably)

Anyone who has ever looked up just before dawn or not long after sunset will have seen the morning and evening star. Venus is the brightest object in our night sky after the Moon, and has inspired much mythology and poetry. Sometimes considered to be Earth’s twin because of their similar sizes, the reality is that Venus is just about as different from the Earth as you can get on a rocky planet. With a thick atmosphere mostly composed of carbon dioxide, the surface pressure would crush you like a tin can and also roast you in temperatures that, like Mercury, are hot enough to melt lead. While the surface is definitely out — there are also clouds of sulphuric acid — building a home in the sky might be just the thing to make this hellish world feel a little homier. Turns out that around 50 km up in the Venusian atmosphere the pressure is similar to that on the surface of Earth. The temperature is also a lot more manageable at or slightly above those altitudes; and as an added bonus, you have similar levels of protection against radiation that we have on good old Terra firma. Because the atmosphere of Venus is denser that that of Earth, you could actually fill a balloon full of breathable air — with a few small, extra compartments of something lighter if you’re higher in the atmosphere — and it would float. The issue of dealing with the clouds of sulphuric acid has a surprisingly easy solution; just coat everything with Teflon, which is highly resistant to it.

Building a floating city on Venus sounds like it might be a great idea; in fact, NASA even considered a smaller version with a floating research lab in a now archived project called HAVOC. However, there are a couple glaring problems that may put a damper on near-term colonization plans. The first is resources: While water and oxygen are obtainable by using abundantly available solar energy to break down carbon dioxide (CO2) and sulphuric acid (H2SO4), finding other materials would be far harder. Any mining of the Venusian surface will need significantly better technology than we currently possess. So, the only real option other than the impractical one of relying on Earth for all imports is asteroid mining. This would likely involve either sending out mining craft and bringing back the extracted resources, or perhaps directing asteroids into orbit where they can be more easily got at. Either way, a significant impediment to a sustained human presence there larger than scientific outposts. The second problem is the winds. Anywhere high enough up in the atmosphere to be livable, the winds are at hurricane speeds and beyond. Now, they do seem to blow around the planet in relatively straight paths, but constructing a floating city that would race along with these winds would be a monumental challenge to overcome. Though I suspect these mountains will eventually be conquered, they knock Venus off the list of initial destinations where we should try to set up a second home.

The Moons of Jupiter

While we may be able to get there with near-term technology, most of these moons look to be pretty uninviting places for humans to set up camp. Assuming that anything significantly smaller than our own Moon is out due to low gravity (our own Moon may also be too low, but let’s just assume it’s manageable for the time being), there are 4 moons orbiting Jupiter that have potential interest to us: Ganymede, Callisto, and Io are larger than ours; then there is Europa, which is slightly smaller. Together these four are known as the Galilean moons because they are large enough that Galileo was able to discover them during his observations of Jupiter. It is interesting to note that being able to see what was in essence a Solar System in miniature helped him come to hold that Copernicus’s Heliocentric view of the Earth orbiting the Sun was correct.

Jupiter has the most intense and dangerous radiation belts of any of the 8 (or 9 if we count Pluto) planets in our system. Unlike our own Moon which is beyond the Van Allen radiation belts, all four Galilean moons are within Jupiter’s. In addition to the problem of intense radiation, Ganymede, Callisto and Europa are all thought to have an icy surface with an ocean below hundreds of kilometers in depth, making access to raw materials for construction difficult for potential human colonists. Mining the rocky moons of Jupiter, or nearby asteroids could be a solution, but that adds a layer of difficulty to any endeavours to settle them. Out of the three, it should be noted that Ganymede may have some rocky material, perhaps left by ancient impacts, at or near the surface. It also has its own magnetosphere, which provides some protection from the strong radiation belts of Jupiter. However, as it has no atmosphere there is really no protection against cosmic rays, and the overall dose of radiation on Ganymede’s surface would be around 8 rems per day, which is about 13 times what a human is exposed to on Earth every year. Not really a healthy environment to stick around in without significant shielding or perhaps living below the surface. Ditto for Callisto and Europa. In addition to the radiation problem, Io, the fourth large moon, is highly active volcanically. The surface is yellowish due the presence of sulphur; overall not a very inviting place.

The Asteroids

Asteroids are like oases in the vast desert of interplanetary space; which we will most likely be going to with increasing frequency to explore and mine. However, we are probably a ways off from tunnelling into the larger ones and building cities inside them as portrayed on the popular show The Expanse. Our probes have already been to several asteroids both large and small, and the Japanese craft Hayabusa 2 and the American OSIRIS-Rex will soon bring back samples from asteroids Ryugu and Bennu respectively. Mining asteroids to get resources ranging from water to platinum may soon become commonplace, greatly accelerating our own expansion into the Solar System. Metals conveniently concentrated in the remnants of protoplanetary cores like that of 16 Psyche — being targeted by the upcoming NASA Psyche mission — may be worth enough to economically mine and ship back to Earth, or perhaps to Mars, the Moon, or anywhere else we set up bases and colonies. Water mined from asteroids or dormant comets may make good refueling depos needed for ships heading back to Earth or Mars from various locale; or perhaps for miners moving from one metal rich asteroid to another.

However, the prospects are not as great for setting up a permanent residency for large numbers of settlers who may want to find a place for a new colony. The biggest problem is gravity, there simply isn’t much of it. At only 1/6th of what our bodies are adapted to, lunar gravity may very well cause problems for long term habitation that will need to be remedied, perhaps by medication or weighted suits (discussed more below). The Moon has plenty of gravity however compared to any of the asteroids, which likely means we physically cannot live on or in them long term until we find a way to deal with this.

Spinning an asteroid to generate artificial gravity is an obvious solution; but unfortunately, our ability to do that is not something we will likely have the technology to accomplish for decades yet. There are problems with that solution as well, for instance, we do not know how structurally competent a lot of asteroids are. While some are probably fairly solid, others appear to be not much more than aggregations of rock or gravel held together by weak gravity and maybe a bit of ice. If spun fast enough to generate sufficient gravity for our bodies to be healthy, such an object would simply fly apart. This does not mean that we will never colonize asteroids; many of them would probably be excellent sites for cities, especially with the protection from radiation all that rock affords. It simply means that the asteroids won’t be on the list of first places we go to build permanent settlements.

. . .

Where can we go now?

For all these places I have put on the ‘no fly’ list there are likely proposals and ideas for how settlement could happen. Maybe they’re right, and one day we will probably settle the Solar System extensively; but I believe that it is important to decide where to go first. To answer that, it is important to know where it is easiest to establish a large population — a city essentially — as opposed to just a research outpost. While not necessarily impossible to settle, these places I have just listed will likely not be the first places we go, because they are more difficult than the easier targets.

Now that we have looked at where we probably will not go first, let’s turn our attention to where we might.

. . .

The Moon

Whether we call it Luna like the Romans and many science fiction writers or simply ‘The Moon,’ our nearest celestial neighbor has lit the nights for us since we were hunter-gatherers. It is the first and so far only world outside our own that Man has ever set foot on. Averaging a mere 384,000 km away, it definitely qualifies as being in Earth’s backyard as far as distances in the Solar System go. Though the lunar regolith (soil) is bone dry, the Moon possesses what may be significant amounts of waster ice near the poles, especially the south pole; in shadowed craters that never see the Sun. This would be an excellent resource for potential colonists, both as a source of water and for making rocket fuel.

A potential downside to building cities on the Moon is the low gravity, only 1/6 that of Earth. Unfortunately, while we know that extended stays in zero g (zero gravity) are bad for human physiology, we do not know how much gravity is required for us to stay healthy. One way around this might be to wear weighted suits of several hundred pounds so that our bodies will be carrying around the same weight we experience here in 1 g. This has been explored in the book Gunpower Moon, and it may be yet another instance of science fiction preceding science fact. A greater challenge is the radiation. Like almost everywhere off the Earth, it is simply one of the inevitable difficulties that will have to be dealt with. For the time being the solution will likely involve building habitations with thick walls and roofs that are able to block small meteorites, cosmic rays and energetic particles from solar flares. Another option is living underground, perhaps in lava tubes which may make excellent places to build a base or even a city. Just remember to check first for giant space worms.

In terms of things we might want to do on the Moon, astronomy is the first that comes to mind. Having a thriving civilization such as ours generates a lot of electromagnetic noise; especially in the form of (visible) light pollution plus a ton of radio waves. Combine this with a rapidly growing number of satellites — such as the constellations proposed by companies like SpaceX and OneWeb — and the night sky is starting to look messy and crowded for astronomers. The far side of the Moon would be an excellent and electromagnetically quiet place to build telescopes to observe the universe with, as the Moon’s bulk would nicely block all that interference coming from Earth. Another potential resource the Moon offers is Helium-3. Deposited by the Solar wind, and essentially non-existent on the Earth, Helium-3 may be a valuable fuel for advanced fusion reactors which could provide us with abundant, clean, energy.

Though there are good scientific and potential economic arguments for why we should settle the Moon, the biggest reason is because we need to start stepping out into our Solar System, and the Moon is literally right there. It’s a short ride back if you get homesick; relatively easy to send supplies to or mount a rescue mission if needed, and offers a platform where we can experiment with living and working off world that may eventually be applied to other moons or even asteroids.


Not so long ago, the Red Planet was thought by some to be the home of intelligent life that built canals crisscrossing the face of Mars in a monumental feat of engineering to provide water to their crops and cities in an attempt to keep their civilization from dying. Since the Mariner 4 space probe disabused us of the idea that there were Martians building canals when it flew by the planet in 1965 we have learned a great deal about our rusty neighbour. In fact, we probably know more about Mars than any other planet in our Solar System, maybe even more in some ways than we do about our Moon.

Mars is in many ways the most Earth-like place in the Solar System, and has the added advantage of being relatively nearby. It possesses an atmosphere, albeit a thin one composed mostly of carbon dioxide, and a surface gravity about 38% of what we have on Earth. Martian day lengths and axial tilt are almost the same as we are used to here, which should make the transition an easy one for our internal biological clocks. Temperatures are often far below 0C, but can get above the freezing in the summer. In fact, summertime near the equator can hit a balmy 20C; practically barbecue weather. Evidence from NASA robotic missions indicates large volumes of water ice exist on or just under the surface of Mars, from the mid-latitudes all the way to the poles. Estimated at more than 5 million cubic kilometers, there is more than enough for any possible needs of future colonists, including for use in terraforming.

One huge advantage Mars has over other possible contenders for a location to set up a second branch of human civilization is minerals. While the Moon is quite poor in some of the minerals we commonly use, Mars appears to have them all. In addition to an abundance of some of the basic industrial metals like iron and titanium, there is also every reason to assume that Martian volcanic activity has left hydrothermal mineral deposits similar to those on Earth, which are often rich in both base and precious metals. Of course, we won’t know for sure until we go and look for ourselves.

As I have written elsewhere, all of this likely makes Mars a more inviting target for large-scale colonization than anywhere else; plus it is the only place in the Solar System that can be terraformed with known or near future technology.

In terms of points against it, Mars has no planetary magnetic field such as the one that shields us from radiation, which may mean the first colonies will be at least partly underground. There are also the challenges of adequate food, oxygen, and heat that are common to any venture we undertake away from our comparatively Edenic Earth. Most of these obstacles however are easier to overcome on Mars than anywhere else, which is really a point in its favour as a destination for expanding humanity to.


Larger than Mercury, and the largest of the moons of Saturn; with diameter of 5,150 km it is the second largest moon in the Solar System (after Jupiter’s Ganymede). Even though considerably larger that our own Moon, due to being less dense Titan actually has gravity only 14% that of Earth, a little less than the 16.7% on our Moon. It is also extremely cold; right around the triple point of methane, also known as natural gas. This means that the temperature is just right for methane to exist as a solid, liquid, and gas on Titan; resulting in bodies of liquid on the surface and something similar to the water cycle on Earth. Except where water is replaced with methane. Titan is the only place other than Earth in our Solar System where the conditions are just right for this sort of triple point behaviour; making it an interesting — though far colder — analogue to our home.

Nitrogen is the major constituent of the atmosphere on Titan, along with about 5% methane. 50% thicker than Earth’s atmosphere, it would provide excellent protection against radiation, and would also allow future explorers and colonists to dispense with worrying about pressure — or lack thereof — which is a problem everywhere else in the Solar System except perhaps floating high in the sky above Venus as mentioned earlier. Your main concern would be the lack of oxygen, which could be easily remedied by oxygen made from the abundant water ice underfoot, or even from cryovolcanoes that spew water instead of lava. Even the temperatures would be fairly easy to deal with given the abundance of raw material and energy in the form of hydrocarbons literally lying around on the surface. While on Earth we are slowly trying to move away from using hydrocarbons as an energy source, on Titan we may find they are exactly what we need to help us set up cities and build a thriving branch of human civilization.

Free Floating Space Stations / O’Neill Colonies

Most visions of the future include enormous space stations; cities in space with tens of thousands, or even millions living in them. One of the first to give some solid scientific and engineering underpinnings to that vision was Gerard O’Neill; a physicist who wrote The High Frontier: Human Colonies in Space, and popularized the idea of massive rotating space stations we now call O’Neill cylinders. He envisioned that they would be built with materials mined from the lunar surface, where it was easier to get them into space than from Earth’s far deeper gravity well. Unfortunately for his dreams, along with those of so many others who thought that America would continue pushing out into space after the Apollo successes, projects involving humans in space beyond LEO have stagnated these last few decades. We are now essentially back at square one, though with better technology in some areas such as computers and materials.

With only the International Space Station (ISS) as a permanently inhabited station, it is clear we are a ways away from building giant cities floating in space. However, it is an option we can start on right now because we already have. We have experience building space stations in LEO already, and of course the ISS is still there orbiting above our heads. There is no reason we cannot start work on smaller stations now, and gradually make them bigger. There are a number of companies with plans to do just that; including Bigelow Aerospace with their inflatable habitation modules, Axiom Space with their LEO station plans, and the Gateway Foundation with a spectacular rotating torus design. For smaller stations in LEO orbit it makes sense to just use material we send up from Earth via rockets; but for larger stations farther out we are going to have to look elsewhere for building materials. Perhaps our Moon, or maybe some near Earth asteroids can be directed into a safe orbit where they can be disassembled. The point is, while we cannot yet build free floating space colonies, we can start right now on smaller stations that will eventually evolve into more sizable structures which will be true cities in space.

Where to First?

The debate of where to go first usually alternates between the Moon and Mars, but this is just a false dichotomy. There is no good reason why we cannot do both at the same time — and we should. Those who argue a Moon first plan often claim that we need to go there first to train for the longer and more arduous voyage to and exploration of Mars. However, the lack of similarities between the Moon and Mars are significant. Mars has similar day length and seasons to Earth — though the seasons are about twice as long — whereas a day on the Moon lasts 29.5 days. The Moon has no atmosphere, while Mars has one, which makes takeoffs and landings on Mars more similar to what we experience here than on any airless celestial body. The Moon has substantially less gravity than Mars, and also lacks the abundant water ice of the Red Planet. In many ways, Earth is a better analogue to prep for missions to Mars than the Moon is. Which is why there are simulations of a Mars mission outpost run regularly by the Mars Society, and another called the HI-SEAS program run by the University of Hawaii.

In addition to the Moon and Mars, there are also excellent reasons why Titan and large space stations will be good homes for people who wish to live and work off-Earth. Right now we have both the freedom, financial heft, and technological ability to spread out into the Solar System. We should do it while we can, because we may not have the option forever. As SpaceX founder Elon Musk said, “Given that this is the first time in 4.5 billion years where it’s been possible for humanity to extend life beyond Earth, it seems like we’d be wise to act while the window was open and not count on the fact it will be open a long time.” We should take the opportunity to establish bases and cities out in the Solar System, and logically start with the easiest places first. Nowhere is truly easy compared to Earth, but as I have tried to argue above, some places are easier to colonize than others. Our Moon, Mars, Titan, and large free-floating cities in space are our best options for the time being.

Dr. Robert Zubrin, founder of The Mars Society, and author of several books, including his latest, The Case for Space said, “If you have it in your power to do something great and important and wonderful, then you should.” We have the ability and the desire to begin expanding out into the night sky that is filled with so much mystery and promise. Hopefully we will do it. Our children will thank us if we do.

Reprinted with permission from the author.

Owen Lewis is a geologist, science and space enthusiast, and founder of Fusion is the Future; which can be followed on Twitter at @is_fusion.

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