UPDATE 2019-03-09. I changed the number of zones from 7 to 8, separating the Sun from Mercury and Venus, as I felt these are really distinct settlement zones since the Sun and the space near it probably can’t or won’t be settled.
In this post, I’ll explain the 8-zone model of the Solar System I’ve developed, which I think is helpful for thinking about space settlement. These settlement zones are regions of the Solar System defined more-or-less by the types of objects in them, which have similar features and might be settled in a similar time frame or in a similar way. The inner Solar System and outer Solar System are divided into four zones each.
The image below (not to scale) shows these zones. I’ve assigned each a colour.
Solar System settlement zones
Zone 1 (Yellow) 0–0.007 AU
In this zone, we find the Sun. It’s a distinct “settlement zone” only in the sense it’s a region of the Solar System that cannot be settled due to the intense radiation and gravitational forces. The outer limit of this zone could be determined by the radius of the Sun, which is approximately 696,000 km, or 698,000 km (about 0.005 AU) if several of the Sun’s outer layers (namely the photosphere, chromosphere, and transition layer) are included. The Sun’s outermost layer, the corona, begins at approximately this radius and extends millions of kilometres into space, but has no upper boundary. Another option is to consider is that the closest a known asteroid comes to the Sun is approximately 0.092 AU. Yet a third option is to consider the Sun’s Roche limit for a terrestrial planet is about 1.1 Gm or 0.007 AU. This is the closest a terrestrial planet could orbit the Sun without being torn apart by tidal forces. I’ve elected to use this value, as it would probably be quite difficult to build a space station closer to the Sun than this.
Zone 2 (Orange) 0.007–0.95 AU
In this zone, we find the hot terrestrial planets, Mercury and Venus. Although very challenging to settle due to the high temperatures and radiation, and lack of water, these two planets have the advantages of proximity to Earth and the Sun, and gravity levels very similar to those of Mars and Earth respectively. There are no moons in this zone, although there are numerous asteroids. The outer limit of this zone is defined by the inner limit of the habitable zone.
Zone 3 (Green) 0.95–1.78 AU
This is the habitable zone, where we find the cool terrestrial planets, Earth and Mars. It’s where at least 80–90% of human activity will probably always be, even once we’ve expanded to different areas of the System. The habitable zone of a star is the region where liquid water is stable on the surface of the planet, which is considered a primary requirement for life. The boundaries of the habitable zone are not well-defined, because a planet’s habitability depends on its size, atmosphere, composition, and temperature, not only the distance from its parent star. However, although calculations of the outer limit of the Solar System’s habitable zone vary significantly, the inner limit has been determined by numerous experts to be 0.95 AU. The outer limit of this zone is defined by the inner limit of the Asteroid Belt.
Zone 4 (Cyan) 1.78–4.2 AU
In this zone, we find the Asteroid Belt, where the vast majority of asteroids are found. This zone represents abundant material resources for building and supplying spaceships, space cities, and infrastructure. The value of these resources to our burgeoning spacefaring civilization suggests we can expect significant exploration and mining activity in this region of the System. Hundreds, perhaps even thousands, of settlements may eventually be established in this zone.
Zone 5 (Red) 4.2–15 AU
In this zone, we find the gas giants, Jupiter and Saturn. This will be the more popular region of the outer Solar System due to its relative proximity to Earth and the Sun, spectacular views, and substantial resources, including numerous major moons that could be settled. The outer limit of 15 AU is somewhat arbitrary, being approximately midway between the orbits of Saturn and Uranus, or approximately half Neptune’s orbit.
Zone 6 (Blue) 15–30.33 AU
This is the zone of the ice giants, Uranus and Neptune. This is an enormous region of the Solar System, which may eventually be settled by some hardy souls; although, doing so will require more advanced technologies, especially in the areas of space transportation, communications, and energy. At this distance, the Sun is just another star in the sky (albeit the brightest) and there is nothing to distinguish day or night. We might consider this zone equivalent to the Arctic or Antarctic of the Solar System. It is very dark, cold, and far from home, and may only ever be inhabited by scientists and robots. Its outer limit is defined by the aphelion of Neptune’s orbit.
Zone 7 (Purple) 30.33–200 AU
This zone extends from Neptune to the heliopause, encompassing the Kuiper Belt and Scattered Disc. Only dwarf planets and small Solar System bodies have been found here so far.
Zone 8 (Black) 200–200 000 AU
This is the zone of interstellar space, from which most comets originate. It extends from the heliopause to the very outer limits of the Solar System, encompassing the hypothetical Hills and Oort Clouds. This is the largest zone and the one about which we know the least. Galactic cosmic radiation here is very high. The outer limit of this zone is defined by the largest possible orbit for an object orbiting the Sun.
Settlement pathway
Zone 3 is where we live now, and is by far the best suited to settlement. Earth and Mars are the most hospitable and habitable planets, and Luna (the Moon) will be settled purely due to its proximity to Earth. Space stations will be built in Earth orbit, and the moons of Mars will probably also be developed into space stations. The asteroids in this zone will be the first to be explored and mined.
Human activity will then spread into Zone 2, since Mercury and Venus are the next easiest locations to reach from Earth, and these worlds have many similarities with Earth, Luna, and Mars.
We will then progressively expand deeper into the System, establishing bases in the Asteroid Belt (Zone 4), followed by the moons of Jupiter and Saturn (Zone 5), especially Callisto and Titan. As technology advances, a few people may even wish to expand to the moons of Uranus and Neptune (Zone 6).
Our pathway into the Solar System will therefore probably look like something this:
Zone 3 (habitable/cool terrestrials)
Zone 2 (hot terrestrials)
Zone 4 (asteroids)
Zone 5 (gas giants)
Zone 6 (ice giants)
It seems unlikely anyone will choose to live in Zones 7 or 8. Only dwarf planets and small Solar System bodies have been found beyond Neptune, and the distances between them are immense. However, these objects represent an enormous store of valuable ices of water, ammonia, methane, and other volatiles. Perhaps mining robots will be put to work out here, creating propellant depots where spacecraft entering or exiting the Solar System can refuel.
This is a longish chapter, but one that I hope readers will find interesting and educational. We’ll review the structure of the Solar System, along with the meanings of the different terms used to categorize the various objects it contains. This will provide a good foundation for the discussions that follow.
Overview
At its simplest, our Solar System comprises the Sun and 8 planets. When I went to school there were 9 planets, because Pluto was included, but Pluto has since been reclassified as a dwarf planet. (I’ll explain the difference between “planet” and “dwarf planet” shortly.)
The 8 planets are organized into 2 groups of 4 planets each. This isn’t a coincidence, and I’ll explain why. The 4 inner planets — Mercury, Venus, Earth, and Mars — are called terrestrial planets. The 4 outer planets — Jupiter, Saturn, Uranus, and Neptune — are called giant planets. Perhaps unsurprisingly, giant planets are much larger than terrestrial planets.
Separating these 2 groups is the Asteroid Belt, which is a large collection of about a million small objects, called asteroids, orbiting between Mars and Jupiter. These objects range in size from about 1 meter up to 946 kilometers in diameter, which is the largest asteroid, Ceres. Ceres is also a dwarf planet.
Beyond Neptune is the transneptunian region, where many small objects are found called transneptunian objects (TNOs). These include Pluto and other dwarf planets, and comets. The transneptunian region is organised into large groups of objects with similar orbital characteristics: the Kuiper Belt, the Scattered Disc, and the hypothetical Hills Cloud and Oort Cloud.
The Solar System is thus organized into 3 major regions:
The inner Solar System, where the Sun, terrestrial planets, and asteroids are found.
The outer Solar System, where the giant planets are found.
The transneptunian region, where all dwarf planets except for Ceres are found, plus the comets and many other small objects.
The Solar System (not to scale) Large-scale structure of the Solar System showing the theoretical Oort CloudThe inner and outer Solar System
Planets
The original meaning of the word “planet” was “wanderer”, referring to objects that looked like stars, but moved across the sky, unlike the stars and constellations that appear fixed in place. In the ancient world, the Sun and Moon were classified as planets, and Earth was not. The 7 planets of antiquity were the 7 wandering celestial objects that could be seen with the naked eye: The Sun, the Moon, Mercury, Venus, Mars, Jupiter, and Saturn.
The current definition of “planet”, according to the IAU (International Astronomical Union), is an object that:
Orbits the Sun.
Is gravitationally rounded, which is to say, its mass is large enough for gravity to have pulled it into an rounded shape.
Has cleared its orbital neighborhood of smaller objects; or, to put it another way, it is not part of a large group of objects in the same region of space.
The planets of the Solar System, to scale
The difference between terrestrial and giant planets is that giant planets have huge atmospheres of hydrogen and helium gas. These are the most abundant gases in the Universe, but also the lightest, and terrestrial planets are not large enough for their gravity to hold onto them.
The terrestrial planets: Mercury, Venus, Earth, and Mars
The terrestrial planets can be thought of as 2 pairs: the hot terrestrials, Mercury and Venus, which are too close to the Sun for water. Both of these worlds are very dry. Then there are the cool terrestrials, Earth and Mars, which have an abundance of water.
The giant planets: Jupiter, Saturn, Uranus, and Neptune
The giant planets can also be thought of as 2 pairs. Jupiter and Saturn are the larger; having formed closer to the Sun, they accumulated a lot more material and especially a lot more hydrogen and helium gas. These 2 are known as gas giants. Uranus and Neptune have less gas, but large icy cores, and are therefore known as ice giants.
The frost line
The reason we have these 2 distinct groups of planets, with the smaller, rocky, terrestrial planets close to the Sun, and the larger, gassy, giant planets farther out, is because of water.
Hydrogen and oxygen are 2 of the most abundant elements in the Universe, and they combine rather easily to form water, which means we find water throughout the Universe. In the vacuum of space, water doesn’t exist as a liquid; only as ice or water vapor.
The frost line (also known as the snow line) is the distance from a star beyond which it’s cold enough for water to freeze. Closer to the star than the frost line, water exists only as vapor, except for in certain places on the surface of planets or moons where the conditions are suitable for liquid water or ice.
The frost line was at about 3 AU from the Sun during the formation of the Solar System, when the Sun was cooler, but is now at about 5 AU. (As a reminder, an “AU” means an “Astronomical Unit”, which is equal to the distance from Earth to the Sun, or about 150 million kilometers.) For context, Mars orbits at about 1.5 AU (below the frost line), and Jupiter orbits at about 5.2 AU (above the frost line).
Closer to a star than the frost line, water vapor is blown away by the solar wind. Thus, as the terrestrial planets were forming, they didn’t accumulate water; just rock and metals.
Farther from a star than the frost line, water freezes to ice, which can be accumulated by planets as they accrete. Because the giant planets were formed from the abundant ice in the outer Solar System, as well as rock and metals, they grew much more massive; so massive, in fact, that they were also able to hold onto the light gases hydrogen and helium. As these gases are super-abundant, the atmospheres of the giant planets grew very large.
The water on Earth and Mars wasn’t obtained during their formation but was delivered to them later by comets, which are icy objects that formed in the transneptunian region. Comets fall into the inner Solar System and sometimes crash into the surfaces of planets and moons, depositing ice. The cool terrestrial planets were able to hold onto this water by having low enough temperatures to prevent it from boiling away into space. On Luna, Mercury, and Venus, however, the temperatures were too high, and almost all of the water brought to these worlds by comets sublimed to vapor and was lost to space. We do find water ice in the floors of permanently shadowed craters at the poles of Luna and Mercury, because comets crashed there, but the temperatures in these places are always so low that the ice has never sublimed to vapor.
Dwarf planets and minor planets
Astronomers used to consider everything orbiting the Sun as either a major planet, minor planet, or a comet. This has varied slightly and everything orbiting the Sun is now classified as a planet, dwarf planet, or small Solar System body (SSSB). Both classification systems exist side by side. The term “planet” is simply shorthand for “major planet”. A dwarf planet is a minor planet that is gravitationally rounded. All other minor planets, plus comets, are SSSBs.
Main classifications of small objects orbiting the Sun
The IAU Minor Planet Center (MPC) maintains a database on all minor planets and comets. Most are numbered. At the time of writing, the MPC database lists 516 386 numbered minor planets, 241 240 unnumbered minor planets, and 4 014 comets. The full designation of Ceres, the first minor planet ever found, is actually “1 Ceres”, and Pluto’s is “134340 Pluto”. Pluto’s number is large because it was only recently classified as a minor planet, and was assigned the next available number.
The distinction between major and minor planets is not based on whether or not the object is gravitationally rounded. For a long time, Ceres was considered a planet, but it became clear that it shared its orbital neighborhood with many small objects (i.e. the Asteroid Belt). This marked it as a different kind of object than the major planets, which had already mopped up almost all of the small objects in their orbital neighborhood. The category of “minor planet” was then introduced, with Ceres as the first member.
Pluto was also believed to be orbiting by itself and not in a group, and was therefore considered a major planet. Being so far from the Sun, no other objects were detected orbiting near Pluto for a very long time. Eventually, however, many objects were found orbiting beyond Neptune, and it was realized that Pluto also shared its orbital neighborhood with a large number of minor planets (i.e. the Kuiper Belt). This raised the question of whether Pluto should also be reclassified as a minor planet, or if Ceres should again be classified as a major planet, or if some new classification was needed for these unique objects.
Ultimately it was decided that a new definition of “planet” was needed (the IAU definition given above) along with a new category of object, namely “dwarf planet”, to describe gravitationally rounded objects that orbit in the same region of space as large groups of other small objects. Dwarf planets are considered a subcategory of minor planet because they’re found among large populations of other minor planets.
There are currently 5 official dwarf planets: Ceres, Pluto, Haumea, Makemake, and Eris. However, there are hundreds of candidate objects that may be classified as dwarf planets once we learn more about them. These include Quaoar, Sedna, Orcus, Salacia, Ixion, and many others, which are as yet unnamed. We don’t yet know how many dwarf planets there are, but there could be hundreds or even thousands of them.
There are trillions of SSSBs. An SSSB is anything in the Solar System bigger than about a meter wide, except for planets, dwarf planets, and moons; therefore, it includes all comets and minor planets except for dwarf planets.
Asteroids
Asteroids are minor planets, typically of the inner Solar System, although trojans and centaurs (see below) are also sometimes considered types of asteroid. Only the largest asteroid, Ceres, is a dwarf planet; all others are SSSBs. The vast majority of asteroids are found in the Asteroid Belt, orbiting between Mars and Jupiter.
Not all asteroids are found in this region of space, however, and many can be found orbiting between and near the terrestrial planets. Those that come close to Earth are called near-Earth asteroids (NEAs). These are an important group primarily because they’re the first asteroids that will be explored and mined, and because some of them have the potential to hit Earth.
The 4 largest asteroids are Ceres, Vesta, Pallas, and Hygiea. These 4 alone comprise almost half the mass of all asteroids combined.
Orbits of the 4 largest asteroids
Trojans
A trojan is a type of minor planet with the same orbit as a planet, leading or trailing the planet by 60º. These positions in a planetary orbit are points of gravitational stability known as the L4 or L5 Lagrange points.
Almost all currently known trojans are co-orbital with Jupiter. It’s estimated there could be about a million Jupiter trojans larger than 1 kilometer in diameter, although it is believed that Neptune may have 10–100 times more large trojans than Jupiter. No Saturnian trojans have yet been discovered, probably because of Jupiter’s powerful gravitational pull.
Jupiter trojans share the same orbit as Jupiter, leading or following by 60°
Planet
# known trojans
Mercury
0
Venus
1
Earth
1
Mars
4
Jupiter
6 515
Saturn
0
Uranus
1
Neptune
13
Centaurs
Centaurs are minor planets with orbital radii between that of Jupiter and Neptune. They have characteristics of both asteroids and comets, hence the name. There are an estimated 44 000 centaurs with diameters greater than 1 kilometer.
Comets
Comets are icy objects with long, elliptical orbits that bring them from the transneptunian region into the inner Solar System. As they come closer to the Sun than the frost line, the icy nucleus of the comet begins subliming to water vapor, causing dust and water vapor to be released into space. The solar wind blows this material away from the comet, giving it a long, visible tail called a coma.
Comets spend most of their time in the transneptunian region, only descending into the inner Solar System for a small fraction of their orbital period.
Meteoroids
There are a great many objects in the Solar System that are too small to be categorized as SSSBs. These are roughly classified as:
Meteoroids, which are between about 1 mm and 1 m in diameter.
Micrometeoroids, which are between about 1 µm and 1 mm in diameter.
Nanometeoroids (also known as cosmic dust), which are smaller than about 1 µm in diameter.
Moons
Objects that orbit other objects in space are called satellites. Those made by humans are called artificial satellites, whereas those formed by natural processes are called natural satellites. The natural satellites of stars are planets, minor planets, and comets; those of substellar objects are called moons.
Planet / dwarf planet
# known moons
Mercury
0
Venus
0
Earth
1
Mars
2
Ceres
0
Jupiter
79
Saturn
62
Uranus
27
Neptune
14
Pluto
5
Haumea
2
Makemake
1
Eris
1
Many SSSBs also have moons. In total, about 330 of these minor-planet moons have been discovered so far.
Some moons in our Solar System are massive enough to have become gravitationally rounded. There’s no official term for these objects, but the term “major moon” is by far the most common. Conversely, natural satellites that are not massive enough to have become rounded are known as “minor moons”.
Our Solar System has 19 major moons:
Orbiting Earth: Luna (the Moon)
Orbiting Jupiter: Io, Europa, Ganymede, and Callisto
Orbiting Saturn: Mimas, Enceladus, Tethys, Dione, Rhea, Titan, and Iapetus
Orbiting Uranus: Miranda, Ariel, Umbriel, Titania, and Oberon
Orbiting Neptune: Triton
Orbiting Pluto: Charon
The major moons of the Solar System
The transneptunian region
Beyond Neptune, we find a vast region of the Solar System aptly named the transneptunian region. This is by far the largest region of the Solar System and the one we know least about. As far as we currently know, it’s entirely populated by minor planets and comets, although astronomers have not ruled out the possibility that one or more major planets may yet be found in this region of space. It extends as far as approximately 100 000–200 000 AU, which is about the largest possible orbital radius for an object orbiting the Sun.
The transneptunian region includes:
The Kuiper Belt, which comprises small objects in stable orbits between about 30 and 55 AU. It includes the dwarf planets Pluto, Haumea, Makemake, and others, their moons, plus lots of SSSBs.
The Scattered Disc, which comprises minor planets in unstable orbits between about 30 and 100 AU. These orbits are unstable because they come close enough to Neptune that its gravity perturbs them. It includes the dwarf planet Eris and others, and their moons, plus lots of SSSBs.
The Hills Cloud (also known as the “inner Oort Cloud”), a hypothetical disc-shaped region that extends from about 250–1 500 AU to about 20 000–30 000 AU. It potentially contains billions of minor planets, plus, potentially, 5 times as many comets as the Oort Cloud.
The Oort Cloud, a hypothetical spherical region that may extend from the Hills Cloud to as far as 50 000–200 000 AU. It is also believed to contain billions of minor planets and comets.
Objects that orbit beyond Neptune are collectively known as transneptunian objects (TNOs). All TNOs discovered so far are minor planets or comets, although it’s believed possible that another major planet may yet be found beyond Neptune. TNOs are further classified according to their group, and thus as called Kuiper Belt Objects, Scattered Disc Objects, Hills Cloud Objects, and Oort Cloud Objects.
Pluto and Eris are the largest TNOs found so far
The heliosphere and heliopause
The heliosphere is a bubble in space defined by the interaction between the solar wind and the interstellar medium. The boundary of the heliosphere is called the heliopause, which is where the force of the solar wind is balanced by the stellar winds of other stars. The heliopause is found in the transneptunian region, approximately 100 AU from the Sun upwind of the stellar winds, and about 200 AU downwind.
This is a logarithmic scale with distances from the Sun shown in AU.
Roughly speaking, the Kuiper Belt and Scattered Disc are within the heliosphere, whereas the Hills and Oort Clouds are beyond it.
Beyond the heliopause is the interstellar medium, where the stellar winds from other stars are stronger than the solar wind. However, the Sun’s gravity dominates that of nearby stars out to a much greater distance, which is why it is believed that billions or trillions of objects orbit the Sun beyond the heliosphere in the Hills and Oort Clouds.
In a similar way to how the Earth’s magnetosphere blocks radiation from reaching the surface, the heliopause blocks about 75% of galactic cosmic rays. Thus, cosmic radiation beyond the heliopause is approximately 4 times greater than inside it, which may be a significant deterrent to space explorers and settlers planning on leaving the Solar System.
Why are we particularly interested in space settlement, rather than simply space exploration, or even just the development of space industry? What are the benefits of actually living there?
Abundant resources
Space represents an unlimited supply of valuable resources, including energy, metals, carbon, water, hydrocarbons, gases, and minerals; the building blocks of every useful resource required by our civilization. If the burgeoning human population on Earth is consuming too many resources for one planet, as some experts claim, then space is the answer. Not only can resources from space be used on Earth, but also in space by the new communities we create there. Having access to abundant resources for constructing and powering space cities and vehicles will give rise to a vibrant space economy and civilization, ultimately spanning the Solar System.
Metals, carbon, silicon, water, oxygen, ammonia, and more are available in abundance from asteroids. There are literally millions of asteroids in our Solar System, many the size of large mountains, composed of valuable materials. Metals, carbon, and silicon can be used to build space stations and ships, and water and oxygen can provide them with propellant and critical life support resources. Ammonia can be used as fuel, fertilizer, a source of nitrogen and hydrogen, and more.
Solar energy can be collected in space, where the Sun always shines, without impedance by atmosphere or clouds. Solar-powered satellites can be placed in the orbits of Earth, Luna, Mars, and other worlds, to provide continuous energy to settlements on the surface.
Hydrocarbons, such as methane, can be accessed from icy asteroids and moons. On Earth, much of our technology is still heavily dependent on hydrocarbons, which we mainly utilize for electricity and heat. However, even as we transition to sustainable energy, hydrocarbons will remain important for the production of plastics, lubricants, and other products, and are a valuable rocket fuel.
Fusion fuels such as deuterium, tritium, and helium-3 (see Table 1, below) can be found in lunar regolith, Martian ice, and in the atmospheres of the giant planets. If fusion becomes a commercially viable energy technology, space will give us access to more fuel than we will ever need. Fusion may be the preferred method of powering settlements in the Outer Solar System, and new forms of high-speed space travel.
Rare earth elements, required for many modern technological products, can be found in concentrated deposits on both Luna and Mars. These elements can be difficult to refine and recover, which is why such ore deposits are extremely valuable.
Accessing the resources of space will enable the building and supply of space settlements. In this way, settlement of space is analogous to the Americas or Australia, when settlers utilized locally sourced wood, stone, water, animals, and other resources, to build and supply new settlements.
Although space exploration and settlement will initially be supported using resources from Earth, once we begin tapping the resources of space it will become much easier and cheaper to build and do more. Luna, Mars, and especially the asteroids, all have much shallower gravity wells than Earth, which means sourcing resources from these bodies for use in space will eventually be much cheaper. Plus, construction on the surfaces of these bodies will be much cheaper than if materials were supplied from Earth.
As our technological capability increases, we are able to access resources everywhere more easily and cheaply, increasing their supply and reducing cost. However, the exponentially increasing demands of our growing population can sometimes be difficult to meet. Access to the resources of space, and progressively improving the efficiency with which we can obtain these resources, will eliminate that problem, perhaps forever. This abundance will mean an improved quality of life for all people and other creatures on Earth, and everywhere else in the Universe where we may spread.
Main isotopes of hydrogen and helium.
Isotope name
# protons
# neutrons
Symbol
Nucleus name
Abundance
hydrogen-1 (a.k.a. protium or light hydrogen)
1
0
1H
proton
99.98%
hydrogen-2 (a.k.a. deuterium or heavy hydrogen)
1
1
2H or D
deuteron
0.02%
hydrogen-3 (a.k.a. tritium)
1
2
3H or T
triton
trace
helium-3
2
1
3He
helion
0.000 2%
helium-4
2
2
4He
alpha particle
99.999 8%
Asteroid defense
Space research will help to ensure the survival of humanity and other Terran species once we learn how to prevent or limit the damage that could be caused by asteroid impacts. It’s possible that Earth will again be hit by a large asteroid, maybe even as large as the one that wiped out most life on Earth 66 million years ago. While major collisions of this type are extremely rare, there are still plenty of large asteroids out there, and impacts will continue to occur. The effect of a major impact could be globally catastrophic.
Advances in astronomy are improving our ability to detect potential impactors in advance, but the development of an asteroid mining industry may enable us to redirect them or break them into pieces.
It’s fortunate that many asteroids are made of highly valuable materials, because if we aren’t sufficiently motivated by the risk of a major impact to develop the technology to redirect or break asteroids, then maybe the potential for vast profits will do the trick.
Space settlement will provide an additional layer of protection, because if an asteroid arrives which is too large, or moving too quickly for us to divert, and it hits Earth, then the people living in space settlements — especially those on Mars, which may have the potential to be self-sufficient and independent from Earth — will survive, even if people on Earth do not. This underlines the importance of establishing a self-sufficient branch of human civilization on Mars.
Asteroids are nature’s way of asking: “How’s that space program coming along?” — Neil de Grasse Tyson
Confidence
Space settlement will greatly improve our confidence as a species. We’ve seen this before; the Apollo program gave humanity (or at least the US) immense confidence. For many years afterwards, people had the attitude of: “If we can go to the Moon, we can do anything!”. Similarly, once we start sending people to Mars, we will again believe that we can do anything. We will start to see ourselves as an advanced, spacefaring species capable of achieving great things on vast scales. When people are living in space, no-one will seriously suggest that we cannot feed everyone, clean the oceans and the atmosphere, or construct a global renewable energy grid. We will just point to our cities in orbit, or on Luna and Mars, and say: “Just look at what we can do! We can do anything!”
A new frontier
Space exploration and industry will open the way to the final frontier, but space settlers will claim it for their own. A physical frontier is arguably essential for human evolution, due to its powerful stimulating effects on the mind. Frontiers break us out of established living patterns. Comfort and safety have their virtues, but too much can be dangerous, potentially encouraging slothfulness, decay, weakness, and complacency. Frontiers, by contrast, can be difficult and dangerous, but they encourage innovation, resourcefulness, and growth.
It will be much more of a challenge to live permanently in space, or on Luna or Mars, than it will be to simply visit, because ongoing supply and maintenance of settlements will require significant local agricultural, mining, manufacturing, and other capabilities. Space settlement entails much bigger challenges than exploration, and will require bigger thinking and better planning. It will mobilize capital and human resources to a much larger degree, and will require significant innovation. The process of settling new worlds will push humanity to higher levels of achievement than we ever dreamed possible.
The frontier also advances and develops the higher spiritual functions of the human mind, not only stimulating new technological ideas, but also philosophical ones. Our physical frontier is thus paralleled by an intellectual frontier, and it may be that expansion into the Universe is necessary for ongoing development of our intellect, consciousness, wisdom, and civilization. The frontier forces us to rethink the established patterns of our culture in the context of new environments and new technologies, producing a new society which is at least partially legacy-free. It encourages reinvention, not only of physical systems, but also of social, political, and economic ones. The space frontier will afford us the opportunity to foster the good aspects of ourselves, and leave the bad ones behind.
More science!
Living in space will give us much more scientific knowledge than can be gained from simply visiting. The more time we spend somewhere the more we learn about it, and this is as true in space as anywhere else. If we really want to understand Mars, we need to be there, on the ground, experiencing its unique astronomical cycles, climate, weather, rocks, dust, sky, landforms, etc., and its relationship with the Sun, its moons, and other celestial bodies. When we make Mars our home, it will become part of us, as we become part of it. We’ll really learn a lot as Mars’ unique environment challenges us in many unforeseen ways. Over time we will unlock more and more of its mysteries, and the increase in our scientific understanding, as well as our technological capabilities, will open up more new worlds for settlement.
More technology!
The frontier, and the freedom and incentive to invent new things from available resources, has always produced new technologies. We only have to look at the history of the frontier in America, Australia, and many other regions of Earth for evidence. Expand into new environments always involves challenges, yet the resources available to meet these challenges may be scarce, different, or unfamiliar. However, if the desire to expand (or, indeed, survive) is powerful enough, then solutions will be found. Such situations bring out the best in human creativity, and is what drives innovation.
It will be much harder to reside permanently in space than to merely explore it. The process of learning how to live on Luna and Mars, and the development of the space economy, will produce tremendous innovation. We will develop robots and telerobotics for tunneling into rock and carving out habitable volumes; efficient and inexpensive renewable energy technologies; efficient food production systems; advanced water recycling and purification systems; super-intelligent computers, vehicles and robots; new technologies for manufacturing and recycling; high-speed space transportation; low-latency, high-bandwidth space communications systems; artificial gravity; and many other new and wonderful technologies, all of which will accelerate humanity to new levels of achievement.
Right now, we’re observing exponential advancements in technological development due to the rapid evolution of the Internet, emergence of a common language, crowd-funding, and an abundance of venture capital. This trend will only continue as we expand into space, and the unique challenges of space development trigger a cascade of new inventions.
Many new technologies that are being developed now, and that will be developed as we learn how to live in space and on Luna and Mars, will find applications on Earth and elsewhere as we expand to new places in the Solar System.
More environmental benefits!
Space settlement will help to create a cleaner, healthier Earth. Living in space requires in-situ production of energy, water, air, food, and materials; efficient recycling and waste management; environment control and life support; mining and manufacturing; advanced robotics and telerobotics; and much more. These same technologies, applied on Earth, will reduce waste and pollution, increase abundance and health, and enable people to live in new regions, while also improving living conditions in many existing regions, and reducing our environmental footprint.
Space habitat design produces insight into air and water recycling, food production, waste management, mass and volume optimization, and more. We know how much volume and energy people really require to live, and it’s much less than what we habitually use on Earth. This suggests opportunities for more efficient use of land, water, energy, and other resources.
Learning how to live in space will teach us how to live, affordably and safely, in more exotic niches of Earth; in particular, underground. This will reduce the need to destroy Earth’s biosphere for the sake of cities or agriculture.
Food systems developed for space will be high-tech, potentially modular, and capable of producing a healthy, nutritionally complete diet, reliably, unaffected by weather, and requiring only a fraction of the usual energy, water, land, and fertilizer. This technology can be used on Earth to feed people while reducing land requirements for agriculture, potentially freeing up cleared land for restoration of the biosphere. It may even be possible to reduce the total amount of land and fresh water we require for food production, even as Earth’s population grows.
Backing up life
Space settlement can help to ensure the long-term survival of numerous Terran species. Species can become extinct due to hunting, predators, climate change, rising or falling sea levels, loss of habitat or food sources, pollution, or disease. Generally speaking, extinction is caused by an inability to adapt to changed conditions or to migrate to more suitable areas.
There have been five mass extinctions in Earth’s history, in which a large percentage of Earth’s species became extinct within a short time frame. These were caused by supervolcanoes, climate change, and asteroid impacts.
When we begin to inhabit other worlds, we will take many species with us, including bacteria, plants, fungi, and animals. Once a species is living across multiple worlds, it will be protected from most possible causes of extinction, because of the extremely low probability that all worlds where that species is present will become uninhabitable at the same time. And, if a species does become extinct on one world, its population can be restored from individuals of the same species from other worlds.
An Earth-compatible planetary biosphere on another world could potentially support millions of Terran species. If a world already hosts a compatible biosphere then it would be unethical or at least unwise to transplant Terran species to that world, due to the risk of unbalancing the local ecosystem and adversely affecting indigenous species, just like when cane toads, rabbits, and other species were introduced to Australia. However, if a world lacks such a biosphere, but can be engineered to support one — i.e. terraformed — then potentially millions of Terran species could be transplanted there and thus protected. Mars is widely believed to be terraformable, which is a major part of its appeal.
Right now we’re losing countless species across the planet due to climate change and habitat destruction. The current extinction rate is estimated to be on the order of 1 000 times greater than the usual background rate, and this accelerated rate of species loss has been called the “sixth mass extinction”. Unfortunately, space settlement is not a solution to this particular problem, and we must learn to value the natural environment more highly and protect it from farming, mining, and industry. Nonetheless, many species on Earth will always be under pressure due to the large human population, and the sooner that some can spread to new worlds, the better.
Population management
Population growth of species is often imagined to be exponential. However, this is only true while resources are plentiful. Limits on available resources cause the growth rate to gradually decrease as the population approaches the carrying capacity. This curve is called a logistic function.
On Earth, we have already passed the “point of maximum growth”, when the growth rate of Earth’s human population peaked in 1969 at about 2.1% (see graph below). Experts predict that Earth’s population will stabilize at around 11–12 billion.
Birth rates worldwide are decreasing in line with economic development.
In developing countries, especially where there’s significant gender inequality, the number of children per family is often higher. This is due to a higher infant mortality rate, lack of social security (because the more children a family has, the more income there is to support the elderly who can no longer work), and because females earn less, which creates an incentive to have more children in order to have more boys.
However, in developed countries, it’s common for couples to have two or fewer children, due to better nutrition, health care, education, and social services, and because women have access to higher education and are free to pursue options other than motherhood.
Death rates are also decreasing. The average human life span is increasing as medical science and technology improve, and life spans of over a century and potentially much longer will be more common in the future (especially when people stop consuming so much animal-based food, refined carbohydrate, alcohol, tobacco, and other drugs). Premature death is also less likely due to a steady decline in war, crime, famine, disease, and dangerous work.
The decline in both birth and death rates produces an aging population, in which a decreasing number of young people are supporting an increasing number of elderly people. This may not be as much of a problem in a highly automated, post-scarcity economy, but a birth rate that is too low is undesirable regardless. In addition to their important contribution to the economy, we need young people for the energy, idealism, and vision that they bring to the world.
The human population, as with any species, has a tendency to increase, because we’ve evolved for survival and reproduction. However, it cannot increase indefinitely on a single planet because of the limitations on natural resources.
To be fair, the carrying capacity of Earth will steadily increase, as technological advancements, especially in areas such as renewable energy, food production, desalination, genetics, and recycling, make it possible for humans to utilize available resources more efficiently and to live in more places, including the polar regions, on oceans, in deserts, underwater, and underground. However, this will only delay the inevitable, and there will always be a maximum sustainable carrying capacity while we remain confined to a single planet.
Space settlement is the only practical way to continue increasing the human population beyond the maximum carrying capacity of Earth. The number of people who can move to or be born in space will increase exponentially as we get better at building space settlements, and especially at planetary engineering. Birth rates can then be maintained at a high enough rate to ensure a sufficient youth population, and deaths due to resource shortages can be avoided as people move off-planet.
Once we start living in space, limits on the human population will then be defined by the maximum carrying capacity of the Solar System. This, too, will increase over time as we improve our technology and learn how to live in more places.
The terraforming of Mars will probably be the most important project we can undertake to increase the carrying capacity of the Solar System, as this will provide land, water, and other resources to enable the large-scale habitation by millions of organisms; not only humans but many other Terran species as well. Perhaps we can terraform Venus, too.
The human population will eventually reach a sustainable maximum within the Solar System, and that may be enough; since, by that time, our long-term survival will be assured, at least until the death of the Sun. However, considering human nature, and the research already being done into terrestrial exoplanets and interstellar travel, it seems far more likely that we’ll expand to nearby star systems and continue to prosper and increase for billions of years to come.
And here’s us, at the beginning of history, dreaming it.
This is the first chapter of my new book “Becoming Multiplanetary”. Please put your feedback in the comments below or email me at shaun@astromultimedia.com. Thank you!
“Nothing is more powerful than an idea whose time has come.” — Victor Hugo
We stand at the brink of a unique and exciting new era in human evolution, as we break the bonds of gravity and atmosphere, and begin our expansion into space. Soon, we will become a multiplanetary, spacefaring species. As Elon Musk said, nothing as significant has occurred in the history of life on Earth since it crawled out of the sea and onto land.
Of all the worlds we might inhabit, none have received as much attention and enthusiasm as Mars. For numerous reasons, this planet stands out above all other destinations, and it’s where we will focus the majority of our attention during the first major phase of human expansion into space.
There’s something about Mars that is incredibly compelling. Perhaps it’s the twenty-four-hour day; the rocky terrain reminiscent of the deserts of Jordan, Arizona, or Australia; or the colorful sky and dusty breezes. Mars feels very familiar compared to other worlds, much less “alien” than any other worlds we know of beyond Earth. It’s a planet we intuitively perceive as one we will eventually explore and inhabit.
Our species currently faces unprecedented environmental, economic, social, cultural, and geopolitical challenges. At the same time, we are evolving into a truly global culture. The growing ubiquity of the Internet, emergence of a global language, proliferation of communication devices, improved living conditions and international relations, and relative ease of air travel, are deeply connecting all humanity and destroying old tribal divisions of nation, race, and religion. We’re witnessing the emergence of a new global culture, with characteristics of tolerance, compassion, empathy, entrepreneurialism, scientific literacy, technological affinity, and care for the planetary environment. These values are most obvious in the young. A new, unformalized system of ethics is tacitly emerging, based on global unity and the sanctity of life. The worldwide interconnection of minds is gradually causing the concept of disparate, competing nation-tribes to disappear.
The rapid evolution of global communications has led to exponential advancement in every branch of science and technology, as well as business and finance. An inspiring and vibrant startup culture is producing disruptive innovation and countless new enterprises every year, delivering a steady stream of new products and services to the global market. This is happening in virtually every sphere: communications, energy, agriculture, construction, transportation, and many others, including space.
For over half a century, the Universe beyond our atmosphere has been the domain of government space agencies, outside the reach of private enterprise. However, we’re now observing the development of a new paradigm where entrepreneurs are creating their own opportunities in space. Companies such as SpaceX, Bigelow Aerospace, Virgin Galactic, Moon Express, Golden Spike, Deep Space Industries, Planetary Resources, Shackleton Energy, Astrobotic, ispace, Reaction Engines, Ripple Aerospace, Saber Astronautics, Gilmour Space Technologies, and many others are embracing the tremendous opportunities in space. Companies like these are leveraging the scientific and technological revolution, and a growing pool of young engineering talent, and developing unique plans for commercial operations in space. This produces network effects: the more space businesses there are, the more opportunities in space become possible, which in turn inspires and empowers other entrepreneurs to create yet more space-based businesses.
Private space enterprise is no longer the domain of the billionaire, either. Thanks to the affluent and optimistic venture capital culture that developed during the Internet revolution, all manner of new technology businesses are finding capital, and even fresh graduates are forming space startups. As the space industry grows, investors become more confident in the sector. Venture capital funds typically reserve a substantial chunk of their capital for high-risk, moonshot ventures, and space is increasingly attracting this money.
The grassroots space settlement movement is playing an important part in evolving and communicating the vision. Although the creation of the Mars Society, Artemis and Moon Societies, National Space Society, Planetary Society, and others in the late 20th century brought together and stimulated discussion among hundreds of scientists, engineers, and enthusiasts in the US and elsewhere, in recent years the social media revolution has engaged technophilic pro-space millennials worldwide, taking the conversation to new heights. The infosphere is bubbling over with ideas, designs, and plans for free space, Luna, and Mars. A number of popular space settlement design competitions have emerged: the Cities in Space Competition, part of the annual New Worlds conference; the Space Settlement Design Contest, part of ISDC (International Space Development Conference); and the Mars City Design Challenge, a worldwide Mars settlement design contest that has captured the imagination of hundreds of enthusiastic would-be Mars settlers worldwide. The growing enthusiasm for Mars has produced countless books, websites, computer games, board games, documentaries, TV specials, and even independent settlement initiatives; and it’s only the beginning.
This influx of ideas, technology, and capital into the space sector, along with the awareness of the tremendous business opportunities in resources, science, media, tourism, sport, manufacturing, property, transportation and more, is ushering in a new era of space development. It will take us from Earth to Earth orbit, then Luna, Mars, the asteroids, and beyond. In the 21st century, the Solar System will be opened up for settlement. This is a major new chapter in human history. In the words of Mike Griffin, former NASA Administrator: “One day there will be more people living off Earth than on it.”
Advancements in science and technology are transforming almost every aspect of global society, but it will arguably be space settlement that produces some of the greatest long-range benefits for humanity, providing us with many of the necessary material, technological and intellectual resources to overcome the present and future challenges we face on Earth. Space exploration and settlement will inspire thousands, perhaps millions, of young people to study STEM (Science, Technology, Engineering, Mathematics) topics, significantly benefiting the global economy and environment, as these young minds apply themselves to problems other than space. This effect has occurred before, during the Apollo program, but in our modern, hyperconnected, global society, it is orders of magnitude greater.
Numerous benefits will result from human expansion into space. Perhaps most importantly, the propagation of humans and other Terran species to new worlds will ensure our long-term survival. Developing the technologies for living in space will also open up new niches on Earth, enabling us to build cities on the surface of oceans, underwater, underground, in deserts, and maybe even in the air. In addition, learning how to more efficiently utilize and recycle the natural resources of Earth will free up more of its surface for supporting a rich and diverse biosphere.
The frontier of human expansion has always stimulated innovation and disrupted many aspects of society; not only technological, but also economic, political, philosophical, and cultural. Because Earth has lacked a physical frontier for some years, the foundational institutions and systems that form the bedrock of our societies have not evolved significantly. We have experienced enormous scientific, technological, and philosophical advancements, with the potential to create better societies, but we need fresh territory where they can be tried. Free space, Luna, and especially Mars, will give us that. The space frontier will rekindle the human spirit of creativity and reinvention, providing countless opportunities for adventurers, entrepreneurs, technologists, and leaders who wish to experience and participate in the development of new branches of human culture and civilization. Perhaps this is why space calls to us, and why it has always been a catalyst for peace, innovation, inspiration, and evolution. It’s simply our destiny.
The 21st century will forever be remembered as the one during which humanity became multiplanetary. Momentum and enthusiasm for space settlement are increasing, as relentless and inexorable as a king tide. We are going, and we are going soon.
When Elon Musk gave his presentation about the SpaceX Mars architecture at the International Astronautical Congress in 2016, he said: “What I really want to try to achieve here is to make Mars seem possible. Make it seem as though it’s something that we can do, in our lifetimes. And that you can go. And there is really a way that anyone can go if they wanted to.” This idea of generating belief within the public about Mars settlement is of critical importance. If people don’t believe that something is possible, then they won’t give it serious attention, contribute their own creative energy, or invest time or money. We must develop a clear and believable vision, in order to attract the attention, enthusiasm, ideas, money, facilities, equipment, people, and other resources necessary to make it happen. The purpose of this book is, therefore, to foster the belief that we can, should, and will settle space.
In the first part of the book, my goal is to make the case why the construction of the first city on Mars is possibly the most important project in the entire program of human expansion into the Solar System, and this is where we should focus our energies, starting now. Subsequent parts of the book will explore different aspects of the city design, and build a clear picture of the various major priorities, and how they can be addressed and the city built.
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