It’s my first cakeday! At least, it was last week. So in honour of the bestest community on the interwebz, I thought you might like a tour of the future of the star system you call home.
You are here. Earth’s best feature is its vast, uncontained biosphere, attributable to its abundance of sunlight, liquid water, carbon, nitrogen, and other elements necessary for biological life, and it’s magnetic field and atmosphere, which protect surface critters from harmful radiation.
The Moon, also known as Luna, is the first place off-Earth we’ll establish a permanent presence, for the obvious reason that it’s close. Luna is not the ideal place to colonise because it has very little water, carbon, and nitrogen, which are essential for life support. However, because it’s super-close, these resources could affordably be imported to a lunar colony once the cost of space travel is low enough.
Most of the water, carbon, and nitrogen that Luna does have is found in the bottom of permanently-shadowed craters at the poles — a fortunate side-effect of Luna’s near-zero axial tilt. However, it will be a challenge to mine these, and we won’t want to build every colony at the poles.
At just over one light-second from Earth, Luna can enjoy real-time conversations with Earth without lag much worse than international phone calls used to have. Colonists on Luna will be able to surf Earth’s internet. Travel to Luna will be cheap because it only takes a few days, and transports won’t have to carry large supplies of food and water, or even spacious cabins.
There are four main reasons to colonise Luna:
- Science. Luna offers unique science, as its surface contains information about early Solar System, solar wind, and space environment, and the far side is possibly the best place in the System for radio astronomy.
- Tourism. More to see and do than Earth orbit, and a lot closer than Mars. Also, a little gravity is better than none.
- Resources and manufacturing. Luna has plenty of light engineering metals such as aluminium, titanium, magnesium, perfect for making spacecraft components, which can then be delivered to construction sites in cislunar space (between Earth and Moon) with much lower launch costs than from Earth.
- Great place to practice for Mars and elsewhere.
Luna is a good place to practice for Mars for multiple reasons:
- Real-time comms between astronauts and mission control.
- Cheaper to send stuff there.
- Spaceships can be smaller, lighter, and cheaper.
- We can go any time, instead of having to wait for launch windows, whereas for for Mars we can only send stuff every 26 months (this is the synodic period between Earth and Mars, when they line up).
- We can also bring people back any time, which means missions can be of any length. Most surface missions to Mars are designed for 18 months, to coincide with the synodic cycle of the two planets. We can run lunar missions of a few days, then weeks to months as we build up confidence, experience, and surface assets.
- If anything goes pear-shaped during a mission, astronauts can be brought back immediately, or supplies can be sent up. Again, with Mars, have to wait for the right launch window.
Summary — risk of LOC (loss of crew) during early missions is much higher for Mars than for Luna.
Mars is the darling of the space settlement community, and with good reason: it’s the planet most like Earth.
- Mars’ solar day is just 2% longer, at 24 hours and 40 minutes. That means humans and other Terran organisms should be able to adapt.
- Its axial tile is also similar to Earth’s, so it has seasons with a similar degree of variation, although they are nearly twice as long.
- It has a transparent atmosphere. An atmosphere is useful for aerobraking and aerocapture, warming and heat distribution, and is an easily accessible source of useful resources. Most atmospheres in the System are either opaque, or so tenuous they’re considered exospheres. A transparent atmo means you can see the surface from space and vice-versa, which is very handy and familiar. Important for planetary observation, navigation, astronomy, satellites communications, psychological reasons, etc. Mars atmo is especially useful, comprising mostly CO2 and nitrogen.
- Mars is the only other terrestrial planet with moons, which are valuable development sites, and sources of useful materials that don’t need to be launched into orbit.
Mars has an abundance of water, carbon, and nitrogen, and all the same minerals as Earth, which means, if it was a bit closer to the Sun, it might also host a planet-wide uncontained biosphere. It is now widely believed that Mars could be engineered to achieve this (i.e. terraformed), if it is warmed up a bit somehow, e.g. by pumping the atmo full of greenhouse gases like SF6 (sulphur hexafluoride) or directing more sunlight onto the planet using space-based mirrors or lenses.
It’s cold, but not too cold, comparable to Antarctica in the winter. The radiation is higher than you would want to live in, about twice what the astronauts in the ISS experience, so we will probably want to spend a lot of time underground. It’s also easier to keep a colony warm underground, and easier to create large pressurised volumes.
Mars is the closest planet with all the resources necessary for life and technological civilisation, and is probably the only world in the System other than Earth that could potentially become fully self-sufficient. As the cost of space transportation drops (thanks SpaceX) and commercial base modules become available, we should see colonies popping up all over Mars.
This creamy egg is our nearest planetary neighbour on the hot side. Venus may have enormous potential in the future, due to its proximity to Earth (it comes even closer to Earth than Mars), similar level of gravity, and abundance of carbon, nitrogen, and minerals. However, it’s insanely hot, with an atmospheric pressure nearly 100 times higher than Earth, which makes it very difficult to explore the surface. It also has very little water (or hydrogen in any form), and doesn’t even offer a moon where we could build a base.
In short, Venus doesn’t make it easy, and some major engineering will be required before it can be colonised. Perhaps the experience we gain from engineering Earth and Mars over the next century or two will teach us how to make Venus habitatble. We would need to:
- Remove most of the atmosphere, either by mining and selling it, blasting it away with meteors, or converting it to biomass. Venus’ atmo is similar to Mars’, mostly CO2 with a few percent N2, but at 10,000 times higher pressure.
- Add lots of water, or at least hydrogen, as Venus doesn’t have much. There’s plenty of hydrogen in the System, though, and we could ship it in from Jupiter or maybe even collect it from the solar wind.
- Cool it down, perhaps with really enormous thin shade-sails in orbit or at Sun-Venus L1.
- Increase the spin rate. This is probably the hardest thing to do and may never be achieved or necessary.
Despite being close, Venus is thus very hard to colonise in the near term, but could still be a useful source of resources and science. It will probably be colonised by robots before humans, although that can be said about any world.
Mercury is much under-appreciated, but will probably be the third world we colonise after Luna and Mars, for the simple reason that it has some important similarities to those worlds. The experience we gain on Luna and Mars will teach us most of what we need to know to colonise Mercury.
- Like Luna, Mercury has almost zero axial tilt, and thus also has craters with permanently-dark floors at the poles, which are also a source of ices containing water, carbon, and nitrogen. The ice mining skillz we develop on Luna can be applied on Mercury.
- Like Luna and Mars, Mercury has a rocky, cratered surface, so the structures, vehicles, construction techniques, etc. already developed for those two worlds may be applicable to Mercury.
- Mercury’s gravity is nearly exactly the same as Mars’. For humans, changing gravity levels is bad. Living in low gravity causes steady loss in bone and muscle mass, due to the body’s policy of “if you don’t use it you lose it”. Returning to a higher gravity places heavy strain on the heart, and it can be hard to adapt. Because Mercury’s gravity is like Mars’, it will be comparatively much easier for people to move between these two worlds. All the engineers, construction workers, etc., who’ve grown up in Mars gravity, will be able to work contracts on Mercury without suffering. Also many of the structures, vehicles, and machinery engineered for Mars gravity will work on Mercury.
Another big advantage of Mercury is that it takes the least time to reach from Earth of all the planets, which reduces the size and cost of any spacecraft headed that way. Also, the Earth-Mercury synodic period is least for any planet, and we can send ships to Mercury or bring them back every 4 months.
Being so close to the Sun, Mercury is hot AF. The radiation is about 7 times as intense as on Earth, and there would be very little warning time in the event of solar flares. Great for solar power, but any colony would be mostly underground, and you would have to stay underground during the day. However, Mercury’s solar day is about 6 months: 3 months dark, 3 months light. You could land and launch spacecraft, explore and construct surface assets during the night, then chillax underground during the day. At high latitudes you don’t have to go far underground at all (like, a few metres) to find comfortable temps.
Ceres is a dwarf planet, only about 1,000 km in diameter, and it’s about twice as far from the Sun as Mars. It’s also the largest asteroid; the only one large enough to have become rounded due to gravity. Apart from the fact that it’s so interesting, Ceres could be important for one main reason: water.
The asteroid mining companies such as Deep Space Industries and Planetary Resources aren’t looking for platinum yet; they’re looking for water. Water can be electrolysed to make hydrogen and oxygen, which is good rocket propellant, and one of the biggest expenses in interplanetary travel is getting propellent into space. When we can obtain propellant in space, out of Earth’s gravity well, exploring the System will get a lot cheaper.
Ceres is estimated to have as much or more fresh water than Earth, making it a useful refuelling station in the main Asteroid Belt. Due to its location, round shape, and abundance of water, Ceres could become one of the main headquarters for the asteroid mining industry.
As you can imagine, the gravity is very low, however, which means, until we figure out how to generate gravity fields, it probably won’t be healthy place to live or spend a lot of time.
The gravity problem can be overcome in some asteroid colonies by spinning them up, although Ceres might be a bit big for that. Some asteroids are basically rubble piles, or are made of weak material, and won’t be spin-up-able because they’ll just fly apart. However, some are made of solid chunks of rock or metal (like 16 Psyche) which could be spun up and converted into massive space stations with healthier gravity than any moon.
The first and biggest of the giant planets is Jupiter, which has 69 moons, 4 of which are major moons, i.e. moons that are massive enough to have become spheroidal. Because they were discovered by Galileo, these are called the Galilean Moons, and, in order of distance from J, are named Io, Europa, Ganymede, and Callisto.
The beautiful moon pictured here is Callisto, the second-largest Galiliean Moon, and the third-largest moon in the System. It’s the pick of the litter in terms of colonising the Jovian System, because Jupiter has a massive radiation belt that Callisto is effectively outside of. The radiation on Io, Europa, and Ganymede would kill a human fairly quickly, but on Callisto the radiation is actually quite low; less than Luna, Mars, or even the ISS. Also, because it’s the farthest from Jupiter, it’s the most geologically stable.
Callisto’s surface is about 50-50 rock and ice, including ices of water, carbon dioxide, and ammonia, which are really useful. There’s also a planet-wide subsurface ocean that could potentially be tapped for free liquid water. Once we get faster ships, we’ll probably see humans on Callisto this century and colonies in the next.
Titan is arguably the jewel of the outer Solar System. It’s the largest of Saturn’s moons and the second largest moon in the Solar System, about midway in size between Luna and Mars. Plus, it’s the only moon with a substantial atmosphere, and the only world other than Earth with liquid on its surface.
Titan’s atmosphere is mostly nitrogen, plus some useful hydrocarbons like methane. Titan actually has oceans and lakes of methane and ethane just laying around for free, so it’s basically a super-abundant planet-size propellant depot for methane-fuelled spacecraft like Elon’s BFS. Titan is therefore perhaps one of the easier destinations in space that a sound business case could be made for, because all that nitrogen and methane are saleable exports to passing spaceships and other space colonies.
On top of this, Titan also has a planet-wide subsurface ocean of liquid water, and orbits arguably the most beautiful of the planets. Saturn has a total of 7 major moons, more than any of the other giant planets, and has much less radiation than Jupiter, so it could be an extremely popular area of the Solar System to colonise. Solar power is about 1% compared with Earth out here, so we’ll probably need fusion by this stage. Fortunately, the atmospheres of giant planets are loaded with fusion fuels like deuterium and helium-3. Just have to make some sturdy self-piloting robots to mine them.
Assuming we’ll be halfway decent at planetary engineering by the time we’re ready to colonise Titan, something we might want to do is make its atmosphere transparent like Earth’s and Mars’. This would require mopping up the organics that give it the orange colour. Property values on Titan would then soar due to the breathtaking view of Saturn overhead. Titan is tidally-linked to Saturn in the same way that Luna is to Earth, which means, on the near side of Titan Saturn remains in virtually the same place in the sky all the time, rotating in place.
Many other moons of the outer Solar System could and probably will be colonised; here I’ve just highlighted the two best. The advantage of the outer Solar System is that there’s a lot more ice, because it’s a lot colder, including ices of water, carbon dioxide, carbon monoxide, ammonia and methane, which are all very useful. Conversely, there’s much less light and heat, being so far from Sol, which means fusion power or something like it will be a requirement. Uranus has 5 major moons, and Neptune has 1, plus there are the many dwarf planets of the transneptunian region. Robots and people will go everywhere they can, but probably most people in the Solar System will live on Luna, Mars, Mercury, Callisto, or Titan, eventually Venus, in addition to Ceres and other minor planets and moons.
FP edit: thanks for the support, beautiful people! Send images of terrestrial exoplanets 🙂