Why Human Beings Will Never Colonize Other Planets

If you're someone who is remotely interested in space, you won't have failed to notice the headlines that pop up every few months about scientist’s latest discovery of a 'potentially habitable planet' – ‘Hey look, we found a planet only twice as large as the Earth, possibly orbiting in the habitable zone of its star, that's only 400 light years away!’

Now yes, I'm not saying we should ignore the latest Kepler discoveries. Nobody has ever done this before, and the spotting of more than 2,000 exoplanets since 1988 is indeed a scientific milestone. Personally, I remain a huge fan of SETI.

But as someone who (secretly) hopes to earn my stripes as a hard science-fiction author someday, I can't help but remain a bit skeptical. Will these far-flung worlds ever really affect the destiny of the human race?

Might it not be the case that, in however many centuries it takes us to reach them, we come up with much better alternatives when it comes to finding a place to live? Might these worlds become obsolete long before anyone has a chance to do their Neil Armstrong impression?

Let’s step back for a moment.

Say all of 21st century humanity, as we know it, was just a tribe from the Stone Age living in a cave somewhere. It’s a cave, but it’s a nice cave, nestled in a hillside, over a river, surrounded by fertile soil. Not a bad place to live.

Now imagine one of the young hunters goes off exploring, and returns a few days later with exciting news.

‘Hey guys! You'll never believe what I found! There's another cave we could live in! I mean...its kinda small, its filled with poisonous moss, oh and its in the middle of the desert...two hundred miles away, but still!’

And indeed, maybe some members of the tribe eventually do make the arduous journey and attempt to live there. Maybe they even find it tolerable, and this sets a precedent, with other bands setting off to colonize caves of their own.

In this manner, Stone Age humans might have crossed entire continents, seeking out more and more caves in which to live. In time, Civilization might have arisen from these underground refuges. Underground wars would have been fought. Cave-based empires would rise and fall. Shakespeare might have written Hamlet under the stalactites.

Indeed, if you had gone back and asked whatever budding science fiction writers may have existed in 10,000BC, this is how they might have pictured the future - a world in which humanity spreads across the Earth, inhabiting every single cave? Whoa!

There's a little flaw in this story though, an unexpected factor that makes this just an alternate history. While humans have sometimes lived in caves, civilization as we know it did not ultimately center on their existence. At some point, someone invented an alternative form of habitation, one that proved superior to caves in most respects.

We call them houses.

The Rise of the Cosmic House

In case it’s not clear what I'm on about, the caves in this analogy are the planets themselves. As for the houses - that's a little harder to describe. So what is the interstellar equivalent of a house?

While the cave analogy is mine, I can't claim credit for this entire idea. We have already seen it in works from authors like Iain M. Banks and Stephen Baxter.

In Banks's 'Culture' universe, most citizens of the advanced, star-spanning Culture live on enormous, galaxy-crossing spaceships or giant, artificial habitats. Planets are considered little more than nature preserves, inhabited by primitive races yet to develop spaceflight.

I also recall a Stephen Baxter story where a character becomes the first man to land on an asteroid. At first, he's little known, with most of the glory going to the first man on Mars. In time however, the Martian cities decline, the planet is largely abandoned, and the bulk of humanity ends up living in space. The protagonist does to the Martian Neil Armstrong what Christopher Columbus did to Leif Erikson.

In the end, it is those who leave a lasting legacy that achieve true glory. Space is where our future lies, not planet-bound like our ape ancestors.

Many studies have been conducted in to what future space habitats may look like. The jumbled mess that is the International Space Station will be looked back upon as the first roughly hewn raft our ancestors assembled on a beach somewhere.

Names like Bernal Sphere, Stanford Torus and O'Neill Cylinder have been floating around for a while now. These consist of different versions of what is essentially the same thing - a rotating space habitat.

The basic idea is that space habitats need to rotate in order to produce centrifugal force, thereby imitating the gravity of a body like Earth. These generally take the forms of cylinders, wheels or spheres. A cylinder a thousand meters across, for instance, would need to rotate about once a minute to approximate 1G.

There's no reason to believe such habitats could not be built in the near future. I'll stress again - this is science, not science fiction.

A very early example would be ‘Space Station One’ - the rotating wheel we see early in 2001: A Space Odyssey. The building of much larger habitats is certainly possible. ‘Rama’ from Rendezvous with Rama (also by Arthur C. Clarke) is a larger example, being a rotating cylinder some fifty kilometers long and twenty wide. 

I can even imagine looking up at the night sky in a hundred years or so and seeing hundreds of cylinders, spheres and wheels the size of cities orbiting the Earth. Probably the best description of this we've seen so far would be Alastair Reynold's 'Glitter Band' of ten thousand space habitats orbiting the planet Yellowstone in his Revelation Space novels.

Some authors have presented us with even larger structures, such as the Halo Ringworlds (about 10,000km across), Iain M. Banks' Culture Orbitals (3 million km across) and Larry Niven's titular Ringworld (300 million km across!)

Of course, the ultimate extension of this principle is a ‘Dyson Sphere’ - a swarm (NOT a solid shell) of enormous solar power collectors partially or entirely surrounding a central star. Such a structure, if placed around our own sun, could collect nearly a million, billion times as much energy as current global electricity consumption.

So one has to ask - amidst all this, what place is there for planets?

Planets vs. Space Habitats: A Losing Battle

When it comes to qualify of life, planets just can't compete.

For starters, planets are horrendously inefficient users of space. The whole point of a sphere is to minimize the surface area of an object after all, making planets literally the worst possible option.

The Earth weighs nearly six trillion, trillion kilograms, but has a surface area of barely 500 million square kilometers. If you disassembled the Earth and used its material to manufacture rotating space habitats, even if the thickness of their shells averaged say, a kilometer (and it could be a lot less) you would increase the available surface area over two thousand-fold.

Not only would you have more space (who needs a mantle or a core after all?) but your new environment would be infinitely more malleable than your old one.

For starters, your new home's gravity can be turned up or down like the volume control on a stereo. Just spin the habitat a little faster and it goes up. Do the opposite and eventually you'll be back in free fall.

This is one of the biggest issues people tend to have with planets. Mars, while many argue it is ripe for terraforming, has the downside of possessing only 38% as much surface gravity as Earth. For us Earthling this adjustment isn't so bad (it could even be viewed as a positive) but for our descendants who may grow up there, returning to the mother planet could be a big issue. Only advanced medical technology (perhaps a pill that can regrow your atrophied bones) might surmount this obstacle.

Even then, a humanity that spreads across the cosmos will surely splinter into innumerable factions. A big divide between them may be gravity. Is a full Earth gravity really ideal? Or would it be better to just live in microgravity? Is there some ideal figure somewhere in between?

Many problems with transportation here on Earth relate to either gravity or air friction, both of which are really quite punishing down here. On Titan for instance, with gravity a sixth of Earth's but a denser atmosphere, people could reportedly flap around with wings strapped to their arms. This will never be possible on Earth.

So I stress - the first problem with planets is the gravity is not malleable. You're stuck with either one or 38% or a sixth of a Gee, or whatever the local constant is.

Other aspects like temperature, air pressure and humidity can all be adjusted in space like the air conditioning in your home. In fact, this brings us to another major flaw that tends to reduce planet-bound property values.

Planets are dangerous. About a hundred thousand people every year die in natural disasters of some sort. So far in the 2010s, the biggest killers have been earthquakes, temperature extremes, floods and epidemics.

With improved technology, these numbers tend to plummet drastically (the vast majority of these casualties occur in developing countries) but it’s hard to see them ever disappearing entirely. The 2004 Indian Ocean earthquake released as much energy as 500 million Hiroshima’s while the average hurricane releases more energy than the global annual electricity consumption. It’s hard to see any civilization dismissing such forces as mere trivial concerns.

While such risks may seem minor to us, I fear our descendants will be much more safety conscious, particularly if we end up curing the aging process and living more or less forever. 

I'm not sure that the average person on 21st century Earth has fully absorbed the implications of geology either. The knowledge that we are not in fact standing on solid ground - that thirty or forty kilometers beneath our feet is a broiling sea of liquid magma the same temperature as the surface of the sun, one on which the plates of the Earth's crust slip and slide like rafts...let’s face it, it’s a terrifying reality we're all just quietly ignoring.

But earthquakes, volcanoes, hurricanes, tornadoes, lightning storms, blizzards - all are optional extras on your brand new space habitat.

Another problem is accessibility. Planetary surfaces lie at the bottom of deep gravity wells. On Earth, you have to accelerate anything up to at least 11 kilometers a second just to even reach space. The building of space elevators may reduce launch costs, but passengers will still be subject to lengthy rides up and down the tether that could last for days.

In space meanwhile, a habitat would generate very little actually gravity. Spaceships could dock with it much like a ship at a pier.

A further difference is one of security. Planets, being massive balls of solid matter, are close to impossible to move. They are destined to follow the same orbit around their parent star for millions of years.

To anyone with a vendetta against your civilization, they make fat, juicy targets. All you have to do is fling an asteroid out of orbit or fire a giant space laser at just the right moment, even from light years away, and your salvo will eventually impact with the planet in question. Any idiot with a giant space laser in the Alpha Centauri system could blast the Earth no problem, and with no warning.

Space habitats meanwhile, are much more mobile (except perhaps some of the larger examples mentioned here). As long as you occasionally fire your thrusters to shift your station's orbit, even slightly, this will foil that dastardly plot by the inhabitants of the Alpha Centauri system. It’s like the difference between trying to hit a ship or a city with an ICBM. This is not a foolproof defense of course, but it does make things light years easier.

One final advantage I'll mention is the availability of resources. Potentially habitable planets are not expected to be found in more than maybe 1 in 10 star systems. If we were to remain a planet-bound civilization, the other 90% would go unused.

With space habitats however, you can construct them pretty much anywhere. There’s no reason to believe that asteroids in some quantity won’t be found around virtually all stars. Our own solar system contains at least a million larger than 1km in diameter. Wherever we go, such raw materials should be abundant. The same off course applies to the solar energy you need to power your mining operations.

So as you can see, your brand new Space Habitat 5000 has numerous selling points - greater living space, adjustable gravity, climate control, geological stability, accessibility, security and availability of resources to name a few.

Honestly - what poor fool would still choose to anchor themselves down on a planet?

So What Good Are Planets?

All this isn't to say of course, that our descendants will utterly ignore planets. I'll admit the title of the article exaggerates slightly.

The first exception I'll concede is that we probably will have a crack at colonizing Mars in the near future. We may even succeed if Elon Musk has his way.

There are an enormous number of technological, political, economic and social challenges to overcome however. Are we really going to redirect some asteroids to slam into the Martian poles in order to melt them for instance? Or will the whole thing be interrupted by a band of Greenpeace protesters who've chained themselves to the top of Olympus Mons?

Once we move beyond the Solar System however, what exactly is the appeal of planets?

Admittedly, planets may have a few redeeming features. Any space habitat will be more vulnerable to sudden decompression in the event of a hull breech for instance, but this is probably a manageable risk. After all, houses have walls much less thicker than caves, but we make do. Space habitats will also require more regular maintenance than a planetary environment, especially with regards to issues like the disposal of waste.

And I suppose that even today, caving is still a sport, and there's plenty of interesting creatures to be found beneath the Earth. No doubt they'll always be some mad scientist willing to plant the flag on whatever bizarre, inhospitable rocks we come across in due course. Some offshoot factions of humanity, perhaps homesick for the feel of real rock beneath their feet or the look of a sunset, may go like Gollum and decide to live in these cosmic caves anyway.

So yes, I'll grant a substantial minority of humanity may still call planets home.

What I'm saying is - just as human civilization has not been focused on caves for something like 10,000 years (if it ever was) interstellar civilizations are unlikely to center around planets. Most people will likely live in space.

I would also like to point out, just in case anyone got the wrong idea, that I am not personally in favor of the idea of dismantling the Earth and turning it into millions of space habitats. The Earth, as our home, will always have a special place in galactic history. I personally would be in favor of keeping it as a nature preserve. Its native wildlife - from beetles to lions to Amish, should be able to roam its surface into perpetuity. The rest of humanity, cruising the galaxy in our vast space arks with our minds uploaded into computers, or come what may, should leave them in peace.

Any planets that we discover with their own native life should also be preserved. Aside from covert scientific expeditions, they will probably be basically forbidden for anyone else to visit. As for tourism, I don't see why it can't be conducted virtually. That's probably where our future lies anyway.

But as for the rest of the universe - go nuts. I'd start with Mercury, Its three hundred million, trillion kilograms of barren, iron-rich rock, ripe for harvesting. Can you imagine the solar arrays we could unfurl with such a goldmine?

I am here merely to point out yet another big lie most science fiction clings to. Sci-fi universes like Star Trek and Star Wars are always filled with warring factions competing over precious, precious planets.

Why? 

Why are they doing that? 

Why not just harvest a few asteroid belts, construct a few thousand artificial habitats capable of housing your billions of excess citizens, and call it a day? The total carrying capacity of the Solar System, were we to convert even a fraction of its usable material into biomass, is easily trillions of times that of the current human population.

As has been pointed out before, a plotline that sees aliens invading Earth to steal our water is like Eskimos invading Central America to steal their ice.

Any interstellar empire focused around planets makes about as much sense.

Why We Haven't Yet Found Aliens

While I have great enthusiasm for efforts like SETI that are seeking to find evidence of alien life, and think we should continue to fund them into the future, I also think its time to bring our expectations down to earth a little.

While I dearly hope we will find such evidence, either in the form of a radio transmission, ancient artifacts of some kind, or indeed a visiting spacecraft, I can think of at least one good reason why we haven't, and probably won't for the forseeable future.

Its an idea I shall call here - the APILE hypothesis.

Now what does this acronym, that I have just made up, stand for, may you ask?

I'll tell you. It stands for the - Anthropic Principle of Intelligent Lightspeed Expansion.

Hence - 'APILE'.

Now what do I mean by this?

It may sound complicated, yes, but allow me to explain.

The 'anthropic principle' is a widely recognized philosophical position. It states that the nature of the universe must, fundamentally, be compatible with the emergence of any conscious life within it.

The point of this is that it seeks to answer the question of why the universe looks so fine-tuned, one might even say 'designed' for the emergence of life - including ourselves.

i.e. - the universe appears so perfectly suited for us, because it couldn't possibly be any other way.

It's also much the same reason the earliest civilizations on Earth tended to evolve in fertile river valleys. It's not just a matter of chance - you wouldn't expect anything different.

I shall now apply the anthropic principle however, to another big question, that of why we haven't yet seen any evidence of alien life. The name for this dilemma is of course the 'Fermi Paradox'.

This basically states the following - given the vast size and age of the universe, why haven't we yet encountered any aliens?

Here's where the second part comes in however - the 'lightspeed expansion' part.

Futurist Ray Kurzweil has written of something he calls the 'Singularity'. This is a time in history, predicted by him to be only a few decades away, when machine intelligence becomes advanced enough that it begins to make actual design improvements to itself.

Just as humanity's technological development has steadily accelerated over time, it is predicted that these improvements will be exponential in nature.

Very quickly within this period, we can expect that computers will develop (possibly the right term is 'evolve') into the most efficient designs possible. This is likely to be many thousands, if not millions, of times more compact and efficient than the neurons in a human brain.

Kurzweil then predicts that, having reached the limits of efficiency, the only way for such entities to grow more powerful will be to make themselves physically larger.

Thus, they will begin to rapidly expand across the universe, devouring more and more of its resources to use for constructing ever larger banks of computers. This great 'waking up' of the universe, turning its 'dumb' matter of stars, planets and asteroids into hyper-efficient computers, will rapidly spread out from the Solar System in all directions.

Quite quickly, such a wave of expansion could approach the speed of light. Within say, a hundred thousand years, the entire galaxy could be consumed by it.

Now while I'll admit I think Kurzweil is often a bit optimistic in his predictions, I can't really fault his logic in predicting this will happen. In principle it simply makes sense. Whether it happens in forty years or four hundred is another matter, and barely of any relevance. Even the question of whether advanced machines do this, or just ordinary humanity, is somewhat beside the point. The key fact is that we begin expanding away from our solar system at near the speed of light, and in the geologically near future.

So here's the real point I am making.

Humanity, it seems, is nothing special in the larger scheme of things. If we can build ever-more efficient computers, than any other intelligent race should be able to as well.

Now we haven't quite gotten to that stage yet, but given the universe's age of 13.8 billion years, there's at least a decent chance (if not virtual certainty) that somebody else has already done it.

Hence, where the anthropic principle comes in, and why we haven't yet seen any evidence of this.

Similar to how you generally don't hear the sound of a gunshot before the bullet whistles past you (as it is travelling faster than the speed of sound) we wouldn't expect to see a post-singularity civilization approaching until it was already almost upon us.

Assuming that the spread of such a civilization is close to the light barrier, there would be virtually no warning of this impending meeting. It could happen tomorrow, or in another billion years.

Admittedly, there may be some warning. Its already been a century or so since the first major radio signals left Earth (think the 1936 Olympics broadcast from the plot of the movie Contact) and we're at least a few decades away from a singularity of our own.

However, you may not realistically expect our earliest radio signals to be detected thousands or millions of light years away, so they may go unnoticed, while our 'singularity wave' could be only a century or two behind.

Our first contact with aliens should be thought of along these lines. We could have similarly little warning, and on a day when this does occur, assuming we are on the receiving end of it, it will be like the Aztecs meeting the Spanish writ large.

However, unlike the Aztecs, I would not say our doom is certain. At least three possibilities could occur.

Firstly, is the most obvious scenario - that we would promptly be devoured by the invaders and turned into computer chips without a second thought.

Secondly, and more ambivalently, we could be ignored by them. It is feasible they may have some long-running policy in place to preserve 'primitive' cultures like our own. This could perhaps spare the Earth, or even the entire Solar System, into a kind of local 'nature preserve' just as we might ignore a remote aboriginal tribe wandering around in the desert.

Thirdly, and most optimistically, the aliens may be benevolent, and attempt to uplift us into their civilization. Presumably this would be the most glorious thing to ever happen to humanity.

So there you have it, an explanation for not only why we haven't seen aliens yet, but also on what the nature of that contact may be.

Have faith in APILE.

Then again, there is a fourth possibility.

That our own expanding singularity reaches them first...

Only time will tell.

How We Should Really Define Planets

Following the discovery of the trans-Neptunian object 'Eris' in 2006 the globe-spanning organisation of stargazing boffins known as the International Astronomical Union decided to finally spell out a definition of what a 'planet' is for the first time. This was done to settle the question of whether Eris (with the fate of similar-sized Pluto hanging in the balance as well) would be categorized as a planet, or given some lesser label instead.

In the end, the IAU decided to create an entirely new class of objects - known as 'dwarf planets' that differ from planets in one key sense. At the moment their definition of a planet means the following -

1. That it orbits the sun
2. That it is large enough to have been rounded by its own gravity (can be anywhere from 200-900 km across, depending on its composition, also known as achieving 'hydrostatic equilibrium')
3. That its gravitational influence has 'cleared the neighborhood' of its orbit of other large objects

A dwarf planet meanwhile, is an object that has only fulfilled the first two criteria.

Given this definition, as of 2014 there are now eight planets and five dwarf planets in the solar system. The planets, which most people should be familiar with - include Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. The so-far designated dwarf planets are generally less well known, and include Ceres (previously the largest body in the asteroid belt) and Eris as well as the more recently discovered objects Haumea and Makemake. It is expected in the coming years that hundreds if not thousands more roughly Pluto-sized objects will be spotted somewhere beyond the orbit of Neptune.

Pluto itself of course was caught in the middle of this interplanetary crossfire, and was thus downgraded to a dwarf planet as well.

Some astronomers however, have disputed the IAU's definition of a planet, and its not hard to see why. Calling these objects 'dwarf planets' is obviously something of a misnomer, as the third criteria for them to fill above is not just based on their size, but also on the distance at which they orbit the sun.

To explain - 'clearing the neighborhood' means that an object is sufficiently massive that its own gravity is able to disrupt any other large objects from sharing its orbit. This is why, for instance, none of the major planets happen to orbit particularly close to each other. Venus orbits about twice as far out from the sun as Mercury, and the Earth and Mars are separated by gaps roughly as large. Saturn is similarly about twice as far from the sun as Jupiter, and Uranus and Neptune continue this trend among the gas giants. It would be impossible for any of these large planets to share orbits remotely close to one another, as within a relatively short while (geologically speaking at least) their mutual gravitational attraction would grow sufficient either for the two objects to collide, or have a near miss in which one will likely be flung out of the solar system entirely.

Originally, models of planetary formation predict, there should have been hundreds of planets orbiting the sun early in the solar system's history, but within a few million years most would have suffered either of these fates, eventually leaving us with the eight major planets we have today, as well as countless smaller bodies of course.

Now how does this upset our definition of  'dwarf planet'? Because whether a planet has 'cleared its orbit' or not is not determined purely by its size, but by the length of its orbit as well. The required size of a 'planet' will increase proportionate to its distance from the sun.

In other words, if we took a small planet like Mars, and plunked it out somewhere beyond the orbit of Neptune (or more precisely, more than 146 AU away from the sun - one AU being the average distance between the sun and the Earth) then according to the IAU, it would no longer be considered a planet at all. Even the Earth, beyond 3,000-odd AU from the sun, would drop from being a planet to a mere 'dwarf planet' without any actual change in its size.

Conversely, if any of the dwarf planets happened to be closer to the sun, they would automatically be upgraded to planet status despite not having gained an ounce of weight. In Pluto's case for instance, this would be approximately within the orbit of Venus (thus for those wishing to have the IAU reinstate its 'planet' definition, all you'd need to do is attach a rope and tow it to somewhere between the orbits of Venus and Mercury...though simply bribing them might be easier).

Therein, lies the misnomer. It is perfectly possible (one may even say likely) that in the near future we will discover one or more 'dwarf' planets beyond the orbit of Neptune that is actually larger than Mercury or Mars (or possibly even the Earth, though we'd likely have spotted it by now).

There are other problems with current nomenclature regarding astronomical bodies as well, ones that will probably not be resolved until such a borderline case is found that the IAU (or whatever future body may make such decisions) is forced to draw a line somewhere. For instance, what is the definition of a moon? There is no minimum size requirement for a moon (i.e. such an object does not have to be large enough to have been rounded by its own gravity) meaning, for instance, that every individual chunk of rock and ice in Saturn's rings could very well be considered 'moons' in their own right.

Even the dividing line between 'planet' and 'star' is vague. The general definition is based on whether an object is sufficiently large to have undergone nuclear fusion. The problem however is that there are several different types of fusion, each requiring a different amount of pressure to begin. Deuterium burning begins at about 13 Jupiter masses, Lithium burning at 65, and Hydrogen-1 fusion (the 'regular' kind) at 75-80. To deal with this problem, astronomers have created a new category between the two called a 'brown dwarf'. This includes objects of between roughly 13 and 80 Jupiter masses (the sun, by comparison, is just over a thousand Jupiter masses).

As for the issues of defining planets and moons however, here is the classification system I would be compelled to use - and which, at the very least, I intend to use in any science fiction stories I may write, and who knows? Terminology used in science fiction can sometimes permeate into the real. Apparently terms such as 'gas giant', 'terraform', 'droid', 'deep space', 'ion drive' and 'computer virus' were all invented by sci-fi writers -

http://hellogiggles.com/10-words-science-fiction-invented-that-we-use-today

Firstly, instead of 'planet' and 'dwarf planet' I would be compelled to put both kinds of objects under the umbrella definition of 'planet'. Only the first two criteria mentioned above (orbiting a star and large enough to be rounded by its own gravity) would be required.

However, I would then divide the definition of 'planet' into two categories - 'primary' planets and 'secondary' planets. The eight 'proper' planets that we currently have - from Mercury to Neptune, would be re-labelled as 'primary planets' while any dwarf planets so found would become known as 'secondary planets'.

I prefer this definition because it does not directly reference the size of the objects, although at the same time it implies primary planets are larger. It references the fact that the eight 'major' planets are privileged to be not only large, but dominant in their orbits. At the same time it still deems secondary planets as planets, as they may be just as large, but also implies that they are usually not.

Simple yes?

And while I do realize that 'primary' and 'secondary' and kinda big words, we are referring to something of a technical definition. Both kinds of objects could merely be referred to as 'planets' until there was a need to distinguish between them. School children for instance, would simply be taught that instead of there being 'eight planets - and lots and lots of dwarf planets' that there are 'lots and lots of planets - of which there are eight primary ones'. In getting them to memorize the planets, I imagine we'd still stick to having them just name the eight primary ones of course. All we're talking about is a different definition, one that is really no more complicated, and at the same time actually makes more sense.

Now as for moons, there is another divide here we should be making.

A term has come into use lately to refer to particularly tiny moons - generally in the context of the relatively small (and often short-lived) clumps of ice that have been spotted forming regularly in Saturn's rings. These have been referred to as 'moonlets'. The term has also sometimes been used to refer to asteroid moons.

I feel the term is underrated however, and deserving of an upgrade.

Currently there are 180 identified moons of the solar system's planets (and dwarf planets) however, there is a distinction that should be made here. Only a small number of those moons are 'moons' of the sort we imagine when we look up into the night sky at our own - at what is obviously an entire separate world. Most of these 'moons' are little or nothing more than captured asteroids. The two moons of Mars for instance - Phobos and Deimos, have dimensions no greater than 27 and 8 km respectively on their longest axes. This compares to the 3,500 km diameter of our own moon, which is several million times more massive than either of them.

Luckily however, there is a solution to this problem.

We can call upon the old definition that separates planets from smaller objects (what used to be called 'minor planets', but were also reclassified by the IAU in 2006 and are now 'small solar system bodies') that is - that a planet has to have been rounded by its own gravity. Objects like asteroids and comets also orbit the sun, but are declared separate to planets based purely on their size, so why not apply the same standard to objects that happen to be in orbit around a planet?

Under this definition, the nineteen of the solar system's 180 moons that have been sufficiently rounded by their own gravities would remain as - 'moons'. These include our own Moon, IO, Europa, Ganymede and Callisto around Jupiter, Mimas, Enceladus, Tethys, Dione, Rhea, Titan and Iapetus around Saturn, Ariel, Umbriel, Titania, Oberon and Miranda around Uranus, Triton around Neptune and Charon around Pluto.

The other 161 discovered moons would be re-labelled as 'moonlets'. Thus -

Of course other labels could be used. The terms 'primary' and 'secondary' moon could be used instead, though I feel that 'moon' and 'moonlet' are more memorable, and less repetitive given that we'd already be using those terms for planets.

As far as our other definitions go, I have fewer qualms. I'm quite happy with the definition of a 'brown dwarf' sitting between planets and stars. Evidently the name is something of a misnomer, as 'brown' dwarfs are predicted to actually have a reddish hue, however 'red dwarf' already refers to low-mass stars with between 75 and roughly 500 Jupiter masses, and would seem even more deserving of that label. Stars are generally categorized by their color, which most closely reflects their mass. Basically, the bigger they are, the fiercer they burn, giving them higher surface temperatures and brighter hues. Stellar evolution is a tricky subject, especially when we start talking about such bizarre forms as white dwarfs, neutron stars and black holes, but the current definitions do sound pretty sensible to me. One problem however, is whether planet-sized objects orbiting around a brown dwarf are indeed 'planets' or merely 'moons'. Personally, I am inclined to think they are the former.

Then at the other end of the spectrum we have objects smaller than planets. These are divided into a number of overlapping categories. 'Asteroid' is a commonly used term, referring to small chunks of rock not large enough to be considered planets. However, the definition sometimes blurs with 'comet' as comets are merely asteroids with significant quantities of lighter elements like water, which sublimates when approaching the sun. The definition wouldn't be too bad, except that many icy asteroids never come close enough to the sun for their surfaces to sublimate at all, making them effectively 'comets-in-waiting'.

Then there is the question of whether there's a lower bound on 'small' solar system objects. Does the definition include meteoroids? That is - objects small enough to burn up in the Earth's atmosphere (generally less than a few meters across, and also begs the question of why the Earth's atmosphere is the benchmark. It is another arbitrary unit, like measuring explosive power in 'Hiroshimas'). What about interplanetary dust as well? Or individual atoms free-floating in space?

To conclude, regardless of these rough edges, the definitions I've outlined above would give us the following solar system - one that consists of one star, eight primary planets and a large number of secondary planets (of which only five have so far been discovered) orbited by nineteen moons and 161 discovered moonlets.

Case closed.

The Children of Space

(The reflections of a psychologist after visiting an orbital habitat around the Earth in about a century's time, when the first generations of children born in space are growing up).

Jason never ceased to be amazed at the limits of people’s naivety. Despite the best education possible, despite even thousands of hours spent in VR, there was still no substitute for physically travelling to and from space. The journey up or down a space elevator was like some mysterious sieve that some perceptions simply couldn’t penetrate.

Newcomers from Earth always needed a period of adjustment in space. Something as simple as walking was naggingly different on the inside of a rotating Orbital habitat. Your sense of balance took some time to adjust; it took maybe a few weeks for people to stop thinking they were falling over with every step. The air, the ground and the watercourses, all were so unfailingly fresh. They lacked the billion year tinge so deeply ingrained in everything on Earth. Your surroundings lacked a fundamental taste your subconscious missed deeply. Something was wrong here!

For those born in space, as had been the case for millions of people over the last thirty-odd years, the change was infinitely stranger. Compared to an Orbital’s sterile environs the Earth was one great big wild sweaty jungle. How could harmless phenomenon like wind or rain or distant quakes in the ground hurt people? Preposterous!

Jason recalled a seemingly innocuous but deeply unsettling visit to an orbital elementary school a few years back. He’d looked in on a few classes with the principal, talking to teachers and students. Nothing was out of the ordinary. Then he’d come across one class, it’s pupils no older than ten, who’d been asked to spend an afternoon drawing scenes from the Earth, a place almost none of them had yet visited. Jason had stood there, fascinated, as their creative little minds went to work.

There were drawings of forests and rivers, towns and beaches. Many were perfectly fine, especially those depicting things that were also dotted about most Orbitals. But some looked truly bizarre to Jason, having been born and lived on Earth for nearly a century and having travelled their frequently since. There was a distinct lack of sunsets, mountains, clouds and oceans, things that were too big or unwieldy to recreate in space. For the most part the children’s knowledge was limited, but not too wildly wrong.

Then there was one boy who’d decided to draw a small armada of earthbound vehicles of various types. Planes, trains, boats and so on. It was so riddled with obvious errors Jason couldn’t help but stand there, rooted to the spot, and watch the boy draw. The train he’d drawn in a transparent tunnel as all modern vactrains were, yet it was a steam train, busily belching out clouds of black smoke. Modern maglev trains could only accelerate to many thousands of K’s a second without great friction by travelling along sealed tunnels with most of their air evacuated, but this detail seemed lost on the boy. He also didn’t seem to notice how quickly his passengers would suffocate.

Then there were the airplanes he’d drawn, which Jason was quite sure wouldn’t be flyable in a terrestrial atmosphere. They didn’t just have a pair of horizontal wings, but a pair of equally large vertical ones as well. This gave them the cross-section of a plus sign when viewed head on. The boy’s thinking quickly dawned upon Jason. While he may never have flown in an airplane, the closest thing he’d experienced was probably an inter-orbital shuttle or maybe even an inter-planetary Liner. Practically all spaceships were vertically and horizontally symmetrical. They had a front and back, but no need for a top or bottom. Space lacked a definite up. Why should planes follow any different rules?

Jason was quickly reminded of the old perceptions people back on Earth used to have of spaceships. Jason was old, but had still been born half a century after Star Wars came out. He’d seen it of course, and at the time couldn’t help but laugh whenever the millennium Falcon banked as it turned, or gaze in astonishment at the glass cockpits and bridges every vessel seemed to have, which while perfectly necessary on an Earthbound aircraft, would have been suicidally vulnerable in any kind of space battle. Ever earlier on was perhaps the most poorly conceived spacecraft of all time, the Space Battleship Yamato, from a Japanese comic series. It was literally a spacecraft built around the WW2-era wreck of the Yamato, and could hardly have been more impractical in space. Space battles in 20^th century science-fiction always seemed to resemble a battle from the Pacific War. Large cruisers and aircraft carriers lumbered about while tiny fighters darted and dogfighted around them. The Battle of Endor might as well have been labelled ‘The Battle of Midway in space’.

Going even further back he had recalled the Martian’s invasion vessels in H. G. Wells' War of the Worlds. They were not rockets, which hadn’t advanced beyond small fireworks in Wells’ time, but large capsules fired by an enormous gun on Mars and aimed at the Earth. The crushing acceleration the inhabitants of such capsules would experience when the gun was fired had seemed lost on Wells, and also Jules Verne now Jason thought about it. Even wise men, Jason had to remind himself quickly, could have utterly impractical visions of the future and other things they'd never laid eyes on.

Jason could perhaps have forgiven the boy for his mistakes so far, but he couldn’t help but crack an oddly horrified smile at the boy’s attempt at drawing a sailing ship. He’d tried to apply the same rules here as he had to the plane. The shape of the hull was about right, and it had a trio of rather well-drawn mainsails on it’s top, but that was where practically ended and the boy’s ignorance began. He’d also tried to drawn the same trio of sails three more times, on the ship’s sides and, best of all, it’s bottom. You could hardly have designed anything more impractical.

While there were numerous pools, ponds, rivers and even lakes on most Orbital habitats and the boy had surely learned how to swim, the vast difference between the skin of Earth’s atmosphere and the depths of it’s oceans seemed to have been completely lost on him. Jason had immediately taken a photograph of the drawing, much to the boy’s confusion and delight, and always brought it out whenever he was giving a lecture on the psychology of children born in space. It was just the kind of snapshot into people’s basic emotions he truly valued.