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Please begin with an informative title:

Throughout the solar system, countless objects of widely varying sizes, bizarre geometries, and diverse compositions orbit the Sun independently or attend gas giant planets in swarms.  Those which overwhelmingly consist of rock and metal are called asteroids, and threaten Earth-bound human civilization at the same time they hold out the promise of a free, unimaginably wealthy, and unbounded future history.  They bear a stark message for mankind: Come to us in hope, before we come to you in fire.  In the culmination of the Asteroids sub-series, we zoom-in for closeups of the surface features of some prominent asteroids, explore the history of our relationship to such objects, examine the threats they pose to us, and peer into the future for what glories they will some day make possible.

Intro

You must enter an Intro for your Diary Entry between 300 and 1150 characters long (that's approximately 50-175 words without any html or formatting markup).

The progress of our adventure so far (current in bold):

1.  The Sun
2.  Mercury
3.  Venus
4.  Earth (Vol. 1)
5.  Earth (Vol. 2)
6.  Earth (Vol. 3)
7.  Earth (Vol. 4)
8.  Earth (Vol. 5)
9.  Earth (Vol. 6)
10.  Luna
11.  Mars (Vol. 1)
12.  Mars (Vol. 2)
13.  Mars (Vol. 3)
14.  Phobos & Deimos
15.  Asteroids (Vol. 1)
16.  Asteroids (Vol. 2)
17.  Asteroids (Vol. 3)
18.  Ceres
19.  Jupiter
20.  Io
21.  Europa
22.  Ganymede
23.  Callisto
24.  Saturn
25.  Mimas
26.  Enceladus
27.  Tethys, Dione, and Rhea
28.  Titan
29.  Iapetus
30.  Rings & Minor Moons of Saturn
31.  Uranus
32.  Moons of Uranus
33.  Neptune
34.  Triton
35.  The Kuiper Belt & Scattered Disk
36.  Comets
37.  The Interstellar Neighborhood
Topics covered in Vol. 3 of the Asteroids sub-series:
I.  Context
II.  Population Groups, Families, and Spectral Types
III.  History
IV.  Properties
1. Orbital and Rotational Features
2. Size and Mass Characteristics
3. Surface Features
V.  Past Relevance to Humanity
VI.  Modern Relevance to Humanity
VII.  Future Relevance to Humanity
VIII.  Future of The Asteroids
IX.  Catalog of Exploration
Just as a reminder, the seven reference asteroids we have been examining are 25143 Itokawa, 951 Gaspra, 433 Eros, 243 Ida and its moon Dactyl (we can consider the Ida/Dactyl system as one asteroid, since Dactyl is almost certainly a piece of Ida), 253 Mathilde, 21 Lutetia, and 4 Vesta.  Once again, the choice of these asteroids is dictated by the thus-far sparse record of detailed asteroid exploration, so these objects aren't necessarily a representative sampling of what's out there.  

3.  Surface Features:

Itokawa is a bent oblong shape with smooth areas near the inner side of the crease dividing the two lobes, and a large boulder the size of a mid-rise office building resting on the end of one lobe.  Because the object is a "rubble pile" consisting of loosely-bound material, the surface is mostly rough and heterogeneous outside the few smooth areas mentioned.  Names for features on this asteroid are Japanese-oriented since the spacecraft that explored Itokawa was from Japan, but there is no particular convention or theme surrounding them.  Maps showing named features:

Itokawa Map 2

Itokawa Map 3

Itokawa Map 4

Itokawa Map 1

Itokawa Map 5

Itokawa Map 6

Itokawa Map 7

You would think they would name the giant boulder on the end, but I guess visual prominence is less important than what a feature reveals about the history of the object.  Below are some more photos of the surface, some showing unofficial working names given by the exploration team while the spacecraft was at Itokawa.  Note in the first two how the play of shadows creates an interesting warping effect on the apparent shape of the asteroid:

Itokawa 5

Itokawa 8

Itokawa2c

A zoom-in of the borderland between a rough and smooth region - you can see the transition in terrain moving from lower left to upper right:

Itokawa 3

A couple of regional shots:

Itokawa1

Itokawa 9

The shadow of the Hayabusa spacecraft as it descended toward Itokawa to collect surface samples:

Itokawa 2a

Itokawa 2

Extreme close-ups:

Itokawa 6

Close-up of Asteroid Itokawa Taken by Hayabusa

surface of asteroid Itokawa, taken by the JAXA Hayabusa spacecraft

All of the above images are monochrome, but a few true-color images were taken - this is what your own eyes would see if you were there:

Itokawa 4 (True Color)

Gaspra has not been explored in the same level of detail as Itokawa, and images of it only come from a single flyby of the Galileo probe that only saw about half the surface.  Nonetheless, there are several named craters, mostly named after famous historical health spas for some reason, while regions are named after people associated with the asteroid or the Galileo mission:

gaspra map-001

gaspra map 2_2-001

Eros feature names are unsurprisingly themed on love and romance, though also a few astronomers associated with the asteroid.  A cylindrical projection map, followed by unlabeled images:

Eros Map

Eros4

Eros 1

Eros6

Eros2

Eros5

Eros16

Eros9

Eros7

Eros12

Eros8

Eros15

Eros21

Eros11

A true-color shot:

Eros10

Some limb shots, craters, and surface close-ups:

Eros24

Eros23

Eros22

Eros20

Eros17

Eros18

Ida was also studied by a Galileo flyby, although it got a much more extensive view of the surface than of Gaspra.  Still, there are no close-ups that I've found.  Most features are named after terrestrial caves:

ida map-001

A closer shot of the "knob" at one end near Vienna Regio:

Ida2

Mathilde features are named after terrestrial areas associated with coal mining due to the dark color of the asteroid.  Its exploration was very limited and brief, amounting to a distant flyby via the NEAR Shoemaker spacecraft on its way to conduct far more detailed work at Eros.  As such, its map is not very well characterized - and I'm having trouble converting the PDF file to a JPEG for display here, so you can view it on the USGS website if you're curious.  However, the most significant feature - the titanic gouge that dominates the few images of the asteroid - is called Karoo.  I will repost the image of Mathilde that has already been seen in previous parts of the Asteroids sub-series for reference:

Mathilde

Lutetia has a nomenclature themed on cities, rivers, and provinces of the Roman Empire.  Crater map followed by region maps:

lutetia crater map-001

lutetia region map_2-001

Some global and regional images of Lutetia, the first in true-color:

Lutetia2 (True color)

Lutetia1

Lutetia3

Lutetia5

Lutetia6

Lutetia4

Vesta is undergoing active exploration by the Dawn space probe, which will remain in orbit of the asteroid until near the end of this month when it will leave for Ceres.  Due to its mass and thus relatively significant gravity (2.5% of g), Vesta is a somewhat warped ovoid like a battered melon - i.e., gravity has played a major role in shaping it rather than just the force and trajectory of random collisions, but not enough to smooth out distortions caused by major impacts.  Like Lutetia, the nomenclature theme is Roman.  Cylindrical maps - first the Eastern and then Western hemisphere:

Vesta Eastern Hemisphere

Vesta Western Hemisphere

Miscellaneous images showing the features of Vesta are below.  Notice the linear features in some that call to mind Phobos, although they may be the result of different phenomena:

Vesta2

dawn-image-071811-800-1040-full

Vesta3

Vesta5

Vesta7

Vesta13

Vesta23

Vesta25

Vesta1

Vesta22

Vesta21

Vesta19

Vesta16

Vesta15

Vesta12

A partly buried crater provides striking contrasts:

Vesta10 (half-buried crater)

Near-true color image of the big triple-impact Marcia, Calpurnia, and Minucia that together are one of the most prominent features of Vesta:

Vesta24

Hopefully as it departs, Dawn will take some true-color images of the full face.  That's part of the purpose of robotic space exploration: To be surrogate eyes for humanity, because we are not yet capable of visiting these worlds in person - not just to gather scientifically relevant data.  Vesta topographic maps (the second is of the South pole - the far-North doesn't appear to have been explored):

Vesta Topo Map

Vesta Southern Hemisphere Topo Map

---

V.  Past Relevance to Humanity

The most profound role played by asteroids in human history occurred before our species even existed, when the dinosaur line was annihilated by a massive impactor.  As described in Volume 1 of the Earth sub-series, reptiles had become so hyper-adapted to the climates of the Jurassic and Cretaceous periods that mammalian ancestors were squeezed into relatively small ecological niches.  

Warm-bloodedness was not especially useful for a significantly-sized creature in a warm, humid climate, so the reptiles got big and diverse while our ancestors shrank and had far fewer prospects.  However, when the dinosaurs were nearly all obliterated in the course of a few hours - and the remainder over the coming months - warm-blooded species were able to move into the vacated niches far more quickly than the surviving reptiles, and thus humanity was made possible by an unimaginable horror that still threatens us.

Although there have been numerous impacts in the ensuing 65 million years, none have played an especially significant direct role in our evolution or ancient societies - neither as lights in the sky nor as meteors (asteroids that enter the Earth's atmosphere).  The ancients recorded incidents of seeing meteors, but interpreted them mythically as "falling stars" or "shooting stars," and in rare cases of an atmospheric entry large enough to create a large fireball, it was usually interpreted as a bad omen.  One example of how the ancients saw these events occurs in the Book of Revelations 8:10-11:

And the Third Angel sounded, and there fell a Great Star from heaven, burning as it were a lamp, and it fell upon the third part of the rivers, and upon the fountains of waters; and the name of the Star is Wormwood; and the third part of the waters (rivers) became Wormwood; and many men died of the waters, because they were made bitter.
Spooky, huh?  They were not fans of seeing objects from the "fixed" and "immutable" heavens suddenly becoming mobile and demonstrative, so it is hardly surprising that a meteor would figure into the apocalyptic delirium of St. John.  Actual meteors witnessed by pre-modern peoples have been far less dramatic in their consequences, although they still tended to freak people out.  A woodcut from 1492 depicting the fall of a meteorite in the town of Ensisheim in what is now Eastern France:

Ensisheim Meteor Fall 1492

The Ensisheim meteorite reached the ground intact - just one of many such cases in history, none of which are known to have caused injuries or fatalities (people were not very numerous relative to Earth's surface area until recently) - and was presented to the son of the Holy Roman Emperor as a gift.  It's now in a museum in the town that first received it from the sky:

museum-ensisheim

One thing, however, has been very significant to human history that is almost entirely the result of asteroid impacts: Gold.  And silver, and platinum, and the rest of the rare metals - materials that have represented money for thousands of years due to their beauty and rarity, and that only recently have been found to have valuable technological uses.  All of the heavier metals that formed the Earth sank into the core billions of years ago via the constant churning of the mantle and plate tectonics, and only the constant replenishment of the surface by external impacts has kept it stocked with them.  If they'd have known where it came from, the ancients would probably have loved gold all the more.

.

So, while most of the same slaughter and bastardy driven by love of money would have happened anyway, perhaps the physical luster of gold - that ineffable quality that makes it so magnetic to the human eye and desire - added an extra push to make historical wars of greed that much more atrocious.  Maybe the ancient superstition that meteors were an ill omen weren't all that wrong.  

In any case, the impact origin of these metals is the reason they're found bunched together in veins and strikes rather than uniformly distributed throughout the crust, so that fact too has played a role in historical events - particularly in the rise of civilizations with a strong metallurgical culture, and the lust of rulers and nations to conquer foreign regions that had more gold and silver than their own territories.  This has been a significant influence even in American history, with the California Gold Rush of 1849 and the forcible removal of Sioux from the Black Hills of South Dakota in the 1870s.  Some time in the distant past, probably before human beings existed, an asteroid deposited gold somewhere that would eventually end up in South Dakota, and as a result there was ethnic cleansing, massacres, and a pretty awesome HBO series.  Such are the distant human consequences of ancient events in nature.

Then, of course, was the Tunguska event of 1908 - a massive explosion and ensuing firestorm in Siberia that leveled over a thousand square kilometers of forest.  The generally accepted explanation for the event is that an asteroid or cometary fragment a few hundred meters in diameter penetrated the atmosphere to a low level and then exploded in the air.  Villagers dozens of kilometers from the epicenter were thrown to the ground by the force of the blast.  A contemporary photo of the aftermath:

tunguska

Had such an event occurred during the Cold War, there would likely have been a civilization-ending nuclear exchange to top off whatever damage had been done by the impact itself.  In fact, there have been several instances where far smaller aerial meteor explosions have triggered US and Soviet/Russian military alerts to possible missile attacks.  Had those events been the size of Tunguska, let alone if they had leveled a city, it seems unlikely that cooler heads would have prevailed.  The US and Russia have some experience with such events, but there remains a danger to this day that other nuclear-armed countries - e.g., China, India, Pakistan - would respond spasmodically to the destruction of a major city before the details could be known.

---

VI.  Modern Relevance to Humanity

Rare metals delivered by asteroid impacts are far more precious today than in the past when their value was simply a factor of their appearance and scarcity.  Today, all of these elements have found increasingly powerful uses - not only gold, silver, and platinum, but such previously obscure materials as palladium, iridium, indium, gallium, and several others besides.  The overwhelming driver of these new applications is the electronics industry, which is finding the electrical properties of the rare metals very useful as processor design evolves.

Unfortunately, much of the same politics that had once driven lust for gold and silver is now based on the actual utility of the materials, which has led to attempts (largely successful so far) by the Chinese government to corner the market on rare metal mining and processing through trade manipulation.  It has also become yet another driver of violence and exploitation in Africa, as global powers have sought control of the vast material riches beneath some of its regions and infused preexisting or simmering internal conflicts with external financing.

However, one beneficial side-effect of the artificially-inflated scarcity of these metals is that significant resources are being invested by a number of extremely wealth people in an unlikely-sounding venture: Going directly to the source - i.e., in-space mining of asteroids.  You may have heard of this when it was first announced, since the resources and credibility of the people launching the venture made it highly newsworthy: Planetary Resources Inc. - which is being launched by at least three billionaires - plans to launch a series of space telescopes into orbit that will scan for metal-heavy Near-Earth Asteroids (NEAs), and commercialize their telescope technology in order to raise revenue for subsequent stages of development.  From there, they intend to send more advanced robotic probes to candidate objects, where more detailed prospecting will occur, and in the final stage unmanned craft that will capture and retrieve small asteroids with the right composition.

Although the up-front costs of establishing such operations are high, the long-term economics appear sound: One asteroid in the size range being targeted, with the composition being targeted, would yield tens of billions of dollars' worth of rare metals for a cost in the low single-digit billions (if not lower, if attempts by SpaceX to radically reduce launch costs succeed).  The timetable for success, of course, is quite long, and we are not likely to see the first asteroid retrieval mission attempted in this decade, but the promise is extraordinary.

A handful of such missions would add more of these metals to the human economy than have ever existed in all of history combined, enabling not only their existing uses but making entirely unsuspected applications practical as the commodity price goes down.  Not only would a key input in the electronics industry become far more affordable, but also materials with properties that are highly enabling to solar panel technology - both of which are powerful technological drivers of the quality of life on Earth.  The company is also interested in mining asteroids for water to be used by other in-space operations, since it's so expensive to launch water from Earth's surface that it would actually be cheaper to get it from an asteroid and send it to anywhere else than to bring it from Earth.  A promotional video for Planetary Resources:

That's the positive side of humanity's burgeoning relationship with asteroids, but the threat they present is still quite ominous.  To this day, humanity has no proven, practical means of diverting or destroying an asteroid found to be on a collision course with Earth, and is still occasionally caught by surprise when significant objects pass close to our planet.  A defensive capability will probably evolve out of the technologies being developed by companies like Planetary Resources and its eventual successors, but for the moment we would be in a very bad position if a major impactor were approaching and we had less than a decade to do something about it.  Less than a year of warning would certainly mean that interception would be impractical, and all realistic responses would be palliative - e.g., moving hundreds of millions of people from coastal regions to higher ground, building shelters, creating national resource stockpiles, etc.

However, there is far more recognition of the threat today than in the past, which has spurred the existence of Spaceguard initiatives seeking to mobilize telescope infrastructure to identify and track potentially threatening objects.  Thousands of Earth-crosser asteroids have been identified through these efforts, and more will certainly be found in the future as remote sensing technology improves.  But it should be noted that these projects are far from comprehensive, and it's reasonable to assume that most such objects out there have yet to be identified and their paths computed.

To enable global collaboration in dealing with potential threats, astronomers created a threat hierarchy called the Torino scale that indicates the level of hazard from 0 to 10.  On this scale, 0 corresponds to asteroids with virtually zero likelihood of colliding with Earth, objects small enough that they couldn't reach the surface through the atmosphere, or else small enough that even if they did hit the surface the likely damage would be trivial.  Ratings 8 through 10 indicate certain impacts, but with increasing potential for devastation - 8 being destructive on the level of a hurricane or earthquake, 9 likely to destroy entire regions, and 10 being an extinction-level event.  A plot of the Torino scale with the x-axis corresponding to impact probability and the y-axis to the force of the impact:

torino_plot_big

The ratings of objects are constantly changing as new data is compiled, but none has ever been rated higher than 4 - a close encounter with a 1% chance of regionally-devastating impact - and none today are higher than 1.  Of course, that doesn't really mean anything as to the likely rating of future threats: A minor perturbation in a low-ranked object could place it on a path that leads progressively to higher and higher threat levels, and possibly an impact, so the point of a warning system is not to breed complacency.

One good thing is that we're very unlikely to be surprised by a very large object like the one that killed the dinosaurs.  The strongest threat is from mid-sized impactors like the Tunguska object, which have the potential to not only obliterate entire cities and kill millions, but to trigger nuclear holocausts between nations if there is no warning whatsoever before a city disappears in a fireball.  Even in the absence of man-made secondary disaster, such events are more than energetic enough to cause substantial and rapid climate changes and regional devastation well beyond the impact zone.  The subsequent political destabilization could itself spark all-out war even if the precipitating event did not.

Despite the vigilance of astronomers, humanity today is actually more vulnerable to asteroid impact than ever because of several converging trends: (1) Exploding population means we occupy an ever-increasing percentage of the surface, (2)most of that increase is happening in coastal areas likely to be devastated by tsunami in the event of an oceanic impact, (3) more countries possess nuclear weapons that may be used in knee-jerk fashion against enemies if they're hit by an impactor, and (4) our economies are increasingly globalized, inter-dependent, and fragile to the point of even minor disruptions having major consequences.  Until Earth is no longer humanity's only home, the fragility of human civilization will only increase with time as we become over-adapted to prevailing conditions and lose our ability to respond when something changes drastically in a short time period.

On a lighter note, asteroids have occasionally been the subject of what passes for science-fiction on TV and in movies.  In The Empire Strikes Back, Han Solo famously flies the Millennium Falcon into an "asteroid field" so dense with objects that they fly in random directions within a few meters of each other and collide on a constant basis for the amusement of the viewer.  This is, of course, nonsense: Newton's 1st Law of Motion states that motion continues in a straight line except as acted upon by an external force, so if those asteroids were flying in all different directions, the field would thin out to nothing within a short period of time.  The only reason for such objects to group together in the first place is that they share an orbital region around a massive body like a star, but under those conditions they're not moving chaotically, and certainly don't come within visual range of each other on a regular basis.

Then there are the two big-budget studio movies dealing with impacts released at roughly the same time, Deep Impact and Armageddon.  Deep Impact is relatively - and I stress relatively - accurate in its depictions, and its special effects are still worth seeing, although the impactor in that film is a comet rather than an asteroid.  Still, the look of the explosion and ensuing tsunami is visually compelling.  

Armageddon, however, is strong in the running for dumbest film of all time, and its premise is that an asteroid "the size of Texas" - i.e., bigger than Ceres, which was so big it was visible from way out in the Main Belt when it was discovered in the 19th century - takes Earth completely by surprise, and NASA discovers that it will hit in 18 days.  The appearance of the asteroid is basically like what a snowflake would look like if it were made of obsidian - jagged, spikey, and totally unlike any large-scale phenomenon that exists in reality, let alone an object that big whose gravity would have made it into a sphere shortly after its formation.  

In those 18 days left to the world an entire Space Shuttle is radically and successfully modified, staffed with an oil rig crew for some unknowable reason, and this object the size of Texas is split in half by a single nuclear weapon placed 800 feet beneath the surface (i.e., 0.00004% of the way to the center).  In other words, the film is a horrifying monstrosity from the deepest Stygian abyss of ignorance, so don't rely on it for anything other than humor.  Bask in Teh Stoopid:

Real planetary defense scenarios would be far less dramatic, and there are very few circumstances where blowing something up would be involved.  The most likely is that we see a mid-sized object coming from a long ways away with plenty of time to get organized - meaning decades - and then land something on it that would slowly push it in a given direction so that it would miss the Earth.  This impetus could be given by a solar sail, or a series of chemical rockets, or an ion engine - there are a number of ways to achieve the same result.  Or, if the object is small enough, it could simply be strip-mined and whittled away to nothing before it ever got near us.

A real emergency scenario, where a significant object is headed for us in a relatively short time period - but still long enough to do something about it - would need a lead time of several years, and still most variants of the scenario would involve gradual diversion (albeit with more power than if there was more time or the object was smaller).  The problem is that you need to know a lot about an object before you do anything about it - if you just lob nukes at something sizable, you would most likely just turn it into a swarm that would sandblast Earth's atmosphere into a superheated cauldron rather than a single impactor with concentrated damage.  And even gently nudging it requires detailed knowledge of the internal structure, because pushing it too hard, too fast might cause it to fracture, or the energy you put into it might just be absorbed by internal jostling if the object is a pile of rubble.

Contrary to what a lot of people seem to believe, the world's ICBM forces are not capable of reaching deep space - those rockets are only built to go up a few hundred kilometers, in order to lob payloads ballistically intended to land back on Earth.  By the time an object got that close to the planet, it would be a split-second before impact, and blowing it up would be meaningless because all the energy of the explosion would still be directed at Earth.  Since you wouldn't have time to rely on the long, slow gravity-assist trajectories that NASA space probes use to get anywhere, every nuclear warhead intended to reach an incoming asteroid would need its own Saturn V-equivalent rocket, and a dedicated spacecraft framework to guide it to the target.  

It takes years to build and test such things, so short of the time to do so, there wouldn't be any last-minute salvation: If the timescale were months, the only practical actions would be ground-based preparations to mitigate the effects of the impact.  And if it were days, governments would probably consider it less disruptive to just not tell people, retreat to their bunkers, and then cleanup the mess after the fact.  Rather than having to account for tens of millions of starving refugees who, forewarned, had fled inland to escape tsunamis and are now desperate and potentially violent, they would only have to handle the much smaller number of people who survived the initial event.  So that's the reality: In the vast majority of scenarios, the response is either anti-climactic or apocalyptic.  But if we can get through this century without facing such an event, the danger will rapidly drop to zero as humanity becomes more competent living and operating in space.

---

VII.  Future Relevance to Humanity

We are at the very beginning of the beginning in the story of humanity's direct relationship to asteroids, as they become prized resources and eventually locations for new history to unfold rather than agents of doom.  Planetary Resources Inc. is planning infrequent operations into the general vicinity of Earth's solar orbit to bag relatively small NEAs a few tens of meters in diameter, but despite the massively positive effect this would have on the terrestrial economy, it would be the tiniest bite out of a trivially small population of objects.  

There are still the Main Belt and the Jupiter Trojans - an unimaginable abundance that would make all the riches of all NEAs combined vanish to nothing in comparison, and very likely take thousands of years for humankind to even make a dent in them.  So I present the following speculative progression:

First Age: Slow, low-level exploitation of inner-solar system asteroids (mostly NEAs) by terrestrial economies and small, nascent operations on the Moon and Mars.  My guess would be that this age would last a couple of centuries.

Second Age: Acceleration and continuous up-scaling of inner-solar system asteroid exploitation, as Moon and Mars economies grow and develop their own taste for resources.  Establishment of what is basically an interplanetary "conveyor belt," continuously harvesting and sending material along slow, low-energy trajectories for delivery at points of demand.  Call it another few centuries.

Third Age: Industrial expansion into and human settlement of the Main Belt and Jupiter Trojans (I explore some of the ways humans can colonize asteroids below).  There is so much material in these populations, and so many objects theoretically capable of hosting human habitats, that as I said above this Age could very easily last for thousands of years without running into any kind of major boundary condition.  Water ice is abundant in this region of the solar system - more than all the oceans of Earth - and it's still close enough to the Sun for solar power to be practical, which is aided even further by the abundance of "rare" metals for use in large-scale photovoltaic systems.  

In other words, even if fusion power is not available or ubiquitous as I think it likely will be at this point, the environment of the Main Belt still provides more than enough of the basic inputs - water, raw materials, and energy.  With those three things, a suitably adapted technology has breathable atmosphere, drinking water, fuel, elements and minerals needed for soil to grow food (or construct it on a molecular level), and energy to power it all.  That isn't to say everything needed would be available on every object where industry or civilization develops, but collectively it would be more than enough - trade between asteroid-based installations would be very cheap, since it requires little energy to move between objects in similar solar orbits with minimal surface gravities.

Moreover, it could be made cheaper over time if - through means already mentioned - asteroid orbits are adjusted to bring them into more convenient alignments.  A promising object with an inconvenient orbit could be moved so that it's easier to go back and forth from it to other objects in the population; high inclinations damped out; inconveniently slow or fast rotations corrected; eccentricities smoothed out or maybe accentuated if there is some advantage to them, and so on.  For instance, it might be useful to operate on an asteroid that migrates between circularized populations, acting as vehicles of slow bulk trade.

In particular, the Hilda group asteroids come to mind.  These objects have a 2.6-year migratory period between the outer edge of the Main Belt and Jupiter orbit, which is 1/3 of their full 7.9-year orbital period.  As such, their relationship to Main Belt objects is constantly changing even though their motions relative to Jupiter are regular.  Imagine, if you would, an entire city on a floating island that migrates regularly between the continents of the world over a 2.6-year period without any need for its inhabitants to use energy or steer it - imagine how rich, prosperous, and culturally dynamic such an island city-state would be.  Just to refresh your memory, here again is the orbital motion of the Hildas posted in Volume 2 (in red):  

Now imagine that instead of migrating among a few dozen coastal nations on a quickly-repeating cycle, this island passes by thousands of them, and the order in which it encounters them changes over time.  The width of Hilda migrations is close to 1 AU - the distance between the Earth and Sun - and thus they experience a significant energy gradient (both in sunlight and gravitational potential energy) moving between perihelion and aphelion: Something that could be potentially useful.  I don't know enough to put this to any kind of quantitative test, but on an intuitive level it seems like these could play a significant role in future human history.

Another major aspect of the Third Age of asteroids would be something I hinted at earlier: The Main Belt is just on the edge of where sunlight can be passively harvested with a practical level of return.  Beyond the orbit of Jupiter, the only practical form of energy to sustain a civilization or even a significantly-sized research installation is fusion - otherwise you would have to sustain it by constantly shipping in massive quantities of fissile fuel or some other form of stored energy from points interior, and there would be no exportable material from these regions that would justify such an expenditure.  

Everything you could send to the inner solar system from Saturn or beyond would be abundantly available from Jupiter and the Main Belt.  This means, quite simply, humanity can't settle beyond Jupiter until we have fusion, and the further inward of Jupiter you are, the less economically worthwhile fusion is because you have a giant ball of natural fusion (the Sun) giving away energy for free.  It's always cheaper to just passively absorb what is freely available than to go through the trouble of manufacturing it artificially,  so unless fusion ends up being a lot simpler, cheaper, and more down-scalable than I think, the Main Belt civilizations would live on the economic boundary between those where fusion is an utter necessity and those where passively harvesting solar energy is trivially easy.  Such a borderland could have interesting and unexpected consequences.

Anyway, as to the particulars of how human beings can settle asteroids, the usual idea in science fiction is that a reasonably strong, stable object is chosen, tunnels and habitats are dug into it, and then the whole thing is rotated with a certain period and along a convenient axis to give the habitats artificial gravity.  Many fictional depictions involve hollowing out the asteroid, filling it with air, running a lighting tube through the rotational axis, and then making the inner surface habitat, but that's probably more work than is necessary.  Settlements would more likely start smaller, carving habitat out of the outer layers of the object and spinning it up, then mining further inward as their material needs and space requirements grow.  Real civilizations expand from small kernels, they don't involve building gargantuan megastructures as the first step.  

Of course, you could go through the trouble of mining the materials from the asteroid, processing them, exporting them, and then importing finished products needed to build a free-floating space station city rather than trying to colonize the asteroid itself, but it would seem a lot more economically convenient to just do everything on the spot with the materials at hand: Bore habitat space into the rock, use extracted material to create structures, fuel, water, and air, etc.  Another benefit of living in an asteroid over building a free-floating habitat from scratch is that the rock "beneath" your feet (closer to the surface than you, since you're relying on centrifugal effects to walk at all) is serving as effort-free radiation and impact shielding.

Given the ability to do this, there is enough habitable volume in the Main Belt and Jupiter Trojan asteroids to sustain anywhere from tens of billions to trillions of people, depending on how many are found and what proportion of them are suitable for such development.  The number is likely higher if we find that human beings can be healthy living their entire lives in gravity substantially less than 1 g, since that would allow people to occupy a greater proportion of an asteroid's volume (further inward = less artificial gravity due to rotation).  And even if otherwise, people could still spend part of their time in 1 g and part of it in lower gravity.

I would characterize the Third Age as the "Aegean of Asteroids," with all sorts of historical scenarios playing out that would be a lot of fun to read as an adventure novel but just awful to live through: An explosive diversity of cultures, religions, political systems, genetic mixes, and all the wonders and horrors that are made possible by it.  There would probably be considerable warfare, piracy, exploration, nth-degree colonization (e.g., a colony founds another colony, which then itself branches out), anarchic economics, brutally oppressive tyrants, corporate and aristocratic oligarchies, democracies, twisted cults, enlightened philosophers, monstrous messiahs, countless artistic flowerings, and ever-so-slowly, the coalescence of a common civilization among people who live under similar circumstances and rely on similar technologies.

Fourth Age: Once the anarchy dies down, asteroid cities could be physically brought together into gargantuan webs, some might move themselves closer to the Sun, some might move themselves farther out to be isolated (perhaps for religious or ideological reasons), and some might leave the solar system altogether on quixotic missions of multi-generational interstellar exploration or colonization.  What began as loose confederacies could fuse into truly bizarre civilizations that people like us, living on the skin of a planet in an environment that we passively enjoy, would find difficult to comprehend.

Suffice it to say that I expect asteroid-based civilizations to be the ones that subsequently colonize the outer regions of the solar system and ultimately make a go of interstellar settlement.  We planet-dwellers of Earth, Moon, and Mars would just be taking it easy at home, basking in our beloved Sun like beach bums, and those colonists who had settled the moons of gas giant planets would be too busy acting out their own adventures in the mini-solar systems they inhabit to be concerned with moving outward.  The relative mobility and wandering lifestyle of an asteroid civilization would be ideally suited to further exploration, both technologically and psychologically.  And once they reached other stars, they would spread among the asteroids of those systems, sweeping both inward to create new terrestrial societies, and further outward to seed still more of their own kind.  Call it 5,000 to 10,000 years from now.

In summary, asteroids are the wealth of the next few centuries; the scenes of explosively diverse history over the next few millennia; and the seed-pods of human dispersal into the galaxy over an arbitrarily large scale of time after that.  They are the fragments of a world that was never allowed to be born, that will give birth to more worlds than have ever been.

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VIII.  Future of the Asteroids

Provided humanity survives this last century trapped on Earth, the vast majority of asteroid material will end up utilized as an economic input for the prosperity and expansion of humans into the solar system over the subsequent millennia, and much of the remainder will become habitat volume.  Unimaginably large quantities of metals will be utilized in expanding the solar-harvesting capacity of the inner solar system, with swarms of colossal arrays orbiting the Sun at various distances.  And when I say colossal, I mean imagine single arrays larger than the distance between the Earth and Moon: Imagine such arrays being mass-produced in space by automated systems that consume asteroid material sent to them and churn out photovoltaic-derived material a hundred square kilometers at a time.  So the material that isn't used by people living out there in the Main Belt or points beyond, and isn't turned into habitat, would be gobbled up by Sunward civilization.  

However, whatever material escapes our usage, or in the unfortunate event that humans go extinct in this century - or if we somehow become extinct despite colonizing the solar system - the asteroids will continue to orbit the Sun long after it's dead, although they'll gradually migrate away from it and be picked up by other stars where the same fates will be decided again later.  And again, and again, and again.  Eventually they would all either end up gobbled by the central galactic black hole or else wander eternally in the intergalactic darkness until the accelerating expansion of the universe causes their atoms to decay and the sub-atomic particles to disintegrate.  

So, somewhere out there in our solar system - somewhere we may eventually see it, and not bother to notice - is an object that will look more or less the same when the Sun is a dead husk as it looks right now; the same when the rest of our entire galaxy is nothing but a black hole; and much the same when every star in the universe is dead, the sky is dark because the universe has expanded beyond sight, and the forces that bind atoms together begin to lose cohesion.  Finally, then, it will change.

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IX.  Catalog of Exploration

1.  Past & Current Spacecraft:

Galileo: USA, 1991 & 1993 flybys of Gaspra and Ida.
NEAR Shoemaker: USA, 1997 & 2001 - flyby of Mathilde, orbit around Eros.
Deep Space 1: USA, 1999 flyby of 9969 Braille.
Stardust: USA, 2002 flyby of 5535 Annefrank.
Hayabusa: Japan, 2005 orbit, landing, and sample return from Itokawa.
Rosetta: Europe, 2008 flyby of 2867 Šteins, and 2010 flyby of 21 Lutetia.
Dawn: USA, 2011-2012 orbit of Vesta.  Planned 2015 orbit of Ceres.

2.  Future Spacecraft:

Hayabusa 2: Japan, intended launch 2014 to arrive at asteroid (162173) 1999 JU3 in 2018 and return sample.
OSIRIS REx: USA, intended launch 2016 to asteroid 1999 RQ36 for arrival at target in 2019 and return sample in 2023.

Extended (Optional)

Originally posted to Troubadour on Wed Aug 15, 2012 at 03:09 PM PDT.

Also republished by SciTech, Astro Kos, and Community Spotlight.

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