On the border between the inner and outer solar system, the dwarf planet Ceres is the largest and most massive object in the Main Asteroid Belt: An embryonic world prevented from growing into full planethood by the brutal gravitational influence of Jupiter. However, its location, nontrivial surface gravity, and overwhelming abundance of water ice suggest that it may play a major role in humanity's expansion beyond Mars, and may itself someday be an abode of civilization.
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
The best image we have of Ceres today - contrast-enhanced, via the Hubble Space Telescope:
I. Context:
Ceres is the largest and most massive object in the Main Belt, the smallest dwarf planet yet known, and the 31st most massive and 33rd largest object in the solar system. It lies roughly in the middle of the Belt, although it experiences significant eccentricity that varies its distance from the Sun by almost half an astronomical unit (AU) - the average distance between the Earth and Sun.
Jupiter is the second brightest object in the Cererian sky after the Sun when the two are on the same side of their orbits, and would be far larger and brighter than we ever see it from Earth - roughly comparable in angular resolution to how we see Venus. Saturn would also be visibly brighter than seen from Earth, although the difference with respect to Uranus and Neptune would be small given the relatively large distances involved. The other inhabitants of the Main Belt, however, would not be visible to the naked eye except on rare occasions of a large, bright asteroid passing relatively nearby - and then it would just be a dim speck. Earth and the Moon blend together into a bluish star, usually lost in the solar glare.
Fortunately, the Sun is still bright enough at this distance for high-efficiency solar panels to be a practical augmentation for nuclear power, so any installation established there would not have to rely exclusively on imported fissile material in the absence of fusion technology. That means there is no absolute economic reason humans couldn't begin personally exploring it as soon as propulsion and habitat technology are up to the challenge, although I would be pleasantly surprised if that happened before the final decades of this century. A rough comparison of solar resources at Earth, Mars, and Ceres:
What the chart above implies is that for every square meter of photovoltaic surface on Earth, you would need about 8 on average in the vicinity of Ceres to generate the same power given the same efficiency. This is not as problematic as it sounds if we grant that mass-production of cheap, highly-efficient solar panels for the space environment will be well underway by the time humans are operating beyond Earth orbit in any significant numbers - an assumption that seems well-justified given the necessity of such technologies for expansion even to the Moon. With metal-rich asteroid raw materials as economic inputs, as described in Asteroids (Vol. 3), and manufacturing occurring in-situ, the cost is likely to become trivial and the production volume gargantuan.
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II. History
Ceres shares the early history of its fellow Main Belt objects, as described in Asteroids (Vol. 1): Shortly after solid rocks and metallic granules began precipitating out of the Sun's protoplanetary disk, Jupiter coalesced into a massive planet that began dumping gravitational energy into objects orbiting in resonance with it. Many of these objects were ejected from the solar system, thrown into the Sun, or perturbed into off-kilter orbits that caused them to smash into each other at such high velocities that their material shattered apart rather than fusing into larger bodies.
Out of that melee only one stunted dwarf planet managed to form, along with a retinue of a handful of large, independently-formed asteroids and millions of significantly-sized impact fragments and primordial relics. Under normal conditions, the vast majority of this material would ultimately have coalesced into a single planet, but the influence of Jupiter distorting orbits has both reduced the frequency of opportunities for collision and increased the destructiveness of those that do occur. Instead of a gentle, persistent rain of material punctuated by an occasional hail stone, the result was an artillery barrage. So the story of how Ceres came to be could be called "Planet, Interrupted," and can teach us a lot about planet formation.
The solar system was once rife with such protoplanets - massive bodies that, sweeping through the dense disk of material, quickly built up their bulk and gravity. But because there were so many of them, most ended up colliding - food for more fortunate brethren, thinning out their respective regions of objects until only one remained: A planet. But in the Main Belt, objects still orbit at quirky angles and eccentricities like they did in the early solar system thanks to Jupiter's interference, so protoplanets Ceres, Pallas, and Vesta still exist - although only Ceres accumulated enough mass to become spherical. Fully a third of the mass in the Main Belt exists in Ceres.
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III. Properties
1. Orbital and Rotational Features
The most significant orbital feature of Ceres is its pronounced 10.6° inclination to the ecliptic. That doesn't sound impressive, and it doesn't look like much on a protractor, but inclinations are very costly in energy terms when traveling in space. As such, the case for Ceres as a waystation for human expansion into the outer solar system is not perfect.
Most people without some background in basic physics don't know the significance of orbital inclinations, and might find it difficult at first glance to understand why objects with significant ones take more energy to reach. The explanation begins with a fundamental fact of Newtonian mechanics: To change the direction of an object in motion to a line some angle off the current path, you have to push it (i.e., input energy) along a line perpendicular to the current path - and the larger the desired angle, the bigger the push needed. So if you're on a rocket moving in a straight line and want to change that line 2°, you don't fire thrusters at a 2° angle - you fire them at a 90° angle for an amount of time calculated to give the right final result: A path that is the hypotenuse of the right-triangle formed by the previous velocity vector added to the force vector of the thruster-firing. An illustration of the principle in linear motion:
The same principle applies to orbital motion: To change the inclination of an orbit, you have to put in energy perpendicular to the current orbital plane - and the faster the linear speed of an orbit, the more energy needed to incline it by any given angle. This is a problem even for orbiting the Earth: To go from an equatorial Earth orbit to a polar one (a 90° transition) takes almost as much energy as it took to get into space in the first place, which is why applications involving polar orbits launch that way from the beginning instead of getting into orbit first and then changing inclination. On the scale of moving between objects in solar orbit, the speeds are much faster than orbiting a planet, so the amount of energy involved is also much larger when inclinations are significant.
For instance, the difference in average orbital speed between Earth and Ceres is 42,825 km/h on top of all the energy needed to get off the terrestrial surface - so the Main Belt is difficult enough to reach even without the added cost of plane changes. Once you reach the area of the Main Belt where Ceres is located, to then incline the orbit you've achieved by 10.6° requires a substantial additional input. As far as I've been able to find, the total change in velocity needed (Δv) is actually slightly greater than getting to Jupiter, although that's far from saying it would be quicker or cheaper to just skip Ceres.
In terms of rotation, a Cererian day is 9 hours and 4 minutes, and doesn't have any special complications because the axial tilt is quite small at 3°. Both of these facts mean that the lighting and temperature profile of the surface is highly regular. It also means that areas of permanent light and permanent dark likely exist at the poles, if either is desired for reasons of energy gathering or scientific observation. You could theoretically build a tower of solar panels straight up from the poles ten kilometers high without having to break a sweat making the base strong enough, and they could be permanently oriented toward the Sun.
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2. Size and Mass Characteristics
The diameter of Ceres is about 974 km, or about 7.6% that of Earth - somewhat larger than France. However, its surface area is comparable to that of India, although I figure volume will be a lot more important to settlers of low-gravity objects than surface. Its gravity is 0.028 g, so a person weighing 150 lbs on Earth would weigh 11.4 lbs on Ceres. It would take several seconds to fall a distance of 1 meter under such gravity, so human inhabitants would likely find it more efficient to crawl hand-over-hand than walk or bounce. Size comparisons - mouse over the image if you don't recognize the other bodies:
The mass of Ceres is about 0.015% of Earth, which - as noted earlier - accounts for a full third of all mass in the Main Belt.
3. Temperatures
Observed temperatures on Ceres are estimated to be as high as 239 K (-34.15 °C or -25.9 °F) and as low as 159 K (-114 °C or -173.2 °F). This is a pretty moderate range for an airless body that far out, and might have something to do with its relatively quick rotation. On the other hand, I'm not certain if these figures are only daytime temperatures across seasonal extremes (due to perihelion and aphelion) - given the likely difficulty of observing the night side from Earth - or if they represent night and day. If the latter, night on Ceres would be over a hundred Celsius degrees warmer than on the Moon, and daylight summers would be comparable to some mid-latitude winters on Earth.
4. Internal Structure
At least some level of internal differentiation is thought to have occurred, causing rock and metal to sink while water ice accumulated into a thick shell. Detailed observations of Vesta by the Dawn probe have strengthened this conclusion by showing evidence of partial differentiation at the less massive body. However, it should be noted that the uppermost surface of Ceres is not icy - it's too close to the Sun for water ice to survive exposure to space under vacuum conditions. Where exposure occurs under prevailing temperature and pressure, the result would be sublimation - a direct phase transition from solid to gas, and escape into space. As such, the surface is thought to be just a coating of dust and rubble over a huge water-ice mantle, with a rocky core a hundred or so kilometers below that. The current hypothesis:
The above model has not yet been confirmed, and Dawn's planned arrival in 2015 will only be able to add or detract from the likelihood of its being correct rather than provide definitive answers: Precisely characterizing the internal structure of a planetary body requires a gravity mapping mission such as the GRAIL mission to the Moon, which involves tandem spacecraft comparing accelerometer readings from relatively tight formation in very low orbit to determine the shape of a body's gravity field. In a weak field like that of Ceres, such a mission would require much greater sensitivity and orbits nearly skimming the surface - assuming that existing instruments would even be capable of measurements that fine.
So the possibility remains that Ceres is less differentiated than thought, in which case there would be no thick icy mantle, but rather hydrated materials (water suffused into solids) and deposits of ice here and there in an otherwise rocky interior. Hydrated minerals have been observed on the surface, but this would be consistent with either hypothesis: With an icy mantle, water that became exposed to the surface via impact would sublimate and leave behind such minerals - something that would also occur if there had been cryovolcanism in its early history. The question is whether water is still there in a coherent subsurface ice shell, or if most of it disappeared early in formation and left behind material like on the surface.
The answer will be decisive in what role, if any, Ceres plays in future human history. If there is indeed a vast icy mantle beneath a thin dusty surface, then its potential as a waystation is profound - although it could still be derailed (or at least delayed) if the mantle is too far beneath the surface, or if other, smaller bodies with different orbits are found to be more convenient. But if the evidence collected by Dawn indicates the water is far less prevalent than thought, or thinly distributed in minerals, that would virtually rule out a major role in the human conquest of the Main Belt.
Given that the spectral type of the Cererian surface is G-type - basically clays and silicates - there wouldn't be much reason to go there for its raw materials. If the current hypothesis is correct, then Ceres possesses more water than all the fresh water on Earth - all the lakes, rivers, aquifers, and glaciers combined - so other than purely scientific interest in its history, water is the only reason to go there. Water in general is the historical pivot around which human activity in the asteroids will likely turn.
5. Surface Features
Until Dawn arrives in 2015, it's slim pickings as far as images of the surface go: The Hubble Space Telescope can see distant galaxies with mesmerizing clarity, but when it tries to look at bodies in our solar system the results are usually underwhelming compared to in-situ probes. In the case of smaller bodies like Ceres and Pluto, Hubble images are dim, blurry exercises in vaguely delineating light and shadow: Not very impressive to the wonder-addicted enthusiast, but instructive enough for scientists. A series of four shots showing rotation - pretty much the best there is for showing surface features:
I would be curious what practicalities stand in the way of building space telescopes specifically for high-resolution visible light observation of solar system objects. Is there any technological reason ruling out an Earth-orbiting telescope capable of seeing the moons of Jupiter or Saturn with the same clarity and resolution of a probe actually present in the those systems? Given declining financial commitment to outer solar system probes - and the ongoing high costs they entail due to the need for nuclear power beyond Jupiter - it would seem like a sensible thing to consider.
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IV. Modern Relevance to Humanity
Eighteenth and nineteenth century astronomers had predicted the existence of a planet where the Main Belt exists due to what they perceived as an orderly geometric increase in the distances of known planets from the Sun - a theory known as the Titus-Bode Law. As a result, concerted efforts were made at the turn of the century to find the hypothesized planet, leading to the discovery of Ceres in 1801 by Giuseppe Piazzi.
Although it was first assumed to be a planet, and kept that status for decades, the subsequent discovery of other objects in the region and their small sizes relative to the other known planets led to the creation of a separate classification: Asteroid, meaning "star-like" - because they were so small that they appeared as mere points of light in 19th century telescopes rather than disks. This was the first instance of a planetary demotion due to population increase, so the 2006 Pluto controversy was only novel for the amount of public interest it generated.
Although most of the attention in the late issue was focused on Pluto, the nomenclature shift also elevated a number of objects that had previously existed in uncomfortable gray areas by creating the new class "dwarf planet," Ceres among them. So it began being classified with the likes of Earth and Jupiter, was debased to the status of potato-shaped rocks and grains of dust, and then redeemed into its proper place as what it is: The stillborn kernel of a planet that was never to be. However, it may still be referred to as an asteroid in terms of its being a member of the Main Belt, although not as a descriptor of its physical characteristics.
Interest in Ceres waned after its demotion to asteroid status in the 19th century, and further declined as new asteroid discoveries became infrequent. It was nothing more than a featureless dot amid a few other featureless dots, and there simply wasn't anywhere to go with it until the latter half of the 20th century when space science was infused with massive government investments. As a result, knowledge of the solar system has exploded in a relatively short time, and interest in asteroids was propelled around the turn of the 21st century by growing recognition of both the threat and potential they represent.
As the largest Main Belt object, Ceres has naturally benefited from the newfound interest, as well as from the growing confidence of robotic space exploration despite its declining financial base. It has also been a subject of some interest in science fiction literature, due in part to the alluringly anarchic sense people typically get from the concept of an asteroid belt - although, of course, the reality is uneventful. The Main Belt may have been a planetary war zone early in its history, but though Jupiter still occasionally throws a curveball, today it's largely a peaceful archipelago of strange and diverse islands living quiet lives of stately celestial motion.
While there have been plenty of fictional depictions, in my personal reading the most significant was by a famous author in a celebrated science fiction series: Larry Niven's Known Space universe. In the early periods of this future history, human beings are still operating within the solar system before the acquisition of hyperdrive technology, and a civilization of asteroid-dwellers called Belters has its government located on Ceres. The setting isn't dealt with in any detail - not much was known about it at the time, and that's still the case - but the idea of Ceres being a focal point for future Main Belt civilization is quite credible.
Of course, nothing says the richest territory ends up with the most people or the political machinery - after all, Manhattan island and the Potomac river were hardly ever epicenters of American productivity. We can just as easily imagine Vesta becoming more important for whatever economic or logistical reasons end up being decisive - or possibly some otherwise unremarkable object (as Manhattan was unremarkable) that just happens to have the right combination of factors.
In any case, we will get a much fuller picture of Ceres - and a glimpse into its prospects in future human history - in 2015 when Dawn arrives. The timing of this arrival is even more significant, since it will be in the same year that the New Horizons probe flies by Pluto: Two dwarf planets in two very different regions of the solar system in the same year. Dawn, of course, will provide much more in-depth information because it will stay in orbit of Ceres while New Horizons will zoom past the Plutonian system at near-record speed, but the results of these two missions together will represent a gigantic increase in the knowledge of a class of poorly-understood objects.
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V. Future Relevance to Humanity
Under the current hypothesis, Ceres is thought to have about 200 quintillion kg (2 x 1020 kg - or 200 "exagrams") of subsurface water ice. To put this in perspective, that's about 10,000 times all the ice in Antarctica. Now, just to be clear, the attraction of such a resource base is for providing water to other locations in space - and in particular as a stopping point for missions headed to the outer solar system - not for sending water back to a planet already covered in it. Even with a drastic reduction in launch costs to Earth orbit, it will remain prohibitively expensive to bring water from Earth on long-term deep space missions, and it will never be worthwhile to bring water back.
Ships headed outward would probably be able to get water from a large number of different objects in various locations, but the economics of scale dictate that it would be cheaper, easier, and safer on an ongoing basis to concentrate the extraction and delivery infrastructure on a few bodies with great abundance rather than each ship bringing its own equipment to do that on whatever suitable objects happen to be near their most convenient path. Flights would thus be planned with the locations and alignments of these few objects taken into consideration, leading to additional efficiencies. So after early exploration gives way to development, the most likely initial use of Ceres would be as a refueling station - mostly for craft operating within the Belt, but also for those headed outward.
Remember, we're assuming that at this point in future history, there are already people swarming around the inner solar system, on the Moon, on Mars, and making tentative forays beyond, so we can say that at first the infrastructure on Ceres and other waystation bodies could just be unmanned infrastructure operated by ship crews as they arrive and then turned off as they leave. But as traffic and volume consumption increases, the capacity and reliability of the systems would have to expand, eventually requiring staff and regular maintenance - and, of course, since we're talking about people, some form of security to prevent "enterprising" elements from making off with the equipment.
If the progression of history I lay out in Asteroids (Vol. 3) plays out for the Main Belt in general, Ceres would at some point accumulate a relatively stable local population and economy in a larger environment that favors dynamism bordering on anarchy. You can never be sure in advance what mix of people will end up settling a given frontier location, so the early history of settlement could be any number of scenarios: A safe, relatively orderly place where prospectors try to build a civilized oasis amid uncertainty; some piratical, Mos Eisley-like hub of every sinister business imaginable up to and including slave trading; or the stifling feudal domain of some robber baron or corporation preventing any organic, self-sustaining society from taking root. I'm sure all of the above would be represented somewhere in the asteroid belt, even if Ceres fails to become significant.
From there, the history of the dwarf planet would depend not only on the overall trajectory of Main Belt civilization, but on what kind of economics had developed around Ceres in particular. On the optimistic side, it could evolve into not only a waystation, but an embarkation point specializing in the outfitting, supply, and perhaps even manufacturing of deep space craft for long-term journeys into the outer solar system - although naturally this would involve more than the resources of Ceres itself, but a larger economic web focused there. However, if it failed to grow a more complex economy, it could eventually wither like some Midwest railroad town if technology and economics evolved beyond its original purpose. Although the significance of the Main Belt is guaranteed if humanity expands into the solar system, the fate of any particular object in it is far less certain.
As such, the potential of Ceres runs the gamut from being virtually ignored and uninhabited to becoming the capital of the most extensive and productive empire human history will have ever seen - but the existing indicators at least lean in the direction of it playing some significant role. Still, we have to get there first before any of this can happen, and there are a number of economic and technological developments that will need to occur before the very first human exploratory mission to Ceres is practical:
1. Radically reduce the costs of launch to Earth orbit. This is by far the largest hurdle to going anywhere in the solar system, and there is a saying that once you're in orbit, you're already "halfway to anywhere." With the gains being made by SpaceX toward this end (they've already reduced costs by between 1/2 and 2/3, depending on the particulars of launch), and the ambitious leadership of its CEO and CTO Elon Musk, I'm pretty optimistic that this step will be achieved in the next couple of decades.
2. Nuclear-powered VASIMR (Variable Specific Impulse Magnetoplasma Rocket) propulsion. VASIMR is an existing but experimental technology that ionizes and expels noble gas molecules as propellant for use in space: It's very low-impulse, so the system can't be used to get into orbit from the terrestrial surface, but the fact that it can be run continuously over the entire course of an interplanetary trip means you can accelerate to speeds unimaginable with chemical rockets and then likewise slow down without having to rely on long, slow gravitational assist flybys. Such a system is estimated to make a Mars run possible in 39 days as opposed to the better part of a year with traditional rockets. A 200 kW test engine operating in a vacuum chamber:
At the moment, VASIMR systems are low-powered and being planned for deployment on the International Space Station as thrusters. For applications on the 200 kW level, solar power is more than adequate to provide the necessary energy, but where a human exploratory mission is concerned, the duration of the trip matters a lot more than the efficiency of the engine - i.e., a lot more power would be needed than solar panels could practically supply. So before VASIMR can take humans to Mars, let alone Ceres, humanity will have to develop megawatt-scale in-space nuclear reactors and evolve the engine technology to handle that much power.
This sounds like a tall order given the relative dearth of resources invested in deep space at the moment, not to mention the high cost and radioactive politics (so to speak) of nuclear power, but the prospects are likely to change considerably once getting to Earth orbit costs a small percentage of what it does today. After all, it's not illegal to build nuclear-powered engines, it's just costly to deal with the level of regulation surrounding it, and it will be a massive challenge all on its own to produce megawatt-scale reactors capable of operating in space - basically, a compact, flight-hardened version of an entire industrial power plant - so there would have to be quite a lot of money invested up-front to achieve it. Fortunately, that's likely to happen once tens or hundreds of billions of dollars are being made beyond Earth orbit in applications that would benefit from nuclear-electric propulsion.
3. Major improvements in habitat technology. Right now human beings can only occupy the ISS with regular resupply from the ground despite recycling air and water, and that doesn't cut it for any long-duration deep space mission. You could, theoretically, keep launching cargo capsules to resupply a manned spacecraft en route, but the expense would be prohibitive. The needed capability is to survive the outward trip using only what is carried, use whatever resources have been landed at the destination ahead of time (as well as naturally-occurring resources like water), then return with the forward-deployed supplies intended for use on the return trip. That means nearly-closed-loop ecology, through whatever means is most achievable: Hydroponics, aeroponics, molecular recycling of organic waste, etc. We're nowhere near this right now, but I think progress will happen fast once Mars is opened up.
Given these technical prerequisites, I'll make a prediction as to when the first human mission to Ceres will occur: 2060s-2080s. Progress could radically snap forward once it passes some tipping point in the next three or four decades and result in early exploration long before anything more involved would be practical, or things could plod along on the Moon and Mars while the economics slowly evolve to make such a mission practical in the next century, but the middle case that I'll go with is that it happens in the middle of the second half of this century.
As to later developments, they would likely track with the changes in the rest of the Main Belt as I discussed in the finale to the Asteroid sub-series, so I wouldn't expect Ceres to be a venue of civilization for several centuries, nor would I expect the ultimate shape of its full significance to be known for a thousand years at least if it did prove influential. But perhaps its most important promise is not what it will mean to the people who settle the region, but what it means to the overall movement and momentum of humankind: Namely, its capacity to serve as a springboard into the outer solar system - worlds vaster, richer, more diverse, more dynamic, and more exotic than anything in human history prepares us to imagine.
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VI. Future of Ceres
Whatever transient roles it plays as a venue for human events - desert oasis, gas station, pirate harbor, suburb, feudal fortress, neutral ground, economic nexus, capital of republic, citadel of empire, or just springboard into the Distant Dark - there are only three probable fates for the body of Ceres itself: Physically joined with other matter to form colossal artificial systems for whatever purpose; scattered to the four winds of the galaxy on some exodus for reasons impossible to predict; or abandoned to its natural course without human intervention - because we no longer exist, or just no longer have a need for rock and ice - wandering almost forever once the Sun has died until eventually swallowed by a planet, a star, or a black hole.
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VII. Catalog of Exploration
1. Past & Current Spacecraft:
(none)
2. Future Spacecraft:
Dawn - USA, scheduled to orbit in 2015.
