Why will we fail creating a self sustaining off world colony?

Question

There have been many (many) questions around here about the how, why & even when will man kind become a multi planet society... this is not one of them.

This question in fact goes the other way around, why would humanity fail to become a multi planet society?

Assume for a minute that all of humanity (yes every single one of us, it may be unrealistic but this is world building after all) is all of the sudden willing to put aside religion, consumerism, pride, ethnicity, politics & all the other silly reasons that divide us and we all agree that settling another planet (at least one) is super important and are willing to sacrifice a lot of comforts to get it (let's say around 15% of the total world GDP as well as mass agreements that needed resources and laws are passed to help make the dream a reality).

Starting tomorrow morning (literally tomorrow morning is the starting point of time) every single man & woman alive will work to create a self sustaining colony off world with the undivided attention, desire & resources of the entire world working together to reach that goal before 250 years pass (why 250? because)

Seems great right? now the question is what is a possible realistic reason (which isn't a natural disaster sending us back to the stone age or wiping us out) that will make us fail that goal?

Answer 1

As every project manager knows, scope creep is the root of all evil.

At the beginning, it was just about sending one ship to another planet. Easy, just a matter of time.

Then someone said something about food diversity, citrus fruits and, for whatever reason, pineapples. So a team was split up to figure out how to grow pineapples on a space ship and in a human colony. Suddenly scientists talked about gene splicing pineapples and strawberries, though no one could figure out why it had to be strawberries. Probably because the wife of the team lead liked them. It took fifty years until someone said "screw pineapples" and stopped the whole thing.

In another meeting another guy talked about how awful dying in space could be, so another team was split off to change the triple redundancies from the current ship design to quadruple redundancies. Just to make sure, they put double redundancies on the quadruple redundancies. The added weight required a redesign of the whole engine system and they had to figure out how to store ten times the previously planned fuel. New propulsion systems were experimented with and abandoned. Eventually someone decided that triple redundancies should be enough after all.

Then someone asked about space pirates, so a team was sent off to develop space weaponry just in case. Nuclear missiles, lasers, railguns, the whole spectrum. They spent forty years just to figure out whether they could build an antimatter bomb. Also armor to defend against all that, which required new materials, so another team was sent off to develop micro-non-newtonic-carbon-tungsten-fluid-nanotube-reactive-reflective plating. And don't forget about shields like in star trek, for which another team experimented with em fields for years. At least someone put a stop to the team that was developing synthetic humans to defend against boarders, though that was more for fear of the AI revolution than because of costs.

In another meeting... well, you get the point.

Answer 2

One problem could be that humanity in fact becomes too committed to the space program and neglects economic sustainability. By concentrating too much on the colonization project, other branches of the economy get neglected and stagnate, which then causes a ripple effect eroding the economic foundation which makes the space program possible in the first place. Examples:

• Budget cuts in health care and sanitation lead to epidemics in the countries which mine the raw materials for the space program, killing all the miners.
• Budget cuts in transport infrastructure make the supply chains between the many manufacturing facilities involved in the space program unreliable and result in huge delays. That's really problematic because interplanetary space missions often rely on planetary constellations which only last for a couple days and only happen every couple years. That means a single part which is a week late can delay your whole space program by several years.
• Budget cuts in education result in a shortage of qualified scientists, engineers and managers to plan the space program and a shortage of qualified workers to execute it. The space program needs to lower their hiring standards. The result of having less competent people in all positions are inefficient solutions which waste resources, unsolved problems which delay project plans and catastrophic accidents which destroy material and cost lifes.
• Budget cuts in agriculture lead to famines and cause the world population to decline, which amplifies all the problems mentioned before.

So while the space program might move really quick at first, the problems caused by neglecting the economy which supports it make it grind to a halt after a few decades. Humanity realizes that in order for the space program to have any chance to succeed, they need to do a 180° turn and focus on economic development again. But all the problems which affected the space program will also affect these economic stimulus programs. So it might take a very long time to get the economy back to what it needs to be in order to sustain itself and an expensive space program. After they fixed the economy, most parts of the space program will be obsolete, repurposed, forgotten, beyond its shelf-life or ruined by years of neglect. They will basically need to start from square one again.

Answer 3

Well... besides the earth being flat and the literal glass dome preventing anyone from escaping... (jkjkjk)

Given enough time, anything is possible. But if we wanted to do this quickly, we would run into some serious road-blocks.

Finding a planet which is Earthlike enough

First of all, we need to find a planet similar to earth. Colonizing Mars might seem like a good idea, but its mass, nearness to the sun, lack of a moon like ours, and the many other differences between Mars and earth, make it likely that plant life won't grow there easily, and long-term exposure to that environment may have significant negative effects on other organisms. There are hundreds of factors involved in enabling life on earth: mass, density, temperature, atmosphere, nearness to sun, type of sun, mass/density of moon, nearness to moon, magnetic field, mineral contents of ocean, ordering of layers in crust, volcanic activity, shape of mountains affecting wind, etc. etc. etc.. For example, we know that long-term exposure to non-earthlike gravity can cause people's bones to atrophy. Every one of those conditions, if not met well, can lead to other serious health conditions. Given the time it will take to get there, we should get this right the first time, so take a few years thinking about where to go, and maybe send out a few probes. (60 years)

Getting there

There have been some theoretical ideas thrown around for how we can get a probe to fly-by a far planet which we think is earthlike in as little as 60 years using massive solar sails, but the probe was tiny, and we still had a problem where it couldn't decelerate quickly enough to orbit. At best, we would get a few flyby pictures at extremely fast speeds. Stopping in a foreign solar system within one lifetime is a problem we haven't figured out how to solve (and might not be able to solve because of the limits on acceleration which humans can tolerate). So, we'll have to first make a space station which can support plant and human life for several generations, which might be impossible even with spinning stations to simulate gravity, because of the inherent differences those stations will have when compared to earth. (120 years or more)

Generation Ship Planning/Construction

Due to the careful planning of the ship-side ecosystem, population control on the generation ship will have to be strictly regimented. This will represent significant social engineering problems which humanity has never faced before. The few experiments with this kind of small isolated population, which I'm aware of, have been steaming failures. We really don't know how humans will handle spending their entire lives aboard a space ship with only a handful of types of food, their romantic interests potentially having been arranged several generations prior to maximize diversity, and the possibility that a tiny, simple mistake made by some kid on the ship could cause a breach or something and kill everyone. Not to mention, you'll have to have warehouses with new electronic components to constantly maintain your ship in-flight. Electronics don't last all that long. (For this, assuming just one minor failure unaccounted-for, I'm adding an extra 20 years. But realistically, we should expect several failures)

Planetside Resources / Terraforming

If we can land the generation ship on the planet and call that acceptable, then skip this part -- but I think a "planet colony" should actually be interacting with the planet somehow, and not just making physical contact with it via ship struts.

Then, once we get there, we'll have immediate resource issues planetside. Earth represents a very distinct mix of materials favorable to life, and we might not be able to survive off-world without either frequently returning for supplies, or quickly gathering resources from several off-world planets simultaneously. All human colonies depend on transformation of one type of matter into another type of matter in order to produce energy, food, etc. Our plants depend on water with very specific ratios of specific minerals in it to bear fruit, and won't live with too much of this or that other chemical in the water.

Suppose there is too much of Chemical A in the water, so we harvest Chemical B to make filters. When it's time to recycle the filter, we can't recover all the B from the dirty filter, so we have to continually harvest more. Suppose B is not abundant in the soil, so we have to dig deep to find stores of it. To make matters worse, the outer crust of this planet is also dense with Chemical C, which is toxic to humans on contact, but the process of removing that requires lots of time, energy, and maybe even some Chemical D, which just isn't present on this planet at all. So we harvest D from a nearby moon or asteroid, use it to terraform a small portion of the surface, and set up deep and very dangerous mines in the planet to acquire B. In the meantime none of our plants will grow, so now we have to land our generation ship safely on the planet, or be constantly shipping supplies back and forth in orbit. Anyway, all that to say, Tweaking the planet to enable it to support life will take several more generations. (At least another 100-200 years before a small part of the planet becomes self-sufficient.)

The Unknown

Lastly, we don't actually know what's far far out in space yet. We just have a pretty good idea, but there's a chance that we'll just be repeatedly killed by the unknown! (60-120 years setbacks, unpredicted events, compensating for unknown)

Conclusion

Like I said, given enough time, anything is possible. But 250 years might be a bit too aggressive of a timeline. Given my estimates, I'd give us a minimum of 360 years; more realistically 500+ years. 360 is not too far from your window, though, so who knows? Maybe we can do it.

Answer 4

Cultural shifts: Once your generational ship arrives at the other planet, your descendants look out of their nice, clean space-ship with its hydroponics bays and gravity controls in every room. They see an ugly ball of dirt, all irregularly shaped, where "down" is always the same direction, and you can't turn off the gravity to float in peace.

Then they'll mine the planet via robots to build a second ship to expand to, and head off elsewhere instead of founding a ghastly "colony" of all things...

Answer 5

You will not go to space today ever

Balancing a biosphere over the timespan of 100+ years hasn't been shown to be possible. Even Biosphere 2 didn't manage to run its entire originally planned duration, "Both attempts, though heavily publicized, ran into problems including low amounts of food and oxygen, die-offs of many animals and plants included in the experiment, group dynamic tensions among the resident crew, outside politics and a power struggle over management and direction of the project."

But lets assume that Biosphere 2 as a baseline, to get a reasonably close lower-bound on the problem at hand. It really doesn't matter if Biosphere 2 was only sufficient for 1 person, 8, or 80. The numbers get so large here in a minute that a single power of 10 equates to a rounding error.

The amount of mass involved is on the level of "absolutely staggeringly preposterous":

The whole structure contains (for the record) approximately 170,000 metres³ of atmosphere, 1,500,000 liters of freshwater, 3,800,000 liters of saltwater, and 17,000 metres³ of soil.

4 million liters of saltwater. Putting nearly 5 million kg of water into space at \$10,000 a kilogram is not economically viable for any reason, full stop. The soil is even worse. A cubic meter of soil weighs in at about 1.5 metric tons. Or about 25,500,000 kg for the amount in Biosphere 2. And another 220,000 kg for the air gives us $310 billion in launch costs, not including the structure of the ship itself (remember, the space shuttle itself is 75,000 kg for a payload of 4,000-16,000 kg depending on destination), or its fuel. That's already double the total cost of the ISS. And we're not even counting the mass of the plants and animals that will be aboard.

And remember, we're calling Biosphere 2 the bare minimum to maintain 8 people. In order to run a generation ship you need a bare minimum of 200 breeding humans under strict breeding regimens in order to maintain heterozygosity. That is, to prevent inbreeding. So, multiply all these numbers by 25 in your head. More if you want to be safe and allow for the occasional soul lost to accident or disease.

Now, fuel.

Getting to the nearest system to ours, Alpha Centauri involves:

• 16.8398 km/s of delta-v to leave the solar system from LEO, including a plane change of 9 degrees.
• 32.935 km/s of delta-v to arrive at, and subsequently not leave, the Alpha Centauri system

Lets round that up to 50 km/s for a bare minimum lowest-fuel trip (and remember that these values do not include launching material into space nor landing on a planetary surface at the other end). And being a generation trip, we don't care if it takes 100 or 100,000 years right now, we just to want to work out the bare minimum needed in order to assess viability: a lowest-fuel-cost trip.

So: 50 km/s, 31 million kg, and 8 people.

8,728,724,371,657,847 (8.7 *10¹⁵) kg of fuel using Space-X's Merlin engines. (Feel free to check that yourself; a Merlin engine's exhaust velocity is 2570 m/s).

That's a small moon. No really, its about as massive as Phobos (10.6 *10¹⁵kg).

That number is so astronomically large as to make any possible improvements in efficiency meaningless. Even if we improve the Merlin engines by a factor of 10 and use fuel with an energy density 100,000 better (say, nuclear), that still knocks the fuel costs down to a mere 8,728,724,371,657 kg, and we'd have to just multiply back up again once we account for the shell of the space ship, extra fuel for course corrections, obstacle avoidance, a shorter trip, more people, a more habitable system, and so on.

Even if you launch your ship using a fuel-less launch system out of the solar system, so that you only need "half" the delta-v (remember you can't count on a laser station at your destination, and in truth, we can only cut out a third of the delta-v not half of it, but its largely irrelevant), that knocks your fuel down to 0.006%. Or 520,350,872 kg of fuel (after the efficiency improvements and before all the other considerations). This is enough fuel to give an (otherwise empty) space shuttle a delta-v budget of 22.73 km/s using Merlin engines. And that original fuel mass? The 8.7 *10¹⁵ kg? The delta-v budget on an empty shuttle is still only 65.48 km/s.

This is the scale of the problem you have.

• Mass is expensive
• Delta-v is exponentially expensive
• Sustaining humans indefinitely requires lots of mass
• Getting anywhere interesting requires lots and lots of delta-v
• Getting there in anything less than "infinite time" requires lots and lots and lots of delta-v

The reason the project will fail will be due to the sheer inability of the human race to gather the raw resources necessary and the impossible engineering necessary to build such a megastructure.

Answer 6

Because you need a biosphere. Humans (and probably most if not all other creatures) can't exist indefinitely in isolation. Now to create a self-sustaining biosphere needs a fairly large area, probably the size of a largish asteroid (or a continent on Earth), but there aren't any such places in the solar system.

So you're left with two alternatives: terraform Mars, which would probably take many generations, or build artifical habitats (moon bases, O'Neill cylinders, &c) that are mostly self-systaining, but still need regular interchange with Earth's biosphere to keep functioning well.

PS: The mention of the Biosphere experiments brings up a point, which is exactly what is meant by "self-sustaining". I'm taking it to mean something that could last indefinitely - thousands of years at a minimum - even if all contact with Earth were to be cut off. The Biosphere experiments were short-term, and I do think it would be possible to build a mostly self-sustaining base that could keep operating for years, even decades. But it'd be more like an Antarctic research station than a place you're emigrate to and expect to raise kids & grandkids.

Answer 7

I can offer a few potential reasons for failure:

A) The problem is harder than expected, and can't be completed in the time limit. It's doable, just not in the time allotted.

B) Many ideas work on paper, but can't work in reality. The socialist communes of the nineteenth century repeated and consistently demonstrated that that economic system wasn't viable despite there being universal acclaim for their merit, rectitude, and fairness. Similarly, when the plans for the self-sustaining colony were being conceived and drafted, some principle or notion or assumption is key to its operation, and no one realizes what that is. It could be an underappreciation for peoples willingness for self-sacrifice or it could be the metallurgical stability of titanium after being exposed to radiation during space travel.

C) Tower of Babel Syndrome. The population of the Earth is too culturally desperate to have common touchstones for practical engineering. That a notion commonly accepted in Germany is not considered important in the USA, and is considered irrational in Asia. This is a real challenge today in transnational engineering projects, and those are just making consumer goods or control modules for automobiles, not a massive and huge system like a self-sustaining colony on a distant planet. Something as simple as engineering units has resulted in comparatively simple space probes to crashing into Mars rather than establish a stable orbit.

Answer 8

It will be something small and unexpected.

Robert Silberberg makes the point somewhere that we may have the technical capability, if we push hard for it, to create an enclosed orbital habitat but that doesn't mean we have the capability to design and maintain the ecosystem therein.

So while there's no reason we couldn't mount an interplanetary or even interstellar colonisation mission and design it to take into account every contingency we can conceive of that doesn't mean that it will succeed. It could be something as simple as the lack of a particular species of ant in the ecosystem of a generation ship, discarded because it carries a potentially harmful bacterium but, unknown to the designers, instrumental in spreading a beneficial fungi. Or the discovery that a particular necessary life-form is sensitive to the minute vibrations of the hull and suddenly stops breeding when the generation ship goes into it's interstellar acceleration phase outside the Kuiper Belt.

The point is that something seemingly inconsequential will result in the project failing, after all the components of it up until that point have succeeded perfectly.

Answer 9

NO REASON TO FAIL

Quite simply, given the scenario at hand, there are no scientific, technical, economic, social, spiritual, medical or any other reasons why the project can not be conceived, planned, implemented and at least brought to its first attempt.

If we could snap our fingers and turn every country in the world into producing something viable for the colony or testing something for the environment or researching something for the project, we ought to be able to plan out the project and put up a working colony somewhere (maybe Mars?) along with the associated infrastructure (Lunar base, orbital construction & flight infrastructure) within the given time period of five to six generations.

You specifically eliminate natural disaster as a means of terminating the colony project. The only other viable option is human error. Even if you could get 7 billion people to focus on this project, you can't make 7 billion people pull off any kind of construction project flawlessly. Somebody's going to mismeasure something or forget to screw something together or misplace the operations manual in a locker back on Earth.

Answer 10

Other planets suck.

Earth is so good for us. You can eat the plants and animals and you can drink the water. You can sleep on the ground under the sky. You can find things to burn and keep warm.

The colony planet will certainly be less friendly. On other planets, the water might be briny or otherwise contaminated; you would need to purify it. Or synthesize it from hydrogen and oxygen. You will not be able to plant a seed in the soil - a lifeless world will have a mineral soil and you would need to amend and supplement it. You would need to make atmosphere to breathe and an enclosure to keep it in. All that can be doable with enormous inputs of energy, and that is sustainable until some crisis interrupts supply of the enormous energy inputs. Then the humans die.

If you find a world with an atmosphere we can breathe, that means there is life that is enough like us that it produces oxygen. If there is life there are decomposers. Then we are at great risk of being decomposed for our raw materials by microbes new to our immune systems - we become the Martians from War of the Worlds.

If humans are lucky enough to stumble into a world very much like ours which has been sterilized completely by a gamma ray burst in the recent past, that might be a good place to live and a fun fiction to read.

Answer 11

Human Tragedy

Let's say that tomorrow everyone will agree that we have to colonize Mars and we devote 10% of our GDP to do it. People will grumble about the increased price of living, but most will agree that it is for the greater good.

Now think about who will handle that large amount of money, corruption and self interest will not vanish overnight and then the beaurocracy and management of such a project it will be a nightmare that has plenty of chances to burn out and start wars and embargoes.

Next up it's the planning itself, the Americans might have a plan, the Europeans one the Asian one, every single one solid and everyone from their respective zones will say that their plan it's the best so you will probably have different major attempts at colonization.

And now comes my final point, let's say they all make their preps and send shuttles towards Mars. Not everything is perfect, human make mistakes, let's say that the American shuttle hits something along the way and goes boom. The Asians get to Mars ,but the landing is not successful and they all crash and die, the europeans land , make base but something happens and an air tank explodes or something goes wrong and they all slowly die, all the while the people on earth see their last moments. Save missions are sent, but they will be somewhat rushed and only one or two will return, now do you think that after all of that people will still try?(maybe after a hundred years or so...) I say no, atleast not in the period of time given, there will be huge uproars from the people, politicians and investors. You know how the old saying goes, the desire for safety stands against every good and noble enterprize.

Answer 12

General

The establishment of (quasi-)planetary colonies will be a much more daunting task than it might appear at first glance due to the sheer amount of resources (physical and immaterial) needed, prime among it energy.

I expect the energy output needed for establishing any kind of truly [1] self-sustaining off-planet colonies to be several magnitudes higher than today's global energy output. To some extent, resource intensity (use per unit of time) can be traded against time, but your premises include a deadline of a few centuries which implies a tight schedule.

On top of it there is the additional constraint that over the period of establishing the outpost, the preconditions for human life on Earth must be preserved, meaning a portion of resources and energy available to mankind must be diverted to support life on Earth.

Note here that currently mankind does not have established a resource-wise sustainable society on Earth. So the OP's proposal might even be more challenging as mankind might be lacking the knowledge under which constraints true self-sustenance is possible.

The OP has not been explicit on the terms under which Earth society would exist while implementing the colonization plan beyond the GDP proportion. Considering the premise of voluntary cooperation, schemes involving atrocity and genocide ( killing off 'unproductive' segments of the population to reduce the resources needed to keep Earth population alive and to reduce burden on the environment ) seem off the table.

TL;DR

A world colony will not be possible within centuries because ...

• Resource requirements are too daunting
• There is no precedent of a truly self-sustaining civilization

Specifics

Some ideas on different colony modes and their respective hardness to follow:

In-system

Apart from Earth, none of the celestial bodies within out solar system is capable of supporting human life. Given your premises wrt resource allocation, the task is thus equivalent to artificially creating a self-sustaining biosphere.

Terraforming

Terraforming a natural habitat into an environment permanently amenable to human life is at best a vision with the current state of human tech, a 'teleological' process in that we only know the baseline and the (possibly) the target conditions but have no idea how to actually perform the transition. If we did know how to tackle it, today's hot (no pun) topic of Climate Change would receive quite a different spin or might not be an issue in the first place.

Existing sketchy terraforming proposals are a couple of steps away from testable engineering. So complete bootstrapping would be needed.

Artificial Habitats

Artificial habitats are in reach of (modestly extrapolated) current technology. However they also come with high resource requirements as the two basic resources of a habitat need to be created at first:

• Physical space
• Gravity

Gravity might be dispensible, though state-of-the-art research in biology suggests that it wouldn't.

Off-system

Colonizing an existing, habitable planet comes with its own resource problems, namely the journey to the destination. The distances and thus the timescales involved make a trimmed-down version of the task a subproblem, namely building a self-supporting habitat aka 'generational spaceship'. On top of it comes what is needed for interstellar travel: Mostly energy again. Lots of.

The first step however is to identify a suitable candidate. Next to basic astrophysical properties ( ie. Earth-like gravity, orbit in habitable zone ), identifying a possible existing biosphere and testing its compatibility with human life is part of the job.

Part of the tasks could be delegated to the travelling colonists, with the possible outcome that the designated target world proves unsuitable for settlement, requiring to prolong the journey, possibly indefinitely. If the incompatibility rests on an existing incompatible biosphere, the sterilization of such a world to prepare a human settlement ( by terraforming or an artificial habitat ) seems in gross violation to the cooperative and constructive spirit of the premises and is ruled out (in fact allowing this option would not significantly reduce the resource burden).

Ways Out

These are mainly scifi - verbatim, as they frequently occur in scifi literature. Most of the solutions would focus on drastically reducing the resource consumption of the colonists, for some time at least, by keeping them alive in some state of 'hibernation' or by assuming that the non-physical self of a human can be separated from and fed into its physical substrate by technical means - 'personality downloads'.

Though these approaches would relativize the resource problems, they are pure fantasies at the moment. Meaning there is no clue whether any of these could be viable.

Moreover they stretch a bit the meaning of what passes for a human colony based on philosophical issues.

Other scifi memes like FTL outrightly deny current physics and are thus even harder to gauge. It is plausible, however, that - if possible at all - their resource (energy) demand would even be higher than that of any other proposal.

--
Remark:

[1] (Self-sufficiency)
Note that i understand 'truly self-sufficient' to mean in principle being capable of supporting human life for an indefinite period of time without any intervention from Earth, which would include any kind of communication. The period may be shortened by the advent of unforseen cataclysmic events (eg. asteroid impact, nearby novae). What would count as 'cataclysmic' obviously is the soft spot of the definition; drawing the limit at events that even an Earth-bound humanity could not thwart might be a suitable heuristics on the basic assumption that the scale of off-planet colonies will not exceed the homeland for a considerable time.

Answer 13

Missing Bootstraping Resources: Since we cannot yet travel to other systems, we are contstrained to use resources from our solar systems. So even if we would throw all are energy/material/humans at the problem but we lack in one we would be stranded here. Same as 50 aircraft engineer stranded on an island couldn't build their own aircraft. Or a smaller country couldn't build its own computer. See here for the Minimal size for self-sufficiency .

Answer 14

Intrinsic risk of civilizational catastrophe from advanced technology

When the Manhattan Project was being developed, some people felt it necessary to check whether a nuclear explosion could ignite the atmosphere, wiping out all life. I think at the time it was pretty clear that this couldn't happen, but the fact that the question even needed to be raised illustrates an important fact: the capability to wield the kind of energy necessary to move civiliizations between worlds, or even just to run our current technologically-advanced (and increasingly energy-hungry) society, may also be the capability to wipe out all life on Earth.

I plead that none of the following disasters are natural :)

Candidate threats

Biotechnology

In order to survive non-Earthlike conditions, I imagine we'll need to (or at least be motivated to) engineer organisms unlike any which already exist. Organisms optimized more purposefully and precisely than evolution could manage, for idiosyncratic tasks that we've not seen done before. This creates a potential for accidentally creating something that could be destructive, or deliberately creating something potentially destructive and failing to appropriately contain it. This is already a concern with medical research today, but substantially more advanced engineering technology and more deliberate and ambitious engineering projects scale up the threat a great deal.

This is not a hypothetical concern: here's a webpage discussing some existing initiatives to reduce pandemic risks created by life sciences research.

Nanotechnology

This one's more speculative. But futurists have raised a concern that the ability to industrially process matter on the microscopic level would unlock the risk of reprocessing matter accidentally that you didn't want reprocessed, like your lab desk and/or vital organs. Self-replicating nanomachines might be hard to stop once accidentally released into the wrong environment.

Artificial intelligence

SF already has lots to say about what can go wrong here. It's perhaps worth mentioning that this risk is taken seriously in the real world as well, though not typically as the "evil AI with glowing red eyes" model but more that it's actually very hard to precisely and unambiguously specify humanity's strategic goals and values, and hard to be sure they've been correctly understood, and most incorrect value specifications in the hands of a sophisticated strategic agent with more computational resources than us are very bad news in the long run.

Speculative extremely-high-energy power sources

Very SF; I have no idea what these will be. But essentially, imagine the next Manhattan Project, but with another order of magnitude of yield.

Dual-use technologies repurposed by bad actors

Mostly I've been talking about accident risk, since your question suggested we were globally united. If you allow even small deviations from that, you allow that many of the advanced and powerful technologies we will develop over the next few decades are also usable by terrorists, death cults, or other groups with an agenda. Wiping out civilization a millennium ago was extremely laborious, required a lot of travel and diplomacy and it was real hard to be completely thorough. Nowadays it can likely be done by unilateral action from any one of several nation states. Maybe in another century or two you'll be able to do it with a device you can make at home from commonly-available parts.

Answer 15

Stupidity

If mankind spent on science half of what it spends on war, We'd already be living on Mars.

Already now people hear the cost of sending a probe to Mars and declare it a waste and demand it get spent on the poor and starving.

Between greed, self interest, corruption and egos, mankind can't work together. There is absolutely no technical reason why we can't. We already have the technology to do it. The only reason to fail is a sociological one.