If you want to turn your science fiction story up to eleven, the traditional trope is to destroy an entire planet (just ask Alderaan). Preferably by blowing up Terra. Though that has gotten to be a bit stale, so there are some stories that destroy the entire universe. In James Blish's fourth "Cities in Flight" novel The Triumph of Time the protagonists make the unsetting discovery that our universe is only months away from merging with another universe — composed of pure antimatter. Egads.
The best list of these on the net used to be Exit Mundi, a collection of end-of-world scenarios. The site has vanished, but can be found archived in the Wayback Machine. Another good source is the TV Tropes entry for Apocalypse How. Also good is the Wikipedia article on Global Catastrophic Risk.
|Focused Destruction||A small localized area undergoes a species-level or higher apocalypse. The rest of the world at large is totally unaffected, maybe not even knowing of the events happening in the affected area.|
|Regional||a part of a continent or landmass, be it a province/state, geographical region, or sub-continent (eg. "California"/"Uganda", "Sub-Saharan Africa", "India", etc).|
|Continental||an entire continent or landmass ("Oceania", "The Americas", "Eurasia", etc).|
|Planetary||an entire planet, or the vast majority of one. (If the given setting does not involve space travel and/or other worlds, then the scale effectively stops here, or skips up to Multiversal if the other worlds are not elsewhere in space, but do exist.)|
|Stellar||a solar system, every planet orbiting a star, the star itself, or the star plus everything in its orbit.|
|Galactic||a galaxy, most or all of its stars, up to all mass associated with it.|
|Universal||the entire universe, all or most galaxies within it, or all major galaxy filaments or equivalent highest-level structures.|
|Multiversal||multiple universes, or whatever that exists outside of the setting's native universe (includes whichever flavour of Another Dimension is on offer).|
|Omniversal||all universes or all possible universes, everything that exists, or reality itself; up to some abstract ontological limit if the setting includes explicit metaphysical stipulations.|
|Societal Disruption||Civilization survives intact, but is forever altered. This may be due to the sheer amount of damage caused lowering the standard of living, or it may be a result of people being forced to adapt to the new threat(s) they face.|
|Societal Collapse||Humanity backslides within the affected area, regressing to pre-industrial level at best and pre-agricultural at worst. Civilization may recover on its own, but not for centuries at the least.|
|Species Extinction||A dominant or major species is either wiped out completely or reduced to such a low population level that its recovery is virtually impossible barring intervention by an outside force.|
|Total Extinction||Life itself ends. No living organism of any kind exists within the affected area.|
|Physical Annihilation||The affected area physically ceases to exist as it did before, but remnants of it can still be found; it's nuked into glass, sunk into the ocean, or blasted into asteroids.|
|Metaphysical Annihilation||The affected area ceases to exist totally, without remainder, or perhaps even to have ever existed; this usually involves erasing it from time. This may go up to the elimination of even the possibility of the existence of anything like the affected area, if for instance the basic system of reality is changed or wiped out. This may get highly abstract, depending on how fundamental the negation is.|
|Class 0: Regional Catastrophe||Regional/Societal Disruption or Regional/Societal Collapse.|
(examples: moderate-case global warming, minor asteroid impact, local thermonuclear war)
Global civilization not eliminated, but regional civilizations effectively destroyed; millions to hundreds of millions dead, but large parts of humankind retain current social and technological conditions. Chance of humankind recovery: excellent. Species local to the catastrophe likely die off, and post-catastrophe effects (refugees, fallout, etc.) may kill more. Chance of biosphere recovery: excellent.
|Class 1: Human Die-Back||Planetary/Societal Disruption.|
(examples: extreme-case global warming, moderate asteroid impact, global thermonuclear war)
Global civilization set back to pre- or low-industrial conditions; several billion or more dead, but human species as a whole survives, in pockets of varying technological and social conditions. Chance of humankind recovery: moderate. Most non-human species on brink of extinction die off, but most other plant and animal species remain and, eventually, flourish. Chance of biosphere recovery: excellent.
|Class 2: Civilization Extinction||Planetary/Societal Collapse.|
(examples: worst-case global warming, significant asteroid impact, early-era molecular nanotech warfare)
Global civilization destroyed; millions (at most) remain alive, in isolated locations, with ongoing death rate likely exceeding birth rate. Chance of humankind recovery: slim. Many non-human species die off, but some remain and, over time, begin to expand and diverge. Chance of biosphere recovery: good.This takes an entire planet back to at least pre-industrial data, if not hunter-gatherer days.
|Class 3a: Engineered Human Extinction||Planetary/Species Extinction (dominant species, engineered).|
(examples: targeted nano-plague, engineered sterility absent radical life extension)
Global civilization destroyed; all humans dead. Conditions triggering this are human-specific, so other species are, for the most part, unaffected. Chance of humankind recovery: nil. Chance of biosphere recovery: excellent. Extinction via unnatural causes (i.e., someone did something, human or otherwise).
|Class 3b: Natural Human Extinction||Planetary/Species Extinction (dominant species, natural). |
(examples: major asteroid impact, methane clathrates melt)
Extinction via natural causes. Global civilization destroyed; all humans dead. Conditions triggering this are general and global, so other species are greatly affected, as well. Chance of humankind recovery: nil. Chance of biosphere recovery: moderate.
|Class 4: Biosphere Extinction||Planetary/Species Extinction (several species).|
(examples: massive asteroid impact, "iceball Earth" reemergence, late-era molecular nanotech warfare)
Global civilization destroyed; all humans dead. Biosphere massively disrupted, with the wholesale elimination of many niches. Chance of humankind recovery: nil. Chance of biosphere recovery: slim. Chance of eventual re-emergence of organic life: good. Not only are humans gone, but most critters with them, leaving only a select few to evolve and refill the biosphere (or, as the name suggests, what's left of it).
|Class 5: Planetary Extinction||Planetary/Species Extinction (all multicellular life).|
(examples: dwarf-planet-scale asteroid impact, nearby gamma-ray burst)
Global civilization destroyed; all humans dead. Biosphere effectively destroyed; all species extinct. Geophysical disruption sufficient to prevent or greatly hinder re-emergence of organic life. The planet may be fit for re-habitation, but in the meantime, there's nothing more complex than bacteria left.
|Class 6: Planetary Desolation||Planetary/Total Extinction.|
The planet is left as a lifeless husk.
|Class X: Planetary Annihilation||Planetary/Physical Annihilation.|
(example: post-Singularity beings disassemble planet to make computronium)
Global civilization destroyed; all humans dead. Ecosystem destroyed; all species extinct. There used to be a planet here. There isn't anymore; it's gone.
|Class X-2: Stellar Destruction||Stellar/Physical Annihilation.|
You know that big ball of hydrogen/helium fusion and the bunch of rocks that used to circle it? Yeah, they ain't here no mo'. This usually happens due to that particular fusion ball doing something unpleasant like going supernova.
|Class X-3: Galactic Scale Destruction||Galactic/Physical Annihilation.|
Via some means, billions of stars, nebulae, pulsars, and so forth, along with the super-massive galactic-core Black Hole(s) at its center are destroyed. Utterly.
|Class X-4: Universal Destruction||Universal/Physical Annihilation.|
Everything that has ever been observed by anyone, anywhere. Eradicated. Or at the very least, not organized into galaxies, stars, and planets anymore. It is the end of all things. Unless there are other dimensions; those are safe.
|Class X-5: Multi-universal Destruction||Multiversal/Physical Annihilation.|
If there are alternate dimensions or different realities or whatever in this fiction, then many of those go away.
|Class Z: Total Destruction Of All Of Reality||Omniversal/Physical Annihilation or Omniversal/Metaphysical Annihilation.|
Not just the universe, and not just other universes, but all places and things that can be said to physically exist get wiped out somehow.
There are a couple of theoretical reasons to expect that an apocalypse is due, even though the exact type of apocalypse is unknown. These tend to keep theorists up at night, staring at the ceiling.
The Fermi Paradox points out that:
- There is a high probability of large numbers of alien civilizations
- But we don't see any
So by the observational evidence, there are no alien civilizations. The trouble is that means our civilization shouldn't be here either, yet we are.
The nasty conclusion is that our civilization is here, so far. But our civilization is fated for death, and the probability is death sooner rather than later. This is called The Great Filter, and it is a rather disturbing thought. For a detailed explanation read the original article by Robin Hanson.
The Great Filter is something that prevents dead matter from giving rise, in time, to "expanding lasting life". The hope is that us humans are here because our species has somehow managed to evade the Great Filter (i.e., the Great Filter prevents the evolution of intelligent life). The fear is that the Great Filter lies ahead of us and could strike us down any minute (i.e., the Great Filter is either a near 100% chance of self destruction, or something implacable that hunts down and kills intelligent life).
The unnerving part is the implication. The easier it is discovered for life to evolve on its own (for example if life was discovered in the underground seas of Europa), the higher the probability the Great Filter lies ahead of us.
The Doomsday Argument is is a probabilistic argument that claims to predict the number of future members of the human species given only an estimate of the total number of humans born so far. Simply put, it says that supposing that all humans are born in a random order, chances are that any one human is born roughly in the middle.
The actual full bown Doomsday Argument does not put any upper limit on the number of humans that will ever exist, nor provide a date for when humanity will become extinct. There is however an abbreviated form of the argument does make these claims, by confusing probability with certainty. You may stumble over it some day, so don't be fooled.
The full blown Doomsday argument only make the prediction that there is a 95% chance of extinction within 9,120 years.
|4.184 × 1021||1 teraton||= 1000 gigatons = 1e6 megatons|
|5.43 × 1023||120 Tt||1 Chicxulub Crater = 1 Dinosaur Killer = 20 Shoemaker-Levys|
|3.0 × 1024||720 Tt||1 Wilkes Land crater = 6 Chicxulub Craters|
|4.184 × 1024||1 petaton||= 1000 teratons|
|5.5 × 1024||1 Pt||total energy from the Sun that strikes the face of the Earth each year|
|3.2 × 1026||77 Pt||Energy required blow off Terra's atmosphere into space|
|3.9 × 1026||92 Pt||total energy output of the Sun each second (bolometric luminosity)|
|4.0 × 1026||96 Pt||total energy output of a Type-II civilization (Kardashev scale) each second|
|6.6 × 1026||158 Pt||Energy required to heat all the oceans of Terra to boiling|
|4.184 × 1027||1 exaton||= 1000 petatons|
|4.5 × 1027||1 Et||Energy required to vaporize all the oceans of Terra into the atmosphere|
|7.0 × 1027||2 Et||Energy required to vaporize all the oceans of Terra and dehydrate the crust|
|2.9 × 1028||7 Et||Energy required to melt the (dry) crust of Terra|
|1.0 × 1029||24 Et||Energy required blow off Terra's oceans into space|
|2.1 × 1029||50 Et||Earth's rotational energy|
|1.5 × 1030||359 Et||Energy required blow off Terra's crust into space|
|4.184 × 1030||1 zettaton||= 1000 exatons|
|2.9 × 1031||7 Zt||Energy required to blow up Terra (reduce to gravel orbiting the sun)|
|3.3 × 1031||8 Zt||total energy output of the Sun each day|
|3.3 × 1031||8 Zt||total energy output of Beta Centauri each second (bolometric luminosity). 41,700 × luminosity of the Sun.|
|5.9 × 1031||14 Zt||Energy required to blow up Terra (reduce to gravel flying out of former orbit)|
|1.2 × 1032||29 Zt||total energy output of Deneb each second (bolometric luminosity)|
|2.9 × 1032||69 Zt||Energy required to blow up Terra (reduce to gravel and move pieces to infinity)|
|4.184 × 1033||1 yottaton||= 1000 zettatons|
|1.2 × 1034||3 Yt||total energy output of the Sun each year|
|SEVERITY||Species Extinction—Total Extinction|
|CLASS||3a: Engineered Human Extinction—|
X: Planetary Annihilation
|CLASS||3a: Engineered Human Extinction|
One way to relatively rapidly render a planet with an oxygen-nitrogen atmosphere uninhabitable by human beings is to somehow induce the oxygen and nitrogen to combine. Since Terra's atmosphere has about four times as much nitrogen as oxygen, all the oxygen can oxidize with nitrogen leaving an atmosphere that is pretty much just nitrogen. Almost everybody will die because the human body is quite insistent that it needs to breath oxygen in order to live.
I would have thought this was a bit far-fetched as a basis for a science fiction apocalypse, were it not for the fact that I remembered not one but two novels using it. These were Hal Clement's THE NITROGEN FIX and John W. Campbell jr.'s THE MIGHTIEST MACHINE.
Since the idea is to create a catastrophe by removing all the oxygen, and the problem that Terra's atmosphere has a mass of about 5 quintillion tons, the mechanism for oxidizing will have to be scaleable. Because of energy limitations, oxygen and nitrogen do not react at ambient temperatures. You have to heat them pretty hot to convince them to combine. Heating up the entire atmosphere hot enough to oxidize the nitrogen will roast to death all humans and other land mammals long before they die from lack of oxygen, which will make for a dull science fiction novel.
THE NITROGEN FIX does non-roasting scale-ability by postulating a genetically created microorganism whose metabolism creates energy by oxidizing nitrogen. The scale-ability comes from the power of single-celled geometric-progression reproduction: i.e., they multiply much faster than cockroaches.
THE MIGHTIEST MACHINE does non-roasting scale-ability by postulating a chemical catalyst. Which as you should know cause a specific chemical reaction to occur at an accelerated rate. The scale-ability comes from the fact that all catalysis are not consumed in the catalyzed reaction and can continue to act repeatedly. Because of this, only very small amounts of catalyst are required to alter the reaction rate.
|CLASS||4: Biosphere Extinction|
Dinosaurs were pretty darn successful. They managed to be the dominant terrestrial vertebrates for 135 million years, while us hairless apes have only been around for 0.04 million years. So why didn't dinosaurs evolve intelligence and create a galactic empire at the end of the Mesozoic Era? Well, they procrastinated just a wee bit too long on creating their space program.
Approximately 66 million years ago the Cretaceous—Paleogene extinction event happened, aka the "Dinosaur Killer Asteroid". Freaking asteroid was 12 kilometers in diameter, blazing along at about 20 kilometers per second had about 5.43×1023 joules of kinetic energy. The blast was approximately the equivalent of a 120 teraton nuclear bomb. You couldn't do more damage with three thousand tons of pure antimatter.
About 75% of Terra's surface is ocean, so it is unsurprising that the asteroid strike was an ocean impact. But this meant it was Megatsunami time. Scientist calculate that the waves were about five freaking kilometers tall. Small islands ("small" as in "Madagascar-sized") would have been totally submerged.
There was a global firestorm, partially ignited by the thermal pulse of the impact, and partially from incendiary fragments from the blast launched into sub-orbital trajectories to all points on the world. The higher proportion of oxygen in the atmosphere back then just made things worse. Scientists examining the prehistoric layer of soot laid down concluded that almost the entire terrestrial biosphere had gone up in flames.
There is also some evidence that the impact was not straight down, with the primary destruction focused at a single impact point. Evidence suggests it was a glancing impact, meaning it was an impact line, creating a flaming path of destruction across the face of North America.
The asteroid also picked a particularly devastating spot to strike: a continental shelf area composed of limestone. The incinerated limestone released huge amounts of carbon dioxide. Some of it led to rapid ocean acidification, spelling doom to ammonites. The rest went into the atmosphere. Note that the Chesapeake Bay impacter of 35 million years ago did not hit a limestone shelf, and apparently did not cause an extinction event.
Between the firestorm and the limestone continental bake-off the amount of carbon dioxide in the atmosphere took a drastic upturn, which started a savage greenhouse effect. Global temperatures skyrocketed.
There was also about twelve years of acid rain, but that was just a flea bite compared to the rest of the catastrope.
The resulting crater was about 100 kilometers wide and 30 kilometers deep.
About 50% to 75% of all species of life on Terra swiftly became extinct. Most of the animals that managed to survived were the ones that ate worms, flies, and carrion, due to the fact that was pretty much the only thing around to eat.
This is because the debris cloud choked off the sunlight for about ten years, which wiped out most of the plants, which caused the herbivores to starve, which caused the carnivores to starve. The only abundant food source was the mountains of rotting animals (fat cooked ones and skinny raw ones) and the maggots who could not believe their own luck. In addition there was mold and fungus everywhere.
Cockroaches and the ancestors of rats survived, of course. Everybody knows how hard they are to kill. Don't sneer at those rats, they were your ancestors too.
|SEVERITY||Societal Destruction—Physical Annihilation|
|CLASS||0: Regional Catastrophe—6: Planetary Desolation|
Forrest J. Ackerman's picturesque term for a nuclear holocaust is "Atomageddon". Andre Norton's colorful term for a planet rendered uninhabitable by nuclear war is Burnt Off. She describes them as having much the same appearance of a charcoal briquette. Except they are the size of a planet. Other authors describe such worlds as looking from orbit much like ordinary worlds. Except there are no green plants, and on the night side the craters marking the former location of cities glow blue with Cherenkov Radiation. Both descriptions are probably more fictionally vivid than they are scientifically realistic.
For the technical details about strategic nuclear weapons, go here.
The initial idea was that the entire continental surface of Terra would be turned into trinitite, with all the plants, cities, and people converted into trace impurities within the radioactive glass. Later the deadly vision shifted to "not with a bang, but a whimper" ending with the formulation of the nuclear winter scenario.
So if a final war which kills everybody on a planet is horrific; hmmmmm, I wonder how to make this several orders of magnitude more horrific?
In reality, a runaway star passing close enough to threaten your planet is an incredibly low-probability event. But it isn't actually impossible. Astronomers estimate there may be billion of rogue planets in our galaxy, wandering in the icy depths of deep space. And lots of them were ejected from their home solar system by an invading star.
Assuming that the planet's inhabitants survive the cataclysmic earthquakes, they can look forward to a lonely frigid death as the atmosphere gradually freezes.
A related concept is when one of these rogue planets happens to score a bulls-eye on some hapless inhabited world. This was arguably invented by the 1932 novel WHEN WORLDS COLLIDE by Philip Wylie and Edwin Balmer, along with the idea of escaping the doomed planet in a spaceship ark. This was used by writer Alex Raymond in his 1934 comic strip Flash Gordon, where the marauding planet Mongo threatens Terra, and our heroes travel there in a spaceship to halt the catastrophe. Jack Williamson's 1934 short story BORN OF THE SUN had the protagonist and his fiancée escaping the doomed Earth in a hastily constructed space ark (though the threat is the Earth hatching like a titanic space egg). In Jerry Siegel and Joe Shuster's 1938 comic Superman the infant escapes the doomed planet Krypton in an even more hastily constructed rocket perambulator. More recently the concept was used in Jack McDevitt 2005 novel SEEKER
Occasionally the astronomers turn out to be mistaken (inadvertenly or on purpose) about the apocalypse-potential of a threatening world or planet. An example is Max Ehrlich's 1949 novel THE BIG EYE.
Stars going boom are pretty apocalyptic. They come in a variety of sizes.
These events are sometimes measured in a unit called a Foe, from the phrase "ten to the power of fifty-one ergs". One Foe is equal to 1044 joules. An average supernova emist one Foe.
Back in the 1950's all the science fiction novels which needed an earth-shattering kaboom would use a Nova. Star goes boom, incinerates the entire solar system, pretty apocalyptic. Astronomers didn't know anything about novae except they were huge, so science fiction author had free reign.
Nowadays we know that novae happen only in binary star systems. A normal star has the misfortune to be orbited by a white dwarf. Over the millennium the dwarf sucks hydrogen out of the normal star like a cosmic vampire. The dwarf starts burning the hydrogen using carbon-nitrogen-oxygen fusion reaction.
Sometimes the the white dwarf suffers a runaway reaction, and you get a nova.
The fact that a white dwarf is required made instantly obsolete all those science fiction stories about the sun going nova.
With each explosion only about one ten-thousandth of the white dwarf's mass is ejected. The point is that a vampire white dwarf can go nova multiple times. For instance, the star RS Ophiuchi has gone nova six times. The mass is ejected at velocities up to several thousand kilometers per second.
Astronomers estimate that about 30 to 60 novae occur in our galaxy per year.
A nova can reach an absolute magnitude of -8.8, or about 42.7 trillion times the luminosity of the sun. Nova emit about 10-7 Foe or about 1037 joules.
Novae to eject enriched elements into the interstellar medium like red giant, supergiant stars, and supernovae do. But only a paltry amount. Supernovae emit 50 times as much, and red giant/supergiant stars emit 200 times as much.
Dwarf Nova are also called U Geminorum-type variable star. Their increase in luminosity is not due to a fusion explosion. Rather it is a vampire white dwarf star whose accretion disk becomes unstable. Part of the disk suddenly collapse onto the white dwarf and releases large amounts of gravitational potential energy.
This only releases a tiny fraction of the energy of a full fledged nova, and would be difficult to detect from another solar system without a telescope. It is hardly apocalyptic.
It is only mentioned here in case you run across the term in your researches and get confused.
Kilonova are caused when two neutron stars or a neutron star and a black hole merge. Kilonovae not only emit intense bursts of light, but also lots of gravitational waves.
Kilonovae emit about 10-5 Foe (1039 joules) to 10-3 Foe (1041 joules).
Luminous Red Nova are caused when two stars collide (probably a binary star whose components spiral into each other).
The only reference I could find said that luminous red novae had luminosities between that of a nova and that of a supernova. Which means it emits about 0.5 Foe (5×1043 joules) if they mean 1 Foe = a supernova, or 50 Foe (5×1045 joules) if they means 100 Foe = a supernova. Your guess is as good as mine.
Novae are impressive but Supernovae are the real deal. A nova will poot off a pathetic one ten-thousandth of its mass in the explosion, with a supernova it is pretty darn close to 100%. The mass will be traveling in all directions at about 30,000 kilometers per second, 10% the speed of light.
Blasted cataclysm will briefly outshine the entire galaxy. In a few months a supernova will emit as much energy as Sol will over its entire lifetime. Type I or type II supernovae emit about 1 Foe (about 1044 joules).
While novae happen multiple times to a star, a given star can only go supernova once. There isn't much left except for a little neutron star or black hole, it is not going to explode again.
The most energetic supernovae are called hypernovae
Supernovae are potentially strong galactic sources of gravitational waves. The expanding gas causes shock waves in the interstellar medium, creating a supernova remnant. Supernova remnants are considered the major source of galactic cosmic rays.
There are two ways a star can go supernova: thermal runaway and core collapse. There are four classifications of supernovae, the first one is caused by thermal runaway and the other three by core collapse.
Thermal runaway is the same mechanism that causes novae: white dwarf vampires hydrogen off its sibling and occasionally suffers from indigestion. The difference is that with a supernova the runaway reaction is not so much like popping a birthday balloon so much as it is like detonating a thermonuclear warhead. Thermal Runaway supernova emit about 1 to 1.5 Foe (1×1044 to 1.5×1044 joules).
Core Collapse. Gravity makes everything fall down, with "down" being defined as the center of gravity of all the objects. So a nebula contracts as gravity tries to squeeze into a tiny ball.
Soon the nebula contracts into what they call a protostar. But at some point the temperature at the center of the protostar becomes high enough to ignite a fusion reaction. A star is born.
The fusion reaction emits lots of electromagnetic radiation, i.e., light. The radiation pressure of the light brings the star's gravitational collapse to a halt. The star's body can no longer fall down, it is "propped up" by radiation pressure.
Core collapse is when one of several mechanisms kicks out the prop. The star then abruptly collapses.
This means that instead of a small steady stream of the star's hydrogen is slowly being burnt in the core, suddenly all of the hydrogen is burnt in a fraction of a second. The supernova explosion obliterates the star, leaving only a small neutron star or black hole. Millions of years from now alien astronomers in an adjacent galaxy notice that our galaxy has suddenly doubled in brightness.
The mechanisms that can cause core collapse are electron capture; exceeding the Chandrasekhar limit; pair-instability; or photodisintegration. Which mechanism does the dirty deed more or less depends upon the star's mass. Details can be found in the Wikipedia article.
Pair-instability supernovae can emit from 5 to 100 Foe of energy (5×1044 to 1×1046 joules). Electron capture, Chandrasekhar limit and photodisintegration supernovae regularly emit about 100 Foe of energy (1×1046 joules).
Supernovae are very important for the formation of planets. When the universe was formed it was composed of hydrogen with a sprinkling of helium. The only reason that elements like carbon, oxygen, iron, and uranium exist at all is because these elements were forged in supernovae explosions and spread into the galaxy at velocities of 0.1c. These elements later condensed into molecular clouds, which formed stars and solar systems.
Specifically a Von Neumann universal constructor, aka Self-replicating machine. These are machines that can create duplicates of themselves given access to raw materials, much like biological organisms. Whatever sabotage they are programmed to do against the defenders is magnified by the fact that they breed like cockroaches.
In the TV series Stargate SG-1, the Replicator are self-replicating machines that are ravaging all the planets in the Asgard galaxy. In Greg Bear's novel The Forge of God and the sequel Anvil of Stars, an alien species systematically destroys planets detected as possessing intelligent life by attacking the planets with self-replicating machines.
Nanotechnology (and it's extension nanorobotics) is the concept of molecule sized machine. The idea is attributed to Richard Feynman and it was popularized by K. Eric Drexler. It didn't take long before military researchers and science fiction writers started to speculate about weaponizing the stuff. A good science fiction novel on the subject is Wil McCarthy's Bloom.
There are many ways nanotechnology could do awful things to a military target. One of the first hypothetical applications of nanotechnology was in the manufacturing field. Molecular robots would break down chunks of various raw materials and assemble something (like, say, an aircraft), atom by atom. Naturally this could be dangerous if the nanobots landed on something besides raw materials (like, say, an enemy aircraft). However, since they are doing this atom by atom, it would take thousands of years for some nanobots to construct something (and the same thousands of years to deconstruct the source of raw materials).
But using nanobots for manufacturing suddenly becomes scary indeed if you make the little monsters into self-replicating machines (AKA a "Von Neumann universal constructor") in an attempt to reduce the thousands of years to something more reasonable. Suddenly you are facing the horror of wildfire plague spreading with the power of exponential growth. This could happen by accident, with a mutation in the nanobots causing them to devour everything in sight. Drexler called this the dreaded "gray goo" scenario. Or it could happen on purpose, weaponizing the nanobots.
Drexler is now of the opinion that nanobots for manufacturing can be done without risking gray goo. And Robert A. Freitas Jr. did some analysis that suggest that even if some nanotech started creating gray goo, it would be detectable early enough for countermeasures to deal with the problem.
What about nanobot gray goo weapons? Anthony Jackson thinks that free nanotech that operates on a time frame that's tactically relevant is in the realm of cinema, not science. And in any event, nanobots will likely be shattered by impacting the target at relative velocities higher than 3 km/s, which makes delivery very difficult. Rick Robinson is of the opinion that once you take into account the slow rate of gray goo production and the fragility of the nanobots, it would be more cost effective to just smash the target with an inert projectile. Jason Patten agrees that nanobots will be slow, due to the fact that they will not be very heat tolerant (a robot made out of only a few molecules will be shaken into bits by mild amounts of heat), and dissipating the heat energy of tearing down and rebuilding on the atomic level will be quite difficult if the heat is generated too fast.
Other weaponized applications of nanotechnology will probably be antipersonnel, not antispacecraft. They will probably take the form of incredibly deadly chemical weapons, or artificial diseases.
Some terminology: according to Chris Phoenix, "paste" is non-replicating nano-assemblers while "goo" is replicating nano-assemblers. Paste is safe, but is slow acting and limited to the number of nano-assemblers present. Goo is dangerous, but is fast acting and potentially unlimited in numbers.
"Gray or Grey goo" is accidentally created destructive nano-assemblers. "Red goo" is deliberately created destructive nano-assemblers. "Khaki goo" is military weaponized red goo. "Blue goo" is composed of "police" nanobots, it combats destructive type goos. "Green goo" is a type of red goo which controls human population growth, generally by sterilizing people. "LOR goo" (Lake Ocean River) nano-assemblers designed to remove pollution and harvest valuable elements from water, it could mutate into golden goo. "Golden goo" are out-of-control nanobots which were designed to extract gold from seawater but won't stop (the "Sorcerer's Apprentice" scenario). "Pink goo" is a humorous reference to human beings.
ACE Paste (Atmospheric Carbon Extractor) designed to absorb excess greenhouse gasses and covert them into diamonds or something useful. Garden Paste is a "utility fog" of various nanobots which helps your garden grow (manages soil density and composition for each plant type, controls insects, creates shade, store sunlight for overcast days, etc.) LOR paste: paste version of LOR goo. Medic Paste is a paste of nanobots that heals wounds, assists in diagnosis, and does medical telemetry to monitor the patient's health.
Example: in Vernor Vinge's classic A Fire Upon The Deep, a team of researchers at the rim of the galaxy experiment with a five billion year old datastore, using computer recipes they do not fully understand in an attempt to activate the software contained. Unfortunately they succeed. The software is The Blight, a malevolent super-intelligent artificial entity. Before it is defeated, it takes over several entire races (rewriting their minds to turn them all into agents of the Blight) and murders several post-singularity trancendent entities.
In Kaleidoscope Century by John Barnes, a rogue artificial intelligence called One True can contact a person on a cellphone, then use rapidly changing audio signals to reprogram the person's brain, turning them into a brainwashed zombie. It then tries to take over the entire world.