Year 2020.

Planet Earth is peacefully turning around the sun. Humans on planet earth, however, do not feel peaceful at all.

Alien spaceships are in the sky. They did not answer, or humans could not find a way to communicate properly. The ships came near earth, stopped, waited a little, went back. Then, after a few weeks, they did it again. (And again, if needed)

In fact, they look like they are scouting. It is pretty sure they are going to attack some day. The spaceships are quite small, they might be waiting for bigger ones.

The spaceships do not last for long near Earth, then all we know about them is a visual aspect, perhaps the kind of metal or the weight of the spaceships (I think these can be calculated or observed with telescopes, comment if you think i am wrong or we can learn much more)

What do we do to defend ourselves ? We don't even know against what we will have to fight !

As the question is very broad, here are a few assumptions to narrow it :

  • Humans chose to stop fighting themselves and cooperate in order to defend Earth (Big assumption)
  • Even if the aliens did not attack yet, scientists and militaries agrees their tactics look like scouting before an attack. I do not ask about politics or peace attempts that will certainly occur in the same time. They are here, now, and we have to prepare a possible war, in the worst case.
  • Alien spaceships did not come in the atmosphere. In a very large orbit, at best.
  • As long as there is no attack, defenses can be improved. Both short terms and long terms plans are available answers.

Bonus : We have no idea what kind of creature/robot/life form is in the spaceships, our defense have to take that in consideration as they might not be organics.

  • $\begingroup$ I'm not a native english speaker, ask if something is unclear. Please don't answer "we're dead", we might have time to prepare ! $\endgroup$
    – Tyrabel
    Commented Sep 20, 2015 at 20:32
  • $\begingroup$ How many spaceships are there, and what size are they? $\endgroup$
    – HDE 226868
    Commented Sep 20, 2015 at 21:09
  • 2
    $\begingroup$ We don't know anything about them. Even though they are advanced on the technology level, what tells us they can even detect metal/steel objects (missiles)? Perhaps their civilization (if there is a structured one) is not even based on the same resources as ours. Do they know about the principle of war? Or is their goal to wander around planets where things move on the surface? We don't even know if they are mentally (assuming they have a brain) able to discuss/plan a strategy or think about those kind of things. We need to stop assuming aliens are human-like. $\endgroup$
    – Jeff Noel
    Commented Sep 21, 2015 at 12:54
  • 1
    $\begingroup$ @JeffNoel partly true. Partly, well, if they come and go, come and go, come and go… it's likely there is some sort of strategy or thinking behind it. Or, well, it might be they are just pissing on a tree, true. $\endgroup$
    – o0'.
    Commented Sep 21, 2015 at 14:31
  • 1
    $\begingroup$ "The ships came near earth, stopped, waited a little, went back." Spaceflight, at least Newtonian or relativistic spaceflight, doesn't work like that. You can't "stop" in space. You can enter an orbit that is identical to that of some other object in orbit of another object (say, place your spacecraft in an orbit around the Sun identical to that of Earth) but that just means that you are stationary with respect to Earth, not relative to anything else. For example, in a heliocentric reference frame, you would still be moving at about 30 km/s. $\endgroup$
    – user
    Commented May 19, 2017 at 13:35

12 Answers 12


Know thine enemy then plan from there. A great deal can be learned about the aliens and their craft, even without being able to talk to them. Listed below are a few parameters to analyze

(I'm going to assume a for this question because it helps provide structure.)

Information Gathering

Propulsion Systems of the aliens

Since the OP says the aliens just appeared in the sky, that means they have coasted into orbit from a long way off. Chemical rocket heat signatures can be seen from great distances and torchships can be seen from even farther away. Whatever they're doing, they took their sweet time doing it. Someone would have noticed a fast moving bright star against the cold of the cosmic background.

From Project Rho:

If the spacecraft are torchships, their thrust power is several terawatts. This means the exhaust is so intense that it could be detected from Alpha Centauri. By a passive sensor...The Space Shuttle's much weaker main engines could be detected past the orbit of Pluto.

If they are using torchships, then passive defenses won't work since they can just burn through them with their exhaust. (Attacks on the torchships must be careful because the exhaust will burn projectiles too. Nuking the propulsion end of the ship won't do anything because th ship is nuking itself.) Further, torchships means that building bunkers for billions of humans will be a waste (and likely logistically impossible anyway).

Or, they found a "reactionless" propulsion system such as the Alcubierre Drive. If this is the case then the alien's understanding of physics far outstrips our own. While we may not be able to directly observe ships using an Alcubierre drive, we may be able to see any lensing effects on the star background when they engage the drive. Capturing such a ship would radically enhance our understanding of fundamental physics.

Temperature of the ships

We can learn a lot about the physical nature of the aliens based on how hot their ships are. If they are about 300K, we know they have biology approximating our own, or at least like living conditions similar to our own.

Shape of the ships

We know what aerodynamic ships look like, so we would be able to tell if the alien ships are intended to fly in an atmosphere. While this information in and of itself may not be incredibly useful, it is a valuable datapoint with regards to what the ships were designed for.

Repurposing Hubble to take pictures of the ships shouldn't be too hard.

Size of the Ships

Larger ships are just harder to build. By determining the size of the ships, we can start making guesses about what kind of material sciences they have developed. A ship a kilometer long is far harder to strengthen and support than a ship a mere 100 meters long.

Ship Emmissions

If the ships are talking to each other over radio, visible light or laser, we should be able to pick that up. Like the shape of the space ships, what kind of radiation these ships emit could tell us something about the occupants. This might tell us why they aren't answering. Maybe.

Active Defenses

Beam Weapons - Dump as much money as possible into beam weapons. The US Navy in 2015 has some powerful ones capable of shooting down drones and missiles. BY 2020, they will likely have stronger ones. They may not be persuaded to share how they got such powerful beam weapons but they may be enticed to deploy such weapons all over the world. Large beam weapon installations could be spread across the planet to cover all approaches.

Nuclear Shaped Charges - Some really crazy things can be done with nuclear charges, especially ones that focus the energy of the nuke into a narrow beam. A barrage of these charges sent into space might destroy the entire scouting force.

  • $\begingroup$ Your answer fits perfectly the part "we should be able to learn something when they're near", thanks for that. $\endgroup$
    – Tyrabel
    Commented Sep 21, 2015 at 14:57
  • $\begingroup$ If we manage to capture a fully functional ship or recover the wreck of a downed craft, then we'll have gained hundreds or thousands of years of progress in a very very short time. $\endgroup$
    – Green
    Commented Sep 21, 2015 at 14:59
  • 2
    $\begingroup$ If they aren't using a reactionless drive, we should be able to get a lot of useful information from spectroscopy: what fuels they use, what trace elements contaminate their fuel, perhaps even information about their building materials (especially if there are ablative components like on an Orion drive). If they still use windows, we might be able to guess a lot of things about their biology and homeworld from the spectrum of light they use inside. $\endgroup$
    – Deolater
    Commented Sep 22, 2015 at 17:20
  • $\begingroup$ Knowing as much as possible about them not only helps us fight them (finding weaknesses, etc), but it also could help us negotiate with them. Odds are that we can't beat them in an outright war, but if they're here for our Rare Earth metals, maybe we can convince them that leasing mining rights (for some technology that's basic to them but incredibly valuable and advanced for us) is a better option than having to fight. $\endgroup$
    – Deolater
    Commented Sep 22, 2015 at 17:23
  • $\begingroup$ @Deolater Or even point out that some rare Earth metals are abuntantly available in other solar system bodies. Just please leave the likes of Titan, Enceladus, Europa and a few others alone, will you? $\endgroup$
    – user
    Commented May 19, 2017 at 13:40

In 2020, humanity would be very much like as it is today, since that is only 5 years in future.

1. How our world would NOT be?

  • There would be no giant robots and flying cars running around our cities.
  • There would be no nanobots fabricating graphene-based complex nano-machines in industrial scale.
  • There would be no cyborgs with broad-band telepathic quantum-digital communication.
  • Genetic engineered mutants will not be warring to displace humans.
  • There would be no AI-based coordinated and efficient all-Earth government.
  • There would be no superintelligent beach crabs who calls Earth their homeworld projecting and building a Dyson sphere or a Death star.

2. How our world would be?

  • The western world will be fighting jihadists in Middle East. UN will be paralyzed and ineffective due to Russia's and China's successive vetos to anything that could have some real effect in practice.

  • People will be talking about global warming. People who earn money by burning or selling fossil fuels will deny it.

  • There will be some people dying from malaria in some poor and disorganized countries.

  • There will be some tensions and hostilities between Israel and Palestine.

  • Obama's successor will be thinking about something to do with Cuba (not something military). Specially if he/she happens to be from the republican party.

  • Apple would be announcing their new iPhone 8.

  • Facebook will be launching some new services to compete with Google Alphabet.

  • Microsoft would be receiving a lot of complaints about their new and buggy Windows 12 and would be preparing a large patch called Windows 12.1. Bing's social-cloud-network integration in the OS raises a lot of complaints about privacy and anti-trust commercial practices, but it will reveal to be a very useful feature to allow botnets to download and install malwares in user's PCs.

  • NSA will be illegally tracking and monitoring your traffic in porn suspicious sites in order to find for interesting nudity criminal evidence.

  • Thanks to many technological breakthroughs, solar and eolic energy are indeed the future and they are very cheap in 2020 - long life to our green energy! But instead, governments and corporations would still prefer to burn oil to keep the money in the hands that it always used to be. Screw up those nasty green ecoterrorists and their stupid solar and eolic technologies!

3. What would happen when the aliens arrive in 2020?

  1. People will suspect that the thing is some sort of secret spacecraft developed in Area 51. US government will (as always) deny. Nobody will initially take US's deny seriously. And this is very ironic, since for tens of years, people who had seen secret airplanes/spacecraft tought that they were alien starships. Now that they are seeing actual alien starships, they will think that they are secret airplanes/spacecraft.

  2. North Korea's government will tell their people that it is a North Korean spacecraft proposed and built by Kim Jong-Un himself and that this proves that North Korea has a tech level much more advanced than the rest of the world.

  3. Islamic fundamentalists will say that the UFO is a sign of Allah's power. Some Christians and Muslims sects would state that it is Jesus coming back in a spaceship directly from God. Some other Christians and Muslims sects would say that it is the antichrist instead.

  4. UN tries to approve some meaningless bureaucratic resolution about the UFO incident. After some months of talking and debating, it is vetoed by China and Russia just for the fun of vetoing.

  5. US will entitle themselves as "the world" and try to establish contact with the aliens. If US concludes that the aliens are friendly, then they came here obviously because they were interested in learning more about the fabulous America's way of life. If they are enemies, it is because they are communists and came to destroy America's liberty. As always, US will forget that there are other countries in the world.

  6. Most countries in Europa, Africa and Latin America will just not give a single [insert a bad word here] about it. Oh, we are being visited by aliens, but who cares, let's just continuing watching stupid commercial TV shows and sharing stupid memes on internet.

  7. Some visionary prophet will start a new religion to worship the newcomers (which he will state that came from Nibiru and Hercolubus) as the returning god astronauts from ancient Egypt and will announce that this is the start of the war against the illuminati. Also, he will show a "proof" that the Mayans miscalculated the world final date by 12 years and that it will end in year 2024. Millions of people would faithful join the religion (for a small monthly fee, of course). The visionary prophet will then quickly get some millions of dollars in his account, some mansions, yachts, airplanes, Ferraris and Lamborghinis. He will always be seen using a Rolex watch and his multiple top-model wives will dress clothes made from gold and diamonds in Caribbean beaches. However, he will always make clear that all his money is for charity, for African hungry child and for saving humanity in those end days and that he never took a single dollar from his followers for himself.

  8. Scientology church, to not get much behind other UFOs religions, publishes a new OT level about Xenu's past plans on planet Teegeeack and link it to the UFOs. But this information would be held secretly and would be released only for those people who had cleansed themselves enough (by providing them a lot of cash), except that it will leak in the internet in the first 5 minutes. Anyway, they would still aggressively insist in trying to sue and persecute anyone that publishes their BS and state that unprepared people who reads it will die from pneumonia.

  9. Then, the aliens impatient and deceived by Earth's people disorganization and stupidly, secretly perform some changes in Earth's society in order to match the OP's expectations. Otherwise, the attack would be too easy for them and they would not have any fun from it.

  10. After the aliens secretly changes Earth's society in some way, all the governments in the world suddenly agrees to coordinate people about investigating the aliens. They are proud that they quickly agreed that undoubtedly the aliens seems to be scouting Earth and that they will attack. However, in truth, the aliens deliberately gave the information to the humans that they are scouting and that they will attack, and did their operations in blatantly open and obvious ways, including by uploading their full and detailed plans in Youtube already properly translated in a lot of human languages. Otherwise, the stupid humans would probably never figure it out what they were doing.

  11. An organized world government finally forms, and perceiving that their time is short, start to heavily and hastily prepare for the war. See part 8 below for more details on some things that they might do.

4. Could humans war against the aliens?

Probably no!

Let's suppose that we choose to attack their ships:

  • They have enough technology to coordinate a lot of spaceships travelling through the galaxy, and who knows how many spaceships are there?

  • If they are sending a lot of scouting ships that travelled though the galaxy, what would be their battleships?

  • They surely have a tech level much more beyond of what we had ever dreamt about, in order to be able to achieve spacetravel.

  • If we could ever harm one of their scouts, an alien counter-attack would probably simply obliterate the humanity, perhaps in a few minutes. Again, they have enough tech for star-travel.

  • Even if they choose to use H bombs just as the same as humans developed during the cold war and in the same number that humans developed, and then land to drop those and explode 'em, humanity as we know it would be gone and we would not have any way to defend against that. We already have enough technology to wipe out ourselves even nowadays (as 2015), so why would a space-travelling alien civilization not be able to easily do it? Also, they probably would have something much more advanced than that if they want to attack us.

So, the conclusion is that attacking them would be like trying to use sticks and stones to defeat an horde of flying Apache helicopters equipped with missiles. No people with at least half a brain seriously thinking about it for 10 minutes would ever conclude that we have even a tiny chance in succeed on driving them away by war.

Earthlings would likely try to insist in communicating. But, goddammit, no matter what, we got no answer, at least no answer that we actually could understand as such. They simply don't seems to want to talk to us. Perhaps they don't want to talk to humanity by the same reason that you don't want to talk with the ants that are nesting in your garage. We are just too primitive.

5. Why could the aliens be here?

Ok, if the aliens would like to just destroy this worthless peace of rock and its stupid Moon, or even simply wipe out humanity from the third planet orbiting the Sun, we probably have no chance against them. So, if this is their ultimate purpose, we are probably doomed.

  1. Maybe they want a new planet to live and are checking if Earth is suitable? If this is the case, we should welcome them and hope that they don't think about exterminating us. But it would help a lot if they actually wanted to communicate (or perhaps, they are already communicating, by uploading their full detailed plans in Youtube).

  2. Maybe they want some mineral, or biomass, or water, or something else from Earth. If so, we could live our lives providing what they want. If they are hostile, but don't want to just kill us, we may be fine by accepting slavery. Of course, this would only be viable if they actually communicate.

  3. Maybe they just will take out of this planet or put it here, or do whatever they want to do undisturbed, without telling anything to us. We could watch them in hope of learning something. Trying to attack or disturb them would probably be useless. They would either don't care and just continue whatever is they work, or they would retaliate.

6. We really need to war against them. How could we have some chance?

If we want to war with them anyway, we could have some chance against them if:

  1. They are severely depleted of resources. And this could be one of the reasons of why they are coming. If the resource is something simple, they can easily go to Mars or the Moon and get it, but if they are solely on Earth, this means that they need something that is abundant only on Earth. Possibly something developed by life activity or by humans theirselves. What we could do to try to resist is try to intercept them as soon as possibly, use anything that we could do to harm them (like missiles), and try to hide or destroy whatever they want to take. We must put our hopes that their resource-depletion makes them somewhat unable to react or defend themselves properly.

  2. They are severely depleted of personnel. Maybe the aliens are just a small couple of pirates or refugees fleeing from their planet coming to Earth in order to hide from their enemies. In this case, if we agree that they are unwelcome, for example, if they are a small number of space locusts that devours all the resources of habitable planets and then just leave for another planet leaving an inhospitable wasteland behind, we should try to locate their members and kill them. Every kill in this scenario is a win even if we lose 10000 humans for each alien kill.

If they are not severely depleted on either resources or personnel, and we could not just adapt to live with them at Earth, then we are doomed. Our only hopes is that they are severely depleted on either those, and even if that is the case, we still have few chances against them.

7. I spotted an alien and want to kill him. What to do?

If the alien is inside a flying spaceship, launching a missile might be doable. They could counter attack, but if they are resource depleted, they might instead choose to flee. They might be able to dodge a missile easily, so lasers might be a better option. Each destroyed spaceship is a score for the humans.

If they are just walking with some spacesuit over the land, we could attack them with lasers, guns, arrows, stones, whatever. If their spacesuit is sufficiently damaged, this could mean their death. However, they are likely to be armoured, so damaging their spacesuit might be very hard, specially if it is done from some virtually alien indestructible material.

An alien walking around could be trapped. We could try to imprison them inside cages or crush them with rocks (probably by strategically positioning dynamites and ambushing them).

If the alien is a robot, EMP pulses might also work in addition to explosives, lasers and traps. If they are not armoured for that and unable to dodge or flee, of course. Also, if they are resource depleted, it is possibly that their robots are fragile and lightly armoured.

Grabbing parts of destroyed spaceships, destroyed robots and dead aliens is very important. It becomes much better if you can capture and safely hold working spaceships and robots or living aliens. The most that we can collect from them in order to learn what they are and how they behave, the better. Kudos, if you can hack their spaceships and robots to make them work for you, or convince some alien to change side and work for the humans instead.

8. What to do to defend/resist them?

If they decide to just blow up this planet, of course, there is nothing to do.

But if they are hostile, but don't want to chase down the humans (or not spend much time and energy in that), other than attacking them with the tactics in the part 7 above (if any of them works), we could:

  • Avoid them. If they just want to do whatever they want to do undisturbed, we could just let them do. Except if those activities are too harmful and untolerable for us, of course.

  • Live inside bunkers, mountains, forests, ships on the oceans and any place far away from the aliens.

  • Learn how to plant yourself, store food and seeds in safe hidden places. If possible, plant in vases that could be quickly moved from one side to the other. Prefer to produce foods that are nutritive, cheap and easy to grow.

  • Anticipate the enemy moves, and take everything valuable that they might want to use or that would be destroyed. This includes, for example, crops, water and machinery.

  • Mass-produce solar cells in a size usable by individual bunkers and that are sufficiently cheap, easy to produce and easy to install. We should do that when we still can.

  • Try to sabotage or steal whatever they produce here.

  • Invest heavily in seawater desalinization in order to provide people who lives in the sea or near the sea a reliable way to get drinkable water. We must invest on that as quickly as possible before our industry collapses or degrade to a level where we are unable to do that anymore.

  • For people living inland, provide ways to get drinkable water from rainwater or from rivers.

  • Invest heavily on communications. If they don't attack our satellites, we should launch as many as possible to ensure efficient communications. Otherwise, we should build the most that we can of ground, underground and underwater communications cables. Coordinating radio transmissions and band-width is also critical. Mass-producing all the equipment for that is also very important. And very important, deploy that f***ing IPv6 for once and all, we already had enough of IPv4.

  • Invest heavily in recycling materials. Avoiding wasting anything that could be useful.

  • Invest heavily in projecting machines that are cheap, easily producible and are easy to fix or replace parts if broken or damaged.

  • Invest heavily in war education. Everyone, including child and elderly people, need to be trained on how to act quickly and correctly in order to don't help the enemy, nor be a victim of them and to attack or sabotage them. The training also helps to improve resistance coordination. Collect as much information as possible from the aliens and teach whatever not deemed too secret you can. A lot of people need to know how to attack the aliens and also teach that to the others. Further, the education should be bended in a way to not tolerate traitors and to make people suspect on any behaviours that a potential traitor could have.

In other words, what humanity need to do to defend itself is similar to what a nation being invaded by an enemy army and trying to resist that does in a war. The economy and people behaviour change directly in accord to that. Think in what was Soviet Russia, UK and Nazi Germany during WWII. The economy of those three countries turned almost entirely into a war economy, trying to frenetically outperform the enemy as quickly as possible and harm it at the same time. Parts of the economy deemed as superfluous suffered or even stopped during wartime.

  • 1
    $\begingroup$ hehe you made my day victor, your analytic is funny and smart, you looked up to everything that effects with aliean , from un to relegion and countries $\endgroup$
    – Moudiz
    Commented Sep 21, 2015 at 12:49
  • 1
    $\begingroup$ Wow, you even went a little far, I tried to narrow the question and you expended it again ^^ $\endgroup$
    – Tyrabel
    Commented Sep 21, 2015 at 14:51
  • 5
    $\begingroup$ +1 for "In 2020, humanity would be very much like as it is today" :-) $\endgroup$
    – user
    Commented Sep 21, 2015 at 17:01
  • 2
    $\begingroup$ "Then, the aliens impatient and deceived by Earth's people disorganization and stupidy, secretly perform some changes in Earth's society in order to match the OP's expectations. Otherwise, the attack would be too easy for them and they would not have any fun from it." +1 $\endgroup$ Commented Sep 21, 2015 at 22:27
  • 2
    $\begingroup$ This answer uses a lot of dot points to very little effect. $\endgroup$ Commented Sep 22, 2015 at 0:35

Massive spoilers for Liu Cixin's The Dark Forest, which has an unusual answer to this.

Threaten to appeal to the galactic community. The idea being that if there's one alien race that can find Earth, there's probably a lot more in the galactic neighbourhood. And however advanced this alien force is, there will always be a bigger fish out there, and these aliens will not want to draw the big fish's attention. Whether these other aliens are benevolent or malevolent would be hard to predict, but there's at least a decent chance they will be utterly terrifying to the invaders. So, prioritise communications technology. Try and get communicators off planet. Launch probes into deep space. Put everything you have into high powered transmitters. Make absolutely clear, that if the aliens do not desist, you will broadcast Earth's presence and what has happened here far and wide.

This will work for aliens even substantially more advanced than humanity, which is what most of the other answers are missing.

  • $\begingroup$ I love Liu Cixin's books and his Three body trilogy is my favorite saga of all time. Indeed ,if the "Dark forest" theory is true, that would be the best strategy. However, not every alien species will know about the existence of the Dark Forest. So they may think they are THE most advanced species in the galaxy. $\endgroup$
    – Mishima
    Commented Jun 28, 2021 at 14:17

Depending on how certain the UN would be, that the aliens will definitely attack, there are multiple options.

In the past NASA and the U.S. military have discussed plans on how to defend against incoming asteroids. In fact the U.S. military has nuclear weapons just in case that would happen. As this seems to be an actual strategy this concludes that a nuclear warhead could be shot into the earths orbit or beyond. I assume that your aliens, even if they are advanced in technology, can't protect themselves on subatomic level, this could be a way to attack them.
It however may not make sense if their spaceship is armed and could in some way destroy the rocket/missile. If we would be able to see the spaceship this could perhaps be spotted beforehand.

What we should consider is, that shooting down the small ships would not help at all. It would rather shorten the time for a counter-attack and we might run out of warheads in case bigger ships would appear later on.

A possibility would be to let them land. It would be perhaps be enough unknown territory for them, that we would have an advantage. This could be for several reasons:

  1. They need to wear protective gear. No matter what shape or size they have, earth's atmosphere is dangerous for them. This might be because they cannot breath our air, their chemical components are based on Sulfur or most certainly they are not immune to bacteria, virus and fungus. This would slow them down.

  2. They come to earth for a reason. Assuming the aliens are not just slaying for fun, they want something from earth and they need it so much that the mobilize an entire fleet of spaceships. If we figure out what it is, we may discover a big weakness.

  3. During landing they likely need to slow down. This makes them an easier target.

A big problem would be if they attack from space, then we would be back to the very first option.

Even if it might not be the most realistic scenario, it could be very effective to use biological warfare against aliens. Assuming they do not wear protective gear inside their spaceships, humanity could try to get astronauts up to one of their spaceships. Maybe they could access their ships, either by force (which would likely not work, because then the aliens inside the ships would die, when the ships hull is broken) or by arranging a meeting is some way. Then they could release the agent inside the ship infecting the aliens. If humanity gets lucky, the aliens are returning to their base like they did before and infect the other aliens. Sounds pretty cruel, but it might work against killer-aliens.

Perhaps you could also do the peaceful attempt and not attack at all. They might get into contact with us humans and they do not want to fight. As far as we know they travel across galaxies just to visit us, so just based on some air maneuvers it might be a little shortsighted to assume the would bring instant doom.

The mayor problem with this scenario is, that it pretty much defies our knowledge about physics, as the aliens would need to travel faster than the speed of light to be returning shortly after leaving for there home planet or base. So if we take this into consideration and assume that they use hard science to achieve that, we are likely not going to stand a chance against them. If they came to destroy humanity their weaponry would be as advanced as their space-travelling and I doubt we could compete.

  • $\begingroup$ Using the astronaut to bring them the virus would be more effective if the astronaut didn't know that: someone else arranged for him to do that without him realizing it. $\endgroup$
    – o0'.
    Commented Sep 21, 2015 at 8:30
  • 1
    $\begingroup$ That being said, it doesn't make much sense: how could we design such a virus without even knowing what the aliens are made of? $\endgroup$
    – o0'.
    Commented Sep 21, 2015 at 8:30
  • $\begingroup$ Finally, we are certainly talking about FTL, it just isn't possible to do seriously space travel without that. $\endgroup$
    – o0'.
    Commented Sep 21, 2015 at 8:31
  • $\begingroup$ @Lohoris we could design several virus, each working on certain cellular models. Scientifically it seems that Hydrocarbon structures are still the likeliest base for life, even on other planets. It seems unrealistic, but maybe it could be done. $\endgroup$
    – John
    Commented Sep 21, 2015 at 9:55
  • $\begingroup$ @Lohoris FTL seems to be the case, but they might as well have a base on a planet within our solar system. $\endgroup$
    – John
    Commented Sep 21, 2015 at 9:56

I'm going to go out on a limb and say do nothing. I'm not joking. War, and preparations for war are expensive, and we know nothing about them and their goals. How can we tell the difference between a cat surveying for prey and a deer looking for plants to eat? The one thing we do know is that their technology is so unbelievably absurdly vastly beyond ours that there is no reason to believe we could even hurt them if we tried (moving so fast they might be teleporting? Do you have any idea how much one of our fighters would kill for that kind of maneuverability to avoid our best of the best interceptor missiles? What would we even throw at them?). We don't even know if we're looking at all the civilization has left in a spacepod, or if they are one of a trillion trillion world empire with powers beyond our imagination. Are they the brilliant creators of their spacecraft, or did they just jimmy the lock when noone was looking and joy ride for a bit?

Frankly, we're so outclassed in every way that our best approach is to use Arthur C. Clarke's rule: Any sufficiently advanced technology is indistinguishable from magic. Rather than wasting our resources at a futile attempt to attack something unimaginably beyond our comprehension, spend those resources on making our life better. The resources spent toward the aliens should be spent similar to how a village might spend its resources towards a wizard who has not actively doing anything to interact with us. Perhaps try to communicate with it. It's certainly no more risky trying to communicate than it would be to try to attack, and it is probably more likely to succeed at gathering intel.

Once we increase our knowledge of what is going on by an order of magnitude, or by an order of magnitude number of orders of magnitudes (or perhaps even an order of magnitude number of orders of magnitude in the number of orders of magnitude), then we could begin to whisper the possibility of preparing for a war. At the point you have, our intel is so unbelievably utterly useless that not even Kim Jong Un would smack the shiny red button to order an attack! The Illustrious Leader is way smarter than that!


First, let me say that a small starship which does not communicate might well be a relatively simple scientific probe. If New Horizons did an Earth fly-by or went into orbit, it wouldn't answer our calls, either. It was made to collect data, not to communicate. As the first probe detected signs of life, they send another one with more specialized instruments. But people on Earth would be preparing for the worst anyway, so the intentions are irrelevant.

The aliens did not develop in our solar system. So they are capable of interstellar travel.

  • Do they have some kind of FTL drive? That's technology beyond our understanding, like a mounted knight who gets hit by a Predator drone. On the plus side, we don't know how good the rest of their technology is. Perhaps the FTL drive is just a few years beyond our technology, and the rest isn't much better, either.
  • If they don't have FTL, it is even worse. They were able to accelerate a ship to a significant percentage of lightspeed. If the next ship decides to fire a missile before it decelerates, they can fry Earth.

My prediction for what happens:

  • Mankind forms some kind of global government in case the aliens want to talk. This government won't be very democratic or very strong. The US and China make a deal, EU, Japan, Russia and India are strong enough to be on the table. A few "rogue nations" are quickly and messily eliminated (North Korea, Syria) to get the "home front" into order. Using that precedent, some countries label their enemies "rogue" and take them out, too (Venezuela, Iran).
  • Any nation which can build kinetic or nuclear-armed interceptor missiles will do so. That's a pretty long list. The global government might try to ban nuclear tests and missile tests to we don't look like a "rogue planet" to the aliens.
  • NATO, China, and Russia will form some sort of Global Aerospace Defense center comparable to NORAD.
  • Ground forces are mobilized and dispersed from their peacetime bases. That's as much for internal security as for defense.
  • $\begingroup$ I wouldn't be so sure Iran wouldn't cooperate, nor that NK wouldn't just play possum and hope to be ignored. Of course ISIS would have to be obliterated, yes. $\endgroup$
    – o0'.
    Commented Sep 21, 2015 at 8:34
  • $\begingroup$ @Lohoris, my speculation for this scenario was that a coalition of superpowers and most regional powers would occupy the most unpredictable "rogue nations" to create a global government. It really wouldn't do to have two or more representatives for Earth. So NK can't "play possum" because it won't be trusted without regime change. $\endgroup$
    – o.m.
    Commented Sep 21, 2015 at 15:48

Passive Defense

  • Bunkers for public

  • Building up doomsday food stocks and cache of seeds etc

Militaristic Defense

  • All airforces on high alert

  • International coordination for assault management. All countries agree on sharing information and sending troops to wherever the aliens assault.

  • Very high altitude aircrafts patrolling the skies. Several space stations focussed on tracking the activities of the alien spaceships.

  • Efforts to track mode of communication between the unknown spaceships. Efforts to scan the interior of the spaceships with ultra-high power x-rays. Coordinated efforts to find out what is the level of technology of the invaders.


"Humans chose to stop fighting themselves and cooperate in order to defend Earth" I am not sure how to take this information but I am going to assume that there is formed worldwide alliance with one "major government" (even tho its not possible so soon, even if there would be alien assault on the earth)

"Year 2020." Assumed by given informations I think that the technology is not developed enough to build space ship for battle or even escape.

Now few questions and answers based on given informations, so from the given info I assume that there is going to be attack for 100% and mankind knows it.

First of all lets think about survival of our kind before we think about fighting them. Some resources have to be moved in to this matter, the best solution would be to create infrastructure under the earth, if the aliens don't intend to destroy whole planet this is our best chance of survival. Best place to start building this if we speak about weather and climate would be England and Australia so from the scratch we should move workers and other needed supplies in to these two "places" and start constructing underground "shelter" with top reinforced with best possible metal.

Now Assuming that this construction has started and we can start with moving and building some facilities in there, we can safely expand and move more in there until the "attack".

This move should give the humanity biggest chance to survive even if we should lose the fight.

Now there are several options of our next priority. Most of the armies around the world even today have special forces and technology in their armies which are "unique", now when the world is under the flag of an alliance we can unite all the forces at our disposal. By creating "military council" from the best of the best in the world we can really move our training and the art of war forward by the speed of light. By combining best techniques considering all + and - of every existing special force with this capacity we should be able to build "ultimate" training program and start with building new army. Biggest drive for first generations of new soldiers are going to be driven by "fear" from the alien assault however this effect is going to fade away the longer the aliens take before the attack. First few generations of new "super-soldiers" should be still ready in short amount of time.

Now speaking about the "tools of war" we have biggest experts from the world available so creating and developing new advanced arsenal adapted to our needs shouldn't be a problem. First matter

If they don't want to destroy whole planet they have to invade the planet with their own troops. Its a wild guess but in the end if we don't have any other chance, since we are moving civil population in to our two shelters we should shatter base of operation in big abandoned cities probably in USA. They are best choice for partisan tactics a.k.a. hit, run and hide.

Now when basics are clear and so is tactics chosen for battle our attention should be moved on developing of AGM/ASM warheads.

IN the time we have left (if there is any) we should keep pushing development of everything above + move some resources and try to develop missiles against their spaceships.


Well any race jumping across the Galaxy would have the tech to destroy our planet, at least kill (almost) all life on it. We can't stop that, not now, probably not ever. Because sending a decent sized projectile at some %c will be almost impossible to detect in time to even think about a defense.

So we will have to assume that if they war with us it is because they want at the very least a habitable planet and worst, Us.

So at this point we would start trying to make defenses that, one we are capable of making and two actually might work against probable attack vectors. Such as, will they be landing troops? If so Rail guns that can launch relativistic projectiles with some accuracy would be high on everyone's development list. if time and money allowed installing some on the moon would be an excellent ring of defense.

Since a ship can only enter the atmosphere at relatively slow speeds they would be sitting ducks if we had a weapon that could throw stuff fast enough, but we would have to catch them off guard, since any ship that can travel between stars will have powerful enough shields to stop or deflect things coming at them. Needing similar smaller weapons also to fight against any ground troops that land.

Otherwise, not knowing what they want, what they need or anything about their biology to prepare poisons, or psychology for psych warfare we are mostly left with designing large powerful guns and hope for the best.

(Lasers are not as good as an options for big guns because they require a much larger power source to produce similar impact, to the best of my knowledge.)


Well, let's send an unarmed, unmanned probe with lots of scientific and spying gadgets, to photograph, ecograph, radiograph, smellograph, x-ray-o-graph, etc-o-graph, them. If they are indeed hostile, they will quite probably destroy the probe. Great, we now know that basic piece of information: they are hostile.

If they don't destroy the probe, we make it attach to one of their space ships, in order to make more tests. We will see if they try to remove it, and whether violently or gently. If they don't, then the probe will go back with them when they withdraw (if they withdraw). In such way, we will discover how they move, at what speed, and in what direction.

They must come from very far away, and use vehicles and drives adequate for this long distance, or we would have detected their radio transmissions long ago. Unless for some reason they do not communicate via radio, but in this case it is difficult to believe that they would have developed space travel.


We would certainly not be able to defend against an invasion aliens if they are more intelligent than us. However, that isn't the only possibility. It's perfectly reasonable to assume that the invading aliens are not intelligent at all. Suppose that civilizations that undergo rapid technological evolution almost always become machine civilizations.

Then, just like most organisms are susceptible to developing cancer, you can imagine that on the long term, machine civilizations are unstable to developing the analogue of cancer; they degenerate into a collection of dumb machines that just hop from solar system to solar system making copies of themselves.

Then if Earth is invaded by aliens are only hope is that they are such dumb machines. They can then be fought off by studying what conditions the machines avoid, e.g. they could stay away from water, and then using that to get rid of them.

  • $\begingroup$ Ah, but the smart machines will be more effective than the dumb ones and will appear as well, although you could have a predator prey dynamic. $\endgroup$ Commented Jun 28, 2016 at 17:03

You've stopped thinking we're alone right? Then stop thinking there's no one else. The technology might not be too far or they would just:

  1. Ascend into the 4th dimension or higher.
  2. Leave the galaxy and find one with a better resource.
  3. Go and compete in a more valuable place, such as the center of the universe or where it began to make scientific research.
  4. Remain in an utopia and/or help others achieve it like them, once we're great enough to contact others. Remember, we're very outdated in contacting.
  5. Rule the galaxy? Just get reconquered and/or rebelled. Only freedom stands strong.

Now if we stay xenophile other xenophiles are likely to help us. If we stay xenophobe noone would help.

Now, if to contact aliens is to ask for help then we can go to war together against threats. Don't discriminate! All sentient beings should be treated equally.

  • $\begingroup$ Hi, tanny, welcome to Worldbuilding! Just a reminder: The question puts the premise in 2020. We're not going to have anywhere near enough technology by then to leave the Solar System, let alone travel anywhere significant in or outside the galaxy. Additionally, though it would be cool if it existed, there is currently no evidence for large extra dimensions; also, the Big Bang didn't happen at a point. Maybe you can consider revising some portions of your answer. $\endgroup$
    – HDE 226868
    Commented Dec 2, 2016 at 13:54

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