Producer’s note: Someone on Quora asked: What would a competent alien invasion force do differently from the normal Hollywood portrayals? Here is one of the best answers that’s been pulled from the thread.
Firstly, although the aliens may be carbon based life as we are and have similar biologies to us they almost certainly do not have compatiblecombinations. So what does that mean? It means that almost all biological material on Earth would be poisonous or indigestible if consumed to them and vice versa.
There are many ways to assemble amino acid chains and life on Earth has adapted to cohabitating with many of these methods. On an alien world with just slightly different elemental makeup in their crust this would likely change the entire spectrum of how those molecules mix and match. Just a tiny bit more of an already semi-rare element on the surface like ammonia or boron (to pick two random ones) might change everything.
There are many foods that are poisonous even to most native species on Earth yet palatable to others. This same concept would be taken to an even greater extreme with aliens. Their biology is likely to be very different from ours.
So the good news is they don’t want to eat us or our tasty cows, chickens, pigs, fruits, vegetables, beans, nuts, or coffee. That is also the bad news. This means they have no need for most forms of life on our planet.
The plan should ideally be done without humanity realizing it is under attack. If they have the chance to irradiate their planet it can make it uninhabitable by any life even their own.
This answer makes a few assumptions.
- The aliens have been traveling a very long time and can potentially survive in deep space for long periods of time. Even at light speed, travel would take many years and they are likely going much slower than that..
- The aliens have learned how to mine and manipulate asteroids to build the colossal ships and fuel needed to travel here.
- The aliens have different biologies to ours and cannot eat our food nor can they catch nearly any of our diseases.
- They are here to colonize the Earth because it is an ideal Garden World in the of its star and has similar gravity to their homeworld.
They want our planet for colonization because it is a rocky body, has a good gravity, is in the zone where liquid water exists in all three states and we have a large moon to help deter meteor impacts. In real estate the three most important things are location, location and location. This goes doubly so in space. They don’t want us or our plants and food, they want the water, air, sun and the land.
Stage one: Kill the most dangerous species, humans.
- Silently take samples of many lifeforms and micro organisms. Small robotic probes can do this without being detected. They could even be as small as an insect.
- Engineered viruses can be created to wipe out most every human on Earth. Remember that amino acid thing? That also means the virus is not likely to have any effect on them if they design it right. While using a bio weapon on another nation would be suicide as you can’t stop the spread, due to their different body chemistry this is not an issue.
- Similar engineered viruses that target key organisms in the food chain could kill most other life. Primary targets could be plankton, bee’s and livestock. Without plankton most of the ocean dies, without pollinators like bees the land animals die. The only reason to kill livestock like pigs and cows is to speed the death of humans.
- As the population dwindles and becomes less stable, wars may break out. Some small numbers will turtle in bunkers and secure facilities that limit the chance of exposure and many weapon systems can be remotely activated. Sophisticated weaponry would need to be ready to place in orbit to destroy any attempts to launch or use nuclear or chemical weapons by the native populace. There is not a high chance of this but preparations need to be made. Radioactive contamination is going to be dangerous to all carbon based lifeforms, even the aliens.
Stage Two: Clean the slate.
- Capture 3 large meteorites and send them on a collision path with the Earth that will take less than 1-3 years to reach the planet. One will be stony and 20-50 km the other two will be 10-20 km and iron-nickel based. Place the two iron-nickel meteorites on a different orbit that will miss the Earth but can be easily corrected to be an impact using thrusters similar to what the ship uses. The larger stony asteroid will be sent directly for the Earth. Ideally, the stony meteor will take less than a year to reach the Earth at speeds just under what the humans would find suspicious.
- This can be done concurrently with the epidemic. While the epidemic is at work there will be too much chaos to counter the impact. Most conservative theories think we would need as much as 3 years to a decade of lead time on a potential large impact to sufficiently stop it. We do not even have any technologies for this scenario prepared beyond pure theoretical concepts.
- When the first asteroid nears the Earth use controlled detonations to break it into smaller pieces which can each strike different regions or continents. 10-20 pieces should do the job. This will eliminate almost all remaining humans and much of the ecosystem.
- Place the second meteorite on a final approach for the Earth. This is the planet killer. Big enough and solid enough to send ejecta (earth, water,) into the sky and cause an but not so big that the Earth’s water is ejected into orbit or the crust is melted. The impact should be a water landing in a fairly deep area to spread the ejecta via steam and water vapor, and reduce the chance of melting the crust. The impact winter will eliminate the majority of life reliant on photosynthesis.
- Repeat step 4 again with the third meteor one year later. At the end of one year most of the ejecta will have returned to the ground and sun levels will rise again.
- Continue repeating this process annually until all photosynthetic reliant life dies out. This will kill all plants, and most animals.
Stage Three: Cleanup and xenoform.
- During the ice age a targeted sterilization program can be fully begun. Any remaining humans in deep bunkers will be found and destroyed. Any nuclear weapons will be collected or dismantled.
- Specific programs will begin that will destroy any hardy or resistant life forms that manage to survive the Impact Winter.
- Those slightly different mixtures of surface materials will be utilized during this stage. Any materials found on the homeworld in greater concentrations will be mined from asteroids and salted on the Earth during the violent storms of the ice age. Chemical compounds will be produced to sequester any chemicals more common on Earth that could be difficult for the homeworld flora and fauna.
- After the last impact winter, begin producing greenhouses to cultivate the seeds native to your homeworld. This will be a massive project and the seeds will be raised in ideal conditions for life on your planet. These will be expanded as needed. These can provide enough food to feed the seeded population.
- A seed population of under 100-200 could fully populate the Earth by using frozen embryos and sperm equivalents from 10-30,000 or more individuals. This would prevent inbreeding and create a viable genetic diversity in a very small space compared to carrying 5-10,000 breeding age pairs alive through the trip. 5-10,000 is the minimum viable genetic diversity theoretically feasible based on Earth creatures.
- Xenoform the environment making any slight changes that are needed to support native life without greenhouses.
- This should carry you through for many, many years until you need to leave again.
Wash, Rinse, Repeat with several hundred vessels each carrying 100-200 colonists and 10-30,000 fertilization samples. In 100,000 years your species could consecutively spread to a large portion of the galaxy by this method.