One of the most fun memes in science fiction is where aliens from an advanced civilization invade Earth. Critics pan these stories because they seem so implausible. After all, if the space aliens can travel across the galaxy, why do they need to attack humanity?
And it’s not like a nuclear war on Earth is a threat to the rest of the galaxy. Every minute, our sun releases more energy than any nuclear war on Earth would release. It’s a fair bet that the Milky Way galaxy’s hundreds of billions of stars are releasing far more energy every second.
Humankind is hardly a threat to the universe. We can’t even break out of our Solar System, much less invade other star systems. The only way space aliens would care about us as a threat is if they live in the far future and decide to come back in time and eliminate us before we become a real threat.
Adapting to Space May Not Be that Easy
We’ve only experimented with space travel for a few decades. The first orbital rocket circled the Earth in the 1950s. Human space travel is less than 65 years old. And to date we’ve only set foot on the Moon, which also orbits the Earth.
Our space probes have visited every planet in the Solar System – and I’m including Pluto in that list. We’ve even sent two probes to the very edge of interstellar space. But neither of those probes is likely to ever encounter a space-faring alien civilization.
The astronauts who serve on the International Space Station return to Earth with weakened muscles and skeletal systems, and possibly compromised immune systems.
We can’t grow food in space, yet. We don’t have proper shielding against Solar radiation or Cosmic radiation. And even though people are planning to colonize Mars, we don’t yet know how they will survive there.
Assuming we keep building bigger and better spaceships, we’ll eventually solve many of these problems. But how will we survive in space for the long term?
It’s not clear that humans will ever adapt to space. And if we can’t do it, then how could aliens on another planet in another star system do it?
They Don’t Need Our Water or Air
Assuming a space-faring civilization finds its way to the Solar System, they won’t stop by Earth for a drink. The Oort Cloud is filled with icy comets and planetoids. They’ll find far more water out there – where it’s easy to harvest and undefended – than they’ll get from Earth.
And if they can mine the Oort Cloud for water then they can mine it for the chemicals they’d need to produce an Earth-like atmosphere. So our hypothetical space aliens should be able to replenish their air and water at just about any star system they visit without starting a war.
The Most Likely Reason to Invade Earth
If space-faring aliens decide to attack Earth, it will be because they need the planet itself. Earth supports a huge amount of life. It will enjoy sufficient sunlight for hundreds of millions more years before it begins to lose its oceans and atmosphere.
It might be less expensive and time-consuming to steal another civilization’s homeworld than to find a planet that can be terraformed in sufficient time to colonize an empty world.
The terraforming process is another popular science fiction meme. But people don’t think about the amount of energy terraforming requires. It’s easy to say you might be able to terraform Mars in 1,000 years – but who’s going to live that long to ensure the program is finished? No human engineering project to date has lasted more than a few hundred years at most.
If we can develop starships capable of taking us to other suns in the galaxy, we may find that the only suitable worlds are already inhabited by species like us. Maybe we can colonize some of their moons – with their help – but sooner or later humanity may decide it needs to steal other species’ worlds.
It remains to be seen whether a space-faring civilization can advance to the point where it’s capable of building permanent habitats or terraforming worlds for less cost (in time and energy) than would be required to seize an inhabited planet.
I think if space aliens are investigating Earth, it may be they are house-hunting. We might not have the right environment for most of them. But sooner or later – if conquering alien planets is the least risky way to expand into the galaxy – science fiction stories about invading aliens may be on the right track.
Conclusion
I’m not saying we should plan on attacking the rest of the galaxy. We can’t even attack the Moon, yet.
All I’m saying is that we’d better be sure we can’t build our own new worlds as we need them before we start exploring the stars. We may need to be the aggressors, because if other space-faring civilizations already know they cannot build the new worlds they need, they’ll be waiting for newcomers to show up. And then they can follow them home and seize their worlds.
That’s a meme yet to be explored in science fiction.