Rishathra, actually.

This is an interesting idea. If neither biologies used the same fuel molecules then they wouldn't compete for resources, but perhaps they would compete for space? But then if both biologies were that different from each other would they be able to even live in the same environment?

Sometimes I think about how so many of us look up at the stars and wonder "if there really are aliens out there, why aren't they colonizing the galaxy as fast as possible, as any intelligent species would naturally do?" like it's the thing just anyone looking at the stars might think. we might be the horrifying biomechanical paperclip maximizer that the other aliens in the galaxy have to band together to defeat or face extermination.

Yep i would expect them to send the blankets down from orbit first.

intergalactic tour guide: now if you look to your left, you’ll see the natural habitats of the Xpheno217 species. This is the only location in the whole universe they can live. And to your right, a brand new residential community fit with Walmart and their very own Chick-fil-A.

I'm sorry. I can't let you do that.

And they’ll bring that shit home too.

Of course, why would you leave your new significant other in outerspace?

Ah, drat! My one weakness! Claws! Oh, and teeth. So two weaknesses.

Oh, and spikes.

Oh, and fire.

Pointed rocks.

Long falls off of cliffs.

Ok, I have many weaknesses!

I heard on the radio that bacteria is what killed the Martians

And risk wiping the life on that planet? Considering something like that happened to native population in America, I guess people don't care.

I'm a microbiologist. I can speak from experience (my grad research required attempting this a few times) that entirely sterilizing anything of microbes is incredibly difficult regardless of technology level. They are tenacious little fuckers. I'll lay this out for anyone interested.

Gotta Kill 'Em All: Most microbes are fairly easy to kill using simple physical and/or chemical means. Some are more difficult, like spore formers, bacteria that produce little personal suspension pods when conditions are rough.

What matters is you start with huge quantities of microbes, they're everywhere, and you can't see them. All you need is one to survive to potentially reproduce into vast legions of descendants. Even NASA's protocol is about lowering the total number, thereby reducing, not eliminating, the probability of causing an issue. Miss the wrong microbe in the wrong environment and you've inoculated a planet.

Checking Your Work: How do you verify that you successfully sterilized your tool? You might say culturing - swab it and grow that on some type(s) of media. That's NASA's protocol! It's just not very effective.

Not all microbes grow on all media. There are an estimated one trillion microbial species on the planet and we only know how to culture less than about 0.5% of them. The rest are a mystery, largely uncharacterized*. Most sterility testing is for known microbes of consequence, not every microbe in existence.

Microbiology is very often a science of slapping your tool or workspace and exclaiming "good enough!", not absolute precision and 100% efficacy, both of which are practically required if you want to be sure you don't inadvertently pull a "smallpox blankets from space".

*Fun fact: Sometimes people get sick with something atypical, that doesn't get IDed through standard testing. I worked for a time identifying these pathogens via gene sequencing. There was a whole lot of "that's a new one" out there.

You don't need FTL travel or multi-generational ships really. If you get close enough to the speed of light, time dilation can make the travel time for the people on the ship be as short as you want. Only for the people that stay on earth would the spaceship appear to take generations. Getting enough energy to reach such high speeds would be the difficult part as well as accelerating quickly without crushing the travellers.

FTL travel implies a mastery over spacetime itself. After that, a practical time machine is a matter of engineering details.

We can be sure that we never get there because we are not flooded with tourists from the future. QED.

Why would your first act on alien life be to launch our vilest weapons of mass destruction?