All the aliens turn up at once, drifting aimlessly around the solar system like a bunch of early party arrivees. They can’t be seen or detected from by any Earth-based observatory. They are cloaked somehow.
An ore freighter en route from Titan to the settlement on Ganymede runs into an alien vessel. Literally. It decloaks, dwarfing the megaton freighter. The Earth ship backs up to take a proper look. The alien ship is a smooth grey cube five hundred miles on a side.
earth ship: we come in peace.
Minutes pass. Finally, a small red light appears near the centre of the surface closest to the freighter.
alien vessel: go away.
earth ship: repeat message. we come in peace.
alien vessel: look, just go. Seriously.
earth ship: request clarification of previous message.
alien vessel: are you serious? piss off!
earth ship: er…why?
alien vessel: you aren’t meant to be able to see us. if the cops notice you hanging around like a bad smell we’ll have our invitation revoked. Now shoo, before we subtract the intrinsic field from your [untranslatable] fusion device.
A pair of idential alien anthropologist tourists in human form, utterly imperturbable and indestructable, insist on involving themselves in a series of major earth-based conflicts and catastrophes. Althrough their profession insists they are ostensibly uninvolved, they are somehow always affecting the outcome of events.
humans: You’re clearly aliens. Why do you look like us? Was there an prehistorical diaspora of stone-age spacemen? Have earth people been breeding in space? Or did you abduct us and steal our DNA?
visiting alien anthropologists: Uh, neither. We had these meatstructures made so we could come down and hang out in your atmosphere. You know, blend in and check out the joint.
humans: But you’re over ten feet tall!
vaa: Ah, that’s just a scaling problem. Maybe we can shave a bit off here and there.
humans: And those, uh, gills? You know, on your neck?
vaa: Ventilation. You got any idea how hot it gets inside one of these?