October 2017: Astronomers in Hawaii discover an extrasolar object shortly after it’s slingshotted around the Sun. They name it ʻOumuamua, which roughly translates to “first distant messenger” or maybe “advance scout”.
June 2018: ʻOumuamua’s trajectory changed without any visible reason for it to do so. There’s probably a mundane explanation. It’s probably not a course adjustment on its way to the next destination. It’s probably not a deliberate wiggle to acknowledge that we were watching it and talking about it. There’s probably a perfectly fine reason for it being ten times shinier than the average comet, and there’s probably a perfectly sensible reason that its slow trajectory change was consistent with what a light sail would create rather than the abrupt change of a breakup or outgassing. There’s probably explanations for all the other interesting facts the chair of the Harvard astronomy department lists in this article, too. It’s probably not aliens.
January 2019: During a lunar eclipse, when humanity is paying a lot more attention to the moon than normal, something flashes on it near the middle of the eclipse. “Just a beachball-sized meteorite”, astronomers are currently saying. “Happens all the time, we just can’t see it when the sun’s on the moon.”
It’s probably just something mundane. Probably something boring. Probably not the work of an extrasolar probe that either knows it’s been seen, or doesn’t care if it is. It’s probably not something dropped on the moon by an alien probe.
Four days later, astronomers (again in Hawaii, with followup observations in London) notice a very low-mass object taking a highly erratic path around the earth outside the Moon’s orbit. Kinda like a trash bag blown in the wind. Looks really light for its size. It’s probably just a piece of junk that fell off of one of our satellites or something. Probably. It’s totally just a coincidence that the Popular Science article about this that I’m linking to compares it to the mechanics behind solar sails. It’s probably not some kind of exploration package ‘Oumuamua dropped off when it zoomed through the “Goldilocks zone” of our solar system. It’ll probably fall into our atmosphere and burn up, or get knocked out of anything resembling Earth orbit in a couple weeks.
I’m probably just stoned and telling myself a story.
It sure works as the opening act of a first contact story, though. In the tiny possibility that it is one, I hope it’s more “The Day The Earth Stood Still” than “The War Against The Chtorr”.