OK, so that novel I'm reading: it's A Sword Into Darkness.
Opening premise: an alien spaceship is on the way, and they didn't call ahead. Their intentions are presumably inimical (else they would have sent radio signals first), and we have only 33 years to respond!
Apparently the aliens' motivations will be revealed later in the book, and will be plausible for their level of technology and the trouble and expense involved in launching such a mission.
But meanwhile, what can I think of?
Well, obviously, they could have discovered that their sun was getting all wibbly-wobbly, and their solar system would soon become uninhabitable. Having decades to respond, they have time for more than "toss some random baby in a tiny probe and send him off to Earth", and embark on a Strangelovian project of packing their top politicians, mad scientists, and supermodels into an interstellar life-pod. (Shades of the Golgafrincham "B" Ark!)
Or - and this was inspired by one of the homophone errors that got past the proofreader of this book - it could be some corporation sending a vast shipload of its assets out-system to avoid home-system taxes.
No? So how about this one: their total civilizational debt has grown to the point where it makes sense to send a hugely expensive expedition across the expanses of space to peddle their government bonds here.
What about: they departed around... oh, never mind. 1937. I was going to suggest that they were pissed off when I Love Lucy went off the air. Or My Favorite Martian. What popular radio show was cancelled in 1916? (Gotta allow 20 years for the next episode to fail to reach them, and a year for them to build and launch their invasion fleet.)
They're finally getting around to checking on their colony here, left during their long-ago Age of Exploration?
(I understand there are space battles yet to come, so none of the explanations I'm coming up with really fit. Whatever. It's fun coming up with them.)
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