It's a fun thought experiment. I've had a similar one, imagining an alternate cold war with the divergent point being the mid 60's Mariner probe visit to Mars revealed canals, cities, vegetation, and a shallow ocean.
A city on Ceres would make NASA's budgetary woes go away instantly. Either there would be a new space race, or the various spacefaring nations would quickly make an arrangement for a multinational crew to visit, with each nation (or group like the EU) reserving the right to make claims later. I'm not sure how the public would react. I think they'd be glued to the news, but there wouldn't be any sort of panic, since both science and science fiction has led many people to expect finding this sort of thing one day.
Of course, it's worth considering that if there was an inhabited alien city on Ceres, the chances of them not knowing we were here would be nil.
So, alternative (a): they know we're here and for some reason don't want us to know about them.
Alternative (b): they know we're here but either don't care / are too alien to want to communicate.
Alternative (c): the city is uninhabited.
I would say that the most immediate reaction on earth would be shouting, initially at each other, but very quickly people would be shouting at Ceres --- everyone who can do some basic electronics and get hold of a satellite dish will be making transmissions. For alternatives (b) and (c) above, we wouldn't get a reply. For (a), well...
I hadn't thought of the aliens actually been indigenous to Ceres. It's an interesting idea. They would most likely have evolved in the water/ice layer, and be based on weird and exotic ice chemistry. That would make the city their equivalent of a lunar outpost, and it's quite plausible that they simply may not have noticed electromagnetic waves.
The chances of talking to them would probably be quite slim --- we would be appallingly alen to each other.
But it's possible that they've lost some of their technology. Or that it's a punitive outpost - a space-jail, where they work off their debt to their society by mining, or performing astronomical observations, or something else.
If you're a space-travelling human-like species, Ceres is pretty hospitable. Lots of water, lots of rock, enough gravity to keep things down while still being easy to get off of, a manageable temperature, and it's in the asteroid belt so there's plenty of nearby materiel for development (not to mention Jupiter not far away, energetically).
Earth's too hot, too far away from anywhere, and at the bottom of a great big hole. The only reason we like it is because we've adapted to its environment. It's not very good real estate. Plus, it's contaminated with life, which means lots of big, complex, wriggly molecules that get everywhere and fiddle with things. Get those in your life system and the alien equivalent of anaphylactic shock is probably getting off lightly.
Depending on where they came from, it may require less Δv to get to Ceres than to Earth. Or who knows, maybe it's an automated surveillance platform set up to monitor the Earth?
> I've had a similar one, imagining an alternate cold war with the divergent point being the mid 60's Mariner probe visit to Mars revealed canals, cities, vegetation, and a shallow ocean.
A city on Ceres would make NASA's budgetary woes go away instantly. Either there would be a new space race, or the various spacefaring nations would quickly make an arrangement for a multinational crew to visit, with each nation (or group like the EU) reserving the right to make claims later. I'm not sure how the public would react. I think they'd be glued to the news, but there wouldn't be any sort of panic, since both science and science fiction has led many people to expect finding this sort of thing one day.