Answer: Light travels really well through space. If you put a telescope outside of the Earth’s atmosphere, you can see objects for millions of miles. So it’s easy for us to map what’s out there.
Light does not travel well through water. We can’t see very far down from the surface, and probes sent deep underwater don’t have a lot of light to work. If they have a light, it only reveals a few meters around them. They can use sonar, but that also only travels a limited distance and gives limited detail. So you need a lot more probes to see a smaller area.
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