The Science of The Callisto Incident
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The distance between Europa and Ganymede, even at closest approach, is nearly 400,000 kilometers: this corresponds to a round-trip lightspeed delay of 2.5 seconds. That's best case: when the moons are opposed this can be over 11 seconds. If you've ever had trouble on a satellite call with less than a second delay, you can imagine the frustration of holding inter-moon conversations in the Jovian system.