> it's not like other nations (the only ones with power to transmit that far, back then) would send rogue commands without getting caught.
How would you catch them?
Also, these spacecraft didn't start off outside the solar system. They weren't always so far away that a lone prankster would have trouble sending them messages.
The antennas used to send signals to probes are very directional. Monitoring the uplink frequency wouldn't detect someone else sending commands to the probe unless the monitoring receiver was very close to the transmitting antenna or within the transmitting antenna's beam.
> Wouldn't they be monitoring whatever frequency it's using?
How would they? They have a huge directional dish antenna for communicating with the probe, they can't intercept every signal on 8 GHz, they would need an omnidirectional antenna which would catch a lot of noise.
How would you catch them?
Also, these spacecraft didn't start off outside the solar system. They weren't always so far away that a lone prankster would have trouble sending them messages.