Mr. Laser Beam wrote:
But note that the ship "disappeared" almost immediately after launch. This would seem to suggest that Earth had a way to keep in contact with the ship until the disappearance.
Most likely explanation for that: Wormhole.
Another explaination could be leaving messages (sometimes for years) at various planets. When other Earth ships arrived, the message was pasted on, a lawyer or some such.
Or using interconnecting ships (like transports) to simply sending mail home.