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.
Meaning: The Valiant
, soon after launch, was caught in one, and found itself at the galactic edge.
I mean, obviously it couldn't have gotten that far under its own power, at a maximum speed of Warp 1 or 2. So therefore some extra intervention would be necessary.