When they get on board Thunderbird 3 after the rescue, Buddy Pendergast flips up his restraints and says "I told you our adventure wasn't over" but in the next frame the restraints are down again.
It would take more than an hour for Brains' response to reach Alan on Europa.
It took the Voyager 1 and 2 probes, two years to reach Jupiter. Although no time is specified, it seems to take Thunderbird 3 just a few hours to reach the Jovian system.