I suspect that your memory may be mixing together more than one story, because part of what you are describing is a very close match with The Lotus Caves by John Christopher (an Englishman), published in 1969.
Per the plot summary on Wikipedia:
Two teenage boys, Marty and Steve, live in a colony on the Moon, "The Bubble", in the year 2068. Exploring outside the dome of "The Bubble" is strictly controlled. The boys grow bored and decide to borrow a lunar vehicle. They discover someone has forgot to remove their key, which makes it possible for them to explore beyond proscribed boundaries without restriction.
The key gives them the ability to explore freely on the moon, whereas normally children and other pleasure riders would only be able to take the crawlers a few mines from the Bubble. Once they get outside that range, the pair become embroiled in the main plot, involving an alien plant that has ensconced itself in a set of caves.
Here are the early covers:
What does not match is their adventures taking place on an interstellar scale. Your memories of that part of the story may actually be from another source.