Learn Portuguese (Portugal): Logo que tiver chegado à órbita, o astronauta dormirá um bocado. - As soon as he has arrived in orbit, the astronaut will sleep a little.

The phrase tiver chegado is the Future Perfect Subjunctive (Futuro Composto do Conjuntivo). We use it because the action of arriving in orbit will be completely finished before the next future action (sleeping) begins. While chegar means "arrives", tiver chegado gives the exact meaning of "has arrived" or "will have arrived".

Does the expression logo que always require the future subjunctive?

Not always, but it does when you are talking about an event that hasn't happened yet. Because the astronaut hasn't arrived in orbit yet, logo que (as soon as) triggers the subjunctive. If you were talking about a past event, like "As soon as he arrived, he slept", you would use the indicative.

Why do we use the preposition a in à órbita instead of em?

In Portuguese, the verb chegar (to arrive) requires the preposition a to indicate the destination, whereas English uses "in" or "at". Here, the preposition a combines with the feminine article a (for órbita) to form the contraction à.

Is um bocado the same as um pouco?