Got it! I think I was not yet aware of the fact that some locative verbs take a destination as a direct object - in which case, (1) we require a transitive verb prefix, and (2) the -Daq suffix is generally not used on the direct object. It's something quite different to learn to see these locations as direct objects rather than following a preposition which may require accusative, prepositional, dative, or even genitive case depending on the preposition, as happens with many languages of Indo-European origin. Oh, those silly Terrans! :-)
In fact, it seems that most of the verbs of motion that we've learned thus far that can take a destination DO in fact take it as a direct object, with the exception of yIt and qet. Is that correct, or are there other (common) verbs of motion that are exceptions to this rule?
Why was "We are traveling to outer space" marked as incorrect?
I'm on the planet. I launch. I'm still in the atmosphere; I'm traveling to outer space. Mousing over 'loghDaq' it very specifically says that 'to' is a possible interpretation; this should be a reasonable translation.
(This is effectively the same answer David has given, but I'm going to approach the answer differently - we are saying the same thing in different ways.) -Daq can be translated as "to" or "in" depending on context (and in some contexts even, "at" or "by"). The context that determines it in this case is the prefix on the verb leng. leng is one of those verbs that actually takes the destination as its object. Since the verb prefix in this case is ma- (we/no object), then the destination is non-existent or at least not defined and we can't be going "to" somewhere. So the locative marked with -Daq must instead be the location at which the action happens, so "in space".
Certain verbs include a locative sense to them. leng is one of these. It means that the object of leng is automatically locative in a to sense. Any other locative noun added before the object cannot include the to sense. loghDaq malengtaH means only We are traveling in space. To say We are traveling to space, say logh wIlengtaH.
This property of inherently locative verbs is explained in The Klingon Dictionary and further described by Okrand here: http://klingonska.org/canon/1998-12-holqed-07-4.txt
I wrote "We are traveling into space" and it was marked incorrect. I don't think there's a grammatical problem here. Even with space-faring civilizations, I think it's generally considered that you have to leave all the various spheres of a planet before you can proceed into outer space. Couldn't this translation work in the context of leaving a planet and traveling into space?
To add to what David has said...
The object of leng is the destination and can be marked with -Daq or not. An additional example sentence, DujDaq vIleng would most likely be understood as "I travel to the ship."
"We are traveling into space" could be either logh wIlengtaH or loghDaq wIlengtaH. But pay special attention to the prefix on leng. loghDaq wIlengtaH might mean "we are traveling into space" or if there is some other destination we know of from context, it could instead mean "we are traveling to it in space". However, loghDaq malengtaH can ONLY mean "we are traveling in (through) space" since the ma- prefix tells us that there is no object and thus, no stated (or implied) destination.
Some nouns take objects that include a locative sense. leng is one of these: The object is the destination. When you use such a verb, any other locative noun you use can't be destinations.
I travel to the ship.
I travel on the ship.
DujDaq yuQ vIleng
I travel to the planet aboard the ship.