The most common and most likely answer to this crossword clue is the 5 letter word FLYBY
I've seen this crossword clue in Independent
F
L
Y
B
Y
More clues leading to the result FLYBY
Buddy explains!

The straight part
We know that "space mission" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case space mission = flyby.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does FLYBY mean?
FLYBY noun- A flight past a celestial object in order to make observations.
- A low-level ceremonial flight, typically in connection with an airshow or a military parade.
- A brief visit.