Investigate deep-space mission (5)
I believe the answer is:
probe
This clue is a double definition.
'investigate' is the first definition.
(probing is a kind of investigating)
'deep-space mission' is the second definition.
(I've seen this in another clue)
(Other definitions for probe that I've seen before include "Question or examine thoroughly" , "Question or examine closely" , "Intrusive device" , "Explore, examine" , "Inquiry" .)