Space traveller not quite awake (5)

I believe the answer is:

comet

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'space traveller' is the definition.
(object travelling through space)

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for comet that I've seen before include "Celestial object" , "Plane" , "One is named after Edmond Halley" , "Halley's space traveller?" , "Rudolph's companion" .)

I've seen this clue in The Sun.

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