The most common and most likely answer to this crossword clue is the 9 letter word ASTEROIDS
I've seen this crossword clue in The Times
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More clues leading to the result ASTEROIDS
Buddy explains!
The straight part
We know that "travellers in space" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case travellers in space = asteroids.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 ASTEROIDS mean?
ASTEROIDS noun- Any member of the taxonomic class Asteroidea; a starfish
- A naturally occurring solid object, which is smaller than a planet and is not a comet, that orbits a star
- In the Solar system, such a body that orbits within the orbit of Jupiter