Latest spacecraft crossing edge of exosphere on time (8)

I believe the answer is:

tardiest

I'm a little stuck... Click here to teach me more about this clue!

'latest' is the definition.
('tardy' can be a synonym of 'late')

'spacecraft crossing edge of exosphere on time' is the wordplay.
I cannot quite understand how this works, but
'time' could be 'est' (EST is an example) and 'est' is present in the answer.
This may be the basis of clue (or it may be nonsense).

Can you help me to learn more?

(Another definition for tardiest that I've seen is " Striated in latest manner".)

I've seen this clue in The Times.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.