I always thought that was part of his underwear not a belt,
Okay . . . it is a "belt" . . . and although there are plenty of jokes and concerns about just what it is that Astro is wearing, they are supposed to be shorts, NOT UNDERWEAR.
Look at how in most images of him, he is "shiny" which indicates he is made of metal (and therefore, his "outfit" is painted right onto his body).
Yes, there are numerous gags (jokes) over the years - and by Tezuka himself - that implied Astro's shorts WERE removable . . . but the truth is: they are not.
This factoid was also reinforced in the 2009 movie when Tenma "created" Astro's body - the boots and shorts (and belt) materialized along with the rest of him.
The only real deviation from "everything" being painted on was in the 1980's anime where Cathy actually gave Astro her boots to wear. This was (IMHO) a flawed decision, because there is NO WAY those boots could withstand the jetfire and punishment Astro would have put them through (but that's for another thread!

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Hope that helps resolve the confusion . . . I do like the "Mood Ring" answer though!
