Bowie is renowned for regurgitating everything (sooner or later), and this includes fictional characters like poor old Major Tom.
I believe that with Ashes To Ashes, Bowie chose Tom, because Tom was an astronaut, an astronaut is an ultra fit ultra healthy ultra squeeky clean living character, an American institution in an age where the Space Program had to be sold to the public in an appealing way. So taking that character and turning him into a useless junkie makes for a very powerful statement. I recall a quote from someone influential in the late sixties heralding in Bowie as a new kind of musician who wouldnt be tainted by drug abuse or sexual excesses.
In reply to:
Space Oddity, his first hit, fits perfectly into that place on the album, when he first becomes aware of and awestruck at this new celebrity (who is him)...
I agree although Bowie wasnt a star then, but perhaps he could almost taste it...."And the stars look very different todaaaaayyy". The astronaut was a new kind of celebrity in 1969, so Bowie draws some comparisons.