There was plenty of “space” imagery at the time, to whit, Ground control to Major Tom, I’m a space cowboy, “Cloud Nine”, not to mention the generic and at the time ubiquitous word “spacey” for just about any dazed and confused condition. AND, virtually any lyrics at the time were apt to be interpreted by just about anybody as being about drugs. However, the balance, IMHO, seems to be in Dave’s favor that the lyrics are not overtly about drugs. There was probably some subtle interplay between heroin experiences, love experiences, and the general slang lexicon. Only the artist knows for sure, and then he may not remember. ;-) John Lennon denied that Lucy in the Sky with Diamonds was “about” LSD, so this sort of thing can never really be settled.
Coleridge was supposed to have written Kubla Khan, or, a Vision in a Dream, a Fragment after wakening from a laudanum-induced sleep during which he composed an entire ouvre of great merit. The problem was he forgot most of it as he furiously scribbled it down on paper—hence “a fragment”. The author of the lyrics to a popular song in the 40’s (can’t remember but it was on Fresh Air with Terri Gross) was a seriously heavy drinker and only composed under the influence of alcohol, and at that usually in his sleep! He kept a tablet by his bed so he could write down anything that occurred to him, lyrically, during the night. One night he woke up and couldn’t find the tablet so wrote the lyrics on the hotel bedsheet and promptly went to sleep. In the morning he rushed to the studio, tablet in hand and only then remembered where he had written it. He then rushed back to the hotel but the linens had already been taken up and sent to the laundry, so he went there, producer in tow, and retrieved them. The story has all the earmarks of apocrypha, but apocrypha related on numerous occasions to his nieces and nephews, drink in hand, by the author himself. His nephew wrote a book about him.
Poetry and lyrics are designed to be unreliable, however suggestive, and there have been a lot of poets and authors with a habit of one kind or another.
(In support of venemousbede, I have little doubt that in some way, shape, or form, the lyrics are influenced by the author’s heroin use. The problem is that it’s like saying “Box of Rain” refers to acid because the Grateful Dead were known to be major LSD users. It could be true.)