With meta-humanity moving out into space how many people would need to live on the Moon or even Mars to create an environment where magic could occur? Is the manifestation of a Giasphere dependent on life only or is there something else in the equation? If it did manifest would it be a toxic environment or the equivalent do to its artificial nature?

Doesn't the Earth have an astral form? If something like that existed on other planets, maybe it would work.

Not many. See the fiction in Street Magic p.111. "a greenhouse full of exotic plants" is sufficient.

I can't really say myself. But one problem I spotted a while back is that we better damn well make sure never to let a single spore of FAB escape our atmosphere. If it ever got a taste of the mana warp in space, it would explode into a veritable plague (since it apparently 'eats' background counts as if it were candy).

Nah lets fill the void with FAB then develop FAB powered space craft....

Street Magic, page 119. Space is a void, not a warp (big difference). BGC is at least -7 in space, and FAB won't be a problem.

Also, FAB III can only feed off BC for "extended periods at a Force equal to the background count" (Street Magic p.127)

It's a fascinating concept from a literary point of view. If the gaiasphere is unique, would the spheres of other worlds be unique as well? And would that uniqueness come with a whole new set of rules (i.e., would the rules for using magic be different)? Lots of possibilities... could the Horrors be lured to, say, Jupiter, and abandoned there leaving the Earth safe?

You should join the Shadowrun Space Technology thread. We are working on answering these exact questions for fun and profit (well, not profit). We were discussing the use of a "Garden of Eden" ship as a way to provide enough biomass so mages could travel in space with only a massive amount of danger as opposed to a death sentence. It's also a directly relevant question for ideas like a Space Prison to keep naughty mages in. How long until the population of mages allows them to work magic together? How many mages would die in a ritual they cast together to affect the prison? Ooooooh the possibilities make me swoon.