I got out the four main books today to verify this thread, and it looks pretty clear-cut:
QUOTE (SR1, p89)
Magical things (spirits, focuses, spells, some magical creatures, ritual teams, magicians using astral projection or perception, and so on) are living entities in astral space. They are visible, corporeal, and capable of action in Astral Space. Beings in Astral Space can see physical objects and assense magical energy given off by the astral forms of all living things. Assensing is the sixth sense that perceives magical energy and astral forms.
Nonmagical living things have dormant astral forms that are visible and corporeal in Astral Space but do not act on other astral forms. The Earth is a living thing in magical terms, and thus corporeal in Astral Space. Vegetation also blocks movement in the same way it does int he physical world. Water, air, and fire are also tangible in Astral Space, but astral beings can pass through them because of their low density. Water and fire do no harm to the astral traveler, but they do reduce his range of assensing.
QUOTE (SR2, p145)
Magical things (spirits, focuses, spells, some magical creatures, ritual teams, magicians using astral projection or perception, and so on) are living entities in astral space. They are visible, corporeal, and capable of action while there. Beings in Astral Space can see physical objects and assense magical energy given off by the astral forms of all living things. Assensing is the sixth sense that perceives magical energy and astral forms.
Nonmagical living things have a dormant astral form, an aura that is visible and corporeal in astral space but does not act on other astral forms. The Earth is a living thing in magical terms, and thus corporeal in astral space. Vegetation, for example, blocks movement just as it does in the physical world. Water, air, and fire are also tangible in astral space, but astral beings can pass through these elements because of their low density. Water and fire do no harm to the astral traveler, but do reduce his range of assensing.
SR1 and SR2 are basically identical, with the exception of some copy editing. Note that, contrary to what I said earlier, the Earth has been solid since SR1! [However, see below.]
QUOTE (SR3, p173)
Astral forms are unaffected by the physical world, allowing you to move through solid objects without slowing your movement. Only astral forms can slow or affect another astral form. The earth is solid on the astral plane, just as it is in the physical world; astral forms cannot pass through it.
As Method points out, Magic in the Shadows provides rules for moving through it anyway, ruling it would just be really, really slow. I wanted to check Street Magic - to make sure Bull's trick still works in SR4 - but I don't have a copy.
QUOTE (SR4, p183-184)
Astral forms are unaffected by the physical world; non-living objects that are solid in the physical world offer no resistance to astral forms and allow a projecting magician to pass freely. Only astral forms can slow or affect another astral form. The earth is solid on the astral plane, just as it is in the physical world; astral forms cannot pass through it.
So SR4 uses the same rules as SR3, again with some copy editing; the rule itself hasn't changed, which is a relief given my familiarity with astral travel in SR3. I can only relearn just so much!