Just wondering if ASFs have the ability to break atmo, whether its from the ground into space or vice versa. I can't seem to find a source that gives me an answer so I'm hoping one of you out there knows.
Yes, aerospace fighters can leave and enter the atmosphere. Just about any novel featuring aerospace fighters depicts them in both air and space.
The current rules addressing this are in Total Warfare. See Aerospace Movement, pg74-93. Specifically:
pg76 details Space Movement, with the Movement Subphases section naming Fighters.
pg79 details High Altitude movement (atmospheric movement on the space map), noting which aerospace units are banned from moving above row 1 (including conventional fighters and fixed wing support vehicles), while airships and VTOLs are banned from High Altitude operations. This leaves other aerospace units (defined on pg74) to maneuver freely on the high altitude map. Yes, that definition includes aerospace fighters.
pg78-79 details crossing the "interface" row of the high altitude map, or as you put it "break atmo," though the interface is really the thinnest and least obtrusive part of the atmosphere.
So, there you go. The rules for getting aerospace fighters into and out of the atmosphere.
For supporting fluff, read any fighter TRO. TW pg71-73 has a nice piece of fiction of an aerospace fighter operating in space. pg178-178 Tech Manual describes aerospace fighter technology and their dual atmosphere/space operations.
