The difference between Earths atmosphere and Space is defined by the Kármán Line. This is 100 kilometers above sea level. Since each Minecraft block is equal to one meter, if you just happened to go to Y-100,000.00 or above, you would be in space. But since you cannot build after Y-256.00, this is pretty much useless. However, you can have entities. If you modded Minecraft enough, you could make an entity, without gravity, simply floating in space. You may even be able to use this mod to "spawn" more blocks onto the entity. Just a thought, and the mod part probably is not possible, but the atmosphere part is true.