(Ar-Rahman) 55 : 35
يُرْسَلُ عَلَيْكُمَا شُوَاظٌ مِنْ نَارٍ وَنُحَاسٌ فَلَا تَنْتَصِرَانِ
A flame of fire and a molten brass will be sent upon both of you; then both of you won’t be helped.
In the heaven, barricades are there for not to pass the boundary of it, and are protected with special type of flames and liquids. They may also be cosmic rays, meteors, nebulas, etc. See explanation 15: 18; and 37: 9-10.