Wahifthan min kulli shaytanin marid (surah Assaaffat: 7)

Tafseer Tafheem-ul-Quran by Syed Abu-al-A'la Maududi

(37:7) and have protected it from every rebellious satan. *6

Wa hifzam min kulli Shaitaanim maarid

*6) That is, "Heaven is not merely empty space so that anyone who likes may enter it, but it has been fortified strongly, and its different regions have been bounded by such strong barriers that it is impossible for any rebel satan to exceed them. Every star and planet in the universe has its own circle and sphere escaping from which is as difficult as entering it. With the naked eye one can only see empty space, but, in reality, there are countless regions in space which have been even more strongly fortified and protected than they could be by steel walls. One can imagine and estimate the strength of these barriers by the difficulties man is experiencing in the way of reaching the moon, which is our nearest neighbour in space. Similar difficulties prevent the other creation of the earth, the jinns, from ascending towards the heavens.