Shadow of Heaven (1974) by Bob Shaw

Those who tried never returned, and those who didn't, always wondered what they were missing by not attempting the perilous journey to 'Heaven'.

The official designation of the giant anti-gravity floating disc was 'International Land Extension U.S.23'. But to the people living like sardines in the fam-apts and dormitories on the ground below, it was known as 'Heaven'.

Government regulations meant that it was almost impossible for a human being to get to 'Heaven' - almost, but no quite, as Vic Sterling found out when he began to hunt for his missing half-brother Johnny Considine. For Johnny, along with over a hundred other rebels, had decided they could no longer stand the living conditions on Earth and had hitched a ride to 'Heaven', a 'Heaven' full of unexpected dangers for Vic when he decided to follow.

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