This article is from the Isaac Asimov FAQ, by Edward J. Seiler ejseiler@earthlink.net and John H. Jenkins jenkins@mac.com with numerous contributions by others.
Asimov introduced the idea of the Earth becoming radioactive in
Pebble In the Sky . It is also a plot element in the other two "Empire"
books, The Stars, Like Dust and The Currents of Space . In these three
books, it is always assumed that the Earth became radioactive as a result
of a nuclear war. These books were all written in the early 1950s, when
it was commonly felt that there would be a nuclear war between the United
States and Soviet Union in the next few years.
Later on, Asimov realized that this explanation wouldn't wash. The
effects he described would not be possible as the result of a nuclear
war. He therefore provides a different explanation in Robots and Empire
and Foundation and Earth .
Within the fictional universe, the explanation is that the *characters* in
the three Empire novels thought that the Earth became radioactive as a
result of a nuclear war, but that they were wrong.
 
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