Jupiter's importance in sf is derived from its status as the
largest planet in the solar system and also the most accessible
– because nearest to Earth – of the Gas Giants. Its four major
moons – Ganymede, Callisto, Io and Europa – were discovered by
Galileo, but it was not until 1892 that the US astronomer Edward
Barnard (1857-1923) discovered the fifth. About a dozen others
were discovered in the twentieth century. The visible "surface"
of Jupiter is an outer layer of a very dense, deep atmosphere
and is thus fluid, though it does have one enigmatic feature
that has endured at least since 1831: the Great Red Spot.
Jupiter was included in various interplanetary tours inspired by
the religious imagination, and is prominent in several
nineteenth-century interplanetary novels, including A World of
Wonders (1838) by Joel R Peabody, the anonymously published The
Experiences of Eon and Eona (1886; by J B Fayette) and John
Jacob Astor's A Journey in Other Worlds (1894), in which it is a
"prehistoric" version of Ear