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Jupiter's Great Red Spot
The main feature of the giant planet, Jupiter, is its Great Red Spot. This is an area in the planet's atmosphere (coloured red!) which Earth could fit into three times! It has been observed on the planet's surface for centuries and what it was remained a mystery until the Pioneer 10 and 11 missions were launched in 1972 and 1973 to take a look at the Gas Giants, followed by Voyager 1 and 2 in 1977.
These four probes sent back pictures of Jupiter and the Great Red Spot, showing us that the area was, in fact, a swirling storm cloud. It is an area of high pressure which gets weaker and stronger over time. This is shown by how red the spot actually appears (the higher the pressure, the redder the spot appears). The Great Red Spot has been observed for over 300 years. Nobody knows how long it has lasted or how long it will last. A similar Dark Spot on Neptune was discovered in 1989 when Voyager 2 arrived at the planet, but it had vanished when the planet was observed again in 1994. However, another mysterious dark spot has since appeared on the planet! It is known that other, smaller spots appear on Jupiter, although these also disappear too. Jupiter's Great Red Spot has shown no sign yet of disappearing.