Short answers, pulled from the story.
The Great Red Spot is a persistent high-pressure anticyclonic storm in Jupiter's southern hemisphere, located 22 degrees south of the equator. It is the largest anticyclonic storm in the Solar System, producing wind speeds up to 496 km/h. Its distinctive red-orange color remains unexplained.
The Great Red Spot has been under continuous observation since the 5th of September 1831. A similar spot was observed by Giovanni Cassini from 1665 to 1713, but a 2024 study suggests that earlier storm was a different system that disappeared, and the modern spot formed later.
The Great Red Spot has been shrinking for approximately a century. Around a century ago it measured roughly 40,000 km across; by the 3rd of April 2017 it was 16,350 km wide. At its current rate it is expected to become circular by 2040, though some astronomers argue the apparent shrinkage reflects cloud coverage changes rather than loss of the underlying vortex.
Juno spacecraft Microwave Radiometer scans taken in July 2017 suggest the Great Red Spot extends roughly 240 km below the cloud level, with atmospheric pressure at that depth estimated at 100 bar. Different analytical methods place the total depth between 200 and 500 km.
The cause is not definitively known. The leading hypothesis, supported by laboratory experiments, is that solar ultraviolet radiation acts on ammonium hydrosulfide and acetylene in Jupiter's upper atmosphere, producing complex organic compounds called tholins that create the reddish hue. The high altitude of these compounds may also contribute.
Voyager 1 transmitted the first detailed image of the Great Red Spot on the 25th of February 1979, when the spacecraft was 9,200,000 km from Jupiter. The images resolved cloud details as small as 160 km across.