Jupiter's Great Red Spot Is Shrinking

Jupiter's Great Red Spot is a storm wider than the whole Earth, and it has been shrinking for more than a century. We start with the real sky over La Palma before dawn on 5 November 2026: Jupiter climbing out of the east, brighter than any star, with Mars just above it and Regulus below. Through a small telescope, Jupiter becomes a striped disc with its four big moons beside it, placed where JPL Horizons puts them that morning, and as the planet turns the spot comes into view. Jupiter itself is the real global map made by the Cassini spacecraft. Then the history: Giovanni Cassini's dark "Permanent Spot" of 1665, lost from the records around 1713; more than a century with no spot reported; and the spot that appears in 1831, the one we still see. A 2024 study concluded that Cassini's spot was probably a different storm. Then the measurements, at true scale beside Earth: about 41,000 km across in the late 1800s (room for three Earths), about 23,335 km when Voyager flew past in 1979, just under 16,500 km in Hubble's 2014 images, more than half its width gone and closer to a circle. Why a storm can last so long on a world with no land; the two jet streams that hold it; the Little Red Spot; winds faster than 640 km/h, up to 8 percent faster between 2009 and 2020; the 90-day wobble Hubble caught in 2024; how deep it goes (Juno: more than 350 km in microwaves, about 500 km from gravity), shown as a true-scale slice; and whether it will keep shrinking. Sources: ESA/Hubble heic1410 and NASA (2014: about 41,000 km in the late 1800s, 23,335 km at the Voyager fly-bys, just under 16,500 km, shrinking faster since 2012); ESA/Hubble heic2110 (2021: winds over 640 km/h, the high-speed ring up to 8 percent faster between 2009 and 2020, diameter about 16,000 km); ESA/Hubble heic2412 (2024: 90 days of observations, the spot squeezing in and out as it speeds up and slows down; Amy Simon: it is over-filling its latitude band and will probably stabilise once it shrinks inside it); Sanchez-Lavega et al. 2024, "The Origin of Jupiter's Great Red Spot" (arXiv 2406.13222: Cassini's Permanent Spot, 1665-1713, is unlikely to be today's spot, first observed in 1831); NASA Science, Jupiter facts and the Juno mission page (11 times wider than Earth, about 1,000 Earths would fit inside, a 9.9-hour day, the Little Red Spot, the spot's depth of more than 350 km from microwaves and about 500 km from gravity, 50 to 100 times deeper than Earth's oceans, jet streams about 3,000 km deep). The coin comparison and the light-travel time were computed from these. Positions of Jupiter, Mars, the Moon and Jupiter's moons over La Palma from JPL Horizons. Images: Jupiter global map: NASA/JPL/Space Science Institute (Cassini, PIA07782); the spot's size at each date is that map resized to the measured widths. Earth: NASA Blue Marble. Stars: Hipparcos. Milky Way panorama: ESO/S. Brunier (CC BY 4.0). The wind tracers, the merging ovals and the depth slice are illustrations. #jupiter #space #astronomy