Although Hubble’s observations of Neptune were confusing at first, a pattern seemed to emerge after 1994. Every few years, this distant gaseous planet appears to readjust the flow in its atmosphere, which then remains stable. This stability is expressed in the pattern of low-contrast bands on Neptune; at the same time, large atmospheric vortexes come and go. When the Voyager 2 space probe visited Neptune in 1989, the most prominent feature was the Great Dark Spot (also called GDS-89), which had a bright companion high in the atmosphere that was also barely discernible in ground-based telescopes. Between 1990 and 1992, the dark spot had disappeared. During 1993, Neptune’s northern hemisphere began to show a number of new white clouds, which still existed when Hubble obtained the first images with its corrected optics. By then, there were clouds from the pole to the equator, and a new Great Dark Spot (GDS-94) had developed near the north pole.
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