Great Dark Spot

Based on observations taken with Voyager 2 and since then with the Hubble Space Telescope, Neptune appears to spend somewhat more than half its time with a Great Dark Spot.

The Great Dark Spot was captured by NASA's Voyager 2 space probe in Neptune's southern hemisphere.

[1] Around the edges of the storm, winds were measured at up to 2,100 kilometers per hour (1,300 mph), the fastest recorded in the Solar System.

Neptune's dark spots are thought to occur in the troposphere at lower altitudes than the brighter upper cloud deck features.

When the spot was to be photographed again in November 1994 by the Hubble Space Telescope, it had disappeared completely, leaving astronomers to believe that it had either been covered up or had vanished.

[6] Beginning in 1994, the Hubble became the only operating facility to detect the presence and observe dark spots on Neptune and is still used to the present day.

[8] [6] In 2015, a Southern Dark Spot (SDS) was discovered by the Hubble Outer Planet Atmosphere Legacy (OPAL) program.

The storm is much smaller in comparison than the one discovered by NASA's Voyager 2, but was found to be larger in diameter than the Atlantic Ocean at approximately 4,600 miles across.

[10] In August 2020, the new Great Dark Spot suddenly stopped its southward motion and reversed direction, contrary to projections that the storm would continue to the equator, where it would have met its likely demise.

Neptune Odyssey is a flagship orbiter mission concept with similar goals as Trident and is targeted for a launch date of 2033.