In 1989, NASA’s Voyager 2 measured winds up to 2,100 km/h around Neptune’s Great Dark Spot, a storm the size of Earth. By 1994, Hubble found it vanished, unlike Jupiter’s Great Red Spot, which has raged for more than a century. Scientists now track Neptune’s storms as transient phenomena, with new ones forming every few years.
Voyager 2’s 1989 flyby of Neptune revealed a storm so massive it could swallow Earth, with winds rivaling Jupiter’s Great Red Spot. The spacecraft’s Imaging Science System (ISS) captured the Great Dark Spot, an oval storm in Neptune’s southern hemisphere measuring 13,000 by 6,600 kilometers. Winds near its edges reached 2,100 km/h, among the fastest ever recorded anywhere in the solar system. But by 1994, the Hubble Space Telescope found no trace of it, raising questions about Neptune’s weather patterns.
The Vanishing Storm: Neptune’s Transient Weather
Since then, Hubble has caught four more, bringing the total spotted so far to six. Researchers estimate these storms last between one and six years, typically fading within two. A 2015 storm tracked by NASA’s Outer Planet Atmospheres Legacy (OPAL) program shrank from 4,990 km to 3,700 km before disappearing. We’ve now watched this happen enough times, Voyager’s original pair, four more caught by Hubble, one tracked all the way through its own death, that it looks less like an anomaly and more like simply how weather works on a planet built the way Neptune is,
said Michael Wong, a planetary scientist at the University of California, Berkeley.
Unraveling the Mystery: Why Neptune’s Storms Don’t Last
Neptune’s storms form in high-pressure regions but dissipate rapidly due to the planet’s internal heat. Unlike Jupiter’s stable system, Neptune’s turbulence erodes vortices within years. The planet radiates far more heat than it receives from the Sun, and that internal heat drives vigorous convection and fast east-west jet streams that appear to erode an isolated vortex over just a few years. This process contrasts with Jupiter’s far larger, more stable storm system.
Scientists also note that methane ice clouds appear years before new storms become visible. In 2015, while the OPAL team was busy watching that storm shrink, they noticed small, bright clouds made of methane ice appearing in a completely different spot on the planet. These bright clouds, it turned out, were an early warning sign appearing years before the storm itself became visible. This pattern was confirmed in 2018, when a new Great Dark Spot formed almost exactly where those clouds had been, nearly identical in size and shape to the one Voyager saw in 1989, just at a different latitude.
Voyager 2: A Mission That Redefined the Outer Solar System
Voyager 2’s 1989 Neptune flyby was part of a broader mission to explore the outer planets. Launched in 1977, the spacecraft initially aimed to study Jupiter and Saturn before a 1981 decision to redirect it toward Uranus and Neptune. The two-spacecraft Voyager missions were designed to replace original plans for a “Grand Tour” of the planets that would have used four highly complex spacecraft to explore the five outer planets during the late 1970s. NASA canceled the plan in January 1972 largely due to anticipated costs (projected at $1 billion) and instead proposed to launch only two spacecraft in 1977 to Jupiter and Saturn. Voyager 2 became the first human-made object to fly by Neptune, discovering five moons, four rings, and a Great Dark Spot.
The spacecraft’s success came after a 1972 decision to cancel a $1 billion Grand Tour plan, opting instead for two cheaper missions. Each of the two spacecraft was equipped with a slow-scan color TV camera to take images of the planets and their moons and each also carried an extensive suite of instruments to record magnetic, atmospheric, lunar, and other data about the planetary systems, including the Imaging Science System (ISS), Ultraviolet Spectrometer (UVS), and Infrared Interferometer Spectrometer (IRIS). The mission’s legacy includes critical insights into Neptune’s atmosphere and the dynamic nature of its storms.
What This Means for Planetary Science
Neptune’s transient storms challenge assumptions about planetary weather systems. While Jupiter’s Great Red Spot has endured for centuries, Neptune’s storms are fleeting, shaped by internal heat and rapid atmospheric dynamics. What I find genuinely odd about this Neptune has spent, by one estimate, more than half the time anyone has been able to observe it hosting at least one of these storms, and yet no single one has stuck around long enough to become a landmark the way Jupiter’s has,
said Wong, adding that it is an entire planet that seems constitutionally unable to hold onto a storm, generating something planet-sized every few years only to dissolve it again before it can become a fixture.
Researchers now focus on understanding how Neptune’s storms form and dissipate. With Hubble’s ongoing observations and future missions, scientists hope to uncover more about the planet’s atmosphere. As Simon has described the disappearance, it was a genuine surprise to a field accustomed to a very different kind of storm.
