<P> The Scooter is another storm, a white cloud group farther south than the Great Dark Spot . This nickname first arose during the months leading up to the Voyager 2 encounter in 1989, when they were observed moving at speeds faster than the Great Dark Spot (and images acquired later would subsequently reveal the presence of clouds moving even faster than those that had initially been detected by Voyager 2). The Small Dark Spot is a southern cyclonic storm, the second-most - intense storm observed during the 1989 encounter . It was initially completely dark, but as Voyager 2 approached the planet, a bright core developed and can be seen in most of the highest - resolution images . </P> <P> Neptune's dark spots are thought to occur in the troposphere at lower altitudes than the brighter cloud features, so they appear as holes in the upper cloud decks . As they are stable features that can persist for several months, they are thought to be vortex structures . Often associated with dark spots are brighter, persistent methane clouds that form around the tropopause layer . The persistence of companion clouds shows that some former dark spots may continue to exist as cyclones even though they are no longer visible as a dark feature . Dark spots may dissipate when they migrate too close to the equator or possibly through some other unknown mechanism . </P> <P> Neptune's more varied weather when compared to Uranus is due in part to its higher internal heating . Although Neptune lies over 50% farther from the Sun than Uranus, and receives only 40% its amount of sunlight, the two planets' surface temperatures are roughly equal . The upper regions of Neptune's troposphere reach a low temperature of 51.8 K (− 221.3 ° C). At a depth where the atmospheric pressure equals 1 bar (100 kPa), the temperature is 72.00 K (− 201.15 ° C). Deeper inside the layers of gas, the temperature rises steadily . As with Uranus, the source of this heating is unknown, but the discrepancy is larger: Uranus only radiates 1.1 times as much energy as it receives from the Sun; whereas Neptune radiates about 2.61 times as much energy as it receives from the Sun . Neptune is the farthest planet from the Sun, yet its internal energy is sufficient to drive the fastest planetary winds seen in the Solar System . Depending on the thermal properties of its interior, the heat left over from Neptune's formation may be sufficient to explain its current heat flow, though it is more difficult to simultaneously explain Uranus's lack of internal heat while preserving the apparent similarity between the two planets . </P> <P> The average distance between Neptune and the Sun is 4.50 billion km (about 30.1 astronomical units (AU)), and it completes an orbit on average every 164.79 years, subject to a variability of around ± 0.1 years . The perihelion distance is 29.81 AU; the aphelion distance is 30.33 AU . </P>

When was the last time neptune orbited the sun