One difference between Neptune and Uranus is the level of meteorological activity that has been observed. When the Voyager spacecraft flew by Uranus in 1986, the winds on that planet were observed to be mild. When Voyager flew by Neptune in 1989, powerful weather events were observed. The weather of Neptune has extremely active storm systems. It's atmosphere has the highest wind speeds in the Solar System, thought to be powered by internal heat flow. Regular winds in the equatorial region have speeds of around 1,200 km/h (750 mph). Winds in storm systems can reach up to 2,100 km/h, near-supersonic speeds.