Voyager 2 proceeded from its encounter with Uranus to fly to Neptune in August 1989. At the time of the Neptune encounter it took radio signals over four hours, traveling at the speed of light, to go from the spacecraft to the large receiving antennas distributed around Earth. The cameras on Voyager 2 took numerous images of Neptune like the one shown here as the spacecraft approached the planet. The large dark oval (the Great Dark Spot, see slide #36) near the western (left) limb of the planet was visible for several months prior to the closest approach. This Great Dark Spot is at a latitude of 22°S and travels around Neptune every 18.3 hours. A second dark spot, at 54°S latitude (near the lower right limb), travels around Neptune every 16.1 hours, a period close to the 16.05-hour rotation rate of the deep interior as revealed by periodic radio emissions. The different rates at which cloud features go around the planet provide information on the winds in the atmosphere of Neptune. The dynamic atmosphere visible at Neptune is in marked contrast to the bland appearance of the atmosphere of Uranus and may be a consequence of the greater amount of internal heat released by Neptune, evidenced by the fact that both planets radiate similar amounts of energy even though Neptune is 1.5 times further away from the Sun.
Voyager 2 image (Press Release P-34611).