Neptune was nothing more than a pale blue featureless disc for more than a century, discovered September 23, 1846. It had only one moon, Triton, prior to Nereid’s discovery in 1949. Large telescopes strained to detect any detail among Neptune’s clouds. The dawn of the Space Age spawned hope of sending a spacecraft to Neptune to answer fundamental questions and make surprising discoveries. It would be a difficult task designing a spacecraft, securing the funding, and surviving a journey that could take decades, but the rewards could be huge.

The Voyager 1&2 spacecrafts launched in 1977 were designed to only explore Jupiter and Saturn; the odds of reaching Uranus and Neptune were low. If Voyager 1 succeeded at Jupiter and Saturn, and succeeded exploring Saturn’s mysterious moon, Titan, Voyager 2 could then be programmed to fly to Uranus and Neptune after its encounters with Jupiter and Saturn since Voyager 1 would leave Saturn heading in the wrong direction to reach Uranus and Neptune. A perfect slingshot trajectory could use the July 1979 Jupiter flyby to propel Voyager 2 past Saturn in August 1981, Uranus in January 1986, and Neptune in August 1989. It was risky as Voyager 2 was ailing with a faulty radio receiver and sticky photo scan platform. Pointing a camera at a dim object at Neptune’s distance would require a scan platform that could move quickly to track the object as it sped by. The amount of data required for a single image was enormous needing round-trip communications of over eight hours, which would strain even a healthy radio receiver. Voyager 2 gallantly headed for Uranus and Neptune, arthritic and nearly deaf. The odds of pulling off a successful flyby at Uranus was 70%, but at Neptune only 40%.

The flight from Saturn to Uranus took over four years but there were swift advances in computer programming during those years. Engineers were able to overcome the sticky scan platform dilemma by programming Voyager 2 to turn slowly to track an object while taking a picture. New programming also allowed images to be compressed before being transmitted back to Earth, thereby decreasing the risk of losing a valuable photograph and increasing the number of images transmitted. These concepts were tested when Voyager 2 flew past Uranus giving flawless results. Further enhancements during the 3½ years between Uranus and Neptune flybys assured scientists that the Neptune encounter had a good chance of succeeding. The balky radio receiver was left on throughout the mission for fear that it might not survive being switched on and off repeatedly. The fuel supply aboard Voyager 2 held up and never became an issue. Neptune was virtually an unseen world, so Voyager 2 was expected to find surprises like it did at Jupiter, Saturn, and Uranus. It did not disappoint.

Neptune was mysterious for years leading up to the Voyager 2 encounter appearing nothing more than a fuzzy blue disc with hints of white spots and darker belts. Occultations of stars by Neptune made it an even stranger world as it might be surrounded by ring arcs. Even stranger was Neptune’s largest moon, Triton. Not only did it orbit Neptune backwards hinting that it is a captured object that may have resulted in Pluto being ejected into its present orbit, but the analysis of the light reflected by Triton hinted at the possibility of it being covered with vast surface seas of liquid nitrogen. Neptune is so far away from the Sun at an average distance of 2.8 billion miles that daylight is no brighter than twilight on Earth about 30 minutes after sunset, and temperatures hover around -350ºF. Neptune is about four times larger than Earth with a diameter of 30,690 miles. It never appears larger than 2.4 arcseconds across or any brighter than magnitude +7.7 as seen from Earth. All the speculation and the approaching Voyager 2 flyby occurred in an era before the Hubble Space Telescope, which launched during April 1990, and before the first discovery of a Kuiper Belt object in 1992.

History was made during August 1989 with mankind’s first, and so far, only close encounter with Neptune when Voyager 2 flew within 3000 of its cloud tops on August 25. Neptune came into focus as a blue world of dark cloud belts, dark spots, and brighter clouds and streaks. Winds howl at an unbelievable 1400 miles per hour, the windiest planet in the Solar System. The soft blue color results from methane in Neptune’s atmosphere that absorbs the red component of sunlight and scatters the blue. There was a large dark spot that gave Neptune the appearance of Jupiter. The Great Dark Spot was the most prominent feature, as large as Earth, but was gone in 1994 when the Hubble Space Telescope took a detailed look at Neptune and its evolving weather/cloud patterns. The Great Dark Spot and other dark spots are areas of high pressure where the air sinks and dries out the atmosphere below. This creates a clearing or hole in the main cloud deck and the darkness of the spots and belts means that we are looking deep into Neptune’s atmosphere.

The white clouds that were visible near the Great Dark Spot and elsewhere were especially active and always changing through the decades since the flyby. The Hubble Space Telescope, James Webb Space Telescope, and the modern super powerful telescopes on Earth have been monitoring Neptune yearly and tracking the changes. Cloud activity has been waxing and waning with the solar cycle during the past several decades and the peak of the summer season in Neptune’s southern hemisphere has increased atmospheric turbulence. The white clouds are formed either from rising columns of air from below, like Earth’s thunderstorms, or from winds forcing gas over a large dome of high pressure such as the Great Dark Spot. The clouds do not appear blue because they are high up where methane is less abundant. The powerful winds can stretch the clouds nearly halfway around the planet. Like Jupiter, Neptune generates more internal heat than it receives from the Sun. This heat, possibly left over from Neptune’s creation as heavier materials sink towards the core, causes huge atmospheric storms with powerful lightning, methane raindrops as big as beach balls, and ferocious supersonic winds.

Triton was the showstopper when mysterious dark streaks seen on the vast nitrogen ice cap turned out to be active geysers! The plumes tower five miles into the sky before the winds carry them up to 100 miles downwind towards the nightside. The geysers are powered by pockets of liquid nitrogen that are kept warm and under pressure beneath the ice cap. As the ice cap thins in the feeble warmth of the summer sun, the liquid nitrogen erupts into a towering geyser of explosive gas. The geysers may have fooled scientists into thinking that Triton had vast seas of liquid nitrogen. Triton is one of the coldest places in the Solar System at a mind-numbing -392ºF, hardly 70ºF above absolute zero, but still has enough heat to make it a very dynamic world. Triton is smaller than Earth’s Moon at 1680 miles in diameter but has an atmosphere of nitrogen that is 500 miles deep! It is nearly a vacuum with an air pressure barely 1/100,000th of Earth’s and the lowest 15 miles contain a fine haze of methane and hydrocarbons. The vast nitrogen ice cap is slowly sublimating, changing directly from solid to gas under the feeble summer sun, and generating a steady wind that blows towards the colder night side where it condenses to create a new polar ice cap. Triton is unusually smooth with flow features consisting of a mixture of frozen methane, ammonia, and water that gives it the appearance of cantaloupe skin with very few craters visible.

Neptune has not been forgotten with several missions proposed, but none approved for funding. The vast distance will keep Neptune from being seen up close again for most of our lifetime. The tight budget and the priority for a Uranus orbiter/probe mission will make any new missions to Neptune and Triton not likely to occur before 2040. The next mission to Neptune that will eventually fly is planned to be an orbiter that will drop a probe into Neptune’s atmosphere and orbit it for at least four years. The mission could also include a Triton lander or rover that could explore one of the geysers. The urge to explore a big blue world and a moon with icy geysers is strong. Eventually, we will return to stay!