The scale of the Solar System is humiliating when it comes to planetary travel. The vast distances to the planets can take years, or more than a decade, for a spacecraft to travel even with the most powerful rocket. A trip to Neptune can take up to two decades unless the powerful gravity of Jupiter is utilized as a slingshot to speed up a spacecraft. There was a golden opportunity in the late 1970s to explore Neptune within a reasonable travel time, but it was not the primary target, and reaching it was not a sure deal.
The two Voyager spacecraft that were launched in 1977 were designed only to study Jupiter and Saturn along with their moons. Flying to Uranus and then to Neptune was considered a long shot. Voyager 1 had to succeed at Jupiter, Saturn and especially at Saturn's mysterious moon, Titan, before Voyager 2, which was following close behind, could be programmed to fly to Uranus and then on to Neptune. Voyager 1 was a huge success at Titan, therefore the mission to Neptune was on, with the flyby set for August 25, 1989, but it was risky. Voyager 2 was aging with a faulty radio receiver and a sticky scan platform. Pointing a camera at a dim object such as Neptune and its moons would require a camera on a scan platform that could move quickly to track the object as it sped by. The amount of data required for a single picture would be enormous and the round trip communication would take over eight hours and be challenging for even the healthiest spacecraft.
Technology was swiftly advancing during the 1980s and Voyager 2 effectively had virtual brain surgery on its journey to Uranus. New programming was designed and uploaded to work around the sticky scan platform. Voyager 2 would then be able to slowly turn and track an object as it flew by while taking a picture. New programming also allowed images to be compressed before being sent back to Earth, thereby decreasing the risk of losing a valuable photograph and allowing for more to be taken. It was all tested during the Uranus flyby in January 1986 and worked beautifully. Further advances in technology and programming as the 1990s neared increased confidence of a successful Neptune flyby. The faulty 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 was never an issue. Jupiter, Saturn, and Uranus were exciting worlds with surprises. Neptune and its largest moon, Triton, were even more remote and less known, and the less that was known of a world, the more it seemed to surprise. This was the understatement of 1989.
Neptune was nothing more than a small fuzzy disc with a few tantalizing bright cloud patches and hints of dusky belts even in the most powerful telescopes on Earth before the Voyager 2 encounter. The soft blue color results from the methane in Neptune's atmosphere, which absorbs the red component of sunlight and scatters the blue. Triton was even more bizarre as the light from it was analyzed and consistently hinted at the possibility of being covered with vast seas of liquid nitrogen. Neptune is far from the Sun with an average distance of 2.8 billion miles. Daylight is no brighter than twilight on Earth about 30 minutes after sunset and temperatures hover around -350F. Neptune is about four times larger than Earth with a diameter of 30,690 miles. It never appears any larger than 2.4 arcseconds across or any brighter than magnitude +7.7 as seen from Earth, which is why it has always posed a challenge to study. Triton orbits Neptune in the opposite direction compared to the other major moons in the Solar System. It was suspected to be a captured moon that possibly resulted in Pluto being ejected into its present orbit.
Neptune finally gave up its secrets in August 1989 when Voyager 2 flew within 3000 miles above its cloud tops. It turned out to be a blue version of Jupiter, but with the fiercest winds of any planet at 1400 miles per hour! Neptune was revealed as a world of bright white clouds, dark belts and spots, and even 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. 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 in Neptune's atmosphere.The white clouds that were seen near the Great Dark Spot and elsewhere have been especially active during the past decade as it is summer in Neptune's southern hemisphere, which has created more atmospheric turbulence. The white clouds are formed either from rising columns of air from below similar to 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 half way around the planet. Neptune generates more heat internally than it receives from the Sun, similar to Jupiter. The heat may be the result of heavier material sinking towards the core from Neptune's creation leading to huge storms and ferocious winds.
Triton stole the show from Neptune when the 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 toward the night side. 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 -392F, hardly 70F above absolute zero, but it still has enough heat to make it a very dynamic world. Triton is 1680 miles in diameter, smaller than Earth's Moon, but has an atmosphere of nitrogen that is 500 miles deep! It is nearly a vacuum; 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 a cantaloupe appearance with very few craters.
Neptune is too far away for any future missions in our lifetimes. The tight budget and a more concentrated effort to explore Mars make any new missions to Neptune and Triton not likely before 2030. The next logical mission to fly would be an orbiter that would drop one or more probes into Neptune's atmosphere and orbit Neptune for at least four years. Technology in that distant future may also allow for a Triton probe to land near one of the geysers and perhaps even have roving capabilities.
The Hubble Space Telescope and sophisticated telescopes on Earth have revealed an increasingly stormier Neptune with numerous bright clouds and dark spots. Methane rain with drops as large as beach balls may fall deep in the blue atmosphere with blinding blizzards of methane ice screaming in the supersonic winds higher up. Triton has been steadily warming during the southern hemisphere summer and it is now a balmy minus 387F. Neptune and Triton may be too far away to readily explore, but their dynamics and constant change every time we observe them beckon us to eventually return and next time to stay.