For over a century Neptune was no more than a small, bluish fuzzy disk and Triton was nothing more than a dim star. Seeing more than only vague hints of bright clouds on Neptune was difficult due to their vast distance. Triton was especially difficult and became even more baffling when in the 1980ís its light indicated the presence of nitrogen lakes or seas on its surface.
Since Neptune can get no closer to Earth than about 2.7 billion miles, everything is bathed in perpetual twilight. Any spacecraft launched from Earth takes at least 12 years to reach Neptune even with an assist from Jupiterís powerful gravity as a slingshot, but that did not stop scientists from planning Voyager 2 to fly past Neptune. If Voyager 1 succeeded with its primary goals of exploring Jupiter, Saturn, and especially Titan, then Voyager 2 could be programmed to fly past Uranus in 1986 and Neptune in 1989. Voyager 1 performed beautifully and achieved all of its goals; therefore, Voyager 2 was programmed for Uranus and Neptune encounters after it flew past Saturn during August 1981. By that time though Voyager 2 was an aging spacecraft with hearing problems and arthritis. It had a faulty radio receiver that had trouble hearing certain frequencies from Earth and a sticky scan platform. The vast distances to Uranus and Neptune meant that radio signals were fainter and light conditions were dimmer than at Jupiter and Saturn, which was not ideal considering the problems Voyager 2 already faced.
The problem with the balky radio receiver was solved by leaving it on for the rest of the mission as switching it on and off created a problem with its tuning frequency; there was fear that it might permanently turn a deaf ear to Earth if it were switched on and off repeatedly. Voyager 2 literally had brain surgery on its journey to Uranus; it was totally reprogrammed to slowly rotate itself in order to track an object while photographing it, and to compress the images for relay to Earth. This lowered the exposure time in photographing a dim object, thereby reducing the risk of losing an image due to smear and it allowed more images to be taken. These maneuvers were tested during the Uranus flyby during January 1986 and they worked beautifully.
Voyager 2 made history on August 25, 1989 when it passed barely 3000 miles above Neptuneís cloud tops. Neptune was revealed to be a blue version of Jupiter with cloud belts and spots including a Great Dark Spot, which turned out to be an area of high pressure as large as Earth where the air was sinking and drying out the atmosphere below. This created a clearing in the cloud deck where the darkness meant that we were looking deep into Neptuneís atmosphere. The Hubble Space Telescope observed Neptune in 1994 and discovered that the Great Dark Spot had disappeared, only to be replaced by other dark spots. Neptune generates gravitational heat due to the fact that its core is still slowly contracting. This heat slowly rises to create a stormy planet of spots and belts along with the most ferocious winds in the Solar System at 1400 miles per hour!
Triton was the star of the mission when mysterious dark streaks on its vast nitrogen ice cap turned out to be nitrogen geysers! The plumes towered five miles into the sky before winds carried them up to 100 miles downwind towards the night side. These geysers, powered by pockets of underground liquid nitrogen, may have been what fooled scientists into thinking that Triton had vast seas of liquid nitrogen. Triton is a frigid world with temperatures as low as -392ļF, hardly 70ļF above absolute zero (-459.67ļF). It is a modest size world that is a little smaller than our Moon at 1680 miles in diameter. Its 500-mile deep atmosphere is composed mainly of nitrogen, but is thin, barely 1/100,000th of Earthís, while the lowest 15 miles contains a fine haze of methane and hydrocarbons. The south polar nitrogen ice cap is slowly sublimating as the warmth of summer in the southern hemisphere creates a steady breeze that blows towards the night side, which condenses to create a new polar cap at the north pole. Triton must have had violent temperature swings in the past as its surface is smooth indicating extensive melting, then freezing into smooth plains of a mixture of methane, ammonia, water, and rock. Ultraviolet radiation from the Sun alters the methane ices on the surface reddening them giving Triton its pinkish tint.
Scientists, excited by the mysteries discovered by Voyager 2 at Neptune, made plans in the early 1990ís to return with an orbiter mission similar to the Cassini orbiter at Saturn. It would have launched on July 25, 2002 and arrived at Neptune on May 13, 2021. It would have released a probe into Neptuneís atmosphere and orbited it at least 40 times during a four-year primary mission with many close flybys of Triton, but it was never funded. It is difficult to get a spacecraft into orbit around Uranus or Neptune because both are further away and smaller than Jupiter and Saturn, making time and gravity a serious issue for an orbiter mission. It is important for a spacecraft to reach Neptune in a reasonable amount of time so the same scientists planning the mission can also experience the encounter and study the results. Using Jupiter as a gravity assist slingshot, a spacecraft can reach Neptune in 9-12 years, but without Jupiter it would take about 40 years. A Jupiter gravity assist works great for a flyby mission, but an orbiter needs more of a looping, slower trajectory in order to be captured by the planetís gravity, therefore flight times are longer and in the case of the never-funded 2002 mission it would have taken nearly 19 years to reach Neptune. When an orbiter fires its deceleration rocket as it approaches the planet, it becomes subject to the planetís gravity, but Neptuneís gravity is not strong enough to capture anything as large as Cassini in a reasonable amount of travel time since it would be traveling too fast, and thus would fly right on by without being captured into orbit. Miniaturization of spacecraft components is now making it possible to reconsider an orbiter mission to Neptune with all its promise of discovery.
A Neptune Orbiter Mission is being planned with the goal of placing a spacecraft into orbit around Neptune that will deploying two probes into its atmosphere at two different locations. The orbiter will study Neptune and Triton for at least three years with at least 40 flybys of Triton. It will make close studies of the geysers, ice cap, and unusually smooth surface by taking razor sharp images. It will potentially fly close enough to Triton to sample its deep, thin atmosphere. An alternate version of the mission will include a small Triton lander that will be deployed after the main spacecraft is safely in orbit around Neptune and after an interesting landing site has been determined from several close Triton flybys. The mission could launch as soon as 2025 and arrive at Neptune 11 years later with a gravity assist from Jupiter.
Another mission being planned for Neptune is Argo. It will be a flyby mission, but has better odds in being funded and flying as soon as 2019 since it would use the same hardware and technology as the New Horizons Pluto spacecraft. Why launch a flyby mission when we already did that and an orbiter mission is being planned? The first answer is that an orbiter to Neptune may not fly until the 2030ís or 2040ís unless there are improvements in rocket propulsion technology. The Neptune Orbiter Mission is a flagship mission, meaning it will cost $1.5 to $3 billion, in competition with flagship missions to Europa, Titan, and even Venus, which will all most likely fly before it will. Argo will do serious science at Neptune for less than $1 billion and obtain crucial information with new technology without the complexity and long time needed to reach orbit. High-resolution cameras will be able to see finer detail in Neptuneís clouds, rings, small moons and Tritonís geysers than Voyager 2 ever could. Argo will also measure and map Neptuneís magnetosphere, and accurately measure the depth, thickness, and composition of Neptune and Tritonís atmospheres. Flight time to Neptune would be cut to hardly nine years if launch occurred on February 21, 2019. Argo would fly past Jupiter on June 1, 2020 and Saturn on February 1, 2022 giving Argo the gravitational boost to fly past Neptune as soon as June 2, 2028.
Argo is indeed a beautiful mission, but at this time it is only a plan. It is like another Voyager 2 mission minus Uranus, but with the added bonus of being able to explore several Kuiperoids after it leaves Neptune behind. It is impressive what the imagination can conceive when planning planetary missions, but is limited only by money. Argo is one mission that fits the bill.