It is May 13, 2021, and a spacecraft bristling with turn of the century high-tech electronics swoops within a few thousand miles of a giant blue globe streaked with white clouds and dark belts. The rockets fire for over an hour slowing it for orbital capture. It is successful and the adventure at Neptune begins. That is what the day would have been like had a new class of planetary spacecraft known as Mariner Mark II been funded.

Scientists were excited by the discoveries Voyager 2 made at Neptune in 1989 and made plans in the early 1990’s to return with an orbiter mission similar to the Cassini mission at Saturn, which had been approved and already under construction. The proposed Neptune orbiter would have launched on July 25, 2002, and arrived at Neptune on May 13, 2021. It would have released a probe into Neptune’s atmosphere and orbited it at least 40 times during a four-year primary mission with several close flybys of Triton. Time and distance make it difficult to plan orbital missions to Neptune along with its weaker gravity being smaller than Jupiter and Saturn. It is important for a spacecraft to reach Neptune in a reasonable amount of time so that 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 get to 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 planet’s 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 planet’s gravity. Neptune’s 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 fly right on by without ever being captured into orbit. Miniaturization of instruments allows for a smaller, lighter spacecraft to be designed making it now possible to consider an orbiter mission to Neptune with all the promise of discovery.

Other missions to Neptune have been considered since Voyager 2’s historic encounter involving flybys such as Argo, which was considered in 2009 and could have launched in 2019. It would have used the tried-and-true technology and hardware of the New Horizons Pluto spacecraft. The logic then was that an orbiter mission still needed improvements in rocket propulsion technology, so any Neptune orbiter would not fly until the 2030’s or 2040’s and would be classified as a Flagship Mission, meaning it would cost a minimum of $3 billion, putting it in competition back then with flagship missions to Europa and the Mars Curiosity and Perseverance rovers. A flyby mission could still do good science for lower cost in a shorter time.

Flagship Missions, also known as Large Strategic Missions, are a vital part of planetary exploration as they allow the pursuit of the most compelling science questions, support workforce development, foster international collaboration, and provide opportunities for investigations and producing scientific discoveries that inspire the public and the next generation of scientists and engineers. A flagship mission to Neptune and Triton would provide many firsts: an orbiter and atmospheric probe to explore an ice giant, its rings, small satellites, space environment, and Triton, a geologically active twin of Pluto. The mission would address important questions such as: how do the interiors of an ice giant planet form and evolve; what causes Neptune’s strange magnetic field; what are the origins of the rings and ring arcs; is Triton an ocean world (underground reservoir of liquid water); what causes its plumes; and what is the nature of their atmospheres? Studying Triton up close would also increase knowledge about Pluto.

Planets discovered around other stars are known as exoplanets. Neptune-size exoplanets are very common in our galaxy, so by studying Neptune up close scientists could better understand them. A thermal imager from ultraviolet to infrared could observe Neptune during the spacecraft’s 12–16-year journey to Neptune along with studying the solar wind and particles. We have a good understanding of the interiors of Jupiter and Saturn thanks to the Juno and Cassini orbiter missions, but of the interiors of the ice giants, Uranus and Neptune, we are totally ignorant. Ice giants are water dominated worlds with icy compounds, but what are the abundances of these and of rocky material? Why does Neptune emit as much heat as Jupiter and more than Uranus? Why is the magnetic field offset 50º from its rotational pole. Probing the atmosphere and studying the gravitational fields may narrow the possible answers. The exciting part of the mission for the public will be the images that focus on the study of its weather and how it evolves. The ever-changing clouds of this blue world will be fascinating to follow as the mission progresses. While most will admire this blue version of Jupiter, scientists will be trying to answer previous questions as well as: what is Neptune’s internal structure and how is heat transferred; why is Neptune’s ratio of emitted/received energy larger than any other planet; what is the thermal structure, composition, and 3D circulation of Neptune’s atmosphere?

Little is known about the rings and satellite systems around Neptune. The observed ring arcs raise questions on how they form, evolve, and interact with other particles and moons. The inner, small moons may have formed with Neptune and may be relics of its creation. Triton will be heavily studied to determine if it has a subsurface ocean and if Neptune creates a strong enough tidal flexing to warm the interior and power the geysers. Triton has a relatively smooth surface punctuated with ridges, cliffs, and knobs up to 3000 feet high and there are several well-defined impact craters up to a mile deep. The surface may be composed of rigid materials such as water ice, ammonia-water ice, carbon dioxide, and/or sulfur dioxide all of which are strong enough to preserve the topography in the near -400ºF cold. While most of us will be in awe of the geysers, scientists will try to answer questions such as: is there an ocean present, and if so, what is its depth and salinity; how thick is the ice shell; what generates the plumes; what seasonal factors influence Triton’s atmosphere; how do Triton’s surface and atmosphere interact with each other and with Neptune’s magnetic field?

A flagship mission to Neptune is important as it will encompass the study of planetary interiors, atmospheric evolution, magnetosphere, rings, small satellites, ocean worlds, Kuiper Belt objects, heliophysics, and exoplanets. It will provide research opportunities for scientists across the planetary community and train the next generation of mission scientists. We have the skill, knowledge, technology, and rocketry to do a Neptune orbiter mission. All we need now is the funding. We are motivated and ready to explore!