![]() ![]() Voyager 2 is the only spacecraft that has encountered Neptune to date, flying by the planet on 25 August 1989 when it was summer in Neptune׳s southern hemisphere (Stone and Miner, 1989). This makes Triton a unique planetary satellite in the Solar System. Triton, by far the largest moon, very likely formed as a dwarf planet in the Kuiper belt (like Pluto) before being captured by Neptune. The planet is surrounded by a system of rings and icy moons (6 regular, 7 irregular). A Neptune day is just over 16 h long, and a planetary obliquity of ~30° leads to seasons over Neptune׳s ~165-year orbit. Neptune orbits the Sun at a distance ~30 times greater than the mean Sun–Earth distance (an Astronomical Unit, AU). The Neptune system is unique, providing opportunities for major advances across multiple scientific fields that cannot be made in any other planetary environment. However, there are fundamental and important differences between the Uranus and Neptune planetary systems, which their common classification as ice giant planets should not obscure. Neptune is classified as one of the giant planets, along with Jupiter, Saturn, and Uranus, and additionally forms a subgroup with Uranus called the “ice giants”, because both planets are primarily composed of “ices” (volatile elements heavier than hydrogen and helium). This review is based on a white paper that was submitted to the European Space Agency (ESA) in May 2013, to inform the selection of the science themes that will be addressed by the second and third large missions in the ESA Cosmic Vision Programme 2015–2025. The primary aim of this paper is to review what we currently know about the Neptune planetary system, and to highlight the many fundamental scientific questions that remain unanswered. Such a mission has significant scope for international collaboration, and is essential to achieve our aim of understanding how the Solar System formed, and how it works today. We present an overview of the results of a European-led Neptune orbiter mission analysis. We discuss all the major science themes that are relevant for further spacecraft exploration of the Neptune system, and identify key scientific questions in each area. Here, we present the high-level science case for further exploration of this outermost planetary system, based on a white paper submitted to the European Space Agency (ESA) for the definition of the second and third large missions in the ESA Cosmic Vision Programme 2015–2025. ![]() However, our exploration of the Neptune system remains limited to a single spacecraft flyby, made by Voyager 2 in 1989. ![]() Triton is geologically active, has a tenuous nitrogen atmosphere, and is predicted to have a subsurface ocean. ![]() Neptune׳s moon Triton is very likely a captured Kuiper Belt object, holding the answers to questions about the icy dwarf planets that formed in the outer Solar System. The ice giant Neptune played a unique and important role in the process of Solar System formation, has the most meteorologically active atmosphere in the Solar System (despite its great distance from the Sun), and may be the best Solar System analogue of the dominant class of exoplanets detected to date. The planet Neptune and its largest moon Triton hold the keys to major advances across multiple fields of Solar System science. ![]()
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