Object 90377 Sedna – a distant transneptunian object best known for its highly elliptical, 11,390-year-old orbit – is currently en-route to perihelion (its closest approach to the Sun) in 2076. After that, Sedna will return to the ‘space. deep and will not return for millennia, making this overview a unique (or, once in ~ 113 lifetimes) opportunity to study an object from the far reaches of our solar system. There are no Sedna missions in the works yet, but astronomers are starting to plan for the possibility and ideal launch date for such a mission is fast approaching, with two of the best launch windows coming in 2029 and 2034.
Sedna was discovered in 2003 by Caltech astronomer Mike Brown and his team, and was one of a series of potential dwarf planets (alongside similar-sized bodies like Haumea, Makemake, and Eris) whose discovery led to the demotion of Pluto in 2006. As best we can tell from a distance, Sedna is about the same size as Ceres, the largest object in the asteroid belt, but its composition and origins are very different. Its chemical makeup suggests it may be covered in deep reddish organic compounds known as tholins, the same material seen on Pluto and other Kuiper belt objects. Unlike Pluto, it is usually too cold for the methane abundant on its surface to evaporate and fall back as snow, though Sedna may briefly gain an atmosphere of nitrogen as it approaches the Sun.
What really sets Sedna apart from the other known dwarf planet candidates is its enormous orbit, which takes it out towards the inner edge of the Oort cloud, the most distant region of the Solar System, where long-period comets lurk. There are several competing theories to explain how Sedna ended up in this position. Perhaps the most high-profile theory is the possibility that a yet unknown ninth planet, perhaps ten times the size of Earth, disrupted Sedna’s orbit and swept it and several other objects out into highly elongated orbits. Visiting Sedna probably won’t solve this particular mystery, but it will tell us a lot about the composition of these extreme trans-Neptunian objects.
The orbit of dwarf planet candidate 90377 Sedna (red) compared to Jupiter (orange), Saturn (yellow), Uranus (green), Neptune (blue), and Pluto (purple). Credit: Szczureq/kheider/NASA (Wikimedia Commons).
