Astronomers are still hoping for another mission to Pluto [space.com], or perhaps another Kuiper belt object:
A grassroots movement seeks to build momentum for a second NASA mission to the outer solar system, a generation after a similar effort helped give rise to the first one. That first mission, of course, was New Horizons, which in July 2015 performed the first-ever flyby of Pluto and is currently cruising toward a January 2019 close encounter with a small object known as 2014 MU69.
[...] Nearly three dozen scientists have drafted letters in support of a potential return mission to Pluto or to another destination in the Kuiper Belt, the ring of icy bodies beyond Neptune's orbit, Singer told Space.com. These letters have been sent to NASA planetary science chief Jim Green, as well as to the chairs of several committees that advise the agency, she added. "We need the community to realize that people are interested," Singer said. "We need the community to realize that there are important, unmet goals. And we need the community to realize that this should have a spot somewhere in the Decadal Survey." That would be the Planetary Science Decadal Survey, a report published by the National Academy of Sciences that lays out the nation's top exploration priorities for the coming decade.
New Horizons 2 [wikipedia.org] was already cancelled due to a shortage of plutonium-238, which still reportedly persists [soylentnews.org]. One proposed target was 47171 Lempo [wikipedia.org], a trinary system. The trans-Neptunian dwarf planets [wikipedia.org] Eris, Haumea, Sedna, Orcus, Salacia, Makemake, and 2007 OR10 [wikipedia.org] (the largest known body in the solar system without a name - with an estimated 1,535 km diameter) have all been discovered since 2002. Several of these TNOs have moons [wikipedia.org] and Haumea was recently found to have a ring system [soylentnews.org].
Now that Cassini is dead, most new NASA missions are focused on Mars and Jupiter [arstechnica.com], leaving the solar system's "ice giants" [wikipedia.org] relatively unstudied:
With instrumentation ever-improving, there were two more worlds waiting for their post-Voyager close ups. It's a promise that has been painfully unfulfilled. Uranus and Neptune, it turns out, are typical of some of the most common planets in our galaxy. Understanding them would not only help us understand the Solar System, its origins, and the interactions that define the outer border of its planets. It would help us make sense of the galaxy as a whole.
Plus those planets have a collection of moons that has the potential to include Enceladus-level surprises. And Uranus is tilted on its side—its axis of rotation is more or less on the plane of the Solar System, rather than pointing perpendicular to it. And its magnetic field is 60º off that axis. It's hard to imagine there's not some amazing stuff to learn there.
The Ice Giants Pre-Decadal Study group [usra.edu] has indicated that only one mission would likely be sent to either Uranus or Neptune [soylentnews.org] in the coming decades, due to costs and the need for plutonium-238. The authors concluded that both Uranus and Neptune are of equal importance, but seem to favor going to Uranus first.
Previously: Return to Pluto? [soylentnews.org] - "Scientists, including New Horizons principal investigator Alan Stern, met in Houston on April 24th to discuss the possibility of a Pluto orbiter mission..."