Pity our poor seventh and eighth planets. Mars has played host to a small army of robotic visitors. Jupiter and Saturn have been imaged in breathtaking detail by spacecraft like Galileo and Cassini. Even Pluto got its moment with this summer's visit from New Horizons. Yet we've visited Uranus and Neptune only once, in 1986 and 1989 respectively, when Voyager 2 flew past.

But plans are in motion at NASA to change that. Planetary sciences chief Jim Green told attendees of the Outer Planets Assessment Group that Jet Propulsion Laboratory is drafting up a flagship mission to one or both ice giant planets.

Uranus and Neptune are the odd ones out.

It makes sense. NASA's M.O. is to visit a planet first via flyby, then via orbiter, and then (if possible) via lander. With the New Horizons mission, NASA completed its initial recon of all the planets and even former planets Ceres and Pluto. We've sent orbiters to study the worlds from Mercury to Saturn in depth, including flagship missions like Galileo and Cassini. Mars has been visited by multiple landers. We've even landed on the hellish surface of Venus, and dropped landers into the clouds of Jupiter and the cold muds of Saturn's moon Titan. Uranus and Neptune are the odd ones out.

There's is a lot to discover out there. Despite superficial resemblances to the gas giants Jupiter and Saturn, Uranus and Neptune seem to be an entirely different kind of planet called ice giants. While the name sounds frosty, the "ice" comes from super-compressed layers in which gasses are squeezed at such a high pressure that they become a solid despite hot temperatures. Compositionally these outer planets are different too. While Jupiter and Saturn are primarily hydrogen and helium, Uranus and Neptune have a core of water, ammonia, and methane, along with comparatively high amounts of those materials in their atmosphere.

The Voyager 2 image of Neptune. NASA

The two worlds also have fascinating and virtually unexplored moon systems. Neptune's largest moon, Triton, appears to be a captured object from Kuiper Belt, the place where Pluto and fellow dwarf planets Eris, Makemake, and Haumea reside. The giveaway that it's a captured wanderer is that Triton is not only much bigger than the rest of the Neptunian moon, but also orbits backwards compared to them. It appears to have signs of cryovolcanism and ice geysers, and there may even be a subsurface ocean. The capture of Triton may have given birth to another moon, Proteus—something a visit to the system would help establish. Two other moons, Nereid and Halimede, were likely one body before an ancient collision.

Then there are Uranus' moons, which, in a charming historical quirk, are named from Shakespearean characters rather than those from mythology. Titania and Oberon are bigger than three of the dwarf planets (Makemake, Haumea, and Ceres) while Umbriel and Ariel are just a shade smaller. Umbriel looks a heck of a lot like Ceres, the biggest object in the Asteroid Belt. This became especially clear during the ongoing Dawn mission to the belt. Miranda and Ariel have deep grooves; there are cliffs with three mile drop-offs on Miranda, the largest in the solar system. Both have evidence of intense past geologic activity, including cryovolcanism.

One of the big obstacles to exploring the more distant planets is power, especially having enough energy to get around once you're there. Until recently, NASA's hands were tied by international nuclear treaties that prevented the agency from refining fuel for radioisotope thermoelectric generators. These nuclear pellets are what spacecraft would use to explore deep space regions where solar power isn't viable. The ban prevented one mission, Argo, from a launch between 2015 and 2020, the ideal window to get out to Uranus and Neptune's neighborhood. But according to Astronomy Magazine, JPL will look toward the Space Launch System heavy duty rocket to shave off decades of travel time to the distant worlds.

Voyager 2 image of Uranus. NASA

It's still a gigantic endeavor. The Voyager 2 visit took advantage of alignments and gravity assists that allowed it to visit all four outer planets in a one-decade period. That was a once-in-a-lifetime event. It took Cassini seven years to get to Saturn. Any Uranus or Neptune mission would take significantly longer, and it would need to keep lots of RTG fuel on reserve for when it's ready to explore, especially to slow down. Orbiters need to go slow enough to insert themselves around a planet, whereas a flyby just zips through.

Getting there may be complicated and it may be expensive, but Uranus and Neptune could further unlock clues to our solar system's formation and of the interactions between Neptune and the "third zone" of the solar system, home to regions like the Kuiper Belt. While the missions are in the proposal stage now, coming up with something on paper is the easy part. Getting it through rounds and rounds of congressional funding is harder, and with no obvious flagship mission on the horizon until 2020, NASA is devoting its resources to Mars and Europa.

There have been dozens of proposals in the decades since since Voyager to get out there. Getting something to the launch pad is a different matter entirely.



Source: Astronomy Magazine

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