The outer Solar System has been a busy place lately, with the ongoing Cassini mission at Saturn and New Horizons’ recent spectacular flyby of Pluto. Literally in-between those two worlds, however, it has been quiet for a long time now—the last time the ice giants Uranus and Neptune were visited was 26 years ago yesterday, when the Voyager 2 spacecraft flew past Neptune. There have been no new missions to these worlds since then, but if a new proposed mission gets the green light, that may change in the not-too-distant future.
Jim Green, head of NASA’s planetary sciences, announced that NASA’s Jet Propulsion Laboratory (JPL) will now be studying a possible flagship mission to return to Uranus and/or Neptune. The news was announced at the Outer Planets Assessment Group meeting in Laurel, Md.
“I’ve asked [the Jet Propulsion Laboratory] to initiate an ice giant study,” Green said.
If it goes forward, this would be the next major Solar System mission since the Mars 2020 Rover and the Europa Multiple Flyby Mission (formerly called Europa Clipper). There is also the Juno mission currently en route to Jupiter, but it is scheduled to end by 2018. Flagship missions are larger and more complex than smaller-scale missions, designed for in-depth, ongoing study of planets and moons. Previous ones have included Cassini, Galileo, and Voyager. The Cassini mission will also end in 2017, when the fuel finally runs out and the spacecraft plunges into the gas giant’s atmosphere, as planned. This new mission, if approved, should cost less than $2 billion, according to Green.
The as-yet unnamed mission could use the massive Space Launch System (SLS) rocket, now being built and tested, to get to Uranus or Neptune faster than was previously possible, although would probably still be later in the 2020s. Even, it would be unlikely to close the so-called “50-year gap”—the time between the last visit to the ice giants and the predicted return to them.
There was one previous mission proposal to return to Neptune, called Argo, but it never got off the ground due to a limited supply of plutonium, which missions to the outer Solar System use since there is not enough solar energy available at such great distances from the Sun. Argo could have launched in a timeframe of 2015 to 2020, but it is now too late for that. Argo would have required gravity assists from Jupiter and Saturn to speed up the spacecraft, but that is no longer an option at this late stage. There is now, however, funding again for plutonium, making such long journey possible again. Of course, all of this is dependant on funding, as well.
Just like the Jupiter and Saturn systems, Uranus and Neptune are of great interest to planetary scientists as they are also like miniature solar systems, with many moons of wide geological variety. They are similar in composition, with atmospheres composed primarily of hydrogen and helium, along with traces of hydrocarbons and possibly nitrogen, as well as water, methane, and ammonia ices. The interiors are mostly ice and rock. Altogether, Neptune has at least 14 known moons and Uranus has 27.
Triton, Neptune’s largest moon, has active plumes or “geysers” of nitrogen erupting from ice volcanoes (cryovolcanoes). Something like the geysers on Saturn’s moon Enceladus, except those one are spewing water vapor and ice particles instead of nitrogen. So far, Triton has only been visited by one spacecraft, Voyager, and only one side was imaged in high-resolution. Even so, scientists have already seen tantalizing glimpses of this fascinating world, with weird “cantaloupe” terrain and the geysers as well as a possible subsurface ocean. The major science objectives of studying Triton closer include the interior structure, surface geology, surface composition and atmosphere, the plumes, and Triton’s interaction with Neptune’s magnetosphere. Due to the scarcity of craters, Triton’s surface is thought to be quite young, only about 10-100 million years, meaning that the moon is still geologically active. Triton is also thought to be a dwarf planet captured by Neptune’s gravity, rather than forming in place, due to its unusual retrograde orbit.
The five main moons of Uranus are Miranda, Ariel, Umbriel, Titania, and Oberon. They’re combined mass would be less than half that of Triton. The largest, Titania, has a radius less than half that of our own Moon. They don’t appear to be geologically active now, but Miranda has obvious signs of past activity, with deep canyons, terraced layers, and a chaotic variation in surface ages and features. It looks like bits of different moons all randomly mashed together. The moon also has the tallest known cliff in the Solar System, Verona Rupes, which is 3-6 miles (5-10 kilometers) in height.
There is an extensive overview of the scientific goals of OPAG, including the continued study of Jupiter, Saturn, Uranus, Neptune, and geologically active moons such as Europa, Enceladus, Titan, Io, Ganymede, and Triton. Many of these moons are known or suspected to have subsurface oceans of water, making them potentially habitable and prime targets for astrobiology and the search for evidence of life elsewhere in the Solar System, even if just microscopic. Some of the primary goals include:
- Study origin and evolution of our Solar System – giant planet migration, with major complementarity with exoplanets
- Investigate habitability of icy worlds – to gain insight into the origin of life on Earth
- Understand the dynamic nature of processes in our Solar System – importance of time domain
- Explore giant planet processes and properties
- Use giant planets to further our understanding of other planets and extrasolar planetary systems
- Determine giant planets’ influences on habitability
Before moving forward, the proposed mission will need to be endorsed by planetary scientists in the 2022 decadal.
“I’m sure they will,” Green said. “They’re very worthy.”
Uranus and Neptune are both jewels in the outer Solar System, just waiting for further exploration; if the new proposal becomes reality, we may get a chance to do just that after all. It was once thought that the outer Solar System was a relatively dead place, geologically speaking, too far from the Sun for much if any activity to be going on. But as the in-depth studies of Jupiter and Saturn have shown, that is not the case at all, with a wide variety of worlds including ones with oceans under their surfaces. Very likely, and hinted at so far, the same can be said about the Uranus and Neptune systems as well.
More information about the Outer Planets Assessment Group (OPAG) is available here.
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