Illustration of Neptune seen from the surface of its largest moon, Triton. Triton has one of the … [+] coldest surfaces in the known Solar System. In this image we see Triton at night, with the Sun below the horizon to the rear. The surface is lit only by the ghostly glow of Neptune itself. The shadow of Triton cast by the Sun can be seen on Neptune’s face.
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Back in 1989 NASA’s Voyager 2 became the first spacecraft to visit and photograph the planet Neptune, the eighth planet from the Sun.
Nothing has been back since. That’s despite Neptune-size exoplanets—planets orbiting other stars—appearing to be by far the most common.
It’s also despite the fact that one of its moons, Triton, is now believed to be a captured dwarf planet from the Kuiper Belt. Essentially, it’s a “Pluto” in orbit of Neptune.
Should NASA send a mission to explore Neptune and Triton? A proper investigation of an “ice giant” planet has been high on NASA’s wants list for a while.
It’s possible that the Neptune Odyssey concept mission could get green-lit by the Decadal Survey for Planetary Science and Astrobiology, a report compiled by the National Academy of Sciences that will set out the priorities for NASA for the next 10 years. It will be published on April 19, 2022.
Here’s why planetary scientists think that, yes, it is time to return to Neptune:
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Triton is a top astrobiology target and may have an ocean
Perhaps the most compelling reason to visit Neptune is to closely study its geologically active moon Triton. It’s a dark and shivering place (around -391° Fahrenheit/-235°Celsius on its surface, according to Voyager 2), but has almost no visible craters, so its surface must be constantly renewing itself. In short, it’s geologically active.
It’s also potentially an ocean world with liquid water under its icy crust; Voyager 2 saw geysers spewing dark material about 8km up, something only seen elsewhere on Enceladus at Saturn, and intermittently at Europa at Jupiter.
However, Voyager 2 managed only to photograph 40% of Triton’s surface. So just like Pluto, Triton is geologically active and may be an ocean world, as well as being a fellow Kuiper Belt object.
Triton is also the largest Kuiper Belt object we know of and the only moon in our Solar System that’s in a retrograde orbit.
Colour photograph of Neptune’s largest moon, Triton. Nearly two dozen images were combined to … [+] produce this comprehensive view of the Neptune facing hemisphere of Triton. Dated 20th century. (Photo by: Universal History Archive/Universal Images Group via Getty Images)
Universal Images Group via Getty Images
We can solve the riddle of how the Solar System formed
“One of the big open questions we have in our Solar System right now is its formation,” said Dr. Cindy L. Young, a Research Physical Scientist at NASA Langley Research Center, who helped design the concept for Neptune Odyssey’s atmospheric probe. “The way the noble gases are distributed can let us know if the ice giants formed where they are now, or closer in before migrating to their current positions.”
Noble gases don’t chemically bind to other elements so they’re the purest tracers of Solar System formation. We have measurements at Saturn and Jupiter, but we need read-outs from Uranus and Neptune to tease-out the formation history of the Solar System. “Knowing exactly where the noble gases are concentrated would help lock-in the formation history of the Solar System, but the only way to measure them in the atmosphere is in-situ using a spacecraft,” said Young.
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Neptune doubles as an exoplanet
Of the 5,000+ exoplanets detected elsewhere in our galaxy, the vast majority are Neptune-sized. We don’t know for sure that they are Neptune-like, but the fact that they are the same radius as a planet in our own solar system surely bolsters the science case for sending a mission.
Conducting complicated scientific investigations of alien planets from many light years away is going to be a lot easier for exoplanet-hunters if they have an analog in our solar system that much is known about.
A mission may be able to de-orbit and visit Pluto
If it’s selected then Neptune Odyssey won’t reach its target until at least 2044, but it could go on far longer than its four-year mission. After all, Cassini at Saturn lasted nine years longer than expected. It could even do something astounding and de-orbit—and become another mission.
“In principle we could de-orbit and continue out into the Solar System and perhaps visit Pluto, which is that direction,” said Abi Rymer, a Program Officer at NASA who acted as Principal Investigator for Neptune Odyssey concept mission. “It would be wonderful to have the same spacecraft study both Triton and Pluto.”
Neptune from Voyager 2 spacecraft, c1980s. The Voyager 2 space probe was launched by NASA in August … [+] 1977. The purpose of the Voyager programme was to study the outer Solar System. Artist NASA. (Photo by Heritage Space/Heritage Images/Getty Images)
Getty Images
We have to go soon … or wait 12 years
Neptune is far, far away—which is why the last Decadal Survey in 2010 appeared to favor Uranus as the top target—but things have changed and rare window of opportunity is about to open for Neptune.
“Orbital mechanics is probably going to decide for us whether we go to Uranus or Neptune because we need to flyby Jupiter,” said Kunio Sayanagi at Hampton University, Virginia, who also worked on the Neptune Odyssey proposal. “To go to Uranus the launch needs to happen by 2033, which is a stretch, but for Neptune it’s 2035, which is more realistic.”
Exactly when a mission can be sent to Uranus, or Neptune, depends on the relative position of Jupiter, which can help give a spacecraft a gravitational slingshot. That drastically shortens the cruise phase.
Since Jupiter takes 12 years to orbit the Sun, that’s how often it’s possible to propose a mission to one of the ice giants.
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Neptune would require a nuclear-powered flagship mission
Either way, missions beyond Jupiter are expensive because it’s not possible to take advantage of solar power, which works efficiently enough to power a spacecraft only to about 10 AU from the Sun. About 29 au from the Sun—so 30x the distance from the Earth to the Sun—Neptune gets a mere 0.001 times the amount of sunlight that our planet does. Voyager 2’s images took four hours to travel through space back to Earth.
That all means that nuclear power is required—and, hence, an expensive billion-dollar flagship mission. Whether NASA is about to be asked to consider going to Neptune is, Until April 19, anyone’s guess. However, if it is then expect a delay to the Mars Sample Return mission. Planetary scientists are on tenterhooks …
Wishing you clear skies and wide eyes.
