In 1989, NASA’s Voyager 2 was the first spacecraft to visit and photograph Neptune, the eighth planet from the Sun.
Nothing has come back since then. This is despite the fact that Neptune-sized exoplanets – planets orbiting other stars – appear to be by far the most common.
This is despite the fact that one of its moons, Triton, is now thought to be a captured dwarf planet from the Kuiper Belt. It is essentially a “Pluto” orbiting Neptune.
Should NASA send a mission to study Neptune and Triton? Proper study of an “ice giant” planet has been high on NASA’s wish list for some time.
It’s possible that the Neptune Odyssey concept mission will get the green light from the Decadal Survey for Planetary Science and Astrobiology, a report compiled by the National Academy of Sciences that will set NASA’s priorities for the next 10 years. It will be released on April 19, 2022.
Here’s why planetary scientists think so, yes, it is Time to return to Neptune:
Triton is a top astrobiology target and may have an ocean
Perhaps the most compelling reason to visit Neptune is to study its geologically active moon, Triton. It’s a dark and trembling place (about -391 degrees Fahrenheit/-235 degrees Celsius on its surface, according to Voyager 2), but has almost no visible craters, so its surface must constantly renew itself. In short, it is geologically active.
It’s also possibly an ocean world with liquid water beneath its icy crust; Voyager 2 saw geysers spewing dark material about 8 km high, something seen only elsewhere on Enceladus near Saturn and intermittently on Europa near Jupiter.
However, Voyager 2 only managed to photograph 40% of Triton’s surface. So, just like Pluto, Triton is geologically active and could be an ocean world, as well as another Kuiper Belt object.
Triton is also the largest Kuiper Belt object known to us and the only moon in our solar system in retrograde orbit.
We can solve the mystery of how the solar system formed
“One of the big unanswered questions we currently have in our solar system is its formation,” said Dr. Cindy L. Young, a physics scientist at NASA Langley Research Center who helped develop the concept for the Neptune Odyssey atmosphere probe. “The way the noble gases are distributed can tell us whether the ice giants formed where they are now or closer before migrating to their current positions.”
Noble gases do not chemically bind to other elements, so they are the purest tracers of solar system formation. We have measurements from Saturn and Jupiter, but we need readings 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 with a spacecraft,” Young said.
Neptune is also an exoplanet
Of the more than 5,000 exoplanets discovered elsewhere in our galaxy, the vast majority are the size of Neptune. We don’t know for sure if they’re Neptune-like, but the fact that they’re the same radius as a planet in our own solar system certainly strengthens the scientific case for sending a mission.
Conducting complicated scientific surveys of alien planets from many light-years away will be much easier for exoplanet hunters when they have an analogue in our solar system about which much is known.
A mission may leave and visit Pluto
If selected, Neptune Odyssey won’t reach its goal until at least 2044, but it could take far longer than its four-year mission. After all, Cassini lasted nine years longer than expected at Saturn. It might even do something amazing and leave orbit – and become another mission.
“In principle, we could de-orbit and go out into the solar system and maybe visit Pluto, that’s that direction,” said Abi Rymer, a NASA program officer who served as lead investigator for the Neptune Odyssey concept mission. “It would be wonderful if the same spacecraft could study both Triton and Pluto.”
We have to go soon… or wait 12 years
Neptune is far, far away – which is why the last Decadal poll in 2010 seemed to favor Uranus as the top target – but things have changed and a rare window of opportunity for Neptune is about to open.
“Orbital mechanics will likely decide for us whether we go to Uranus or Neptune because we have to fly past Jupiter,” said Kunio Sayanagi of Hampton University, Virginia, who also worked on the Neptune Odyssey proposal. “To get to Uranus, the launch has to be 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 starship a gravitational slingshot. This drastically shortens the cruise phase.
Since Jupiter takes 12 years to orbit the sun, it is as often possible to propose a mission to one of the ice giants.
Neptune would require a nuclear-powered flagship mission
In any case, missions beyond Jupiter are expensive due to the inability to utilize solar energy, which operates efficiently enough to power a spacecraft only up to about 10 AU from the Sun. Located about 29 AU from the Sun—30 times the distance from Earth to the Sun—Neptune receives just 0.001 times as much sunlight as our planet. Images from Voyager 2 took four hours to travel through space back to Earth.
It all means nuclear power is needed – and with it a multi-billion dollar flagship mission. Whether NASA will be asked to consider a trip to Neptune is anyone’s guess until April 19th. If this is the case, however, a delay in the Mars Sample Return mission can be expected. Planetary researchers are in suspense …
I wish you clear skies and big eyes.