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Here in the 21st century, we’ve learned so much more about our Universe than we could have imagined even just a single generation ago. Back in 1990, we hadn’t discovered any planets orbiting stars beyond our own Solar System; today, we’re closing in on 6000 confirmed exoplanets. Back then, the only prospects for life beyond Earth were potential microbes on planets or moons in our backyard; now, we know of scores of stars that may host living worlds around them. And a number of missions that didn’t just image the other planets and/or moons from afar, but orbited, probed, or even landed on them to explore them have occurred, teaching us about the complex chemistry and composition of worlds that are wildly different from our own.

Over all the time that’s passed since the Voyager missions, however, there are two major planets in our Solar System that remain unexplored from up close — unvisited — since the late 1980s, when Voyager 2 flew by them. Uranus and Neptune, the smallest gas giants in the Solar System and the closest analogues we have to the most common type of exoplanet found thus far: the mini-Neptune. In fact, in recent months, many have speculated that some of these mini-Neptunes may be potentially inhabited, advancing the science case for studying the worlds we do have that are like them, Uranus and Neptune, up close.

Here in the JWST era, we’ve seen these worlds better than ever since Voyager 2’s flyby, but haven’t designed or flown a mission to go visit them in all the time since. 2034 will be the perfect opportunity to change that, however. Here’s the science of why.

Pioneer 11, following in the footsteps of Pioneer 10, actually flew through Jupiter’s lunar system, then used Jupiter’s gravity as an assist maneuver to take it to Saturn. While exploring the Saturnian system, a planetary science first, it discovered and then nearly collided with Saturn’s moon Epimetheus, missing it by an estimated ~4000 km. Newtonian gravity, alone, was capable of calculating these maneuvers.

The Solar System is a complicated — but thankfully, regular — place. The best way to get to the outer Solar System, which is to say, any planet beyond Jupiter, is to use Jupiter itself to help you get there. In physics, whenever you have a small object (like a spacecraft) fly by a massive, stationary one (like a star or planet), the gravitational force can change its velocity tremendously, but its speed must remain the same.

But if there’s a third object that’s gravitationally important, that story changes slightly, and in a way that’s particularly relevant for reaching the outer Solar System. A spacecraft flying by, say, a planet that’s bound to the Sun, can gain-or-lose speed by stealing-or-giving-up momentum to the planet/Sun system. The massive planet doesn’t care, but the spacecraft can get a boost (or a deceleration) depending on its trajectory.

This type of maneuver is known as a gravity assist, and it was essential in getting both Voyager 1 and Voyager 2 on their way out of the Solar System, and more recently, in getting New Horizons to fly by Pluto. Even though Uranus and Neptune have spectacularly long orbital periods of 84 and 165 years, respectively, the mission windows for getting to them recur every 12 years or so: every time Jupiter completes an orbit and lines up with Earth, Uranus, and Neptune once again.

By either passing inside of a planet’s orbit while plunging toward the Sun (as shown), or outside of a planet’s orbit while moving away from the Sun, a spacecraft can get de-boosted via the gravity assist/gravitational slingshot mechanism. The two opposite maneuvers would increase the spacecraft’s speed, resulting in a velocity boost, rather than a de-boost. Both are used in navigating spacecraft across the Solar System.

A spacecraft launched from Earth typically flies by some of the inner planets a few times in preparation for a gravity assist from Jupiter. A spacecraft flying by a planet can get proverbially slingshotted — gravitational slingshot is a word for a gravity assist that boosts it — to greater speeds and energies. If we wanted to, the alignments are right that we could launch a mission to Neptune today. Uranus, being closer, is even easier to get to.

Bac in 2009, the Argo mission was proposed: it would fly-by Jupiter, Saturn, Neptune, and Kuiper belt objects, with a launch window lasting from 2015 to 2019. However, performing flyby missions are relatively easy, because you don’t have to slow the spacecraft down relative to the planet you’re targeting.

Inserting it into orbit around a world is harder, but it’s also far more rewarding. Instead of a single pass, an orbiter can get you whole-world coverage, multiple times, over long periods of time. You can see changes in the atmosphere of a world, and examine it continuously in a wide variety of wavelengths invisible to the human eye. You can find new moons, new rings, and new phenomena that you never expected. You can even send down a lander or probe to the planet or one of its moons. All of that and more already happened around Saturn with the Cassini mission, which came to a planned end in 2017.

A 2012 (top) and a 2016 (bottom) image of Saturn’s north pole, both taken with the Cassini wide-angle camera. The difference in color is due to changes in the chemical composition of Saturn’s atmosphere, as induced by direct photochemical changes.

Cassini didn’t just learn about the physical and atmospheric properties of Saturn, although it did that spectacularly. It didn’t just image and learn about the rings, although it did that too. What’s most incredible is that we observed changes and transient events that we never would have predicted.

• Saturn exhibited seasonal changes, which corresponded to chemical and color changes around its poles.
• A colossal storm developed on Saturn, encircling the planet and lasting for many months.
• Saturn’s rings were found to have intense vertical structures and to change over time; they’re dynamic and not static, and provide a laboratory to teach us about planet-and-moon formation.
• And, with its data, we solved old problems and discovered new mysteries about its moons Iapetus, Titan, and Enceladus, among others.

In other words, we discovered all that we discovered about Saturn — along with its system of moons and rings — because we dared to go there with a high-tech, radiation-hard dedicated orbiter mission, and because we equipped it with a suite of instruments that could probe so much about this planet and the moons it encountered. It was loaded with discovery potential, and that enabled it to find out what was previously unknown about the Saturnian system: to the benefit of all of humanity.

Over a period of 8 months, the largest storm in the Solar System raged, encircling the entire gas giant world. The storm itself was large enough that it was capable of fitting as many as 10-to-12 Earths inside. Cassini, although it wasn’t expecting this to occur, was equipped with instrument technology that was more than sufficient to discover and study this unprecedented feature.

Without a doubt, there’s no question that we absolutely want to do the analogous things for Uranus and Neptune. Many orbiting missions to Uranus and Neptune have been proposed and made it quite far in the mission submission process, but none have actually been slated to be built or fly. NASA, the ESA, JPL, and the UK have all proposed Uranus orbiters that are still in the running, but no one knows what the future holds.

One of the major, flagship-class missions proposed to NASA’s planetary science decadal survey in 2011 was a Uranus probe and orbiter; it was ranked #3, but in the most recent planetary science decadal, it was ranked as the highest-priority planetary flagship mission. Uranus, as well as its outer neighbor, Neptune, are both suspected (based on modeling and Voyager 2 data) to have enormous liquid oceans beneath their atmospheres, which an orbiter should be able to discover for certain. The mission could also include an atmospheric probe, with the potential to measure cloud-forming molecules, heat distribution, and how wind speed changes with depth.

In all the time since Voyager 2, which flew by Uranus in 1986 and Neptune in 1989, we’ve only studied these planets from afar. The most recent views that we have of these worlds, however, are indeed the most spectacular ones obtained in all the time since: visions of Uranus and Neptune from the James Webb Space Telescope.

Numerous features surrounding Neptune, as identified in the JWST images. All 7 of Neptune’s inner moons can be seen here, along with the two main rings and two dusty, more diffuse rings seen here. Triton, although captured by JWST, is too far away to be a part of this cropped JWST image.

JWST’s views of Uranus and Neptune showed us features we only ever had hints of from Voyager 2 data, with Uranus in particular making a very interesting test case. You see, Neptune got its JWST close-up more than halfway through 2022, and so many features were revealed in a visually stunning way. They include:

• Neptune’s disk,
• its highly reflective clouds,
• all seven of its known inner moons,
• its four major rings (Adams, Lassell, Le Verrier, and Galle),
• and its highly reflective largest moon, Triton,

in our best view of all of these features since 1989’s visit.

Uranus, on the other hand, has already been viewed twice by JWST, with its second, superior looking coming in late 2023. Uranus is a bit special: of all the planets in the Solar System, it’s the only one that rotates primarily on its side, with its rotational axis oriented at a nearly 90° tilt (at around 98°) to the “vertical” rotation of the other planets. With an 84 year orbit around the Sun, this means that every 21 years, it undergoes transitions from Uranian solstice, where one pole points directly at the Sun and the other point directly away, to Uranian equinox, where each part of that world receives equal night and daylight, and then back again in the next 21 years.

When Voyager 2 flew by Uranus in 1986, the planet was near solstice, with its southern hemisphere facing the Sun and its northern hemisphere facing away. In 2007, Uranus achieved equinox, and now heads toward its next 2028 solstice. It won’t reach equinox again until 2049, when JWST will likely be out of fuel and defunct, but when an orbiter mission could be present.

When Voyager 2 flew by Uranus in 1986, it was at Uranian solstice. It appeared bland and featureless due to the Sun heating one of its poles, not the entire, rapidly rotating planet. Then, in 2007, Uranus was at equinox, displaying rapidly evolving atmospheric features and auroral activity visible remotely: from Hubble and from the world’s flagship ground-based telescopes. Now, however, it’s approaching Uranian solstice once again, which it will reach in 2028. This time, the opposite pole from 1986 is starting to face the Sun, and the planet, again, overall, will soon become largely featureless in appearance.

Therefore, when JWST takes its looks at Uranus, we’re seeing it as it’s finishing its transition from equinox-to-solstice, illuminating one pole, preferentially, but only obliquely: at an angle. What JWST saw was spectacular, and again, unprecedented since Voyager 2.

• Uranus currently displays a polar cap, although those high-altitude ices and clouds are beginning to dissipate due to their continuous exposure to sunlight.
• Surrounding that cap is a less dense region, where the cap is evaporating.
• Dark lanes indicate further evidence of evaporation, punctuated by bright spots: Uranian storms.
• Then the inner Zeta (ζ) ring, followed by the α and β rings, the η ring, the thin δ, and the thick ε ring.
• After that, nine of Uranus’s prominent, inner moons appear: Bianca (#3) through Puck (#12), excluding only the too-small Cupid.
• And finally, the five major moons Miranda, Ariel, Umbriel, Titania, and Oberon can be seen.

The five largest moons of Uranus, in order from the innermost to the outermost, are Miranda, Ariel, Umbriel, Titania, and Oberon, with the latter two being the largest and first-discovered among Uranus’s moons. All of these moons and the innermost one rotate within a single degree of Uranus’s orbital plane except for Miranda, which is inclined by 4.3 degrees.

While JWST can continue to image Uranus for approximately the next 20 years or so from afar, the ideal goal is to go there, in situ, during the opposite conditions from when we were last there. We went during solstice last time, with Voyager 2, and therefore the next time, ideally, we’ll go to coincide with equinox. And it just so happens that the travel-time to Uranus, to enter an easily-insertable orbit around it with the appropriate gravity assists on the way there, involves about a 13 year travel-time. Under ideal conditions, after leaving Earth, you’d get a gravity assist from Jupiter, and then you fly past Uranus, dropping off (and inserting) an orbiter and possibly an atmospheric probe as well, and then you’d continue on, assisted by Uranus’s gravity, to Neptune, where you’d then have a second orbiter and possibly atmospheric probe, too.

Most orbiters that have been proposed, with or without probes, typically are slated for about 5 year science lifetimes. What should give us all tremendous hope for a future mission is that there will be a launch window to reach both worlds with a single mission, Uranus and Neptune alike, that align at once: in 2034. That’s when the conceptual ODINUS mission would send twin orbiters to both Uranus and Neptune simultaneously: arriving at Uranus in 2047, just two years before the next (2049) Uranian equinox, and then allowing an orbiter to arrive at Neptune about three years later: in 2050. The ODINUS mission itself, as originally proposed, would be a spectacular, joint venture between NASA and the ESA.

Uranus and its five major moons are depicted here in this montage of images acquired by the Voyager 2 mission in 1986. The five moons, from largest to smallest, are Ariel, Miranda, Titania, Oberon, and Umbriel. Puck, the 6th largest moon, is interior to all of them, and appears in the first and second JWST images of Uranus alongside these five. An orbiter and atmospheric probe, combined, could revolutionize our knowledge of this world.

In order to get the maximum amount of science possible out of the mission, you’d have to design your instruments properly. The orbiter would require multiple separate instruments on it designed to image and measure various properties of Uranus, its rings, and its moons. Uranus and Neptune should have enormous liquid oceans beneath their atmospheres, and an orbiter should be able to discover it for certain. The atmospheric probe would measure cloud-forming molecules, heat distribution, and how wind speed changed with depth. Originally, missions were focused on just one world at a time: Uranus as the higher-priority one (because it’s closer and has been studied for longer), and Neptune as the secondary one.

As proposed by the ESA’s Cosmic Vision program, the Origins, Dynamics, and Interiors of the Neptunian and Uranian Systems (ODINUS) mission goes even farther: expanding this concept to two twin orbiters, where we would send one to Neptune and one to Uranus. A launch window in 2034, where Earth, Jupiter, Uranus, and Neptune all align properly, could send them both off simultaneously. The scientific advantages of orbiters over a flyby mission are tremendous: longer observing times over much longer temporal baselines, the ability to focus on multiple targets over time, and the ability to discover features you may not have even anticipated would be there. Its proposed suite of six-to-eight instruments would not just take images and spectra, but seismic, magnetic, and ion measurements. The only additional costs are in terms of fuel, and enabling your orbiting spacecraft to perform burns, slow down, and enter and maintain stable orbits. The deluge of science that you get from remaining around a planet, long term, more than makes up for those increased costs.

A Plutonium-238 oxide pellet glowing from its own heat. Also produced as a by-product of nuclear reactions, Pu-238 is the radionuclide used to power deep-space vehicles, from the Mars Curiosity Rover to the ultra-distant Voyager spacecraft. It is most useful very far from the Sun, and Pu-238 also powered the Cassini and Galileo missions.

The current limitations on a mission like this don’t come from technical accomplishments; the technology exists to do it today. The difficulties are a combination of:

• political, arising from NASA’s finite, limited, and threatened budget,
• physical, because even with low-cost, heavy payload launch vehicles, we can still only send a limited amount of overall mass to the outer Solar System,
• and practical, because at these incredible distances from the Sun, solar panels will not power a sustained mission.

That practical limitation requires a power source of radioactive isotopes, with the radioisotope thermoelectric generator (RTG) Plutonium-238 serving as our preferred source for such missions.

However, most places in the world stopped producing Pu-238 back in the 20th century, and if we want enough to power a dual orbiter mission to Uranus and Neptune by the time the launch window arrives in 2034, we should really start producing it now. For the New Horizons mission to Pluto, an orbiter would have been a much more challenging mission strategy; New Horizons was too small and its speed was far too great, plus Pluto’s mass is quite low for attempting an orbital insertion. But for Neptune and Uranus, particularly if we choose the right gravity assists from Jupiter (and possibly Saturn), this could be feasible.

A unique, dual mission to both Uranus and Neptune could be launched in 2034, allowing us to fill in the biggest gaps in our knowledge of the Solar System: the gaps of what’s truly happening on and around our final two planets. The only way we’ll find out is if we dare and go look at what’s out there.