When Will We Return? The Clock Ticks as the Launch Window for a Mission to the Ice Giants Nears.
No spacecraft has visited Uranus and Neptune since the historic Voyager 2 almost 40 years ago. But our chances of launching a new mission in time will soon slip by. Despite saying that a mission to the ice giants is the greatest priority, problems already delay the launch.
Eyes on the outer Solar System
From the buzz and warmth of the inner Solar System, we quickly forget what lies beyond the rocky planets that surround our neighborhood. A quick look through a telescope reminds us of the protective gas giants Jupiter, with its vibrant clouds, and Saturn, with its distinctive ring system.
But beyond that, silence. Emptiness. Ice.
As the amount of time it takes sunlight to reach the outer Solar System increases to hours, the colder it gets, with temperatures approaching near total zero.
In this icy realm, two giant blue worlds emerge: Uranus and Neptune, travelling around the Sun in 84 and 164 Earth years, respectively. Their blue color makes these planets look like they are covered in deep oceans — and that is true, only that these oceans consist of clouds and water-ammonia. We call them the ice giants, marking the frontier between the eight planets and a lonely world inhabited by comets and dwarf planets.
Since the first ice giant was discovered in 1781, we haven’t learned much about Uranus and Neptune, and only one spacecraft took a quick look at these interesting planets.
This is why we have to go back. This is why the ice giants are calling for us.
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| Uranus as seen by Voyager 2; Source: Wikipedia |
An oddly tilted world named Uranus
In Greek mythology, Uranus is the deity of the sky — only fitting that a planet as blue as the sky would get the same name. However, despite its heavenly name, Uranus is a world of extremes.
Orbiting the Sun 20 times the distance between the Earth and Sun, Uranus’s axial tilt is 82° — that means that it orbits on its side, the poles being where you’d expect the equator. During solstice, each pole is exposed to 42 years of continuous sunlight, followed by 42 years of eternal darkness in its 84-year-long journey around the Sun.
It’s clear that Uranus is an alien world to us short-lived Earthlings, looking at a planet where one year lasts an average human life span.
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| This photo taken by the Webb Telescope shows Uranus' rings in detail; Source: Wikipedia |
In 1986, Uranus received its first spotlight when Voyager 2 went on its Grand Tour through the outer Solar System and reached the ice giant after nearly ten years of travel. Voyager 2 only had a few hours during closest approach to study the planet.
During the entire observation phase, Voyager 2 beamed 7’000 photos back to Earth, revealing 10 new moons and two new rings. The spacecraft also found that Uranus has a very odd magnetic field, tilted by 59° from its rotational axis, so it varies greatly in strength.
We know of 28 moons orbiting Uranus today, but our knowledge relies on careful observation by astronomers and photographs taken by Voyager 2.
Among the new moons Voyager 2 discovered are Cordelia, the innermost moon in the Uranian system, and Puck, the biggest undiscovered moon.
Photos taken by the spacecraft reveal intricate surface features on Miranda that make it look as if it was stitched together by an amateur. The moon has deep canyons and terraced craters — but what stands out most is that there are regions that appear very old, while others seem relatively young.
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| The strange Uranian moon Miranda; Source: Wikipedia |
It has been theorized that a meteorite collided with Miranda, melting some of its subsurface ice and allowing water to seep through the surface, where it froze over again.
Titania, the largest Uranian satellite, is thought to have a subsurface ocean (might be frozen), and is also geologically active. Scientists were able to study Titania in more detail in 2001, when it occulted the bright star HIP 106829, leading to better estimates of Titania’s physical properties.
As noted above, Voyager 2 found two more of Uranus’ rings. Astronomers first noted Uranus’ ring system the year Voyager 2 was launched, in 1977, and today we know of 13 rings, the second most complex after Saturn’s.
Voyager 2 came as close as 81’000km above the cloudtops of Uranus, but after months of studying the syetem from near and far, it was time to venture on to other worlds: Neptune.
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| True color image of Neptune as seen by Voyager 2; Source: Wikipedia |
Stormy approach to the corners of the Solar System
Where the planets make way for the icy realms of the Kuiper Belt, Neptune resides, 30 times the distance between the Earth and Sun. Here, the temperatures drop below -200°C and the Sun’s light barely illuminates any surfaces. It was only in 2011 that one year elapsed in Neptune-time since its discovery in 1846.
Neptune’s name already plays at its appearance — a deep blue planet with dynamic cloud systems. At least that’s how we thought of Neptune based on the only images we had when Voyager 2 took them in 1989.
However, the blue of Neptune as we know it is only an exaggeration of color to better contrast the bands of clouds streaking the planet. Just like Uranus, Neptune is a pale blue dot.
Scientists re-evaluated data collected by Voyager 2 to create a more representing image of Neptune as a light-blue world. So even decades later, we still have a lot to learn from the spacecraft.
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| Color exaggerated photo of Neptune; Source: Space |
Uranus and Neptune have a lot in common: they are similar in size, shine in the same blue, and they have ring systems.
However, while Uranus has 28 moons, Neptune has only half as many, Triton being the biggest of them. It measures 2720km across and is extremely cold — its temperature lies at about -235°C and, just as we see it in the Uranian system, Triton is geologically active, with impact craters eroded by water flow.
Interestingly enough, Triton was once a Kuiper Belt dwarf planet before it was captured by Neptune!
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| Triton as seen by Voyager 2; Source: Wikipedia |
When Voyager 2 approached Neptune, it would be the closest encounter to any planet during its Grand Tour, venturing as close as 4’900km above cloudtops. Therefore, Voyager 2 was able to study Neptune’s dynamic atmosphere in detail, including the planet’s supersonic winds that blow opposite the planet’s rotation.
Most notably, Voyager 2 discovered that Neptune had a storm similar to Jupiter’s Great Red Spot, dubbed the Great Dark Spot. But while the Great Red Spot has been around for hundreds of years, the Great Dark Spot only makes an occasional appearance.
Radio silence: why we have to go back
If there’s one thing that makes our Solar System so unique, then it is that we have no super-Earths or mini-Neptunes — planets intermediate in size between Earth and Neptune. However, based on our data from the Kepler space telescope, these types of planets are the most common in our galaxy!
When astronomers want to learn more about a specific planet type, it offers to take a look at worlds closer to home, to understand those that are so difficult to spot. But what do we know about our own ice giants to understand exoplanetary Uranuses and Neptunes? Not a lot, since all our data is based on observations from home or from Voyager 2.
Going back to the ice giants means learning more about the Solar System and exoplanets that are ice giants.
For example, there are many hot-Neptunes, which orbit very close to their star, and we also know of a few actual exoplanetary ice giants. One was discovered 25’000 light years from home, orbiting its star at a similar distance as the Solar System’s Uranus.
Keep in mind that the fact that we have so few ice giant exoplanets in our database is mainly because they’re so difficult to detect — at their distances from their stars, they barely have any gravitational effects we may detect, and because their orbits take so long it’s a lucky chance to find a transiting ice giant. Our best case is gravitational microlensing, a rare alignment chance.
The reason to go back to the ice giants is simple: we don’t know enough. While other worlds can pose with numerous missions launched to them, Uranus and Neptune can only boast one visitor, which was only able to study the planets for a few months.
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| Description of the instruments on the UOP; Source: Wikipedia |
In 2022, it was announced that a mission to Uranus should be NASA’s next big priority for a flagship planetary mission within the next decade. One concept is the Uranus Orbiter and Probe (UOP), which shows quite some similarities to the Saturn mission Cassini-Huygens.
As its name suggests, the UOP consists of a multi-year orbiter, as well as a probe that would descend into Uranus’ atmosphere to collect valuable scientific data. The UOP would also be able to take a closer look at Uranus’ odd moons.
There are many questions the orbiter would answer about the ice giants, such as how atmospheric circulation works in ice giants and why both Uranus and Neptune have such misaligned magnetic fields.
And NASA is in quite a hurry to launch a Uranus mission. The initial plan was for the UOP to be launched by 2031 — the spacecraft would then arrive after a 13-year cruise in 2044.
Voyager 2 studied Uranus during the southern summer, so it’s best to observe Uranus during one of its different, long-lasting seasons on the next mission, and the Uranian southern spring starts in 2049.
In other words, arriving in 2044 would be ideal… if only NASA were able to launch in time. Last year, the space agency announced that due to plutonium production shortfall it would be more likely for the UOP to be launched by the mid to late 2030s. NASA also said it would rather delay the UOP than the Mars Sample Return mission.
So, we still have a long time to wait before our dear Uranus mission arrives.
If you’re stressed to see a Uranus mission relatively soon, then look out for Tianwen-4 to arrive at the ice giant in 2045 — of course, only if the spacecraft is launched on time.
China’s goals for Tianwen-4 are ambitious: the mission would be launched in 2029 with two spacecraft in one launch! One of the missions will head to the Jovian moon Callisto, while the second spacecraft will travel to the Uranian system.
That’s all great… but what about Neptune? Technologically and economically, a mission to Uranus is more feasible than the distant Neptune, so a lot of upcoming missions to the ice giants focus on the nearer Uranus.
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| Neptune's Great Dark Spot; Source: Wikipedia |
But Neptune is equally interesting for sciecne — after all, we want to learn more about its moon Triton and its seasonal Great Dark Spot. Another advantage for studying Neptune is that we’d also learn more about the Kuiper Belt, since this is the place from where Triton was captured.
After all, we have to learn how Triton exactly ended up in Neptune’s orbit.
One mission proposal is the Neptune Odyssey orbiter, which would, just like the UOP, have an atmospheric probe descending into the Neptunian clouds where supersonic winds rage.
The mission would be launched in 2031, receive a Jupiter gravity assist, and arrive at the Neptunian system 12 years later. Without Jupiter acting as a gravity slingshot, which would be the case were the spacecraft launched after 2031, the cruise would take 16 long years.
Scientific objectives for the Neptune Odyssey mission would be similar to Uranus’, only that we’d also be able to expand our knowledge on the Kuiper Belt thanks to Triton. In fact, it has been suggested that once the Neptune study ends, the spacecraft could de-orbit and head on to Pluto! We’ve only seen such a mission once so far, when Dawn visited the asteroids Ceres and Vesta.
The ice giants wait for us — but not forever. Launching a mission in time is crucial to ensuring that we maximize what we can learn about these distant and mysterious worlds of ice and wind.
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