The science of becoming "interplanetary": How can humans live in the outer Solar System?

With the right strategies and technologies, humans could live to the very edge of the Solar System someday!
Matthew  S. Williams
Triton’s summer sky of methane and carbon monoxide.ESO

Welcome back to the final installment in our "Going Interplanetary" series! In previous installments, we explored how humanity might someday live on Mercury, Venus, the Moon, Mars, the Asteroid Belt, and the largest moons of Jupiter and Saturn. Today, we will look at Uranus, Neptune, and the outer Solar System, and how humans could establish homes at the very edge of the Solar System!

Beyond Saturn and its system of moons, there lies a region of space that is the least explored and understood by humanity. Nevertheless, what we have discovered over the centuries has intrigued and mystified us and inspired us to learn more! This region of space includes the gas/ice giants of Uranus and Neptune, Pluto, the Kuiper Belt, and the many "dwarf planets" and icy bodies (many of which become comets) it contains.

Our general lack of knowledge concerning bodies in this region of space arises from how distant they are from Earth. Only one mission has conducted a flyby of Uranus and Neptune, the Voyager 2 spacecraft, which passed through the systems in 1986 on its way to the edge of the Solar System. During its flybys, Voyager 2 took the only close-up pictures we have of these planets, their largest moons, debris rings, atmospheres, and magnetic fields.

The New Horizons mission was the only other mission to visit any bodies in the outer Solar System, flying past Pluto in the summer of 2015. The images it acquired of Pluto were the sharpest ever taken and finally gave astronomers a close-up look at its surface features. Data obtained by its various instruments also led to breakthroughs in our understanding of Pluto's tenuous atmosphere, interior, geology, and dynamic landscape.

The science of becoming "interplanetary": How can humans live in the outer Solar System?
Source: NASA/JPL-Caltech

One of the main recommendations made in the Planetary Science Decadal Survey (2023 - 2032) was for a "flagship" mission to be sent to Uranus. This mission would include an orbiter and probe and launch by 2031 at the earliest. The China National Space Agency is also considering a mission to explore Neptune and its largest moon Triton, and it is planned for launch by 2034.

These missions could pave the way for future spacecraft that rely on more advanced propulsion. These spacecraft could rapidly transport robotic workers that would build the necessary infrastructure to extract resources and maybe build habitats for people seeking a new life in the outer Solar System. Someday, if we're fortunate enough, people could be listening to messages like this:

"Good morning, passengers! We are closing in on Sky Station, the gateway to the Uranian system and beyond! If you look out your left window, you'll see good ol' Sky Blue, the Cobalt Marble. Or, as you probably know her, Uranus! Isn't she lovely? We certainly think so! Be prepared to see plenty more of her in the coming days and weeks! Once you're unpacked and ready to go, our shuttles will escort you to your destination of choice.

"Enjoy skimming through Uranus' beautiful atmosphere, witness our famous aurorae and  'diamond rains' - from a safe distance, of course! Visit the poles of Titania, Oberon, Ariel, Umbriel, and Miranda, and see what it is like to witness constant daylight. And plot a visit to the Outer Rings, where our expert guides will teach you the fine art of Ring Surfing!"

"Those bound for Neptune are in for a treat. After a brief stay aboard Poseidon Station, you'll be free to explore Neptune and Triton, which await only the most adventurous of adventure tourists! Skim the Azure Marble's atmosphere, see the Great Dark Spot, and witness the diamond icebergs that float across the liquid interior. Walk among the icy surface of Triton, see its powerful cryovolcanoes, and experience what it's like to witness the psychedelic landscape!

"Anyone who isn't stopping there and will be proceeding to Pluto and the Kuiper Belt, we wish you a bon voyage and tip our hats to your adventurous spirit! Once you reach Lowell or Adlivun Station, the treasures of the Outer Rim will be free for you to experience! Walk the nitrogen glaciers of Pluto's Heart, Tombaugh Regio. Book a speeder tour and drive across the bright reach of Sputnik Planum or take a slow ride through Pluto's 'darker' region, Cthulu Macula. Or climb the peaks of the floating ice mountains, and stare into the depths of Virgil Fossae!

"Research crews bound for the outer marker must have their identification and bioscans ready for presentation at all security check-points. Make sure your passes are up to date. Uplink services are available for those who require access to planetary servers located deeper within the inner Solar System. The Extrasolar Research Council reminds you that the security of our stations and the health of our researchers are paramount!

"A reminder to all our passengers that transitioning between the full gravity of the respective stations to the local gravity can be challenging. On the Uranian satellites, Triton, Pluto, and Charon, you will be subject to gravitational forces significantly less than that of Earth. But once you've acclimated, we encourage you to take full advantage of the low-gravity environments."

Gas/Ice Giants

Like Jupiter and Saturn, Uranus and Neptune are predominantly composed of hydrogen and helium but contain more volatile elements like water, ammonia, methane, and various hydrocarbons. The presence of methane is what gives them their characteristic blue color. Based on current planetary models, scientists believe that the structure of ice giants is differentiated between a gaseous outer envelope, an icy mantle in the middle, and a rocky, metallic core. 

Because of their differences in composition, Uranus and Neptune are often referred to as "ice giants" to differentiate them from Jupiter and Saturn. This refers to the fact that conditions in the planet's interior cause the volatile elements they possess to be compressed to the point where they become solid. Another interesting feature is how the extreme pressure in their atmospheres will cause carbon atoms to crystalize, which results in "diamond rain."

The science of becoming "interplanetary": How can humans live in the outer Solar System?
Source: IGO/NOIRLab/NSF/AURA, J. da Silva/NASA /JPL-Caltech /B. Jónsson

The name Uranus is derived from the ancient Greek god Ouranos, the primordial god of the Sky in the Greek pantheon who mated with Gaia (the primordial Earth goddess) to produce the Titans. Cronos (Saturn in the Roman pantheon) was the chief of the Titans, who overthrew his father and mated with Rhea to produce the Olympian gods. While Uranus is (at times) visible to the naked eye, it was long thought to be a star and was not recognized as a planet until the 18th century.

Sir William Herschel first observed Uranus on March 13th, 1781 (it was identified as a planet two years later) and later observed two of its largest moons (Titania and Oberon). Unlike any other Solar planet, Uranus' axial tilt is almost parallel with the Solar plane, meaning it orbits on its side. This results in seasonal changes that are different from any other Solar planet, where one pole will experience 42 years of continuous sunlight while the other experiences 42 years of continuous darkness.

Neptune is invisible to the naked eye and was therefore unknown to ancient astronomers. German astronomer Johann Galle first observed it in 1846. In the Greek pantheon, Neptune was the god of the sea (the equivalent of the Roman Poseidon), and the planet's intense blue color inspired this choice of name. 

Dwarf Planets

While astronomers predicted the existence of Pluto (Planet X) during the late 19th century, it was not observed until 1930 by Clyde Tombaugh at the Lowell Observatory in Flagstaff, Arizona. The name Pluto (the Roman god of the underworld) was proposed by Venetia Burney, an eleven-year-old schoolgirl. Pluto measures an estimated 1476 mi (2376 km) in diameter and has a mass of roughly 17.7% of the Moon, resulting in a surface gravity of 6% that of Earth (0.063 g).

Between 2002 and 2007, astronomers discovered multiple Kuiper Belt Objects (KBOs) that are comparable in size to Pluto. These include (in order of discovery) Quaoar, Sedna, Orcus, Haumea, Eris, Makemake, and Gonggong. These bodies range in size and mass from an estimated 1,445 mi (2326 km) in diameter and 22% the mass of the Moon (Eris) to 570 mi (910 km) and 0.8% (Orcus).

Large Moons

Uranus' largest moons include Titania, Oberon, Ariel, Umbriel, and Miranda - which range in diameter from around 980.5 mi (1578 km) for Titania to 293.3 mi (472 km) for Miranda. Whereas William Herschel discovered Titania and Oberon in 1787, Ariel and Umbriel were discovered in 1851 by William Lassell. Miranda, the smallest and innermost of the five largest moons, was discovered in 1948 by Gerard Kuiper (who theorized the existence of the Kuiper Belt).

These moons were named based on suggestions made by John Herschel, the son of William Herschel (who discovered Uranus). Instead of figures from Greek mythology, John Herschel named them after characters from Shakespearean plays and the works of Alexander Pope. The tradition continued as more moons were discovered well into the 20th century.

Whereas Oberon, Titania, and Puck were named after faeries and sprites from William Shakespeare's A Midsummer Night's Dream, Mab was named after the fairy queen from Romeo and Juliet, and Ariel, Umbriel, and Belinda were derived from Alexander Pope's The Rape of the Lock. Miranda and several smaller retrograde moons were named after a character in Shakespeare's The Tempest, while the only known outer prograde moon, Margaret, is named from Much Ado About Nothing.

The science of becoming "interplanetary": How can humans live in the outer Solar System?
Source: CactiStaccingCrane/Wikimedia Commons

The Uranian satellites are believed to be composed of roughly equal proportions of water ice and rocky material that are likely differentiated between icy surfaces, rocky interiors, and metallic cores. For Titania and Oberon, there's even the possibility of a liquid water layer in between (assuming there is enough ammonia or other antifreeze compounds). The surface gravity on these satellites ranges from around 3.7% (0.037 g) of Earth-normal for Titania to 0.8% (0.008 g) for Miranda.

Neptune's largest moon, Triton, measures around 1,680 mi (2,710 km) in diameter and is the only satellite massive enough to achieve hydrostatic equilibrium (become spherical). It was discovered on October 10th, 1846, by English astronomer William Lassell, just 17 days after Neptune itself was discovered. The name Triton refers to the son of Poseidon, the Greek god corresponding to the Roman Neptune.

Its structure is differentiated between a surface composed predominantly of nitrogen ice, a water-ice crust, an icy mantel, and a large core of rock and metal. It also has a tenuous nitrogen atmosphere resulting from cryovolcanism, where liquid nitrogen and water erupt through the surface. This feature is relatively unique to Triton since it is one of a few moons in the outer Solar System known to be geologically active.

Like Jupiter's moons Io and Europa, and Saturn's moons Enceladus and Titan, this is believed to be the result of tidal flexing in the interior caused by the moon's gravitational interaction with its parent planet. Unlike the other large moons in the outer Solar System, Triton has a retrograde orbit - it orbits in the direction opposite to the planet's rotation.

Between its orbit and its composition (similar to Pluto), many scientists believe that Triton is a dwarf planet that was ejected from the Kuiper Belt and captured by Neptune's gravity. This theory would also explain why Neptune has no other large spherical satellites today. It is possible that it did it at one time, but these were destroyed when Triton was captured, causing collisions and orbital perturbations.

Charon, Pluto's largest satellite, was discovered in 1978 at the United States Naval Observatory Flagstaff Station (near the Lowell Observatory). It measures 753 mi (1,212 km) in diameter, slightly more than half that of Pluto, and is about 12.2% as massive as Pluto. Because of their size and mass, some consider Charon and Pluto to be co-orbiting objects rather than a planetary body and a satellite.

Benefits & Challenges

Having access to the outer Solar System and its many planets, moons, and other bodies presents numerous benefits. Like Jupiter and Saturn, Uranus and its many moons have a tremendous resource base, which includes vast amounts of silicate minerals, metals, and frozen volatiles like water, methane, and ammonia. These resources could not only be leveraged to build infrastructure locally but could be exported to facilitate human settlement elsewhere.

The science of becoming "interplanetary": How can humans live in the outer Solar System?
Source: ESO/L. Calçada

Metals could be used to fashion space stations and surface habitats, while silica could be harvested to make building and paving materials and soil for agriculture. The local water ice would provide abundant drinking and irrigation water and could be chemically dissociated to produce oxygen gas and hydrogen fuel for reactors. Methane could also be processed to create fuel and enrichen soil. Ammonia could be used to create fertilizers and nitrogen gas - an important buffer gas and the main constituent (78%) of Earth's atmosphere.

In terms of exports, ammonia could be shipped to Mars to assist in efforts to thicken and convert the atmosphere to something breathable. Methane would provide Mars with an essential greenhouse gas for warming the planet. And water, which is always in demand, could be exported to ensure that people all across the Solar System have enough to drink, water their plants, and establish biomes within habitats.

On the other hand, establishing settlements among the ice giants and their moons presents similar challenges, not the least of which are distance, airlessness, and low gravity. In all cases, Uranus, Neptune, Pluto, and other Kuiper Belt bodies are extremely distant from Earth. As a result, the variations in their distance from Earth are largely insignificant.

When they are closest - i.e., both planets are on the same side of the Sun - Uranus is about 1.6 billion miles (2.57 billion km) from Earth. When they are on opposite sides of the Sun, Uranus is about 1.957 billion mi (3.15 billion km) from Earth. Neptune ranges from around 2.67 billion mi (4.301 billion km) at its closest to 2.829 billion mi (4.553 billion km) at its farthest from Earth.

Pluto is an outlier considering the extremely elliptical nature of its orbit, which varies from 2.734 billion mi (4.4 billion km) at perihelion (closest) to 4.8 billion mi (7.375 billion km) at aphelion (farthest). This eccentric orbit also leads to a variation in its distance from Earth, which ranges from 2.6 billion mi (4.2 billion km) to 4.66 billion mi (7.5 billion km).

To put these distances in perspective, it took the Voyager 2 spacecraft roughly eight and a half years to reach Uranus (in 1986). This was with the help of several gravity-assist maneuvers, where spacecraft will slingshot around a planet (or the Sun) to pick up a boost in velocity. Its flyby of Neptune and Triton took place on August 25th, 1989, roughly ten years after launching from Earth.

As noted, the New Horizons mission is the only spacecraft to make a close flyby of Pluto and a Kuiper Belt Object (KBO) - Arrokoth. These occurred in July 2015 and January 2019 (respectively), a flight time of roughly nine and a half and thirteen years. These transit times make transporting crews and cargo to the outer Solar System highly impractical, meaning more advanced propulsion systems must be developed first.

While their respective distances from the Sun mean that they are exposed to less solar radiation, Uranus, Neptune, and bodies in the Trans-Neptunian region (like the Kuiper Belt) are still subject to significant amounts of galactic cosmic rays (GCRs). The fact that these satellites are airless or have a tenuous atmosphere (in Triton's case) means that their surfaces receive very little protection from GCRs.

The science of becoming "interplanetary": How can humans live in the outer Solar System?
Source: NASA/JHUAPL

Last but not least, the surface gravity on every satellite and body in the outer Solar System is significantly less than on Earth (or Earth's moon). The long-term effects of living in these conditions are likely to include muscle and bone density loss, deteriorating cardiovascular health, organ function, and eyesight, and could have serious repercussions on child-rearing.

Solutions for living

To live in the outer Solar System, would-be settlers will likely have to forgo living on the surface of planets and moons and rely on rotating stations in space - a la Pinwheel Stations or O'Neil Cylinders. These would provide the potential inhabitants with all the amenities they need while also addressing the greatest barrier to long-term living - low gravity.

These stations could be built in situ using readily-available resources in the outer Solar System, including minerals mined from asteroids and smaller bodies. At the same time, water ice and volatiles could be harvested from moons, iceteroids, and comets. Solar radiation could be focused into these habitats using solar mirrors, providing adequate sunshine for plants to grow (with the option of being switched off to create a day/night cycle).

Energy needs could be met through space-based solar, nuclear power, and fusion reactors. The proximity to Uranus and Neptune would ensure a sufficient supply of hydrogen and helium fuel. Functioning biomes could be created within the habitats by combining asteroid regolith, organic molecules (derived from methane), and nitric fertilizers to create soil.

Oxygen and nitrogen gas could be fashioned from water, ice, and ammonia and pumped in to create an atmosphere. Plants, shrubs, trees, and animals could then be introduced to create a fully-functional and regenerative ecosystem, where plants would scrub carbon dioxide from the air and produce oxygen gas, and the soil and plants would establish a nitrogen and water cycle (like we have on Earth).

These stations would provide access to each system, Uranus, Neptune, Pluto, and the Kuiper Belt. From these hypothetical stations - suggested names include Sky, Poseidon, Lowell, and Adlivun - residents could regularly travel to the local moons or asteroids for a little fun and adventure. Robotic or crewed haulers could also use the stations as a base to harvest resources, either for local use or transportation to the inner Solar System.

Assuming diamonds are still a valuable commodity, "skimmers" could also dive into the atmospheres of Uranus and Neptune to scoop up "rain diamonds." These could also be used as supermaterials to build rotating habitats in space and surface habitats, especially where transparent sections are needed to admit focused sunlight.

Over time, a thriving economy could be built based on resource exploitation and tourism. Bases in the outer Solar System would also make ideal places for many types of scientific research. In addition to the treasure trove of research that could be performed locally, research facilities in the outer Solar System would be able to conduct deep astronomy surveys and SETI research that would be unparalleled.

Interstellar probes could also be launched from these stations and not worry about navigating the perils of the inner Solar System. You might say that living close to the edge of our Solar neighborhood would increase the odds of us discovering if we have neighbors! On top of that, humans could be listening to parting messages like this:

"To all adventurers returning from Uranus, Neptune, and beyond, we welcome you back to Sky Station! Over the next days and weeks, we encourage you to take your time reacclimating to Earth-normal gravity. Be sure to take advantage of the many relaxing accommodations we have here. Visit the bioluminescent baths and experience what it's like to bathe in mineral-rich pools as beautiful and colorful as the Cobalt Marble!

"We also recommend a quick visit to the interior ring for one last taste of low-gravity before heading home. Don't stay too long, as your full recovery requires that most of your stay be spent in a one-gee environment. But that still leaves plenty of time for a little gliding and free jumping, doesn't it? We certainly think so!

"Be sure to pick up some souvenirs at our many gift shops before leaving. For those returning to the inner Solar System, nothing says 'I went to the Outer Reaches' than a rain diamond! Sure, you can find the same items in the Jovians and Cronians, but these ones are unique to the Cobalt and Azure marbles. Why not pick up some 'ice' from the 'ice giants'?

"From all of us here, we wish you a relaxing time and a fond farewell. Always remember, there's always something new and exciting going on in the Outer Reaches. So be sure to come and visit us again!"

Thank you all for reading the "Going Interplanetary" series!
We hope it was as much fun to read as it was to write!

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