Why Haven’t We Returned to Neptune?

Nv09 Lonely Neptune

I feel bad for Neptune. Only one spacecraft (Voyager 2) has ever been there, and it didn’t stay long.

Neptune is the planet farthest from the Sun, so getting there is an extra special challenge. It’s not only a question of distance. It’s also a question of gravity. Spacecraft require ludicrous amounts of energy to travel that far, given that the Sun’s gravity is constantly trying to pull them back.

But right now (between 2015 and 2019), the planets are aligned to make a trip to Neptune easier. With a gravity assist from Jupiter and another from Saturn, a small spacecraft could build up enough momentum to fling itself toward Neptune, using relatively little fuel in the process.

Like Voyager 2, the Argo spacecraft (as NASA called it) wouldn’t be able to stay long. By the time it reached Neptune, Argo would have built up so much momentum that it wouldn’t be able to slow down enough to enter Neptunian orbit.

But hey, a flyby mission is better than no mission at all. As added bonuses, Argo would also have opportunities to collect data about Jupiter and Saturn while performing its gravity assist maneuvers. Mission planners also designed an orbital trajectory that would allow Argo to get some extreme close-ups of Triton, Neptune’s largest moon. Argo would also be able to visit a Kuiper belt object some time after its primary mission ended.

All of this could be done within the strict spending limits of a New Frontiers-class mission ($1 billion or less). That’s a real bargain, at least for NASA. So why aren’t we doing this?

The Great Recession hit at just the wrong time for Argo. The mission was still in early development when Congress slashed NASA’s budget. And now, even though NASA has some money again, there isn’t enough time to develop and construct a spacecraft before the 2015-2019 launch window closes.

Nv09 Neptune Understands

Sorry, Neptune. Better luck next time.

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Today’s post is part of Neptune month for the 2015 Mission to the Solar System. Click here to learn more about this series.

Sciency Words: Centaur

Sciency Words MATH

Today’s post is part of a special series here on Planet Pailly called Sciency Words. Each week, we take a closer look at an interesting science or science-related term to help us all expand our scientific vocabularies together. Today’s word is:


When it comes to large, rocky objects drifting through space, naming conventions can get tricky. Many are called asteroids. Others go by stranger names. Some of these objects are called centaurs.

Nv08 Centaurs

Eh… no. It has nothing to do with horses, although these objects are sort of half one thing and half another. Most if not all centaurs could be classified as both asteroids and comets.

One definition of centaur is an object orbiting the Sun between the orbits of Jupiter and Neptune. More technical definitions involve an object’s semi-major axis, relative to the semi-major axes of Jupiter and Neptune, and non-resonant (i.e.: unstable) orbital periods. But I think a better way to describe centaurs is this: they are objects in a state of transition.

It is believed that most centaurs used to be Kuiper belt objects or objects from the scattered disk: basically, they were those large, icy things hanging out beyond Neptune. Due to gravitational interactions with Neptune and other gas giants, these objects have been pulled into increasingly eccentric (non-circular) orbits.

Eventually, most of these objects will transform into comets, with orbital paths that cut through the inner Solar System and bring them within melting distance of the Sun (allowing them to form cometary tails).

The Interanational Astronomy Union (I.A.U.) originally wanted to name all centaurs after actual centaurs from Greek mythology. However, some have been named after other hybrid or shape-shifting creatures, which I think is perfectly appropriate. Examples include Typhon (a snake or dragon hybrid), Ceto (a sea monster goddess), and Narcissus (a man who transformed into a flower).

In the future, centaurs could conceivably be named after the minotaur, the sphinx, or perhaps even Dracula. If the I.A.U. wants to have some fun, they could also use names like Ariel from The Little Mermaid, Constable Odo from Star Trek: Deep Space IX, or any of the Animagus characters from Harry Potter.

In fact, throughout fiction and mythology, there are plenty of hybrids and shape-shifters to chose from. So what names do you think the I.A.U. should give the next centaur asteroids?


Centaurs: Cross-Dressing Comets That Go as Asteroids from Discovery News.

Centaur (minor planet) from Wikipedia.

JPL Small-Body Database Search Engine: List of Centaurs from JPL Solar System Dynamics.

Neptune: Which Way Is North?

Something’s wrong with Neptune’s magnetic field.

Nv07 Neptune's Compass

Not all planets have magnetic fields. For the planets that do, we can generally expect two things:

  • Magnetic north and south should roughly correspond to geographic north and south.
  • The center of the magnetic field should run through the planet’s core.

Neither of these statements are true for Neptune (or Uranus, by the way). In 1989, when Voyager 2 visited Neptune, it found a magnetic field skewed 47º from the planet’s axis of rotation, with the source of the magnetic field offset by 0.55 Neptune radii (placing it not in the planet’s core but in a seemingly random point in the mantle).

Nv07 Neptune's Magnetic Field

More recent computer simulations suggest that the situation on Neptune could be even more complicated than Voyager 2 observed, with the magnetic field in constant flux, rotating and changing in ways that planetary magnetic fields ought not to do.

But new research, published earlier this year in Nature Communications, might help explain what’s going on. Remember those exotic forms of water ice, like ice VII, ice VIII, and ice X? Those weird “hot” ices believed to exist in the high-pressure interiors of Uranus and Neptune? Now we can add superionic ice to that icy mix.

In superionic ice, water molecules are under so much pressure that the oxygen and hydrogen atoms dissociate from each other. Oxygen forms a tight lattice structure which ionized hydrogen atoms can then move through freely. Much like free flowing electrons, these free flowing hydrogen ions (also known as protons) would generate an electric current.

Superionic ice located in the mantles of Neptune and Uranus would conveniently explain why these two planets have such wobbly, lopsided magnetic fields. Yes, that would be very convenient. Too convenient, perhaps, to be the whole story.


Scientists Predict Cool New Phase of Superionic Ice from Science Daily.

Magnetic Fields: On Other Planets from Joly Astronomy.

Neptune’s Badly Behaved Magnetic Field from Astronomy Magazine.

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Today’s post is part of Neptune month for the 2015 Mission to the Solar System. Click here to learn more about this series.

Molecular Monday: Hot as Ice VII

Welcome to Molecular Mondays! Every other Monday, we examine the atoms and molecules that serve as the building blocks of our universe, both in reality and in science fiction. Today we continue our investigation of:


Water is basically everywhere in our universe. On Uranus and Neptune, most of the water is in the form of ice.

Nv06 Ice Ice Baby

But this ice isn’t cold. It’s hot. Like thousands of degrees Celsius hot.

The ice you and I are most familiar with is called ice Ih (pronounced ice one-h). The I is a Roman numeral one, and the h stands for hexagonal, because the water molecules form hexagon-shaped crystals.

Another form of ice is called ice Ic (ice one-c). In this case, the c stands for cubic.  The water molecules crystalize into cube shapes.

Nv06 Ice Cube

Ice Ic requires extremely cold temperatures (-50 to -140ºC) and is believed to form in the uppermost reaches of Earth’s atmosphere.

There are at least fifteen more crystalline forms of ice, ranging from ice II to ice XVI. Some have been created in the laboratory. Others remain purely hypothetical. Water molecules will line up to form ice crystals of wildly different shapes, sizes, and complexities depending on various combinations of pressure and temperature.

Nv06 Snowflake

Somewhere deep inside the planets Uranus and Neptune, water molecules probably “freeze” as ice VII and ice VIII. I’m not sure how to describe the geometry of these types of ice; just click the links to see some diagrams.

Pressures on Uranus and Neptune may even be great enough for ice X to form. In ice X, water molecules are so tightly packed that they lose their identities. It would be better to think of oxygen and hydrogen atoms squeezed together, not separate water molecules. For this reason, ice X is sometimes called non-molecular ice.

Of course, all of this seems very strange and exotic to us Earthlings. And yes, things like ice VII or ice X aren’t exactly common in our universe. But that doesn’t mean ice Ih is normal either.

In the next edition of Molecular Mondays (two weeks from today), we’ll meet yet another kind of ice. A kind of ice that doesn’t play by the rules and doesn’t need no Roman numerals.


Is Salt the Key to Unlocking the Interiors of Neptune and Uranus? from Science Daily.

Phase Diagram of Water from Wikipedia.


Sciency Words: Cantaloupe Terrain

Sciency Words PHYS copy

Today’s post is part of a special series here on Planet Pailly called Sciency Words. Each week, we take a closer look at an interesting science or science-related term to help us all expand our scientific vocabularies together. Today’s word is:


This is a cantaloupe.

Nv05 Not Actually a Cantaloupe

And this is Triton, Neptune’s largest moon.

Nv05 Not Actually Triton

Wait, I think I got those mixed up…

In 1989, Voyager 2 became the first (and so far the only) spacecraft to visit Triton, and it sent back some weird pictures of Triton’s surface. Pictures like this one:

Nv05 Cantaloupe Terrain

This heavily dimpled surface topography, which bears an uncanny resemblance to the skin of a cantaloupe, is unique to Triton. It may have formed due to a geologic process called diapirism, whereby blobs of warm material (called diapirs) force their way upward through layers of solid rock.

We know this process occurs on Earth and possibly a few other places in the Solar System. However, diapirism does not generally produce a cantaloupe-like appearance. That only happens on Triton, and no one’s entirely sure why.

So research continues on what scientists have officially named “cantaloupe terrain.”

Nv05 Cantaloupe Terrain vs Chaos Terrain

Today’s post is part of Neptune month for the 2015 Mission to the Solar System. Click here to learn more about this series.

And click here to find out more about Europa’s chaos terrain.

The Strange Story of Triton

Meet Triton, the largest of Neptune’s moons.

Nv04 Meet Triton

Triton is in a retrograde orbit, meaning that as Neptune rotates in one direction, Triton orbits in the opposite direction.

Nv04 Retrograde Orbit

This weird orbit strongly suggests that Triton is not one of Neptune’s “natural children” but rather an “adopted child.”

So who knows where Triton originally came from? As usual, scientists have two competing theories.

A Lunar Collision

One day, as Triton was going along minding its own business, it happened to pass a little too close to Neptune. Neptune would have has a few moons already, and one of those moons happened to be on a collision course with Triton!

The collision would have altered Triton’s momentum and trajectory through space, allowing Neptune’s gravity to capture it.

The problem with this explanation is that the hypothetical moon that hit Triton had to have been just the right size: big enough to change Triton’s orbit but not so big that it could destroy Triton in the collision.

And this just-right-sized moon would have to have been in just the right place at just the right time in order for the collision to occur. Stranger coincidences have happened in our universe, but this scenario is a little too unlikely for my taste.

A Binary Planet

Instead, let’s imagine that Triton was once part of a binary planet system, similar to the Pluto-Charon binary. Such binaries appear to be common in the Kuiper belt, as are binary asteroids in the asteroid belt and elsewhere.

One day, Triton and its binary partner strayed too close to Neptune. This resulted in a gravitational tug-of-war, with Neptune pulling in one direction and Triton’s companion pulling in the other. Neptune won, and due to Newton’s whole equal and opposite reaction thing, Triton’s companion was flung off into deep space. It may have even been ejected from the Solar System.

These sorts of gravitational tug-of-wars happen far more frequently than collisions, and this scenario does not depend on Triton’s hypothetical partner (or any other object in space) being perfectly sized or perfectly positioned.

That doesn’t prove that the binary planet hypothesis is correct, but it does sound a lot more likely than the alternative.

Life on Triton?

Whatever happened, Triton’s introduction into Neptune’s family would have had interesting effects. Triton is large for a moon (almost twice as massive as Pluto), and its initial orbit around Neptune would have crossed paths with many of Neptune’s much smaller moons. That means more gravitational tug-of-wars, creating chaos in the Neptune system.

Gradually, tidal forces pulled Triton into a more circular (and less disruptive) orbit, but those tidal forces would also have caused Triton’s interior to heat up dramatically. Today, Triton still shows evidence of active cryovulcanism and may have subsurface liquid water, just like Europa, Enceladus, or many other moons in the outer Solar System.

Which means we can add Triton to the list of places in the Solar System that may support extraterrestrial life.


Neptune’s Capture of Its Moon Triton in a Binary-Planet Gravitational Encounter from Nature.

Is Triton Hiding an Underground Ocean? from Universe Today.

Life Beyond Earth: A Day on Triton (Neptune’s Largest Moon) from Quarks to Quasars.

Sciency Words: Flagship Mission

Sciency Words BIO copy

Today’s post is part of a special series here on Planet Pailly called Sciency Words. Each week, we take a closer look at an interesting science or science-related term to help us all expand our scientific vocabularies together. Today’s word is:


There is a growing need among planetary scientists to study an ice giant up close. We keep discovering ice giant size planets orbiting distant stars, but we know next to nothing about the two ice giants in our own Solar System: Uranus and Neptune.

Nv03 Uranus and Neptune

To get to know these two planets better, NASA will have to launch a robotic mission of some kind. But which kind? There are three mission classes, defined primarily by their price tags:

  • Discovery Missions: Proposals for discovery-class missions are submitted to NASA and go through a highly competitive selection process. Approved missions must cost less than $450 million (a real bargain! Well, for NASA at least). Examples include the Mars Pathfinder Mission, the MESSENGER mission to Mercury, and the Kepler Space Telescope.
  • New Frontiers Missions: Like discovery missions, new frontiers missions go through a highly competitive selection process. Total costs (not including the launch vehicle) are capped at $1 billion. Examples include the New Horizons Mission to Pluto and the Juno Mission to Jupiter, due to arrive in 2016.
  • Flagship Missions: Unlike the other two mission classes, there is no regular submissions process for a flagship mission. Instead, NASA develops these missions internally, with costs ranging between $2 and $4 billion. NASA tends to launch only one flagship mission per decade. Examples include the Curiosity rover on Mars, the Cassini spacecraft orbiting Saturn, and the Voyager 1 and 2 probes that are currently exploring the very edges of the Solar System.

In relation to the ice giants, everyone seems to agree that a discovery-class mission could never reach Uranus or Neptune. A new frontiers mission could work, especially if it’s just a flyby mission like the recent New Horizons flyby of Pluto.

But to really get up close and personal with an ice giant, we need to send an orbiter. That will be expensive, and it will require NASA to commit to a flagship mission.

Upcoming flagship missions will focus on Mars (another Curiosity style rover) and Europa (potential home to alien fish). So despite the growing need among planetary scientists to study an ice giant up close, there probably won’t be a Uranus or Neptune orbiter any time soon.

Molecular Monday: Water on Neptune

Welcome to Molecular Mondays! Every other Monday, we examine the atoms and molecules that serve as the building blocks of our universe, both in reality and in science fiction. Today we continue our investigation of:


Meet Neptune, the other blue planet.

Nv01 Neptune Blues

Neptune may be named after the ancient god of the sea, but the planet’s striking blue color is caused by a high concentration of atmospheric methane, not water. Methane absorbs red light and reflects blue light back into space. So Neptune has nothing to do with that most precious commodity in space: water.


Okay, this sort of statement should no longer come as a surprise to me or anyone else, but the planet Neptune does in fact have water. In fact, water is sort of everywhere in space, which makes sense because the water molecule is made of two of the most common elements in the universe: hydrogen and oxygen.

Furthermore, water is the simplest possible combination of hydrogen and oxygen. So of course we’re going to find it everywhere. Even on Neptune. Why would anyone think water is somehow special to Earth? (Why did I think that for so long?)

But water on other planets rarely behaves the way it does here on Earth. As we’ve seen in two previous Molecular Monday posts (click here and here), the freezing and boiling temperatures of water change depending on pressure and salinity. At a certain point, known as the triple point, water’s freezing and boiling temperatures are the same, so water skips over its liquid phase, transitioning directly from gas to solid and vice versa.

Nv01 Water Water Everywhere

So the planet Neptune might actually live up to its sea god name. Not only does it have water, but there is a slim possibility that deep in the planet’s interior, the pressure and temperature are just right to support an ocean of liquid water. If Neptune were just a bit colder, the odds of such an ocean forming would improve dramatically.

Discovering liquid water on Neptune would be pretty cool, but there’s something even cooler (or perhaps hotter) about Neptune’s water. In the next edition of Molecular Monday (two weeks from today), we’ll take a closer look at Netune’s “icy” interior.

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Today’s post is the beginning of Neptune month for the 2015 Mission to the Solar System. Click here to learn more about this series.

Discovering Uranus’s Rings (Sciency Words: Occultation)

Sciency Words MATH

Today’s post is part of a special series here on Planet Pailly called Sciency Words. Each week, we take a closer look at an interesting science or science-related term to help us all expand our scientific vocabularies together. Today’s word is:


You know that thing when the Moon passes in front of the Sun, completely blocking the Sun from our view here on Earth. That specific event, known as a solar eclipse, is an example of a more general phenomenon called an occultation.

The term is related to the more vernacular word “occult” in the sense that they both refer to things that are hidden. When a planet, moon, or other celestial body passes in front of a distant star, for example, the star is “occulted” in the sense that it is briefly hidden from sight.

Occultations are a rare and wonderful cosmic coincidence, and they also provide astronomers with an incredible opportunity. Whenever an occultation is predicted to occur, a great many powerful telescopes all across the globe swivel around to watch.

And sometimes amazing discoveries are made.

In 1977, the planet Uranus occulted a star with the unimaginative name of SAO 158687. After setting up their telescopes, astronomers presumably got their popcorn ready and waited to see what would happen. They were hoping some of the occulted starlight would pass through Uranus’s atmosphere, revealing the atmosphere’s structure and chemical composition.

Surprisingly, the show started early and ended late. SAO 158687 dimmed exactly five times before the occultation and exactly five times afterward. This provided the first evidence that Uranus has rings. At least five of them (we now know of thirteen Uranian rings).

And it’s a good thing we discovered those rings too. Given Uranus’s otherwise bland appearance, how else could I depict the planet’s sideways orientation without the help of those sideways oriented rings?

Oc09 Uranus Without Rings

P.S.: On a personal note, I’ve been feeling a little under the weather lately, which is why this edition of Sciency Words is a day late, and I want to apologize in advance if I don’t respond to comments as quickly as usual.

Is Uranus Boring?

Planetary scientists can say some pretty mean things abut planets, especially when they’re competing over limited funding and telescope time. That’s probably why Uranus has frequently been described as “boring.”

Oc08 Uranus Visible Light

Aside from the whole spinning sideways thing, Uranus doesn’t look particularly exciting. The entire planet is just a uniform cyan color with no distinguishing features. At least that’s how it appears to the unaided human eye.

Oc08 Uranus Ultraviolet Filter

Observations in non-visible wavelengths of light reveal a more complex and interesting world, with atmospheric belts and zones that look similar to what we’ve seen on Jupiter, Saturn, and Neptune.

And in 2014, the seventh planet from the Sun became a lot more interesting. Both professional and amateur astronomers began reporting anomalous bright spots in Uranus’s atmosphere. These bright spots were interesting enough to deserve observation time on the Hubble Space Telescope, which confirmed in October 2014 (one year ago this month!) that they were storm clouds.

It seems these storms are seasonal. Because of Uranus’s 98º tilt, the changing of the seasons are much more dramatic than would otherwise be expected, but since a Uranian year is over 84 Earth years long, we don’t get to see this happen very often.

As part of the 2015 Mission to the Solar System, I’ve spent almost a whole month studying Uranus (stop laughing). I kind of feel bad for this poor planet. Uranus is stuck with an embarrassing name, and it’s been too often neglected and misunderstood. But like many things in life, there’s a lot more to it than at first meets the eye.


Uranus in True and False Color from NASA.

Giant Methane Storms on Uranus from Phys.org.

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Today’s post is part of Uranus month for the 2015 Mission to the Solar System. Click here to find out more about this special series.