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.

Sorry, Neptune. Better luck next time.

* * *

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

Something’s wrong with Neptune’s magnetic field.

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).

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.

Links

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.

* * *

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

Meet Triton, the largest of Neptune’s moons.

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

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.

Links

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.