Tuesday, August 11, 2015

Beyond Our Solar System's Plutonian Shore: Whither Pluto After New Horizons?

An artist's conception of New Horizons passing Pluto and its satellites (NASA)
It has definitely been a big year for one little world in our solar system. 

The flyby of Pluto by the New Horizons spacecraft on July 14th of this year has spawned renewed interests in the king of the Kuiper Belt, lord of the dwarf planets

From reigniting the discussions over Pluto's designation as a dwarf planet to revealing that the surface of Pluto holds geological mysteries for us to explore, New Horizons has been an amazing success. As the spacecraft continues on its mission and leaves Pluto behind, many of us wonder what might come next for the 17th largest object in our solar system.

This image has probably been the widest shared image from the New Horizons mission thus far (NASA)

Pluto has long fascinated many of us. Since the accidental discovery of Pluto by Clyde Tombaugh in 1930 (he was definitely looking for a planet, but it's very lucky he actually found Pluto) and its being named for the god of the underworld in Roman mythology by an eleven year-old girl, Pluto has made many of us wonder about what lies beyond the planets in our solar system and has inspired storytellers of all sorts to question what remote regions of star systems might be like. 

H.P. Lovecraft included Pluto in his fictional mythologies of the "ancient evil ones", it's believed by many that Disney named their famous cartoon dog after Pluto, Doctor Who visited Pluto in a fictional future, and in the Mass Effect video-game universe Charon, Pluto's largest moon, is the locale where an alien device for faster-than-light travel is discovered. In fact, there have been a large number of science fiction stories that have included mention of Pluto. Part of the allure of Pluto for storytelling has been the uncertainty about what kind of world it is.

We've often talked about Pluto as being a frozen world touched only by the dimmest light from the Sun; a little, icy ball enshrouded in mystery. But, thanks to the New Horizons mission, we now know so much more about Pluto: we know that there are icy mountains on Pluto that rise as high as 3.5 km above the surface, there are variations in the composition of surface ices (most notably causing the "heart" on Pluto; see above image), and that the moons of Pluto have their own surprises in store.

Also, it's great to know that scientists from the New Horizons team have been naming the features on Pluto after various science fiction and fantasy stories as well as from the history of exploration. There are the Cthulhu Regio, Vader Crater, Sputnik Planum, Viking Terra, and Uhuru and Spock Craters, just to name a few. 

Some of the surface features of Pluto
New Horizons' mission is continuing on now that the spacecraft has screamed past Pluto. The current plan for New Horizons is to try to fly by some other Kuiper Belt Objects (KBOs) before continuing on and away. Much like the Pioneer 10 and 11 and Voyager 1 and 2 spacecraft, New Horizons will continue sailing away from us, leaving our solar system behind in the decades and centuries to come. It will have long-since lost the capability of communicating with us or even operating, but maybe thousands or millions of years from now it will bump into some alien spacecraft and present a mystery to whoever finds it.

I used to think that New Horizons was going fast enough to overtake Voyager 1 at some point in the near future, but it turns out that New Horizons will never catch up with Voyager 1. This means that Voyager 1 will continue to be the furthest stretch of humanity in the universe for quite some time to come.

"Goodbye Pluto!" A look back from New Horizons (NASA)

Now that New Horizons has left the Plutonian system, a lot of people have wondered what else lies in store for Pluto?

Of course, there's the still the debate as to whether or not Pluto should be called a planet. You may know that Pluto is currently classified as a dwarf planet due to a vote amongst the International Astronomical Union (IAU) back in 2006. This decision caused a lot of public backlash, mostly for sentimental reasons. A lot of people felt like Pluto had always been a planet in their lifetimes so it should stay that way (of course, that's not how science works). There are certainly some good scientific reasons to call Pluto a planet, but, as many people point out, if we call Pluto a planet, then there are a lot of other worlds in the solar system that we'll have to call planets as well. These other worlds are also currently known as dwarf planets, and include the likes of Makemake and Eris (it was really the discovery of Eris that became the impetus for reclassifying Pluto). 

I personally tend to be on the fence about calling Pluto a planet. It most certainly shouldn't be classified along with the terrestrial worlds, like Venus and Earth, and definitely doesn't fit with the gas giants, like Jupiter and Uranus. Yet, the word planet hails from the Greek for "wandering star" (aster planetes) and that original concept could be fit to just about any body in the solar system. Also, to change the classification of Pluto required finding a definition of "planet" that fit the other eight large worlds, but would cut out Pluto and other dwarf planets and moons. That's what led the IAU to come up with their three requirements for a "planethood":

1. A planet is in orbit around the Sun

2. A planet has sufficient mass to assume hydrostatic equilibrium (a nearly round shape)

3. A planet has cleared its orbital neighborhood

The first part takes care of moons that orbit around other bodies, but also fails to include exoplanets (which do not orbit our Sun!). The second part makes a planet anything that is massive enough to draw itself roughly into a spherical shape, which fits with the planets and the dwarf planets, but leaves out many of the smaller asteroids and some moons. Finally, the third part is where they got Pluto. Pluto is a member of the Kuiper Belt and has not "cleared its neighborhood" of other bodies. Pluto is also weird in a lot of other ways (for instance, it's orbital plane is nothing close to that of the eight planets in the solar system), but there are many of us who still love Pluto, regardless of what it's called. 

It seems to many of us that what we choose to call Pluto should be based on science. There are several other dwarf planets and smaller bodies that were once considered planets (including Vesta, Juno, Ceres, and Pallas), so the idea that Pluto should be a planet because it was once a planet makes little to no sense. If we make Pluto a planet, then that means we have many other planets as well (which is not necessarily a problem, but does bother some people). Of course, there is also the potential that we could abandon "planet" as a scientific word and find something else, leaving the word "planet" to be something of a public matter. I suppose the debate over Pluto's status will continue on. How long that debate will last and what its outcomes will be, who knows...

As for what comes next for Pluto: there are no current missions in the works that will visit Pluto. We've learned a lot from New Horizons and will continue learning more as the data stream in over the next 15 months. However, although we'll have gained a lot more knowledge about Pluto, there will surely be many more mysteries to ponder. I would love to see a future where we could afford to send missions to the outer solar system more often, but, for now, we have to hope that there might be another mission to Pluto within our lifetimes.

If you'd like to know more about Pluto and the New Horizons mission, the video below has a lot of great information. It was released before the New Horizons flyby, but still serves as a fantastic resource for interested people:

Also, if you'd like to know more about the New Horizons flyby of Pluto, Space.com posted a Complete Coverage article for following the news as it was coming up online.

An image from New Horizons shows a mountain range of ice on Pluto (NASA)

As I close out this post, I leave these questions for you: 

-What comes next for Pluto? 

-Would you like to see another mission to Pluto? A lander this time, maybe? 

-Where do you stand on Pluto's status as a dwarf planet? 

-Finally, if you could name some of the new features on Pluto after science fiction and fantasy or famous exploration stories, which names would you choose and why?

This image shows what appear to be swirling ices of different compositions on Pluto (NASA)

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