New Horzions Closes In On Pluto

Our current best images of Pluto, combining multiple images taken with the Hubble Space Telescope. Not very good.
Image Credit: NASA, ESA, and M. Buie (Southwest Research Institute)

When people find out I study astronomy, I generally get one of two reactions: blank looks and an awkward end to the conversation, or a barrage of questions (I prefer this reaction). And one of the most common questions is some variation on "Why isn't Pluto a planet any more?"

Now, I do have an answer for that, which I'll discuss at the end of this post, but I don't want to dwell too long on it as I don't find it particularly interesting. What is exciting is that, in just a few months time, we are finally going to see Pluto up close for the first time.

On 14th July this year NASA's New Horizons spacecraft will fly past Pluto at a relative velocity of 14 kilometres a second (about 31 thousand mph), becoming the first spacecraft to visit the distant space rock. The event will be fleeting, as at that speed it will be impossible to slow down into orbit without using roughly sixty times the spacecraft's mass in fuel. But for a few months, starting in May, the tiny spacecraft will be close enough to Pluto to get better data than we've ever obtained before. 

What will it see? I think that the interesting thing about this mission is that we don't really know. Our best images of Pluto at the moment, obtained using the Hubble Space Telescope are shown above. The mess of light and dark patches ask more questions than they provide answers. Pluto has one of the most varied surface brightnesses in the solar system- those dark patches really are very black, whilst the lighter patches reflect most of the light that fall on them. But we have no idea what the patches are. Mountains and craters? Frozen lava seas like those on the Moon? We will know very soon.

Even less is known about the moons. So far we've discovered five moons around Pluto. The largest moon, Charon, is the biggest moon in relation to its parent body in the solar system- so big, in fact, that it doesn't really orbit around Pluto, instead they both orbit around a common centre of mass, or barycentre. Some of the first images from New Horizons have nicely shown this peculiar cosmic dance:  

Pluto and Charon form a binary system, orbiting around each other in 6.4 days. Image Credit: NASA/APL/Southwest Research Institute

The four other moons, called Nix, Hydra, Kerberos and Styx, are much smaller than Charon and were only discovered recently. Their small size has lead to suggestions that they might be the largest remnants of a cloud of debris, a hypotheses that New Horizons will be able to test. Although hopefully not by flying through it at 14 kilometres a second...

In addition to the first good images of Pluto and its moons, New Horizons will return a wealth of other science data. The spacecraft carries seven instruments, including spectrometers to observe Pluto's surface composition and investigate its tenuous atmosphere, solar wind detectors to see how Pluto interacts with it's distant star, and a student-built and -operated dust counter.

With a whole new world to explore, NASA and the International Astronomical Union (IAU) have asked the public to suggest names for the surface featured New Horizons will find. Until the end of April you can nominate and vote for names here, as long as they fit into a set of categories like historical and fictional explorers or spacecraft. The IAU will then come up with a list of approved names based partly on your votes.

(My favourite fictional spacecraft is probably the Falling Outside The Normal Moral Constraints, which I don't think will make the list somehow. Points if you know where it's from.)

I will almost certainly write more about this mission as the results come in, but I suppose I better address the planet thing now.

For years after Pluto's discovery in 1930, it was the only object bigger than a comet we knew about in the outer solar system, so although it's significantly smaller than any of the other planets  (less than half the diameter of Mercury, the next smallest) its planet status was secure. Problems began to arise with the detection of more objects in what we now know as the Kuiper Belt, with things really becoming unstuck in 2005 when an object larger than Pluto, Eris, was discovered far beyond the Kuiper Belt.

At this point it was clear that the old system of nine planets couldn't last- either Eris and several other objects, including maybe even some moons, had to reclassified as planets, or Pluto had to go. In an infamous meeting in 2006, the IAU decided on the latter. Pluto was stripped of its planet status, joining Eris and a handful of other objects in the newly created dwarf planet class.    

Was this the right decision? Whilst I agree that something had to be done, I'm not sure that the right thing was done. The definition of dwarf planet has many problems, and doesn't work at all when applied to the other solar systems that we're now discovering by the bucketload.

Perhaps we need to stop trying to define things into rigid categories, as it increasingly looks like the Universe isn't like that. Recently we've found plenty of things that don't fit into the old definitions. We've spotted planets bigger than Earth but smaller than gas giants, brown dwarf stars that are small enough to be planets, and asteroids that look like comets. Rather than arguing about arbitrary definitions, maybe we should just keep looking and see what's out there.

The images and data that New Horizons will be far more exciting and interesting that any argument over whether it's a planet or not. With this in mind, I propose that we refer to Pluto from now on as a "Mysterious Round Shiny Dark Ice Rock Thing".

Like my favourite spaceship, I doubt that will get past the IAU.

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