Pluto “Fans”

The recent fly-by of Pluto by the New Horizons spacecraft has reignited a debate that should have stayed buried forever. I’m not saying the IAU’s 2006 definition of planet wasn’t lacking, it’s just that this specific debate should have died and stayed dead.

Plutesters, hehehe.


The problem is that it is entirely unclear why we’re defining “planet” to begin with. Categorization of phenomena is supposed to help us organize them epistemologically. This is why we have a taxonomy of species. Any definition of space objects should be designed to help us classify and study them, not contrived for cultural reasons. We shouldn’t try to exclude KBO’s or other minor bodies because we don’t want to have 15 planets, and we shouldn’t try to include Pluto because we feel bad for it. The classifications we come up with should mirror our current understanding of how similar the bodies are. On the other hand, our precise definitions should produce the same results as our imprecise cultural definitions for well-known cases. As evidenced by the outrage caused by the IAU’s “exclusion of Pluto from planethood”, people don’t like changing how they think about things.

Images of Pluto and Charon.


Which brings us to the current debate. Fans of Pluto seem to be hinging their argument on the fact that Pluto is geologically active, and that it’s diameter is actually larger than that of Eris. Previously it was thought that Eris was both more massive (by 27%) and larger in diameter than Pluto (with the flyby of New Horizons, we now believe Pluto has the larger diameter). This is what moved the IAU to action in the first place; if Pluto is a planet, then so is Eris. There is no world in which we have 9 planets. We either have 8, or 10+.

Then you have Makemake, Haumea, Sedna, and Ceres. How do those fit in? It’s possible we would end up having far more than 15 planets, based on current predictions of KBO size distributions. This illuminates a fundamental problem: what is the use of a classification that includes both Sedna and Jupiter? These two bodies are so different that any category that includes both is operationally useless for science within our solar system. But continuing that logic, the Earth is also extremely dissimilar to Jupiter. The Earth is more similar to Pluto than it is to Jupiter. So having Earth and Jupiter in the same category but excluding Pluto also seems weird.

Unless we consider our definition of similarity. There are two ways to evaluate a body: intrinsic properties (mass, diameter, geological activity, etc), and extrinsic properties (orbit, nearby bodies, etc). One would be tempted to define a planet based on its intrinsic properties. After all, at one time Jupiter was still clearing its orbit, and in the future Pluto will eventually clear its orbit. Does it make sense for the same body to drop in and out of statehood. Well… yes. The fact that a human stops being a child at some point doesn’t make the category of “child” any less useful for a huge range of societal and cultural rules.

In fact, “intrinsic properties” is sort of a gray area. Rotation rate doesn’t really count, since tidal locking is common yet caused by extrinsic forces. Geological activity is also not necessarily intrinsic. Io has extreme internal activity caused by tidal heating. One can imagine the same for a planet close to its parent star. Composition can change as atmosphere is blown away by the parent star, and even mass and diameter can change through planetary collisions.

Regardless, defining a planet only on its intrinsic properties means that moons are now technically “planets”. “Moon” becomes a subcategory of “planet”. This is actually a great definition, but too radical to get accepted currently, so thus functionally useless.

So we must define a planet at least partially based on extrinsic properties. The rocky inner planets and the gaseous outer planets are similar in that they make up the VAST portion of the mass within their orbital region. Earth is 1.7 million times more massive than the rest of the stuff in its orbit. On the other hand, Pluto is 0.07 times the mass of the rest of the Kuiper Belt. Yeah, it makes up less than 10% of the Kuiper Belt. This is a pretty clear separation.

After that revelation, everything falls into place. We have large, orbit-clearing objects, and we have smaller objects that are still in hydrostatic equilibrium but are part of a larger belt of objects.


It turns out, this definition is already in place. For all the hub-bub about the IAU’s definition, most everybody agrees with the splitting of bodies via two parameters that measure likelihood of a body ejecting other bodies in its orbit (the Stern-Levison parameter Λ), and a body’s mass relative to the total mass of bodies in its orbit (planetary discriminant µ). The split occurs at a semi-arbitrary Λ=1 and µ=100.

What everybody is really arguing about is whether or not we get to call both types of bodies planets, or just the big ones.

Stern and Levison propose the terms überplanet and unterplanet, but I think major planet and minor planet is more adoptable.

Finally, just plain old “planet” should refer by default to major planets only, but can contextually refer to both classes in some cases.

Problem solved.

Why The Next 5 Years Are Going To Be Awesome (In Space)

To close out 2014, I’d like to talk about why I’m very excited for the next 5 years in space travel.

Dawn renderingDawn being built

Early next year we’ll get to see two extremely cool missions returning pictures: the Dawn spacecraft, and New Horizons. In April 2015, Dawn will be the first spacecraft to enter orbit around body that isn’t the Earth or the Sun, then exit orbit and enter orbit around another body. We’ll get to see high-res photos of Ceres; expect a lot of articles about old theories being overturned by the data Dawn returns.

New Horizons renderingNew Horizons being built

Second, New Horizons will be performing a fly-by of Pluto in July 2015. This will be our first good look at a trans-Neptunian dwarf planet. Observations could provide a lot of insight of the Kuiper belt, as well as other structures like the (potential) inner Oort cloud. Between Dawn and New Horizons, we’ll be getting our first close-up look at dwarf planets.

Trans-Neptunian dwarf planets

There are other fascinating missions that are either already launched, or on schedule to be launched. ExoMars is a joint mission between the ESA and Roscosmos with the single purpose of searching for bio-signatures (past or present) on Mars. This is exciting because all current NASA missions very pointedly don’t have this scientific objective. The last NASA mission to search for bio-signatures was the Viking landers in the late 1970’s. I’m a little concerned that Russia will have trouble with their end of the mission; after all, the Russians don’t have the best track record when it comes to Mars.

Hayabusa 2 launchHayabusa 2 rendering

Also exciting and potentially more fruitful is Hayabusa 2, launched earlier this fall. Hayabusa 2 is interesting because they plan to shoot an asteroid with a space gun. Leave it to the Japanese to put cannons on their spaceships (technically the Russians did it first, but they didn’t actually shoot at something). After blowing a crater in asteroid 1999 JU3, Hayabusa 2 will float down and take samples from the newly exposed subsurface. The mission will finally return the samples to Earth in December 2020.

A bit closer to home is another interesting mission: the Chinese plan to launch a Moon sample return mission in 2017. The mission architecture is interesting; unlike early sample return missions, the lander will rendezvous in lunar orbit with a return craft. I might be wrong about this, but I think this will be the first automated rendezvous and docking around a body that isn’t Earth. I think it’s great that China is making leaps and bounds in its space program; earlier this year, they launched a test mission for the upcoming sample return mission and took a German payload along for a ride. The more the merrier, I say!

Speaking of which, the competition for the Google lunar X-prize is going to draw to a close in a few years. The deadline was recently extended to the end of 2016, and at least one team already has a flight reserved in 2015. There are only a few teams still seriously in the running, but if even two of them actually get off the ground, the Moon could become a very crowded place indeed.

One of teams at the forefront, Astrobotic, has booked a launch with SpaceX on a Falcon 9. And SpaceX really has come to prominence lately. Expect a lot more out of them in the next few years. For example, in 6 days they are going to attempt to land the first stage of a Falcon 9 on a barge for the first time. Although this has a pretty low chance of working (Musk estimates 50%, so who knows how low it actually is), it is a huge step towards their long-term goal of rapidly reusable rockets. In fact, if they do get a barge landing to succeed, we might even get to see a used stage re-fly as early as 2015!

Rendering of the Dragon V2

And on that front, SpaceX will be finishing up the Dragon V2 by 2016 or 2017. Besides launch abort tests and propulsive landing tests, we will also no doubt be seeing manned commercial launches in a few years. Remember the excitement when SpaceX became the first company to dock a spacecraft with the ISS? The celebration will be ten-fold when SpaceX becomes the first company to put a human in orbit.

But Spacex will also perform the maiden launch of the Falcon Heavy, and facilitate ground-breaking tests for both VASIMIR engines (if funding for that doesn’t run out) and an inflatable habitat on the ISS.

We might even see more action from Bigelow Aerospace. They’ve manifested a number of flights from SpaceX, presumably to start launching components for a commercial space station. Now that cheaper orbital crew transportation is just a few years away, Bigelow is ramping up production again; hundreds of new positions have opened open at Bigelow.

Finally, the wildcard. Will the SLS actually launch, or will it get cancelled before its first flight due to a change of presidency or loss of support in Congress? If it does launch, it will be spectacular. Unfortunately, I pretty much doubt any of the potential missions for the SLS (Europa Clipper, ATLAST, or Uranus orbiter) will get funded, so it is almost guaranteed that the SLS gets shelved even if does fly in 2018. So there’s that to look forward to.

The Falcon Heavy and SLS preparing for launch