So Is Pluto a Planet or Not?

John Derbyshire alerts us to the fact that the International Astronomical Union will be meeting in August 2009 for the first time since it demoted Pluto from the status of "planet", and that the issue of Pluto's planethood may come up yet again.

But he makes a couple of mistakes in his rush to condemn creeping New Ageism. First, he says: "The deciding factor was the opinion, held by a majority of delegates, that merely being massive enough to form a sphere under your own gravity is not enough. (If it were, we'd count the Moon as a planet.)" Not true, of course; the Moon has never been counted a planet. There is the important requirement that the body in question must orbit the Sun, which eliminates the Moon from consideration.

Second, he quotes Mark Sykes of the Planetary Science Institute in Tucson, Arizona as saying:

We are in the midst of a conceptual revolution. We are shaking off the last vestiges of the mythological view of planets as special objects in the sky — and the idea that there has to be a small number of them because they're special.

OK. But Derb's comment to this is where he goes wrong: "Just beneath the surface of those words you can spot the egalitarian sensibilites of our age. Nothing is special! Everything is equal to everything else!" That's a serious reach, and it's surprising coming from a man who understands and appreciates science and the scientific method as much as he does.

It's important in science to classify things. But classification is complicated by two factors: first, many things fall naturally into multiple categories (e.g., my telephone is both "a piece of office equipment" and "a communication device", but there are instances of each class that do not belong in the other); second, the dividing line between categories can be vague (is a futon a "bed" or a "couch"?). That doesn't mean that everything is equal to everything else, though. It's plausible to call a futon a bed or a couch, or both, but no one would call a futon a type of car.

Biologists have been frustrated by this problem for ages. The definition of "species" has historically been "a class of living things which can interbreed to produce viable offspring". On the surface, this seems simple enough. Are animals X and Y in the same species? Breed them: if their offspring is viable, then the answer is "yes". Unfortunately, Nature has a few tricks up Her sleeve. There are cases where three classes of animals exists, X, Y and Z, where X can breed with Y and Y with Z, but not X with Z. By our definition, then, X and Y are the same species; so are Y and Z. But X and Z are not. The definition made an implicit assumption, transitivity, which does not hold. And this causes the scheme to break down.

But it does not, as Derb surely understands, mean that biologists say that any two living things are in the same species, or that the concept of "species" is meaningless. It's more complicated than we thought a century ago, but in a way similar to the way our knowledge of gravity is more complicated than we though a century ago. Newtonian gravitation still works in most cases, but you need to keep in mind the Einsteinian correction in certain cases. When we find that an old classification system does not work because of new facts, we have to find a new one. (I'll return to this point in a bit, though.)

In the case of Pluto, there are several problems with its planethood. On the case for the defense, clearly it does orbit the Sun and has sufficient gravity to be spherical. But the prosecution is forced to point out that the same is true of other known bodies in our Solar System, such as the asteroid Ceres. Not wanting to add additional planets, astronomers looking to keep Pluto in the list of planets sought other criteria that were true of Pluto but not other bodies. This proved difficult. Eccentricity doesn't work: Pluto's orbit is more eccentric than Ceres'. Nor does orbital inclication: Pluto's orbit is highly inclined to the ecliptic. Pluto has moons of its own, and that proved briefly interesting, but some asteroids were found to have moons, too.

In the end, the IAU was left with two choices: either demote Pluto, or promote lots of other bodies. The least distruptive option was the former, and that's what they did.

Much as it may bother Derb, what Mark Sykes had to say is simply true: the Solar System is pretty complicated, and coming up with a pure classification system for every body in it is hard. The size of bodies is a smooth curve up from millimetric grains to centimetric pebbles, to meter-scale boulders, to kilometer-scale mountains, and on up to really large asteroids that have enough gravity to become spherical (a dividing line, by the way, which is also imprecise), continuing up to large moons and the smaller planets. Up to Earth size, all the solid bodies in the Solar System fall nicely into a curve. It's the four gas giants (Jupiter, Saturn, Uranus and Nepture) which are the exceptions. (But are they? Maybe they have rocky planetlike cores and just have huge gas envelopes because of how they were formed.)

The fact is that if we classified Pluto as a planet, we'd be doing it for purely historical reasons. Now that doesn't make it wrong, of course. We can still talk about species A and species B and biologists won't immediately start bringing up the problem with species classification (unless it happens to matter in that particular case). But if we're doing real science, we have to accept that Pluto isn't particularly exceptional. If we want to call it a planet, let's just give it an asterisk and say: It's traditional to call it a planet, but from here on it to be a planet you have to satisfy all these criteria. Tradition does have value, of course. Just as we didn't drop Newtonian gravitation from high school curricula after Einstein came along, it's not absolutely necessary to demote Pluto in the same of purity.

But doing so isn't creeping postmodernism, either. It's an effort to promote the classification project that is at the heart of science.

More Lunar Exploration

On National Review Online today, Charles Krauthammer opines that:

For the first time in history, the moon is not just a mystery and a muse, but a nightly rebuke.

Poetic words. But poetry holds the only answer to the question: why? Why go to the Moon? More Krauthammer:

Why do it? It’s not for practicality. We didn’t go to the moon to spin off cooling suits and freeze-dried fruit. Any technological return is a bonus, not a reason. We go for the wonder and glory of it. Or, to put it less grandly, for its immense possibilities. We choose to do such things, said JFK, “not because they are easy, but because they are hard.” And when you do such magnificently hard things — send sailing a Ferdinand Magellan or a Neil Armstrong — you open new human possibility in ways utterly unpredictable.

I'm as tempted as the next space nut to jump on this bandwagon. And if CK were calling for a massive private initiative to go, I'd already be with him. Unfortunately, of course, he isn't:

Yes, we have a financial crisis. No one’s asking for a crash Manhattan Project. All we need is sufficient funding from the hundreds of billions being showered from Washington....

Obviously, CK views space exploration as a separate, unique government project that should be funded in its own right. Fine. But hundreds of other government projects are viewed the same way by other people. If you can get your own pet project - an admittedly romantic, impractical one! - funded, then you undermine your rational, principled objections to those other projects.

One reason government seems to grow without bound is that different interest groups trade projects: I will help you get your project funded if you will help me with mine. This dynamic works in corollary with the fact that the few recipients of a project's funding work like mad to keep it funded - it is their livelihood - but the many taxpayers providing that funding individually have less incentive to cancel it. Each project may only cost the average taxpayer a few dollars; it is only collectively that the burden is felt.

The only way conservatives will actually shrink the size of government is to resist these romantic temptations. The fact that this is so difficult surely provides the pressure that inexorably inflates government.

UPDATE: Unsurprisingly, John Derbyshire is with me on this one.

Lunar Exploration

What if a NASA's lunar budget hadn't been axed in 1967? That's the question an article in New Scientist asks and answers.

It's July 2009, and in Johnson City, America's permanent colony on the moon - named after Lyndon B. Johnson, the president who authorised it - they are celebrating the third generation of lunar Americans: the first child born to parents themselves born on the moon. With just 5000 inhabitants, "city" is perhaps too grandiose a term.

(I guess that would make it too small to qualify in CNN's Best Places to Live database. But I digress.)

The New Scientist article is a wonderful fantasy. It's what all us space nuts - and I certainly am one - wished for when we read Heinlein and Clarke as kids. But is it remotely realistic? What the article keeps in very soft focus is any talk of whether money is being made out there in space.

There's talk of a trade in mined helium-3 for use in fusion reactors on Earth - if fusion technology ever gets established on the mother planet.

Cool. But even in this rose-tinted alternate history we still don't have fusion technology, much less a technology specifically based on helium-3. So there's merely "talk".

Shipping costs of material from Earth are still astronomical, and local extraction of water and oxygen is a boom business.

Right now, the cost to lift a kilogram into Low Earth Orbit is about $10,000. That goes up if you used man-rated vehicles, which presumably they've used a lot of if there are 5,000 people on the Moon. And it goes up even more when you realize that it's much more costly to go to the Moon that to LEO. Getting to the Moon requires about 40% more delta-v just to reach a transfer orbit, plus another chunk of delta-v for landing on the Moon. The Apollo mission did this in three stages: the whole Command Module/Lunar Excursion Module stack performed a translunar orbit insertion burn, then the LEM undocked and performed a deorbit burn, and finally the LEM used braking rockets to reach the surface at a survivable speed. The lack of a Lunar atmosphere, while a boon to launching from there, is a problem for landing: you can't aerobrake or use parachutes. So while the Apollo Command Modules could get back to Earth essentially unpowered ("essentially" because they reserved a little maneuvering fuel to make sure they were in the right window for re-entry), a Lunar lander has to use big engines the whole way in.

Adding delta-v adds tremendously to cost, because the rocket equation is exponential. Staging helps, of course - that's how Apollo did it. But the cost of lifting one kilogram from Earth to the Lunar surface would still be factors more than $10,000.

But let's say for the sake of argument that in the New Scientist scenario, rocket technology has improved enough that cost to LEO is $1,000/kg and cost to the Moon is $10,000/kg. How much would Lunarville cost to resupply? The ISS has a current crew of 6, is resupplied about 2.5 times a year, and the average resupply mission brings in 2.5 metric tonnes of supplies (this all comes from Wikipedia, so take with the appropriate amount of salt, but I trust the numbers are close enough for our back-of-napkin calculation). That's about 1 metric tonne per person per year. Lunarville, then, would require about 5,000 metric tonnes per year. At $10,000/kg, that's about $50 billion per year. Even if we assume that the Loonies are highly efficient recyclers, mine much of their own water locally, etc., just the resupply cost is still going to be huge.

Granted, a mere $50 billion/year may seem like chump change compared to the kinds of numbers bandied about today, but it's worth noting that NASA's peak budget - in 1965 - was only $33.5 billion (2007 dollars). So this scenario proposes that the popularity of a Lunar base would be so high that merely resupplying it at an annual cost higher than NASA's entire budget, when NASA was as popular as it ever was, would be politically feasible for 40+ years. And this does not factor in the rest of the scenario about Mars missions and the like. I find this unlikely, to say the least.

I wish that we'd gone into space as much as the next guy. Maybe more. I can even give you sociological reasons to go (more on this another time). I just don't see how either the science or the politics works out, though. Maybe if we do develop He-3 fusion technology on Earth, we can make Lunar mining of it profitable. But even if that were to happen, I think it's more likely we'd end up with a mostly-automated mining outpost than a full-fledged city.

What would induce people to actually emigrate in large numbers to outer space? It would have to make more sense (in cost, in political will, in scientific value, etc.) to go in person than to send robots. Or the cost would have to drop so far that other factors come into play - for example, it doesn't make "sense" by any of the above factors for my family to travel to London for a week, but we might do it anyway just for the fun and the experience. There's some indication that some forms of space flight might be in this range for the super-rich in the near future. But these short hops are a far cry from emigration.

We space nuts have to keep hoping for some game-changing technology, I think. We won't see large-scale manned space flight otherwise.