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James McCanney's Nonsense: Comets Turn Into Planets

Table of Contents
Comets are not "dirty snowballs" The solar wind is electrically charged Comets don't lose mass, they gain it
Comets are huge, not dinky Miscellaneous nonsense  
Links of Interest

McCanney Claim #4: Comets are not small, but huge, planet-sized objects.

McCanney claims that comets gain mass, and become huge. On many occasions he has said that when the comet Hale-Bopp approached the Sun, it was the size of the Moon (about 3400 kilometers, or 2100 miles across), and when it left it had gained so much mass it was the size of Mercury (4900 km or 3000 miles across)!

Well, in another page I showed that comets lose mass, not gain it. But his claim is silly for two other reasons: one is that if comets were this big, it would be pretty obvious. The second is that if comets gained that much mass, it would destroy them in short order.

McCanney claims that comets turn into planets, and that some are huge. But a comet nucleus that big would be pretty obvious! For one, it would be big enough to see. The nucleus of a comet is actually tiny, only a few kilometers across in general. I once heard McCanney claim in a radio interview that comets are obviously huge; just look at them them through a telescope, he said. You can see that they are not tiny points but discernible disks.

HST image of Comet Hyakutake In reality, his claim is silly. The apparent size of a comet is due to the expanding material that has sublimated off the surface, destined to become the comet's tail. That forms a cloud around the nucleus, called the comet "head". As an example, the image here shows the great comet Hyakutake as seen by Hubble. The actual comet nucleus is so small it would be a dot in this image. The big fuzzy part you see is actually sunlight reflected from the head. It's like seeing a lit cigarette with a cloud of smoke around it. McCanney is in essence saying the smoke is the cigarette. Clearly, the cigarette is smaller than the cloud of smoke. It's the same with comets.

But since McCanney says comets don't lose matter, he must deny this obvious fact that comets are small. But like always, McCanney is wrong.

Also, a planet the size of Mercury would alter the orbits of other planets. This would be pretty obvious to the hundreds of astronomers who painstakingly plot the positions of the planets. We send probes to planets, remember, and astronomers would notice if the planets were not where they are supposed to be. McCanney loves to cry foul about NASA, but we're talking astronomers from around the world, and not just Americans. Once again, I have to ask: how big is his conspiracy, anyway?

Okay, so comets aren't planet-sized because we'd notice. But there's a physical reason as well. Bear with me, there are some numbers here, but it's not too bad.

Imagine McCanney's scenario: an asteroid the size of the Moon is moving through the solar system. It gains mass, so much so that in roughly one year (the time Hale-Bopp spent in the inner solar system) it gains enough mass to equal the mass of Mercury. It does this by having small particles slam into it as it plows through the solar wind and other material.

OK, so let's think about this. What happens when a particle hits the surface of that object? The particle is moving pretty fast, and that motion has energy (called kinetic energy). That energy has to go somehwere, and in a collision like this the energy is released as heat. Kinetic energy depends on the mass of the object and its velocity. The mass might be small for each particle, but there are a lot of particles; enough, according to McCanney, to more than quadruple the comet mass! Also, the velocities of collision are quite high. Near the Earth, such collisions are typically 40 or 50 kilometers per second. But let's be generous to McCanney, and say the velocities are much lower, say, 10 km/sec. You'll see why this is generous in just a minute.

The amount of energy released as heat is easy to calculate in this case; it's roughly 1038 ergs. An erg is a small unit, but 1038 is an awful lot of them. The total energy released by the Sun every second is only about 4 x 1033 ergs, so the energy the comet "feels" from impact is more than 25,000 times the Sun's total energy output! Another way to think about it: a one megaton nuclear bomb (about 50 times the explosive energy of the bomb dropped on Nagasaki) releases about 4 x 1022 ergs, so the amount of energy absorbed by the comet as it gains all that mass is the same as dropping 2,500,000,000,000,000 nuclear bombs on it. Since the mass is gained in less than a year, that's the same as exploding 80 million nuclear bombs per second on the comet.

Maybe it's just me, but I'm thinking a comet wouldn't do so well under such treatment.

Obviously, that's so much energy that it would easily vaporize the comet. The amount of energy it takes to totally destroy an object can be calculated in a number of ways. One way is to use what's called its gravitational binding energy. I won't go into details, but I'll point out a terrific page that describes it (using the Death Star from Star Wars as an example!). It turns out that to vaporize a comet of the Moon's mass, it would take about 1036ergs, or one-hundredth the heat released by the impacts. So, ironically, the heat caused by McCanney's mass gain is actually enough to destroy the comet itself!

I'll note that a comet is not held together by just gravity, but also by molecular bonds and other forces. This means it would take more energy to vaporize one. It could conceivably be a much closer contest between the amount of energy holding the comet together, and the amount trying to tear it apart. However, this amount of heat generated is still enormous (enough to make the comet shine as brightly as 80 million nuclear bombs per second, remember), and I already showed comets are not hot, but cold. And of course, the solar wind is neutral, and comets lose mass. Don't forget those! So McCanney is wrong on all these counts.

Remember too I was generous with the collision velocity. The higher the velocity, the higher the kinetic energy, and the more heat generated per impact. In reality, the velocities are much higher, resulting in a heat energy more than ten times what I calculated! So that's what I meant by being generous. The numbers are even worse for McCanney's theory than I calculated, making him even more wrong. If that's possible.

Conclusion: if Hale-Bopp had gained mass the way McCanney claimed, the heat of this would have torn it apart. And if they were as big as he claims, we'd know it. McCanney is wrong.

©2008 Phil Plait. All Rights Reserved.

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