Get Your Straight Skinny Right Here

I used to study comets for a living.  They’re iceballs, mixed with a little dirt.  They’re pretty small as solar system bodies go — often not more than a couple of miles (or kilometers across). They orbit the Sun just like planets do, and once you know a comet’s orbit (or any solar system object’s orbit), you can predict it pretty well.  They don’t suddenly change their orbits without reason (see Kepler’s laws of Planetary Motion, which apply to comets and asteroids in general (see discussion under “First Law”) as well, to understand why).

To really “get” what a comet is and does, the next time it snows in your neighborhood, take a handful of snow and mix it with some dirt.  If it doesn’t snow, then go get a snowcone or get some chipped ice and mix it with dirt to make an iceball.

Heft it in your hand. Look at it.  It’s not very dangerous on its own, is it? Common sense tells you that it doesn’t have much mass, it doesn’t have a strong gravitational pull.  If you could build a snowball maybe a mile or two across and put it into orbit around the Sun, you’d have a comet.  Most comets are made of water ice, with traces of other ices mixed in (carbon dioxide ice, methane ice, stuff like that that we know the physical principles of).  They orbit the Sun, often in very long orbits that take them out beyond the orbits of Mars, or Jupiter or even Neptune.  There are many, many comets and each one has the same basic makeup and long orbits.  I findthem fascinating because of what they are and where they came from, and what they tell us about the solar system’s history.

The true value of comets is really what they tell us about the conditions in which they formed.  that’s what always kept me interested in the comets we studied. Each one carries a treasure trove of chemical information about the elements in and conditions prevailing in the early solar system.  In the original solar nebula, the cloud of gas and dust from which the Sun and planets formed, gases such as hydrogen, oxygen, nitrogen and so forth were pretty abundant. So were grains of dust and water and other molecules. Because space temperatures are cold, many of the molecules existed as frozen ices.

As the conditions at the center of the nebula warmed up (where the Sun was forming), the hot bright radiation of the protosun destroyed any icy material that existed nearby.  Only the icy materials and gases in the far reaches of the solar system (mostly out beyond Jupiter,where the temperatures were cold enough to support icy objects) survived.

Comets come from a reservoir of icy chunks that has existed beyond Neptune’s orbit since the very earliest epochs of solar system history.  All these objects — collectively grouped as Oort Cloud objects — orbit the Sun, but at very huge distances.  And, as I mentioned above, they carry the chemical evidence of what it was like in the early solar nebula. That makes each comet a treasury of information.

So, how do comets get to the inner solar system?  Their orbits are changed by entirely normal and scientifically understandable circumstances. Since they’re small, it doesn’t take much to nudge a cometary nucleus from its orbit into a slightly different orbit — one that takes it closer to the Sun. The most logical and commonsense suspects for such gravitational nudges would be nearby planets (dwarf or otherwise), or possibly a passing star (and yes, stars can do that) at the very edges of the solar system.  Spacecraft (alien or otherwise) would not be big enough to nudge a cometary nucleus, but a close pass with a body the size of Pluto, for example, would.

Anyway, once nudged, the cometary nucleus is on a new orbit — and often times that orbit is one that takes it in toward the Sun and through the orbital paths of other planets and asteroids.  As a comet gets closer to the Sun, it feels more of the Sun’s gravitational pull, and—at that point, you can see how Kepler’s laws really do work—a comet’s orbit is shaped by the gravitational tug of the Sun and any planetary bodies it flies close to.  If it happened to get close to Earth, it might be affected by that, for example.

This is all very natural and, if you understand what orbits are and how they evolve over time due to natural forces, then you “get” what comets do. They’re frozen chunks of ice and dust, following paths set in motion a long time ago

So, there’s this comet called Elenin doing its closest pass to the Sun during its elliptical orbit.  It’s doing what all things in orbit around the Sun do—which is completely normal and nothing to be worried about.   Its path will take it close enough so that we could spot  it, but not close enough that it’s going to do anything to us.  Even if it passed really close to Earth, its mass is so small and its body so inconsequential that nothing would happen. Really.

So, here’s the skinny on Elenin’s appearance in our skies. On October 16 of this year, it will be approximately 22 million miles (35 million kilometers) from Earth. That is 90 times the distance between Earth and the Moon (which lies around 238,000 miles (~333,000 kilometers) away).  It’s probably not going to be very bright in the sky, and you may need binoculars to see it. So, it’s not really the brightest comet to come into the inner solar system. Certainly many amateur and not a few professionals will take a look at it, and measure its tail and gas out put to help understand its chemical makeup. But, that’s about it.  Another entirely normal cometary appearance in the solar system.

There are a LOT of people out there, posting on the Web about how Elenin is going to blot out the Sun, or align with some other celestial body and cause trouble for Earth in some other way. Some of the stuff I’ve read even invokes unknown aliens, UFO fleets (that nobody except the Bravo Sierra vanguard can see), suddenly appearing and disappearing mysterious spacecraft, and other ad hoc fantasies. It’s like reading about the Bermuda Triangle or voodoo economics—lots of Bravo Sierra, few (if any) provable facts.

It really is all nonsense. There’s no other polite way to put it.  These fantasies are written by people who haven’t taken the time to learn the basic laws of physics and Kepler’s motions. It’s kind of like reading financial news from people who don’t understand how money works or soccer stories written by people who don’t know the rules of the game.

How an object as small as Elenin could blot out the Sun from a distance of 22 million miles makes me laugh. This is a really small comet. If you were looking directly at the Sun (never a good idea though—since it would burn your retinas in a few seconds, so don’t even think about it) and the comet passed between us and the Sun, I doubt you’d even see the difference.  That is, if you could see at all after staring at the Sun that long. Do you really want to trust your eyesight to idiots on the Web who post such nonsense?  So, why trust their “scientific knowledge”?  That’s right. You wouldn’t.

You probably should read all the nonsense though—it’s always good fine-tune your Bravo Sierra Detector_(TM), especially as we head into an election year in the United States. And, in these tough economic times, a little laughter at silliness can be a good thing, as long as you know it’s silliness.  I know that logic and the laws of science are sometimes less enticing and entertaining than out-and-out nonsense.

Before you do wade through the Web-enabled fantasies about this comet, arm yourself with some scientific facts.  Check out the Comet Elenin FAQ, written by people who know the science of comets. The more you know, the less likely it is you’ll be taken in by purveyors of Bravo Sierra.