Well, I'm no great shakes at geology. Astronomy is more my bag. So if no one else is going to tackle it, I'll present another armegeddon guide. I call it:
Ahh! Meteor!
There are billions of rocks floating around out there, and one of them has our name on it!
So Just What's Out There, Anyway?
Space is big. Really, really big. Heck, our solar system is bigger than most people can conceive of. And there's lots of stuff floating around out there.
Most people have a vague idea from grade school astronomy lessons that there's a bunch of asteroids floating out somewhere between Mars and Jupiter. Fewer, I think, know about the Kuiper belt (although everybody knows at least one Kuiper belt obejct; we'll get to that later). Fewer still know about the (hypothetical) Oort Cloud, and fewer than that have studied astronomy in enough depth to be aware of the concept of LaGrangian points and the Trojans. Let's go through these in order from nearest to farthest, shall we?
1. The (Main) Asteroid Belt
Where Is It?
Floating in the space between Mars and Jupiter, spanning from approximately 1.5 astronmical units to just over 4 astronomical units (from 47 to 280 million miles from Earth) is the asteroid belt. This group of objects in recent days has actually come to be known as the Main belt, due to the discovery of other similar groups of objects within our solar system.
What Is It?
The Main belt (as the name implies) is a grouping of asteroids that orbit the sun in a mostly continuous belt. Currently there are hundreds of thousands of known objects there, ranging in size from a few hundred metres to just under 1000 kilometres in diameter. There is also quite a few objects that are smaller still, along with a large amount of dust; however, these pose no threat and are extremely difficult to detect and catalogue. The largest object, known as Ceres, is large enough to be considered a dwarf planet under the new rules.
These objects tumble through space, frequently colliding with one another. This has two effects; one is that the geography of the Main belt is constantly changing, as asteroids break apart or fuse together to form new objects. The other effect is that debris is occasionally thrown off.
What Kind Of Threat Is It?
Moderate. As previously noted, high speed collisions do occasionally produce debris that will be flung out of the Main belt. Some of it comes our way and occasionally even impacts us. Most of that is too small to get worked up about; it either burns up in orbit or is slowed and broken up sufficiently that when it does impact it doesn't do any serious harm. However, every once in a while something big enough to be a threat does get through. It was an object of that nature that is theorized to have wiped out the dinosaurs.
2. The Trojans
Where Are They?
There are two groups of Trojans, known as the Trojans and the Greeks (fans of Homer will get the reference). They occupy the
Lagrangian points around Jupiter (the explanation of what Lagrangian points are is fairly involved; suffice to say that the ones we're talking about are two stable points where objects may share an orbit with another object or planet). This puts them between 5 and 5.5 AU out (approximately 370 to 420 million miles from Earth).
What Are They?
As mentioned above, the Trojans are a group of asteroids sharing an orbit with Jupiter. Hypothetically Trojan objects may occupy Lagrangian points and thus share an orbit with any planet, and Martian and Neptunian objects have been identified as well. It is even theoretically possible to have Trojan planets that share orbits, although there are none of those in our solar system. Regardless, the Jovian Trojans are the most widely recognized ones.
Currently there are 600 known Trojans sharing space with Jupiter, and split between the two 'camps'.
What Kind of Threat Are They?
Negligible. Unlike the Main belt, the Trojans are relatively stable. They're smaller on average than Main belt objects and tend to throw off less debris. A large amount of what they
do throw off is captured in Jupiter's gravity and poses no threat to us.
In short, the odds of one of these things killing us are longer than the odds of me winning the powerball (keeping in mind that I am Canadian and thus ineligible).
3. The Kuiper Belt
Where Is It?
The Kuiper belt lies on the fringes of our solar system, just past Neptune's orbit. Much larger than the Main belt, it spans from about 30 astronomical units to roughly 55 astronomical units (approximately 2.5 billion to 5 billion miles from Earth).
What Is It?
The Kuiper belt has a consistency very similar to the Main belt, with asteroids ranging in size from hundreds of metres to a bit over 2000 kilometres in diameter. The largest object in the Kuiper belt is very well known; Pluto was until very recently the ninth planet in the solar system. However, when Eris was discovered, definitions were changed and Pluto was downgraded to a dwarf planet (which, despite the name, is not actually a planet in the strictest sense of the word). The objects of the Kuiper belt do contain more ice and less mineral content than objects in the Main belt, which is useful when attempting to detemine what came from where.
What Kind Of Threat Is It?
Minimal. Like the Main belt, objects in the Kuiper belt do collide frequently and have been known to throw off debris. However, between them and the Earth are the gas giants. These four planets act a bit like guards for the inner solar system and block a large portion of the stuff that comes from the outer reaches. Kuiper belt objects are less stable (and therefore pose a greater threat than) Lagrangian objects, but the chances are far greater of a Main belt object hitting us first.
4. The Scattered Disc Objects
Where Are They?
The Scattered disc objects lay on the very outer reaches of the solar system. The inner edge overlaps with the Kuiper belt, approximately 50 astronomical units out. It extends out to about 100 astronomical units (approximately 10 billion miles from Earth) and possibly further.
What Are They?
Honestly, we don't know a whole heck of a lot about them. These things are on the very outer fringes and are quite difficult to detect. We can postulate that they are probably objects similar in composition to the Kuiper belt objects and we can say that the largest currently known Scatted disc object is Eris, with a diameter around 2500 or 3000 kilometres. As noted above, it was the discovery of Eris that forced the IAU to finally create a firm definition of what exactly a planet is, and thus lead to the demise of Pluto's status as such. How many are there? Where are they? What do they do for fun? We can't say.
What Kind of Threat Are They?
Miniscule to mild. It's hard to say for sure just how threatening these things are because we know so little about them. We can note with a bit of relief, though, that any Scattered disc object with evil intent faces all the same obstacles the Kuiper belt objects do, plus the added obstacle of navigating the Kuiper belt itself. Thus, we're probably pretty safe.
5. The Oort Cloud
Where Is It?
Hypothetically, the Oort cloud is a spherical cloud of objects enveloping the solar system, about 50 000 astronomical units out (nearly 5 trillion miles from Earth).
What Is It?
In short, conjecture. Nobody even knows for sure if it exists, at this point. The Oort cloud was advanced as an explanation for where long-period comets come from; we have no direct evidence of it whatsoever.
What Kind Of Threat Is It?
Unknown. With it's very existence still in doubt, whether or not the objects in the Oort cloud pose a threat to us is anybody's guess.
6. Long-Period Comets
Where Are They?
It's impossible to give one location or distance for these things, because they're highly eccentric. They possess very long elliptical orbits which makes it difficult to even catalogue them all, let along know where they all are at any given time.
What Are They?
As the name implies, long-period comets are objects which take a very long time to complete an orbit. Their orbits, as noted above, are highly elliptical and not always completely stable. This, unfortunately, makes it nearly impossible to track them all with any degree of accuracy.
What Kind Of Threat Are They?
Moderate to high. Our methods of detecting these things simply aren't good enough for us to even be completely sure that there isn't one bearing down on us right now. To make matters worse, once such object is currently projected to actually be on a collision course with Earth. The good news is that current estimates place that collision somewhere around 2900 AD, so nobody reading this will be around to see it.
So What Do We Do?
Unfortunately, there isn't really very much that can be done to prepare for the worst. In a worst case scenario, it's possible that one of these objects could throw enough dust and debris into the air to start another ice age. Plants would be unable to survive, and the whole food chain would collapse on itself. Smaller objects may only cause local extinction events or throw up a thinner dust cloud. To prepare for these eventualities, one could stock up and canned and frozen goods, so as to have food to survive the disaster. Aside from that, all we can really do is try not to think about it.
So smile, Because we could all be dead tomorrow!
(All information is backed up by links to wikipedia, for simplicity on behalf of both the author and the readers. For more information on comets of doom, consult your local library.)