DEATH FROM THE SKIES! OR SOMETHING.

First of all, the first sentence of this post’s title is taken from Phil Plait’s book of the same name, which is awesome.  It looks like this.

You should buy it and read it, and then make all the people you know do the same thing.  It’s a really good book and Phil is really cool and here is his website also, from which I will be drawing heavily for this post as well.  Be internet friends with Phil Plait, is what I’m saying.

ANYWAY, I assume by now that you’ve all stopped crapping your pants over the recent spike in falling-space-object-related news, so I thought I’d offer you a quick recap of what happened, why it happened, and the likelihood of it happening to you.  And stuff.

(Non-Threatening) Space Rock #1

On Friday, February 15, a little rock called 2012 DA14 passed within about 18,000 miles of Earth, which is closer than some of our satellites.  It’s not the closest pass we’ve ever had, and we knew about it a year in advance, but it’s still pretty damn close and is cool because it allows scientists to refine their models about what these things look like and how they move.  Here’s a video taken by NASA of 2012 DA14 as it passed by.

It’s a pretty grainy video, but if you’ve ever tried to take a video of a hundred-foot rock from 18,000 miles away — and I can confidently say that you haven’t — you’ll understand.

Now, this is sort of boring news for a non-scientist, so lots of news outlets tried to spice it up with stuff like “if it had hit us, it would have released the energy equivalent of detonating 2.4 megatons of TNT!” This is true, and that would have sucked a lot, in the same way that being in Hiroshima on August 6, 1945 would have sucked, only about 150 times worse.  But it didn’t.  And we knew it wouldn’t.  Orbital dynamics are not all that complicated.  What’s hard is to figure out where the thing is and where it’s going in the first place.  Once you have that, extrapolating future motion is really quite simple.

There was also talk that since it would be traveling in the area that satellites frequent, it might hit a satellite.  It didn’t, nor was there basically any risk that it would.  Put it this way: satellite manufacturers are always concerned about the risk of strikes from micrometeorites: sand-size rocks zipping around in space that would punch right through the delicate instrumentation that satellites contain.  That almost never happens, and there are millions of times more of those than there are big rocks like this.  The sheer amount of space out there is just too vast.

(Non-Threatening) Space Rock #2

The second space rock on our list is called 2012 YQ1.  Here’s how a “news” site called The Voice Of Russia decided to report this particular story.

There is so much wrong with that headline and subhead that it’s hard to know where to start, but let’s strap on our big boy science boots and wade out into the sucking quagmire of idiocy that is this story.  Somewhere in this swamp of hyperbole and magnificent ignorance squatteth the toad of truth.

First, the main headline.

We have 93 years left till the next End of the World: killer asteroid to hit Earth in 2106

Wrong.  There’s no reason to think it’ll hit Earth at all.  The fact is that it was discovered by one telescope that took a couple of pictures, and then tried to extrapolate where it’ll be 93 years from now.  I’ll let Phil Plait explain why you can’t do that.

Trying to project the orbit of an asteroid like this too far in the future starts getting difficult. Any small uncertainty in the initial measurements get amplified as you project the position into the future, making its predicted location murkier and murkier. Eventually, it gets so fuzzy you can’t make any claims at all.

Think of it this way. Imagine you’re an outfielder in a baseball game. You see the pitcher throw the ball, and the batter swings. It’s a hit! But one-tenth of a second after the batter makes contact, you close your eyes.
Now, based on the fraction of a second you saw the ball move, can you catch it?

Obviously that’s a rhetorical question, and no you cannot.  Not even Bryce Harper can, and he’s a rat-tailed freak of nature.  But Phil’s analogy doesn’t do justice to the uncertainty involved.  See, when a baseball takes off, there aren’t that many forces acting on it, and it’s just going to go up and back down again.  That’s what makes baseball so exciting.  There’s air resistance, wind, and gravity.  Given precise enough measurements, you could probably look at a 0.1-second clip of a baseball and predict where it’d land with enough precision that you could put a trash can under it.

The asteroid, on the other hand, is WAY more complicated.  When I said that orbital dynamics aren’t that complicated, I meant for one orbit.  In this case, the Earth will complete 93 more orbits and the asteroid will complete about 30 before the supposed point of impact.  And on each orbit, the asteroid is going to get nudged.  Its orbit is extremely eccentric, so it’s possible that it would swing close enough to Mars, Venus, or even Jupiter to get pulled one direction or another.  It’d be a fairly minor effect, like a tiny puff of wind on a flying rhinoceros (I don’t know why the rhinoceros is flying), but it doesn’t have to be much.

Let’s say the asteroid is on target to hit the Earth dead center, which is completely impossible to predict.  It only has to miss by about 5,000 miles to not hit us at all.  And between now and January 2106, it’ll travel over 30 BILLION miles through space.  So let’s update the baseball metaphor.  Imagine that rather than standing in the outfield, 300 feet away, you’re standing in San Francisco and the ball’s coming from New York.  Same situation.  The ball gets hit, you close your eyes, and you have four minutes before it comes screaming through the atmosphere and you have to guess where it’s going to land to within about eighteen inches.

The point is that it’s basically impossible to guess where this thing is going to be 93 years from now.  Sure, it might hit the Earth, but it is stupendously more likely that it will not, and it is recklessly irresponsible to say that it will before MANY more observations have been made.

(Actually Somewhat Threatening) Space Rock #3

This is the one you’ve heard of.  It was in Russia and we’re calling it the Chelyabinsk meteor because that’s the city it came down near and also everything sounds cooler if you name it after Russian words (more on that later).  By now I’m sure you’ve all seen the dashcam footage, the footage of the thunderous shockwave hitting windows, and possibly some fake shit which I will also summarily dismiss shortly.  But I’m going to break this down very simply, as a series of questions.

1. What The Fuck Happened?

What happened was that a roughly 10,000-ton chunk of rock hit the Earth at around 40,000 miles an hour.  It then experienced what’s called an “air burst.”  This is what people mean when they keep saying that the asteroid “exploded,” but I find that term misleading.  First, a brief summary of how these collisions work.

Rocks that hit the Earth come from leftover crap from the formation of the solar system, and there are thousands of them hitting us every day.  Most of them are minuscule — the ones you see as “shooting stars” at night are about the size of a pea.  They’re all orbiting in the same direction as the Earth around the Sun because everything does (due to something called the accretionary disk model), so when they hit us, it’s not a head-on collision.  Think of merging into someone on the highway rather than hitting someone in the opposing lane.  The reason they’re going so fast is that once they get pulled into Earth’s gravitational field, it’s a many-hundred-thousand-mile freefall to the surface.  They build up speed and build up speed, accelerating toward us as they get closer and the pull of gravity gets stronger.

Then the atmosphere happens.  The atmosphere is exponentially thicker near the surface of the Earth, to the point that by the time you’re only 19,000 feet off the surface, half of it is below you.  There is as much air contained in the first three and a half miles of atmosphere as there is in the next three hundred. So a rock that’s speeding up meets an atmosphere that’s getting thicker, and it’s like hitting a wall.  The air can’t move out of the way fast enough, so it gets enormously compressed.  When it gets compressed,  it gets extraordinarily hot and that heat burns the rock up.  Not friction.  By the way, that process is called “adiabatic compression.”  Use that at parties.

Now, most rocks will just burn up until there’s no more rock, and that’s it.  But sometimes the heat of the rock causes it to break in half.  Now you have a lot more surface area exposed to the heat of the air and two small rocks, which heat up faster.  They break apart again.  And again and again, in a rapidly accelerating fashion, which turns all the kinetic energy of a very big rock moving very fast into heat, very quickly.  That’s what happened here.

That is NOT what hurt people in this case.  What hurt people was the shockwave from the sonic boom.  A sonic boom happens when an object goes faster than the speed of sound and all the sound waves it produces bunch up in a cone coming out from it, and when it goes by you and the cone catches up, you get ALL THE SOUND at the same time and it’s enough to blow the living shit out of some windows.  Like this.

Those windows just sort of broke and fell down.  But imagine an object 700 times bigger and moving 50 times faster, and you get a shockwave that will blast glass into every room like a goddamn Claymore mine, which would mess you up a lot, especially if, I don’t know, you’ve just run over to stand right next to the window to see what the hell just lit up the entire sky like a military-grade spotlight.  Lots of people — between 900 and 1500, depending who you ask — got stabbed a lot by flying glass, and none of them died and that’s what happened.

2. Why Didn’t We See It Coming?

That’s a good question.  The simple and slightly embarrassing fact is that it was too small and came from the direction of the Sun, so we literally didn’t see it coming because the Sun was in our eyes.

The problem is that there are bajillions of asteroids out there, most of them really small, not shiny, and very hard to see.  We have cameras in space looking for them, but to quote Harry Truman from Armageddon, “it’s a big-ass sky.”

3. Did it make this huge flaming hole in the ground?

NO.  The title on that video is wrong.  That’s a well-known landmark called the Door to Hell, and it’s in Turkmenistan.  What happened was that the Soviets built a natural gas drilling rig on the site and started storing it underground, and then the whole thing went to shit and collapsed into the ground, releasing a lot of methane that poisoned a lot of people nearby.  Scientists, concerned about more huge amounts of methane leaking into the atmosphere, decided to light it on fire and burn it off, figuring that that would take a few days and be easier and safer than trying to extract it from what was obviously unstable ground.  That was 41 years ago, and it’s still on fire.

But there’s another reason you can be absolutely sure that that crater didn’t come from the Chelyabinsk meteorite: meteorites are not hot.  Space is extremely cold; it hovers around -270º Celsius.  The rocks floating around in it are equally cold.  If a rock falls from space and hits the ground, the outside of the rock may get burned off, but the inside is still extremely cold.  To be honest, if you ran over and touched it right after it fell, you’d get frostbite, not burned.

4. Has This Happened Before?

Yes.  A lot.  Many thousands of times.  In fact, NASA estimates that such air bursts happen in the upper atmosphere roughly every year.  But we don’t see or hear those.  They’re too far away and the air up there is too thin to transmit sound.  But we’ve had our close calls.

Remember 2012 DA14?  Space rock number one from up above?  In 1908, a rock about that size came through the atmosphere over Tunguska (remember what I said about Russian names?) and exploded about five miles above the Earth.  That’s ten times the size of the Chelyabinsk meteor and a third the distance from the ground.  It detonated with a force a thousand times stronger than the bomb dropped on Hiroshima, and probably looked something like this.

Let’s try to put the size of that kind of explosion into perspective.  It flattened 80 million trees in a circle 32 miles wide.  The explosion created a shockwave that knocked people off their feet and blasted out windows hundreds of miles away.  Here’s an eyewitness account from a guy forty miles away:

The sky split in two and fire appeared high and wide over the forest. The split in the sky grew larger, and the entire northern side was covered with fire. At that moment I became so hot that I couldn’t bear it, as if my shirt was on fire; from the northern side, where the fire was, came strong heat. I wanted to tear off my shirt and throw it down, but then the sky shut closed, and a strong thump sounded, and I was thrown a few metres.

And it didn’t even hit the ground.  Luckily, it came down in the middle of nowhere, so it killed zero people, but if an asteroid like that hit today, it would have enough force to level a city the size of…well, just pick a city because it doesn’t even matter, millions would die.  If it hit the ground, it’d be even worse.  So on that note…

5. Will It Happen Again?

Also yes.  With absolute certainty.  There’s an algorithm for the frequency of impacts by various sizes of rock, based on what’s likely to be floating around out there, and we can use that for estimates, but big impacts are a matter of when, not if.  Pea-sized rocks hit us every few minutes, ones the size of the one that killed the dinosaurs are supposed to happen every hundred million years or so.  Tunguska-sized impacts are likely to happen roughly every three hundred years, but obviously we don’t know.  That’s why we track large objects as best we can to see if any of them are likely to hit us.  We still don’t know exactly what we’d do if we found one that was likely to hit us (other than pooping ourselves and calling Bruce Willis), but the first step is finding it.  On that front, there’s good news and bad news.

In Armageddon, they say that the asteroid is “the size of Texas.” There are very few rocks that big floating around out there, and they’re pretty easy to see, and we’re very confident that we’ve found them all and they’re not going to hit us.  They’re in stable orbits, and they’re very hard to deflect.  Of course, the one that killed the dinosaurs was only about six miles across, so that may not be super reassuring.

The bad news is comets.  Comets are much scarier for three reasons.  First, they’re bigger.  A lot bigger.  The Hale-Bopp comet is roughly 50 miles across and, despite being made of ice instead of rock, is about five times the mass of Tunguska.  The second problem is that their orbits are weird as shit.  Asteroids orbit in the same plane as everything else in the solar system, so we have a relatively narrow part of the sky to look in.  Also, they orbit in normal, somewhat elliptical orbits.  Comets do not.  They have extremely stretched out orbits that take thousands of years, and they can come from basically any direction.  So we don’t know when they’re going to show up or where to look.  That brings us to the third problem.  Since they have such odd orbits, a comet would hit the Earth a LOT harder.  Asteroids generally hit the Earth at between 40,000 miles an hour, a comet would probably hit at more like 140,000.  Given all those parameters, it’s estimated that if Hale-Bopp hit Earth, it’d be 44 times more powerful than the asteroid that killed the dinosaurs.

So the downside is that a comet might hit us and we can’t do anything about it, but the upside is that it will definitely kill everyone and everything in the world and we’ll never see it coming, so…you know…you don’t have to find a hiding place.

And that brings us to…

(Also Not Threatening) Space Rock #4

This one is named as romantically as the rest of them: Comet C/2012 S1.  People are saying it’ll be the “Comet of the Century,” but that’s not necessarily true.  The comet was discovered in September, and its orbit is fairly well-established.  We know that the comet is going to come very close to the Sun (800,000 miles), which is very promising for it to have a huge tail that will look very shiny and awesome from Earth.  For perspective, a comet in 1910 was bright enough to see for months, even in the daytime, and in 1882 a comet passed that was bright enough to see in the daytime sky right next to the Sun.  That’s very cool, and this one could be that bright too.  The problem is that comets are very unpredictable objects.  They’re mostly ice and some rock, but we rarely know the proportions of those two materials until we watch it start to burn off.  If it’s a lot of ice and not very dense, it might all melt away as it gets closer over the next few months, before we even get a good look at it.  If it’s too much rock, the tail won’t be as bright.  We just don’t know.  What we do know is that it’s almost half a billion miles away now and already has a 40,000-mile tail, so as long as it’s big enough not to burn out before it gets close, it should be pretty awesome.  Keep an eye out.  You’ll hear about it.

(Not Threatening (To Us)) Space Rock #5

I just nested some parentheses.  That’s how real this just got.  You see, there’s another comet out there called C/2013 A1.  It’ll make its way into the inner solar system sometime in the middle of 2014, and it may be as bright as the one I mentioned above.  For us.  If you’re on Mars, it’ll be WAY brighter, because it’ll be right in your goddamn face.

Now, we’ve only been observing it for a few months, so the uncertainty of its path is still high, but the best estimates right now put it about 63,000 miles from Mars at its closest pass, which is 150 times closer than a comet has ever been to Earth.  On the outside edge of the estimate, it’ll be as far away at 650,000 miles, but on the inside estimate, it’ll hit Mars.  

We don’t really know what that would look like.  We’re not entirely certain what Mars is made of and how thick the various layers of the planet are, so we can’t really picture how the seismic force would propagate through the planet.  It doesn’t have much of an atmosphere or magnetic field, so we don’t know how the debris would spread over the surface.  You can’t have a fireball in an atmosphere that’s almost purely CO2, but you can have a shockwave.  The truth is that we just don’t know what happens when a comet hits a hard object.  The only clue we have is Shoemaker-Levy 9.

Shoemaker-Levy 9 was a comet that broke up near Jupiter and slammed into it at roughly 135,000 miles per hour.  The effect was mind-blowing.  Once we realized it was going to hit the planet, we pointed basically all the telescopes that exist at Jupiter and got these images.

Those brown spots are the impact sites of the various pieces that hit Jupiter.  They may not be able to truly convey the scale of this though, so I’ll give you some numbers.  Instruments on one satellite detected a fireball that reached a temperature of 24,000 Kelvin — ten times hotter than what the Space Shuttle encounters on reentry — and a height of eighteen hundred miles.  THAT IS A FIREBALL THE SIZE OF AMERICA.  THE COUNTRY.  And those big brown spots in the photo above?  They’re the size of the planet Earth.  Each one of them released the energy equivalent of six hundred times the entire nuclear arsenal of the combined nations of the world.  And there were three of those.

The shockwaves traveled across the surface of Jupiter at 280 miles per second.  The scars were visible from Earth with even small telescopes for weeks.  The impact raised the temperature of the entire planet by ten degrees, which lasted for almost a month.

And that was Jupiter.  Jupiter is the heavy hitter of the solar system.  Jupiter weighs more than twice as much as all of the other planets in the solar system combined, and it was absolutely fucking rocked by a punch like that.  Mars is 3000 times smaller and much harder.  SML9 exploded in Jupiter’s atmosphere at a density only slightly thicker than that of Earth’s air.  Mars is a rock, and has no atmosphere or strong gravity to break the comet apart, so it would hit in one piece and it would hit a lot harder.  How hard, we don’t exactly know.  But it would be fucking spectacular and might literally break Mars open like a goddamn soft-boiled egg and I hope it happens.  Although I’d be a little sad for the Curiosity rover, because it’s very expensive and awesome and adorable.

So that’s that, kids!  Lots of space rocks are flying around in space and crap.  And now you know enough to bore the shit out of lots of people all the time or, if they’re actually cool people, they’ll be fascinated and like when you tell them these things.  I have one friend who calls me her official personal astronomer (actually I call myself that on her behalf), and when I told her why the full moon is a lot brighter than the nearly full moon, she said it was her favorite fun fact I’d ever told her.  That felt good.  Anyway, now you know some more facts.

It’s good to be back.