The following exchange appeared on Speculations Rumor Mill (a message board for spec fiction writers; you don't have to subscribe to the quarterly spec fiction market gold mine known as Speculations to participate, but usually people who become involved in the RM end up subscribing, because it's so valuable for a writer).

WHAT DO YOU DO WHEN A METEOR HITS YOU?

***Message #244 left by Mary on Apr 22, 1999 at 16:53

In my story, a meteor hits a planet not far from my characters. It weighs about a couple hundred pounds. How loud is this likely to be?

***Message #245 left by Naomi Kritzer on Apr 22, 1999 at 17:48

Mary, I believe that when I read about the many scientific travesties perpetrated by the movie "Armageddon," I heard that the meteor believed to have killed the dinosaurs was only about the size of a VW bug. I could have that wrong, but I'm guessing the answer is, "real loud."

(For potential comparison: when I was 17, a friend of mine -- without warning -- smacked a small piece of impact explosive with a sledgehammer in a garage. I was also in the garage. The resulting sound deafened me completely for at least a minute. My hearing then slowly returned, but it was only approaching normal the following day. I suspect I have retained permanent hearing damage thanks to that one @#*& kid.)

--Naomi

***Message #246 left by Mary on Apr 22, 1999 at 18:41

Err, actually between the dinosaur-killing types (which are a few hundred tons) and the one I'm looking at, air resistance makes a significant difference. I have been able to track down that my meteor will not make a large crater or be very hot; a one-ton one can be lukewarm when found right after impact. But I haven't been able to find out how loud it would be. :(

***Message #247 left by RSWebb on Apr 24, 1999 at 18:03

I think the meteor question has some variables. A comet or stony asteroid might explode in mid-air, and in that case the noise I think would be quite loud indeed, like a high explosive bomb of comparable size.

On the other hand, a couple hundred pound iron asteroid (something over a foot in diameter) falling into soft ground wouldn't make much noise, just thump, though presumably there would be an ungodly shriek as it fell through the atmosphere.

Keep in mind what the meteor hits. Any tree or house or whatever hit by a two hundred pound iron asteroid would be destroyed with a most satisfying crash.

Ecology busting asteroids are measured in the millions, if not billions, of tons, and are a mile or more across. Check your research there.

***Message #248 left by Karina on Apr 24, 1999 at 22:52

Since we're talking about meteors, here ... What, exactly, would happen if a bunch of fist-sized meteors fell into a body of water? More specifically, I want more than 10 but less than 50 (this is specific?!) meteors to fall into Lake Ontario. Can anyone tell me what this would sound like and/or what it would do to the water?

***Message #249 left by Mark Rapacioli on Apr 25, 1999 at 8:28

Fist-sized meteors? I'm guessing it would sound the same as throwing a very large rock into the water, kind of a "bloop" sound followed by little splashes from the displaced water.

***Message #250 left by RSWebb on Apr 25, 1999 at 11:05

The equation for drag force is F=.5 * C * D * A * V^2

C is a dimensionless coefficient based on the shape of the object, usually between .5 and 1.0.

D is density of air, 1 kg/1 m^3

A is cross sectional area, Pi * R^2 for a sphere.

V is velocity

What we're left with is approximately F = R^2 * V^2

We divide force by mass to get deceleration. Since mass is density * volume, and assuming a sphere V = 4/3 * Pi * R^3 and the density of the (iron) meteor is 5000 kg/m^3

Dec = 5 * 10^-5 * V^2 / R

With a typical meteor entry velocity of 15,000 m/s

Dec = 11,000 / R

Things get a little tricky here. For one thing, the density of the atmosphere changes with altitude, and even worse, that force dependency on V^2 and not just V makes integrating the calculation rather difficult, which is why they don't talk much about air resistance in introductory level physics courses.

But we can see as a rough estimate that a meteor less than a foot or two across will decelerate to terminal velocity, while a meteor a kilometer across won't decelerate much at all.

Surprising as it sounds, you might actually survive being conked on the head by a fist size meteor, though I don't recommend conducting the experiment.

***Message #251 left by Karina on Apr 25, 1999 at 14:55

Thanks! I was also wondering if I needed to make my meteors bigger or smaller, but if all they're going to do is "bloop" and not kill people, then they sound just right to me.

***Message #252 left by jAY aRR on Apr 25, 1999 at 17:20

Actually, this has happened. In 1956 a meteor came through a roof and hit a woman in the shoulder. Damage to the woman was minimal, but the roof was a write-off. It's been years since I read this, but I believe it's mentioned in one of Arthur C. Clarkes's books for one of his television series. Clarke's World of Strange Powers, or Strange Occurrences or something like that.

Also, while your fist-sized meteors wouldn't be incredibly hot (not enough cross-section to create the sort of air resistance that makes for real friction around the edges), they might be hot enough to make a little steam on hitting water. A loud hhsssss! would be appropriate, or at least believable.

***Message #253 left by Paul Pence on Apr 26, 1999 at 12:21

I'd go with the explosion sound, myself. The shriek of the incoming artillery, then a boom as the volume of water is displaced at supersonic speeds. If the water is shallow enough, the hot rock would boil some of the water as it cooled. If the rock is very hot and it shattered, then a huge amount of heat would be transferred to the water quickly, so there would be a second explosion of steam, imagine a pressure cooker lid exploding or someone opening a hot radiator.

In all, I'd expect the noise to be about the level of a shotgun blast.

***Message #254 left by Kenny on Apr 26, 1999 at 13:53

Okay, a large part of my professional work is bringing reality to bear on business decisions so maybe I've become too cynical, but I'm having a real problem with surviving being hit by a fist-sized meteor. Terminal velocity or not, there has to be enough kinetic energy in a hunk of metal or rock falling from a few miles up. I know I can drop a chunk of brick off a building and kill someone by splitting their head open; heck, I can throw it hard enough to split someone's head open. So I think I'd forget about meteors bouncing harmlessly off our heads. Or have I missed something here? If so, clue me in because I can't find the flaw in my thinking:)

***Message #255 left by Mark Rapacioli on Apr 26, 1999 at 15:04

Well, for one, terminal velocity is related to the relative density of the object to the density of the medium (i.e., the density of a brick divided by the density of air). As RSW infers above, the ratio of the densities is proportional to the velocity squared. Thus, denser objects falling in air will reach a higher terminal velocity than less dense objects falling in air. So, if a brick is more dense than your typical meteorite, then a brick at terminal velocity will be traveling faster than a meteorite at terminal velocity.

Another factor is the shape of the object. In this case, a spherical object would encounter less air resistance than an object with corners, or an object with irregularities in smoothness. An irregularly shaped object, such as a meteor, would have a lower terminal velocity than a spherical ball of the same volume and density.

Experiments many, many years ago tried to show that objects of different weights, when dropped from the same height, would fall at the same velocity. This is only true in a vacuum (no air resistance) or at velocities much smaller than the terminal velocity. Once you approach an object's terminal velocity, then a heavier object of the same shape and size will fall faster.

Also, Jay Arr's anecdote doesn't mean that if you dropped the same asteroid directly on a person that they would suffer similar, non-catastrophic injuries. A lot of energy is lost in putting a hole in the roof, which should slow down the object considerably. Had the roof not been there, for all we know the woman's arm may have been torn off.

***Message #256 left by Mary on Apr 26, 1999 at 17:41

Thanks for the noise information.

(This is weird. I start out needing a sizeable iron meteor, and when I research the details, they couldn't be more convenient for the story if they tried. I better enjoy it this time; I dare say it won't repeat.)

***Message #257 left by RSWebb on Apr 26, 1999 at 19:39

Wait a minute- I didn't say harmlessly bounce, I said "might survive" being hit on the head by a fist sized meteor. How big a fist are we talking about? How close is medical help? Where on your head does the meteor hit?

Terminal velocity for a baseball, according to Halliday and Resnick, is 42 m/s (that's 95 m.p.h.). Yes indeed- a good baseball pitcher can throw a baseball FASTER than that same baseball will fall if dropped from orbit. Surprising I know, but true.

A spherical rock 64mm (2 1/2") across, with a density of 3500 kg/m^3, will have a terminal velocity of 64 m/s (144 m.p.h). But the terminal velocity could be as low as 50 m/s (112 m.p.h.) if the drag coefficient is favorable. OK, I'm not an expert on head wounds. But if people can survive being shot in the head, I would think people could survive a 112 m.p.h rock.

***Message #260 left by Tim on Apr 27, 1999 at 7:22

I need to know about chickens.

Specifically, I need to know about eggs. How long after conception does the hen lay the egg? How long after that before it hatches? How many eggs can a hen lay at a time?

If anyone knows that stuff offhand, or can point me to a website, I'd appreciate it.

***Message #261 left by Barry on Apr 27, 1999 at 9:56

A few chicken answers.

If incubated the eggs will hatch in 21 days, but a hen can lay an egg a day and not sit on them at first and accumulate a batch before she sits them, the correct word there is broods them. Then the chicks will hatch at about the same time.

A large hen could brood ten to a dozen eggs. By herself she would have trouble laying that many before they would need to be brooded, but usually the other hens will lay their eggs in with hers so she can accumulate quite a batch of eggs.

***Message #262 left by Barry on Apr 27, 1999 at 9:59

About the conception part, the rooster keeps track and takes care of everything.

***Message #263 left by RSWebb on Apr 27, 1999 at 10:45

I've been using a spreadsheet to refine my calculations. That V^2 dependency really makes it tough to integrate. But a spreadsheet can make an approximation.

Keep in mind there are some variables here, the initial entry velocity, the angle of entry (which determines how much atmosphere the asteroid passes through), drag coefficient, and density.

A foot diameter asteroid would not decelerate to terminal velocity, if fact it would still be supersonic under all but the most optimistic assumptions.

I still think a "fist-sized" asteroid will decelerate to terminal velocity, under most circumstances. The transition from supersonic to subsonic speeds usually occurs around 6", and below about 3" terminal velocity is reached. A 1" asteroid would surely reach terminal velocity.

***Message #264 left by Jay Arr on Apr 27, 1999 at 13:41

Also, remember that these things break up in the atmosphere. I'm assuming that all sizes refer to the object at the point of impact with the earth. (Is everybody making that assumption?) So a foot-diameter object in space might be a single five-inch object at impact or it could be several fist-sized objects. Depending on where any particular piece broke away from the main body, two pieces from the same object could have wildly varying trajectories and velocities. There's a lot of freedom here for interesting effects.

***Message #265 left by Tim on Apr 27, 1999 at 15:09

Thanks, Barry! That was just what I needed.

***Message #266 left by Barry on Apr 27, 1999 at 15:31

You're welcome.

Did you notice that everyone else grabs those easy thermodynamic asteroid questions, but leaves the difficult chicken work to me?

***Message #267 left by Kenny on Apr 27, 1999 at 15:50

Ah, I begin to understand the logic. Now if we drop an igneous (sp) rock from the empire state building and it hits someone on the head, the odds of survival would exist for sure, but probably not to any significant extent. Mind you, I've been know to draw to an inside straight just to irritate the other players:)

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1. I spell-checked this before posting it, to avoid outraged cries from my fellow writers, but you should know that you don't have to have perfect spelling/grammar/punctuation to join the fray at the Rumor Mill.
2. If you try some of these ideas, email me and let me know how they worked!
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