Recent supercomputer simulations of the Tunguska collision made at Sandia National Laboratory (www.sandia.gov/news/resources/releases/2007/asteroid.html) produced movies of a simulated Tunguska impact that look like works of modern abstract art. These simulations have helped scientists understand what happens during such an impact.
Scientists figure that a rock measuring fifty meters in diameter was pulverized in the atmosphere high above the ground. This explains why no one finds large pieces of the Tunguska object scattered over the ground, although tiny pieces have been found embedded in trees.
The rock's energy was released as an explosion that was the equivalent of a five-megaton nuclear bomb high in the atmosphere. Originally, scientists had estimated that the explosion at Tunguska was closer to twenty megatons. They revised their estimates as they gained a better understanding of how the energy of the blast spread. A five-megaton explosion requires a smaller rock than the twenty-megaton explosion. This is an important distinction since small rocks in space are much more common than larger ones. So Tunguska-like collisions may be more common than we thought. That's one reason we are glad that NASA pays the folks at the Catalina Observatory to watch the skies for us. (Paul suspects that government agencies are keeping watch for nuclear-sized explosions around the Earth, and hears rumors that they may pick up one large nuclear bomb equivalent blast per year due to rocks from space striking the atmosphere.)
What about Mars?
To understand what happens when an asteroid hits Mars, you need to remember that Mars has only 1/100 the atmosphere of Earth. You have to go up forty kilometers above the surface of the Earth to enter air with the same density as gas at the surface of Mars. Without the atmosphere to break them up, much smaller rocks can make it to the surface of Mars.
The spacecraft with high-resolution cameras that orbit Mars have documented more than twenty new impact craters that have formed in the last decade. These Martian craters tend to be twenty-five meters or so in diameter, which means that they were made by falling rocks as small as one meter in diameter.
The creation of twenty-five-meter-wide craters on Mars is common. Scientists say that if you lived on Mars for a decade you would hear or feel the shockwaves from an impact. Thank goodness our atmosphere protects us on Earth.
Throwing Rocks,
Making Craters
Before we describe the actual impact on Mars, let's pause to make some craters. (We know that all this talk about falling rocks has made you want to throw some.)
At NASA, scientists study cratering made by hypervelocity impactors by using an extremely high-speed gun. Paul develops activities for middle school science students and decided rather quickly that arming students with a hypervelocity gun might be unwise. So he did some experimentation and discovered a safer model of crater formation.
The key ingredient to Paul's model is table salt. Take a plastic tray at least five centimeters (two inches) deep and a foot in diameter. (Garden supply stores sell trays like this to place under potted plants.) Fill the tray full of table salt. (In an interesting tangent, Paul has discovered that salt costs almost the same regardless of the size of the package indicating that the price of the salt is almost entirely due to packaging and advertising.) Get a spherical ball bearing or fishing weight as big as a fingernail (at least one cm or 1/2 inch in diameter). Drop the weight into the salt from about a meter (a yard) above the tray. Watch what happens.
The crater will be much larger than the impacting object—as much as ten times larger. The same thing happens in real cratering events. (Of course, the real cratering events happen in solid rock and your event occurred in loosely packed salt.)
Notice also the pattern of the ejecta, the material thrown out of your crater. Some salt grains were probably thrown entirely out of your plant tray.
Rocks are found surrounding craters in real life as well. To study ejecta patterns, cover the salt in the tray with black paper. Cut a fist-sized hole (ten cm or four inches in diameter) in the middle of the paper. Drop a weight into this hole and observe the ejecta pattern. There will be more ejecta near the crater than farther away, and the ejecta will spread to a distance of many crater diameters from the point of impact. In fact we know that impacts on Mars have given some Mars rocks escape velocity. We know this because scientists have found Mars rocks on Earth.
You can also experiment with throwing your weight into the salt at an angle. Even when an object impacts at an angle far from vertical the impact crater is still circular. It takes an impact at an angle that skims the surface (under twenty degrees from the horizontal) to create an elliptical crater. (Meteor Crater Arizona is more circular than elliptical even though the main body of the impactor is located underground, beyond the rim of the crater.)
When the odds were in favor of the asteroid hitting Mars, Paul decided to create a simulation of the impact in Second Life (www.secondlife.com, Exploratorium (132, 163, 251)). Alas, the full scale model was too large even for a virtual world. The resulting crater would have been one km in diameter and the virtual land for such a simulation would have cost over $16,000. So Paul made a smaller crater impact, one that created a crater that was only fifty meters across. The cratering event portrays the same details you will see in your hands-on model of cratering.
Meanwhile, Back on Mars
With your new understanding of cratering, let's return to our consideration of WD5 on a collision course with Mars. It would have gone something like this: WD5 enters Mars atmosphere at hypersonic velocities, glowing incandescent and breaking up a little, shedding debris as it falls. It hits the surface at twenty kilometers per second, taking only three milliseconds from the time the bottom of the asteroid hits the surface until the top passes the level of the surface.
The energy of the impact spreads down and outward as a shock wave compresses rocks and actually changes the minerals in the rocks through shock metamorphism. (Finding shock-metamorphic rocks is one way to make sure you are dealing with a meteorite crater on Earth.) Because the crater is excavated by the shock waves and is larger than the impactor, the crater is hemispherical to start. The compressed rocks rebound, throwing the ejecta up and away from the crater at high speed. The ejected blocks fall back to the planet surface where they may create secondary craters. The ejecta that hits nearest the rim has been thrown out at the lowest speeds and so creates smaller secondary craters. If the surface is made of layered rocks, the layers of rock near the rim are flipped over creating a rare instance where older rock is on top of younger rock. The flipped-over rock and ejecta near the rim create a raised rim for the crater. In all but the smallest craters, the bottom of the crater rebounds since the rock above has been removed.
Ejecta falls back into the crater and avalanches roll down the sides of the crater immediately starting the process of filling in the crater. The result is a crater that is wider than it is deep. Seismic waves spread out from the crater and sound waves too. Even the most enthusiastic cratering expert would agree: A meteorite impact is definitely something you want to observe from the side or above at a distance of many miles.
What Next?
Calculations made shortly after Boattini spotted WD5 gave the asteroid a 1/25 chance of actually striking Mars. As data accumulated, the odds dropped to 1/10,000.
Scientists are fairly confident that the asteroid missed Mars, although there is a chance that it struck the planet without being captured on camera. WD5 has not been seen since its close approach. If it did pass Mars without colliding, its close encounter with the planet threw it into a new orbit.
Fortunately for Paul and other cratering enthusiasts, WD5 will remain an Earth-orbit-crossing asteroid even in this new orbit. In four years, Paul will be watching the skies and dusting off his lecture notes.
* * * *
The Exploratorium is San Francisco's museum of science, art, and human perception—where science and science fiction meet. Paul Doherty works there. Pat Murphy used to work there, but now she works at Klutz
(www.klutz.com), a publisher of how-to books for kids. Pat's latest novel is The Wild Girls. To learn more about Pat Murphy's writing, visit her web site at www.brazenhussies.net/murphy. For more on Paul Doherty's work and his latest adventures, visit www.exo.net/~pauld.
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Short Story: Evidence of Love in a Case of Abandonment: One Daughter's Personal Account by M. Rickert
Mary Rickert won a pair of World Fantasy Awards last year, one for her story “Journey Into the Kingdom” (from our May 2006 issue) and one for her first collection of stories, Map of Dreams. Now she gives us a chilling glimpse of how the near future might be.
"When I, or people like me, are running the country, you'd better flee, because we will find you, we will try you, and we'll execute you. I mean every word of it. I will make it part of my mission to see to it that they are tried and executed."
—Randall Terry, founder of Operation Rescue
* * * *
It took a long time to deduce that many of the missing women could not be accounted for. Executions were a matter of public record then and it was still fairly easy to keep track of them. They were on every night at seven o'clock, filmed from the various execution centers. It was policy back then to name the criminal as the camera lingered over her face. Yet women went missing who never appeared on execution. Rumors started. Right around then some of the policies changed. The criminals were no longer named, and execution centers sprung up all over the country so it was no longer possible to account for the missing. The rumors persisted though, and generally took one of two courses: Agents were using the criminals for their own nefarious purposes, or women were sneaking away and assembling an army.
When my mother didn't come home, my father kept saying she must have had a meeting he'd forgotten about, after all, she volunteered for Homeland Security's Mothers in Schools program, as well as did work for the church, and the library. That's my mom. She always has to keep busy. When my father started calling hospitals, his freckles all popped out against his white skin the way they get when he's upset, and I realized he was hoping she'd had an accident, I knew. The next morning, when I found him sitting in the rocker, staring out the picture window, their wedding album in his lap, I really knew.
Of course I am not the only abandoned daughter. Even here, there are a few of us. We are not marked in any way a stranger could see, but we are known in our community. Things are better for those whose mothers are executed. They are a separate group from those of us whose mothers are unaccounted for, who may be so evil as to escape reparation for their crimes, so sick as to plan to attack the innocent ones left behind.
I am obsessed with executions, though there are too many to keep track of, hard as I try to flip through the screens and have them all going on at once. I search for her face. There are many faces. Some weeping, some screaming, some with lips trembling or nostrils flaring, but I never see her face. Jenna Offeren says her mother was executed in Albany but she's lying. Jenna Offeren is a weak, annoying person but I can't completely blame her. Even my own father tried it. One morning he comes into my room, sits at the edge of my bed and says, “Lisle, I'm sorry. I saw her last night. Your mother. They got her.” I just shook my head. “Don't try to make me feel better,” I said, “I know she's still alive."
My mother and I, we have that thing some twins have. That's how close we've always been. Once, when I was still a little kid, I fell from a tree at Sarah T.'s house and my mom came running into the backyard, her hair a mess, her lipstick smeared, before Mrs. T. had even finished dialing the cell. “I just knew,” Mom said, “I was washing the windows and all of a sudden I had this pain in my stomach and I knew you needed me. I came right over.” My wrist was broke (and to this day hurts when it's going to rain) and I couldn't do my sewing or synchronized swimming for weeks, but I almost didn't mind because back then I thought me and Mom had something special between us, and what happened with my wrist proved it. Now I'm not so sure. Everything changes when your mother goes missing.
I look for her face all the time. Not just on the screens but on the heads of other women, not here, of course, but if we go to Milwaukee, or on the school trip to Chicago, I look at every women's face, searching for hers. I'm not the only one either. I caught Jenna Offeren doing the same thing, though she denied it. (Not mine, of course. Hers.)
Before she left us, Mom was not exactly a happy person, but what normal American girl goes around assuming that her own mother is a murderer? She even helped me with my project in seventh year, cutting out advertisements that used that model, Heidi Eagle, who was executed the year before, and I remember, so clearly, Mom saying that Heidi's children would have been beautiful, so how was I to know that my own mother was one of the evil-doers?
But then what did I think was going on with all that crying? My mother cried all the time. She cried when she was doing the dishes, she cried when she cleaned the toilets, she even cried in the middle of laughing, like the time I told her about Mr. Saunders demonstrating to us girls what it's like to be pregnant with a basketball. The only time I can ever remember my mom saying anything traceable, anything that could be linked from our perfect life to the one I'm stuck in now, was when she found a list of boys’ names on my T.S.O. and asked if they were boys I had crushes on. I don't know what she was thinking to say such a thing because there were seven names on that list and I am not a slut, but anyhow, I explained that they were baby names I was considering for when my time came and she got this look on her face like maybe she'd been a hologram all along and was just going to fade away and then she said, “When I was your age, I planned on being an astronaut."
My cheeks turned bright red, of course. I was embarrassed for her to talk like that. She tried to make light of it by looking over the list, letting me know which names she liked (Liam and Jack) and which she didn't (Paul and Luke). If the time ever comes (and I am beginning to have my doubts that it will) I'm going to choose one of the names she hated. It's not much, but it's all I have. There's only so much you can do to a mother who is missing.
My father says I'm spending too much time watching screens so he has insisted that we do something fun together, “as a family” he said, trying to make it sound cheerful like we aren't the lamest excuse for family you've ever seen, just me and him.
There's plenty of families without mothers, of course. Apparently this was initially a surprise to Homeland Security, it was generally assumed that those women who had abortions during the dark times never had any children, but a lot of women of my mother's generation were swayed by the evil propaganda of their youth, had abortions and careers even, before coming back to the light of righteous behavior. So having an executed mother is not necessarily that bad. There's a whole extra shame in being associated with a mother who is missing however, out there somewhere, in a militia or something. (With the vague possibility that she is not stockpiling weapons and learning about car bombs, but captured by one of the less ethical Agents, but what's the real chance of that? Isn't that just a fantasy kids like Jenna Offeren came up with because they can't cope?) At any rate, to counteract the less palatable rumor, and the one that puts the Agents in the worse light, Homeland Security has recently begun the locks of hair program. Now they send strands of a criminal's hair to the family and it's become a real trend for the children to wear it in see-through lockets. None of this makes sense, of course. The whole reason the executions became anonymous in the first place was to put to rest the anarchist notion that some women had escaped their fate, but Homeland Security is not the department of consistency (I think I can say that) and seems to lean more toward a policy of confusion. The locks of hair project has been very successful and has even made some money as families are now paying to have executed women's corpses dug up for their hair. At any rate, you guessed it, Jenna shows up at execution with a lock-of-hair necklace that she says comes from her mother but I know it's Jenna's own hair, which is blonde and curly while her mom's was brownish gray. “Tha
t's ‘cause she dyed it,” Jenna says. I give up. Nobody dyes their hair brownish gray. Jenna has just gone completely nuts.
It seems like the whole town is at execution and I realize my father's right, I've been missing a lot by watching them on screen all the time. “Besides, it's starting to not look right, never going. It was different when your mother was still with us,” he said. So I agreed, though I didn't expect much. I mean no way would they execute my mom right here in her home town. Sure, it happens but it would be highly unlikely, so what's the point? I expected it to be incredibly boring like church, or the meetings of The Young Americans, or Home Ec class, but it wasn't anything like any of that. Screens really give you no idea of the excitement of an execution and if you, like me, think that you've seen it all because you've been watching it on screen for years, I recommend you attend your own hometown event. It just might surprise you. Besides, it's important to stay active in your community.
We don't have a stadium, of course, not in a town of a population of eight thousand and dwindling, so executions are held on the football field the first Wednesday of every month. I was surprised by the screens displayed around the field but my father said that was the only way you could get a real good look at the faces, and he was right. It was fascinating to look at the figure in the center of the field, how small she looked, to the face on the screen, freakishly large. Just like on screen at home, the women were all ages from grandmothers to women my mother's age and a few probably younger. The problem is under control now. No one would think of getting an abortion. There's already talk about cutting back the program in a few years and I feel kind of sentimental about it. I've grown up with executions and can't imagine what kids will watch instead. Not that I would wish this on anyone. It's a miserable thing to be in my situation. Maybe no one will even want me now. I ask my dad about this on the way to execution, what happens to girls like me, and for a while he pretends he doesn't know what I'm talking about until I spell it out and he can't act all Homeland Security. He shakes his head and sighs. “It's too soon to say, Lisle. Daughters of executed moms, they've done all right, maybe you know, not judges’ wives, or Agents, or anyone like that, but they've had a decent time of it for the most part. Daughters of missing moms, well, it's just too soon to tell. Hey, maybe you'll get to be a breeder.” He says it like it's a good thing, giving up my babies every nine or ten months.
FSF, October-November 2008 Page 21
