Science: asteroid vs. hero physics

[Ovinomancer]

another wrinkle in the heroics is catching up to this thing and matching speed to then get behind it to push.

that seems like it could be action packed, but also complicated. unlike superman, where smashing headlong into it would be easy, if she flies toward it, she's got momentum in the wrong direction once she gets there. I assume she could zip out to some midpoint, then reverse direction and bring herself back up to speed as the asteroid catches up

I'm sure the mass-driver system she uses defies conventional physics, but...

Space is BIG. This asteroid moving at 50km/s still takes two hours to go between the moon's orbit and Earth. You could slingshot the moon to come up behind a la Armegeddom but flip the expectation by shoving it past the Earth rather than blowing it up.

By the by, the close pass to Earth would likely fling the asterood out of a passing orbit, so no need to destroy it before it comes back -- it most likely won't. An easy, and not unlikely, out is that it's thrown into the Sun after the near miss.

 

[MarkB]

If you want to play up the sniper angle, maybe make the asteroid particularly elongated, maybe even barbell-shaped so it has a weak point in the middle.

The way it's oriented coming in, it's going to hit the atmosphere end-on and carve right through, like a bullet punching through armour, losing only a little mass to friction before striking the ground at almost full orbital velocity, causing catastrophic damage.

But if, instead of trying to divert it, she re-orients it so that it slams into the atmosphere side-on, maybe with some additional tumble, the stress of atmospheric entry will shatter it. Some of the mass will still make it to the ground, but at a shallower angle, and the majority of the asteroid will be broken down into small enough chunks that it burns up in the atmosphere.

Not a clean win, but enough to reduce it from a dinosaur-killer to, say, a really bad day for several hundred square miles plus a side-serving of nuclear winter.

But the only way to give it plenty of spin and still ensure that it hits atmosphere at the right orientation is for her to ride it all they way down to the edge of space, making last-minute adjustments as she goes - thus the extra peril.

 

[Mustrum_Ridcully]

I am not sure the distances could possibly work out with the 5 minute time frame, but maybe instead of trying to deflect it to avoid hitting Earth, you just need to deflect it so it hits the Moon?

 

[Eltab]

I am not sure the distances could possibly work out with the 5 minute time frame, but maybe instead of trying to deflect it to avoid hitting Earth, you just need to deflect it so it hits the Moon?

Our Heroine adopts this plan, begins pushing. She tells herself that she'll get out of the way when the heat of re-entry makes her uncomfortable. Only a few minutes later does she realize: the Moon has no atmosphere so there won't BE any 'heat of re-entry' … and there's almost no room or time to get out from underneath the onrushing Really Big Rock.

 

[Janx]

I am not sure the distances could possibly work out with the 5 minute time frame, but maybe instead of trying to deflect it to avoid hitting Earth, you just need to deflect it so it hits the Moon?

I might need to expand the time frame to an hour or so, depends on how close to math correctness I need for verisimilitude.

Technically, I bet it's less distance from ground to "space" than it is from "space" to rock, so even in the original 15 minutes, I bet I can shorten how much the initial flight is to get a few more minutes on the secondary stage.

Mostly, I chose a time that seemed urgent and could be divided into stages for purposes of story telling. This is all act 3 of a 5700 word story, so it's got to move somewhat quickly and keep the action pace up. It's not quite like the usual asteroid movies as the first two acts are about other wrong views of why the world is like it is (post impact 100 years ago). Pacing wise, I've got it where I want it, but I'd like the science to be closer to right so if y'all read it, I wouldn't look like lousy science fiction writer

 

[freyar]

I haven't bothered with the numbers, since I think those have probably been covered. But assuming a "glancing blow" scenario, hitting the asteroid perpendicularly is the way to go. What really matters is change in momentum, not really energy, though your heroine will need a lot of energy to have enough momentum to transfer to the rock.

Treating Earth as a fixed point completely ignores the geometry of the problem. The asteroid is essentially a bullet fired leading Earth, on a rendevous to meet it, not a bullet fired directly at a stationary Earth.

I did want to address this point. Over the timescales we're talking about (say an hour, even up to a day or two), the earth is moving at close to a constant velocity. So it is actually perfectly fine and even easier to think about everything from the earth's point of view (ie, "rest frame of the earth"), where the earth is sitting still and the asteroid is coming straight at it. This is in fact the way the characters in the story, coming from earth, would think about it. There's no need to worry about the earth's orbit. Think about the asteroid like a bullet shot at a stationary earth all you like.

 

[Ovinomancer]

I haven't bothered with the numbers, since I think those have probably been covered. But assuming a "glancing blow" scenario, hitting the asteroid perpendicularly is the way to go. What really matters is change in momentum, not really energy, though your heroine will need a lot of energy to have enough momentum to transfer to the rock.



I did want to address this point. Over the timescales we're talking about (say an hour, even up to a day or two), the earth is moving at close to a constant velocity. So it is actually perfectly fine and even easier to think about everything from the earth's point of view (ie, "rest frame of the earth"), where the earth is sitting still and the asteroid is coming straight at it. This is in fact the way the characters in the story, coming from earth, would think about it. There's no need to worry about the earth's orbit. Think about the asteroid like a bullet shot at a stationary earth all you like.

This is... not even wrong. To attempt to explain, again, both the Earth and the asteroid are orbiting the Sun. The Earth is in constant motion along it's orbital path, as is the asteroid. The point of impact is where those two lines cross. In other words, the asteroid isn't moving towards the Earth, it's moving towards where the Earth is going to be.

Imagine trying to hit a moving car with a baseball from the field next to the road the car is travelling along. If you throw the baseball at the car, you will miss because by the time the baseball arrives at the road, the car will have already traveled past that point. Instead, you throw the baseball ahead of the car so that when the baseball gets to the road, it meets the car there.

In this scenario, the hero is like a wind pushing the baseball after you throw it. The car is moving so fast that it takes less wind to slow the thrown ball so that the car zooms past the rendezvous than it does to push the ball to the side so that it gets there at the same time but far enough off to one side. Largely this is because, for reference, the car is huge and moving really fast so that a small change in speed leverages the speed of the car to cause the miss rather than having to push the baseball off target by half the length of the car from zero lateral speed.

If you insist on treating Earth as the center point, then the asteroid is going to appear to move under constantly changing acceleration (because it has to include Earth's orbital movement, which is elliptical, alongside it's own highly elliptical orbit, which is going to do weird things -- other planets appear to occasionally do loop-de-loops in the sky, for instance). You're essentially recreating the problem of predicting planetary motions in an Earth-centric universe, a field of rather complicated mathematical modelling. There's a reason everything got a lot easier (but not exactly easy) to predict when we moved to a heliocentric model. Don't ignore Kepler!

 

[tglassy]

So, didn't bother reading all 27 posts, so forgive me if this has been said.

How big is your asteroid? Cause if you're looking at something, say, the size of Texas (like in Armageddon), then what you have done is create a scenario that you are unable to win. You are constraining your hero to be unable to simply push it, and simply "flying real fast around the earth and hitting it perpendicularly so it goes off course" wouldn't work. She's too small. Even a few tons of mass would simply be too small. It wouldn't push something massive like that off course, it would puncture a hole through it and go through the other side.

If Superman really tried catching a falling plane by grabbing the nose, he wouldn't be able to. The amount of strength needed to catch the plane would be FOCUSED on his FINGERS, focusing all that power into minute points, which would simply rip through the plane's skin like paper, not affecting the rest of the plane at all. The plane isn't strong enough, structurally, to be able to withstand that level of force.

In the same way, depending on the make up of the asteroid, I don't believe it would simply impact on the surface, diverting the asteroid. It would be like a bullet. She'd rip right through the thing.

Now, if it was smaller, and going faster, I still don't see her diverting it, but potentially shattering it. If she hit the most dense portion of the asteroid, punching a hole through it, it could potentially disrupt the structure enough to make it break apart.

Also hitting the larger asteroid with something of equal size, or of enough size to do anything, wouldn't push it off course. The sheer mass and energy of both objects colliding would cause both objects to shatter, sending most of that debris to earth. Or, well, it might actually give Earth a ring. That would be cool.

If the big asteroid would NOT shatter at something near its own mass slamming into it at those speeds, then the thing is so damn solid that the earth probably wouldn't even stop it. It would punch through atmosphere, earth and mantle and rip straight through the core and out the other side without even slowing down.

It's possible I'm not as scientifically accurate, but perhaps the answer lies in what the asteroid is made of. If its mostly rock, granite and the like, have her analyze its structural weak point and shatter it with pinpoint precision. If it's mostly metal... I don't know what to tell you, there.

 

[tomBitonti]

This is... not even wrong. To attempt to explain, again, both the Earth and the asteroid are orbiting the Sun. The Earth is in constant motion along it's orbital path, as is the asteroid. The point of impact is where those two lines cross. In other words, the asteroid isn't moving towards the Earth, it's moving towards where the Earth is going to be.

Imagine trying to hit a moving car with a baseball from the field next to the road the car is travelling along. If you throw the baseball at the car, you will miss because by the time the baseball arrives at the road, the car will have already traveled past that point. Instead, you throw the baseball ahead of the car so that when the baseball gets to the road, it meets the car there.

In this scenario, the hero is like a wind pushing the baseball after you throw it. The car is moving so fast that it takes less wind to slow the thrown ball so that the car zooms past the rendezvous than it does to push the ball to the side so that it gets there at the same time but far enough off to one side. Largely this is because, for reference, the car is huge and moving really fast so that a small change in speed leverages the speed of the car to cause the miss rather than having to push the baseball off target by half the length of the car from zero lateral speed.

If you insist on treating Earth as the center point, then the asteroid is going to appear to move under constantly changing acceleration (because it has to include Earth's orbital movement, which is elliptical, alongside it's own highly elliptical orbit, which is going to do weird things -- other planets appear to occasionally do loop-de-loops in the sky, for instance). You're essentially recreating the problem of predicting planetary motions in an Earth-centric universe, a field of rather complicated mathematical modelling. There's a reason everything got a lot easier (but not exactly easy) to predict when we moved to a heliocentric model. Don't ignore Kepler!

Changing from the earth’s Frame of reference and a sun centered frame of reference, a vector perpendicular to the velocity of the moon in the earth frame is no longer perpendicular in a solar frame. The shift is from 30-60 degrees, with the earth moving perpendicular to the moon in the solar frame, and varying the relative speeds from 1/2 to 2.

Over the course of a day, the earth’s motion shifts by one degree. That doesn’t seem to change the situation much, in the sense of how you would want to adjust the asteroid’s movement.

In all of this, is the deflection caused by the earth’s gravity big enough to matter? The effect in the earth frame puts the asteroid on an arc instead of a line, with (I’m thinking) more curvature closer to the earth.

Thx!
TomB

 

[tomBitonti]

So, didn't bother reading all 27 posts, so forgive me if this has been said.

How big is your asteroid? Cause if you're looking at something, say, the size of Texas (like in Armageddon), then what you have done is create a scenario that you are unable to win. You are constraining your hero to be unable to simply push it, and simply "flying real fast around the earth and hitting it perpendicularly so it goes off course" wouldn't work. She's too small. Even a few tons of mass would simply be too small. It wouldn't push something massive like that off course, it would puncture a hole through it and go through the other side.

If Superman really tried catching a falling plane by grabbing the nose, he wouldn't be able to. The amount of strength needed to catch the plane would be FOCUSED on his FINGERS, focusing all that power into minute points, which would simply rip through the plane's skin like paper, not affecting the rest of the plane at all. The plane isn't strong enough, structurally, to be able to withstand that level of force.

In the same way, depending on the make up of the asteroid, I don't believe it would simply impact on the surface, diverting the asteroid. It would be like a bullet. She'd rip right through the thing.

Now, if it was smaller, and going faster, I still don't see her diverting it, but potentially shattering it. If she hit the most dense portion of the asteroid, punching a hole through it, it could potentially disrupt the structure enough to make it break apart.

Also hitting the larger asteroid with something of equal size, or of enough size to do anything, wouldn't push it off course. The sheer mass and energy of both objects colliding would cause both objects to shatter, sending most of that debris to earth. Or, well, it might actually give Earth a ring. That would be cool.

If the big asteroid would NOT shatter at something near its own mass slamming into it at those speeds, then the thing is so damn solid that the earth probably wouldn't even stop it. It would punch through atmosphere, earth and mantle and rip straight through the core and out the other side without even slowing down.

It's possible I'm not as scientifically accurate, but perhaps the answer lies in what the asteroid is made of. If its mostly rock, granite and the like, have her analyze its structural weak point and shatter it with pinpoint precision. If it's mostly metal... I don't know what to tell you, there.

Yeah, Superman would simple travel straight on through. But, a high speed impact of normal matter would turn into an explosion.

Thx!
TomB