# Science: asteroid vs. hero physics

#### Janx

##### Hero
I'm working on a short story due in a few weeks (for an anthology, not school). I need all my physicsful friends ideas to adjust the plan.

The big climax is the hero has just bonded with her new super sci-fi space suit that will let her fly up and help protect the earth from asteroids and stuff. She can fly, propel matter, and using pico-printer-gel*, disassemble and reassemble matter.

In the current draft, she has about 15 minutes to fly up and prevent an asteroid that's going to splatter the earth. This is the "last one", since the previous about a 100 years ago (which aligned with Asteroid Bennnu's description and timing). Mankind can't survive another hit.

There's limits to how much mass she can move, we don't have numbers, but the asteroid is too big. She's a sniper (back on earth) and tends to deal with problems distantly. I decided she'd have to get close for this one (and thus risk her life).

Since she can't just superman shove the asteroid off course, I thought she'd try aikido. Let's say she just got up to space and there's 5 minutes left. The asteroid is on her left (relatively) and coming in to kill us all. She flies right, around the planet, picking up speed (a lot of speed), gathering up matter to build a lattice that she can ram/cushion with. She rejoins the asteroid from behind and pushes with all her super-pushy-power. Thus adding speed along a vector the asteroid is mostly going, and thus, causing it to skirt past the earth instead of slaughtering the last of humanity.

I don't need exact math, but I think there's a hole in the idea. To get around a planet of 24K miles circumference in under 5 minutes is a whole lot of speed and acceleration. Assuming she can now handle higher g-forces, wouldn't her new speed be very much faster than the asteroid, causing her to splat, instead of line up to shove?

Is there a better solution for dealing with an asteroid in 15 minutes (5 assuming it takes 10 to get up there)?

I don't think Nukes has the right vibe and lots of articles disproved that as a solution.

Is the original plan workable if I adjust the effective starting point for the asteroid so it will be far enough away and have just the right speed for her to join it by orbiting the planet once?

I can play with the parameters a bit since it's just fiction, but I need a short timeline and personal risk. Her flinging an object at it might work, but how would she herself be at risk to add tension?

This is the big finish to my post-Ragnarok coming of age biker cultist story about a fallen angel.

Thanks!

#### Umbran

Staff member
Hm. This is a good one.

The kinetic energy of a non-relativistic object is 1/2 M V^2. So, there are two basic ways for it to have a lot of KE. One is for it to be big, and the other is for it to be fast. Note that the energy goes linearly with the mass, but goes like the square of the velocity. Double the mass, you double the KE. Double the velocity, and you *quadruple* the energy.

And, part of making this work, or at least be cinematically plausible - make the asteroid less small, but faster. That, overall, makes it easier to push around.

Now, not, pushing "along a vector it is mostly going" doesn't help. Any portion of her effort that moves along the line of the asteroid's current motion makes it reach Earth faster, and hit harder. She needs to add velocity more or less perpendicular to its current motion.

We can set up things to minimize how much she needs to do. If the thing is aimed dead center, she needs to move it 4000 miles in order to be a clear miss. If we set it up as an off-center, almost glancing blow, then she only needs to move the thing a few hundred miles. Mind you, she needs to move the thing a few hundred miles in 5 minutes. If it is, say, 400 miles in 5 minutes is like 4800 miles per hour. She has to add 5000 mph *to the mass of an asteroid* and that's if she takes the asteroid from its current path to +4800 MPH instantly. If she takes time to accelerate it, the first moments are slow, and the end speed must be even higher to succeed.

Note - this is basically saying that she, alone, can create an impact roughly equivalent to the asteroid's impact herself. She is capable of ending civilization. Is that the power level of your hero?

#### Nagol

##### Unimportant
The problem with dealing with asteroids with a (very) short timeframe is the any asteroid that can harm the denizens of Earth has to have a lot of mass and since you have so little time to react, tremendous forces need to be brought to bear.

If you want to keep the time pressure, perhaps you could move the asteroid further out and unless its vector is changed in the next 5 minutes, the impact will be unavoidable? Assuming the answer is no, let's do some back of the envelope math!

I'll assume the rock is about 100m in radius and has a density of about 3, that means it masses 12 million tonnes.

Because the timeframe is short, you have to adjust the vector by a substantial amount. That mass is moving at quite a clip (~11 km/s). If you have 300 seconds, you're intercepting the asteroid about 3500 km from Earth. Earth has a radius just over 6,000 km. Worst case is a centre strike which would mean you'd need to change its vector by 45 degrees. But let's assume the impact is projected to be a grazing hit about 1000 km from the edge which means you'd need to shift its angle by about 20 degrees.

Now that shift needs to be instantaneous. Every second of delay/acceleration means the distance is closing and the angle needs to become more extreme. Vectors at these speeds are mostly additive so you need to impart a velocity of about 3 km/s which would have the rock just skim the atmosphere -- assuming we ignore gravitational attraction. Let's bump that up to 3.5 km/s for a basic safety margin. That change is what you'd need if you could impart it instantly. Let's assume you spend a whole minute of acceleration and thus need to achieve a higher velocity of about 6 km/s to simplify the math. A = v / t so 6km/60 seconds = 100 m/s-squared or about 10 g of acceleration. F = ma, so 12 billion kg x 100 m / s-squared is 1.2 trillion Newtons of force which is the same as 1.2 terawatts needs to be applied for the whole 60 seconds.

Regardless of how our hero applies the force -- whether directly to the asteroid threat or through a passive intermediary, the amount of energy remains the same. Since you want the hero to use an intermediary, there needs to be something that acts as a force multiplier. You don't have sufficient time to use the Earth's gravity well so let's look elsewhere.

One simple force multiplier would be a secondary large rock -- say a half the radius of the threat that is moving in a different orbit that is closer to the preferential vector that can be adjusted to impact the threat. With a mass of about 10% of the target rock, the energy required to change its vector is equally reduced. If the asteroids are mostly metallic then you could even potentially set up an elastic collision. In an perfect scenario, the larger mass would have its velocity changed by ~5% of the closing velocity between the rocks.

Last edited:

#### Janx

##### Hero
thanks for getting back to me so quick.

To umbran's question about her power level: not sure. The space suit was part of a fleet of these for defending the earth from ballistic attack (mass drivers pushing big rocks from Mars). So using math for evil would probably mean yes, she could. I want her constrained that she can't just fly up and superman nudge the threat away.

I was trying to come up with a different approach to solving the problem (instead of brute force), and aikido inspired me to have her use the rock's momentum/direction to her advantage. Sounds like for that to work, she needs to push perpendicular, not "with" the general direction of the rock.

I can change how long it takes to get from ground to space, and then from space to the rock. Mostly, I wanted a short enough time to be urgent, she she had to leave right away, and be able to exposition what's going on to the folks back home over radio for the first part of it, because people have a totally wrong understanding of what happened.

the rock should be aimed off-center, it's a misfire anyway, and 100 years later, we've come back around to meet it. I can change the 100 years to whatever sounds plausible. The first strike was timed to align with Asteroid Bennu (a real supposed danger). Since this was pushed toward earth, it can be smaller, with a higher velocity.

Other than space debris (dead satelites, etc), I'm not sure if there's another rock floating around for her to grab. Ironically enough, a named asteroid like Bennu coming by at just this same time would make for opportunity to be used as ammo.

Or she could just grab enough matter to make a space bullet and fly with it so she can maintain acceleration on it, thus killing or deflecting the asteroid with a bullet/herself. It's heroic, and it's the kind of idea a sniper would come up with.

So, it sounds like she needs a bullet that is about half the mass of the threat. Umbran suggests a smaller, faster rock. Not sure how small would work, to justify how much mass she could feasibly gather (think all that wrecked stuff from Gravity). Maybe there's 1-2 tons worth of movie-metal to grab in a hurry? Space is big and that's kinda bullcrap, but let's pretend that's possible for her to scoop up while zooming along the right path. I'm guessing that should be the limit of her power to move/manipulate anyway.

Is it feasible to launch a 4 ton rock from the asteroid belt toward earth, miss (let's say somebody had second thoughts after the first launch and interfered) and have it eventually come around a century later and be a threat that our last Valkyrie can solve? Would that 4 ton rock be a threat?

#### Nagol

##### Unimportant
thanks for getting back to me so quick.

To umbran's question about her power level: not sure. The space suit was part of a fleet of these for defending the earth from ballistic attack (mass drivers pushing big rocks from Mars). So using math for evil would probably mean yes, she could. I want her constrained that she can't just fly up and superman nudge the threat away.

I was trying to come up with a different approach to solving the problem (instead of brute force), and aikido inspired me to have her use the rock's momentum/direction to her advantage. Sounds like for that to work, she needs to push perpendicular, not "with" the general direction of the rock.

I can change how long it takes to get from ground to space, and then from space to the rock. Mostly, I wanted a short enough time to be urgent, she she had to leave right away, and be able to exposition what's going on to the folks back home over radio for the first part of it, because people have a totally wrong understanding of what happened.

the rock should be aimed off-center, it's a misfire anyway, and 100 years later, we've come back around to meet it. I can change the 100 years to whatever sounds plausible. The first strike was timed to align with Asteroid Bennu (a real supposed danger). Since this was pushed toward earth, it can be smaller, with a higher velocity.

Other than space debris (dead satelites, etc), I'm not sure if there's another rock floating around for her to grab. Ironically enough, a named asteroid like Bennu coming by at just this same time would make for opportunity to be used as ammo.

Or she could just grab enough matter to make a space bullet and fly with it so she can maintain acceleration on it, thus killing or deflecting the asteroid with a bullet/herself. It's heroic, and it's the kind of idea a sniper would come up with.

So, it sounds like she needs a bullet that is about half the mass of the threat. Umbran suggests a smaller, faster rock. Not sure how small would work, to justify how much mass she could feasibly gather (think all that wrecked stuff from Gravity). Maybe there's 1-2 tons worth of movie-metal to grab in a hurry? Space is big and that's kinda bullcrap, but let's pretend that's possible for her to scoop up while zooming along the right path. I'm guessing that should be the limit of her power to move/manipulate anyway.

Is it feasible to launch a 4 ton rock from the asteroid belt toward earth, miss (let's say somebody had second thoughts after the first launch and interfered) and have it eventually come around a century later and be a threat that our last Valkyrie can solve? Would that 4 ton rock be a threat?

A 4 ton rock would need so much velocity to be a threat that if it missed on the first pass it would exit the solar system.

#### Nagol

##### Unimportant
The only way an intermediary helps is if it already has energy that can be used against the target. If you have to pick up the target and accelerate it, it is only helpful if you have extra time or if energy output of the suit is not the limiting factor for the acceleration. In other words, if the suit has the force to accelerate a 100 kg human at 50 g, but humans go unconscious at 3 g then the hero could afford to pick up and accelerate about another 1500 kg and stay conscious.

Let's change the asteroid's composition to have density of 8 (solid iron) and set the orbit to a ~100 year elliptical. The math for calculating the instantaneous velocity isn't hard, particularly, but I'm rusty enough not to do it on the back of an envelope. Let's guess the velocity at 1 AU would be about 3 times the base velocity of a circular orbit with the same period or 20 km/s and further assume its going counter to the planetary motion so we can increase the closing velocity to 50 km/s.

To achieve the same kinetic energy of my original assumption 12 million tonnes at 11 km /s, we have ~600,000 tonnes at 50 km/s and it has about a 25 m radius.

Let's push the rock further out to say the hero intercepts it 1 hour from Earth (about 180,000 km out). The necessary deflection is much smaller. We still need to shift about 1,000 km, but we have 3,600 seconds to complete the shift or merely about 350 m /s.

If she picks up 1,500 kg and accelerates at 2 g for the whole trip, it takes a bit over an hour to arrive (4200 seconds) and will be travelling a bit over 40 km/s compared to the Earth. That gives a relative closing velocity on the target of about 80 km/s. The 1,500 kg mass in an elastic collision would cause the target to gain about 400 m/s vector (and thus miss Earth) and the 1,500 kg slug would bounce back at nearly 80 km/s. Since it unlikely the slug could survive those g-forces, the hero would have to evade the expanding shrapnel cloud.

Actually, this assumes a side strike appropriately perpendicular to the target's forward motion. So toss in another hour or so of travel time to line the shot up.

Last edited:

#### tomBitonti

To be accurate, be careful to include both energy and momentum in the physics of the collision. If I have the right sense of it, a very fast small object hitting a larger relatively still object either, with an elastic collision, bounces off at high speed (transferring momentum and a little energy) or with an inelastic collision does a lot of local damage and transfers almost the same momentum. (This is a classic physics test problem, with the key observation being whether the collision is elastic or not, which tells you what analysis to do.)

As what Umbran said: The push should be perpendicular to the line of motion of the asteroid. A perpendicular push will create the greatest deviation. There is a case of a skimming trajectory, where the straight line motion of the asteroid would miss, but gravity curves the asteroid path into the earth, where it seems that speeding up the asteroid should work.

Some other considerations: How far away the asteroid starts from the earth, and at what speed, compared with escape velocity (11.2 km/s) and orbital velocity for a circular orbit (about 7.6 km/s): If the asteroid is left with less than escape velocity, the asteroid may go into orbit, or may impact the earth at a later time because of the particulars of the changed orbit the asteroid.

Thx!
TomB

#### Janx

##### Hero
the suit is made of pico-printer-gel (tinier than nanites) and it can modify/repair her such that she can probably handle twice the G. I made that up of course, but thus far, it's in specification for what I'd already figured out. Calling it a suit is really a simplification for our discussion.

So that might mean that she can go twice as fast, turning that hour long flight into aprox. 30 minutes (or as you added an hour, an hour instead of two).

I can still make it tense and "guys I need to go now" because she really does need to get there while it's still farther away. Add in radio fade out after the Q&A with the folks back home , a bit of time passes and as she lines up for the shot, bring the radio back in for final words.

I can even have the science guy explain it to the rest of the party, "of course, she needs to achieve a speed of 80km/s and impact at a perpendicular angle to blah-blah" or some such.

I don't want to cite too many numbers, but I wanted to have the strategy be interesting, heroic, and plausible if one of y'all crunched numbers.

I think I can make this work and be closer to plausible, since it is a sci-fi story.

Glad I asked y'all. Let me know if there's any holes with my recap of what I think you said, or a different way to approach the problem (besides let everybody die, which I already dealt with

#### Janx

##### Hero
To be accurate, be careful to include both energy and momentum in the physics of the collision. If I have the right sense of it, a very fast small object hitting a larger relatively still object either, with an elastic collision, bounces off at high speed (transferring momentum and a little energy) or with an inelastic collision does a lot of local damage and transfers almost the same momentum. (This is a classic physics test problem, with the key observation being whether the collision is elastic or not, which tells you what analysis to do.)

As what Umbran said: The push should be perpendicular to the line of motion of the asteroid. A perpendicular push will create the greatest deviation. There is a case of a skimming trajectory, where the straight line motion of the asteroid would miss, but gravity curves the asteroid path into the earth, where it seems that speeding up the asteroid should work.

Some other considerations: How far away the asteroid starts from the earth, and at what speed, compared with escape velocity (11.2 km/s) and orbital velocity for a circular orbit (about 7.6 km/s): If the asteroid is left with less than escape velocity, the asteroid may go into orbit, or may impact the earth at a later time because of the particulars of the changed orbit the asteroid.

Thx!
TomB

so are you saying, speeding up the asteroid might actually work?

And you raise the other elephant in the room, if the asteroid doesn't hit the earth (she wins), it's still out there, ready to be a problem again...

Any other solutions to that, besides let it hit the earth? Or is there a way to hit the earth safely? Or build a parachute for it? or wings to land it like a shuttle?

I don't think she can drill it fast enough or disolve it with her pico-printer-gel (think my version of nano-tech, but smaller)

#### Eltab

##### Is this a moon, or is it a space station?
Flying up to the Really Big Rock and doing a Superman trick sounds like a bit of a cliché. (Let her best friend tell her so.) Let's make her smart and clever as well as tough.

She wants to plant a motor on the asteroid and let THAT push it off collision course. This could be a solar sail (very fragile thing provides very low-G push over a very long time) or a Saturn V moon rocket (tough robust structure provides high-G push over a short time). Either way, she does the math and realizes she must go NOW. Her friends, the parts suppliers, &c all think there's plenty of time no hurry. If she proposes the solar sail and shows the physics but everybody blows her off because they watched Armageddon as kids and now they think a moon rocket full of nukes will do the job, we set up that everybody acknowledges the seriousness of the situation but not the urgency. That allows some people to change their minds and help her out once they think it through.

"Victory" for Our Heroine may be that she breaks up the Really Big Rock into several smaller parts that mere mortals can handle, plus a lot of gravel and fragments that make a very spectacular meteor shower as they burn up on re-entry. This may look too much like that old movie, though...

As the writer you can align everything as you want. The asteroid is arriving from a direction such that, if you push it to one side enough, the Moon's gravity does some more of the work for her. (Hmmm, her computer simulations account for the Moon's gravity but the official simulation doesn't; she finds out the Asteroid is going to hit THIS time, not NEXT time, it crosses Earth's orbit. Must hurry!)

Or … the incoming Really Big Rock is actually a Really Big Iceball (comet). If you can make only one side melt, the natural jets (which normally create the tail and halo) will push it off its original course. The solar sail can perform double duty IF it is in place early enough and can be installed in the right position.

Our Heroine may have to hack NASA's supercomputers along the way, to figure out how to place the sail so all the physics tricks can work together.

#### Tonguez

##### A suffusion of yellow
So could the pico-gel be used to collect up all the space debris in orbit and use that as her bullet?

Of course there’s the power of narrativium which would allow cool effects like having the asteroid graze the atmosphere until her suit channels te increase energy to expel the asteroid in fiery aurora explosion the lights up the sky around the world (ie every thing works through the power of cool)

Last edited:

#### tomBitonti

so are you saying, speeding up the asteroid might actually work?

And you raise the other elephant in the room, if the asteroid doesn't hit the earth (she wins), it's still out there, ready to be a problem again...

Any other solutions to that, besides let it hit the earth? Or is there a way to hit the earth safely? Or build a parachute for it? or wings to land it like a shuttle?

I don't think she can drill it fast enough or disolve it with her pico-printer-gel (think my version of nano-tech, but smaller)

Yes to speeding up the asteroid ... in very specific initial conditions. (Think of a putt that grazes the cup going too fast.)

Yup, it's not just enough to perturb the initial trajectory. Unless the asteroid is booted out of orbit entirely, it either needs to get into a stable orbit, or it will hit earth anyways, say, after looping away for a short bit.

A solution might be to dissolve the asteroid into a dispersed mist (or maybe a very fine gravel?). I'm not sure what will happen in those cases. Say, have the pico-gel go van-neumann (start self replicating, by doing an ultra dangerous removal of safeguards against such,"But the manual said never every enable full self replication!") and have it burrow through the asteroid and break it into millions of fragments.

Thx!
TomB

#### Nagol

##### Unimportant
Yes to speeding up the asteroid ... in very specific initial conditions. (Think of a putt that grazes the cup going too fast.)

Yup, it's not just enough to perturb the initial trajectory. Unless the asteroid is booted out of orbit entirely, it either needs to get into a stable orbit, or it will hit earth anyways, say, after looping away for a short bit.

A solution might be to dissolve the asteroid into a dispersed mist (or maybe a very fine gravel?). I'm not sure what will happen in those cases. Say, have the pico-gel go van-neumann (start self replicating, by doing an ultra dangerous removal of safeguards against such,"But the manual said never every enable full self replication!") and have it burrow through the asteroid and break it into millions of fragments.

Thx!
TomB

Considering the thing is in a century orbit, the perturbation is likely to be insufficient to yank it into a secondary pass. It's more likely to fly away and become a threat again in a few thousand years (maybe). If the hero is a particularly lucky math genius then the perturbation could either (a) reduce eccentricity and keep the asteroid outside Earth orbit or (b) have it hit the moon as it moves away from the Earth.

Converting the asteroid to dust would have potential if there was time for the dust to start to disperse. Dust or a single solid lump, it contains the same kinetic energy -- dust would just tend to release it faster as it hit the atmosphere. More *boom* less splat.

#### Ovinomancer

##### No flips for you!
Hmm. Earth is moving pretty quickly in space, so, depending on approach vectors, speeding the asteroid up or slowing it down a bit should generate enough 'miss' if you're far enough out.

For example, if the asteroid is crossing the orbit of Earth perpendicularly, the window to strike Earth is only 13,000 km / 30 km/s or about 435 seconds (about 7 and a quarter minutes). Assuming the asteroid aimed precisely at the center of Earth, you'd need to speed up or slow down the approach by half that 7 minute time, or about 4 minutes to be safe (240 seconds). A 4 minute delta in arrival time depends on the velocity of the asteroid and how long you take to start the acceleration. Assuming a 50 km/s speed on the asteroid, 4 minutes at 1 hour distance is a ratio: needed speed over current speed = needed time over original time. Or needed speed = original speed x (new time/old time). In this case, needed speed = 50km/s x (56 minutes/60 minutes) = 50 km/s x (.933) = 46.67 km/s. You'd need to generate a delta-v of 3.33 km/s at exactly 1 hour out (or earlier) to cause a complete miss. I think this is lower than the necessary delta-v to accelerate the mass sideways 6500 km over an hour (6500km/360 seconds = 18 km/s, so, yep, 6 times less).

Now, if your geometry has the asteroid coming in much closer to the orbital direction, this changes, as earth's relative movement is much less with respect to the path of the asteroid. However, a rock launched from the asteroid belt some time in the past should have a high angle of intercept to Earth's orbit, as it would be on a steep elliptical around the Sun. But, assuming a near head on collision, say at about a 20 degree approach, combined delta-v would be 30km/s (Earth) plus cosine(20)x50km/s or about 77 km/s. Earth relative speed on the perpendicular would be sin(20)x30km/s or about 10km/s. This puts a Earth on-target time window of 1300 seconds or 21.7 minutes. Again assuming a center strike, that's a time window of 11 minutes you have to modify. Needed speed up/slow down delta-v at 1 hour would be 14.7 km/s. This is still less than the needed perpendicular delta-v to cause a miss, although not much.

Orbital mechanics are funny. Often speeding up or slowing down along your path causes huge changes in orbit, while orthogonal thrusts change orbits more slowly. For the purposes of your story, gathering up a bunch of debris and smacking the asteroid just enough to slow it down would be sufficient for a high angle of intercept. This is the most likely scenario given the orbital geometries of a rock launched from the belt that's coming around again.

A neat way to tell this story might be that the angle of impact is such that meeting the asteroid and speeding it up is the right call (this would be the case if the asteroid was going to hit on the forward half of the Earth). Timing could be such that the push means the ateroid scrapes the atmo and skips, causing an awesome light show and placing the heroine at risk.

#### Janx

##### Hero
So could the pico-gel be used to collect up all the space debris in orbit and use that as her bullet?

Of course there’s the power of narrativium which would allow cool effects like having the asteroid graze the atmosphere until her suit channels te increase energy to expel the asteroid in fiery aurora explosion the lights up the sky around the world (ie every thing works through the power of cool)

adding more details (ain't like y'all are gonna spoil it for readers). The suit is a Vid Liv Kostym, a swedish acronym I made up with Google Translate to spell VLK which thus smells like Valkyrie. It's got mass driver bracelet/anklets, solar panel wings bonded to the back and a happy coat of pico-printer-gel for general protection and utility. Plus BalDR, the Ballistic Defense Ring, satellites hosting an AI that does all the heavy lifting, math-wise.

So the mass drivers can move matter around up to some range and weight limit. They'd be how she scoops up space debris to make a shield or something. The pico-printer-gel does the fine detail work of re-organizing that matter when it's brought into range to make electronics, repairs, heal injuries by re-organizing atoms (hence why I went tinier than nano-tech).

All of it limited by the power of narrativium as Tonquez surmises. Basically if it sounds good or plausible within what I described above, but not too powerful. Much like Umbran asking if she could kill the planet. I guess, but I already got an asteroid on the way to do that so it ain't OP right now...

Tom's idea to replicate the pico-gel is good. I had similar thoughts but ruled them out for purposes of just dissolving the asteroid to save the day was "too easy" Presumably, something about the total mass vs. time isn't possible.

Now I could have it that Mars has been sending these every year or something, rather than looping around a century later. Mars doesn't play directly into the story, it's just the big final reveal of how nobody understood what really happened to Earth.

So to recap the ideas:
perpendicular attack to deflect
speedup to skip might work
dissolve it - not allowed per original restriction of too easy unless we think of some complication
put a rocket on it and push (hauling a rocket up might be too heavy or slow)
build a mass-driver on it and move itself (variant of rocket idea)

Did I get everything? There's a lot of posts since I got back, sorry if I missed something. I'll be re-reading these after I finish my night-shift of work...

I'm hoping for one that is risky to her, and being non-obvious. Extra points for if its something a sniper would think of OR if it required her to get close and personal (the opposite of a sniper's mentality). Also, in the current draft, I only have 300 words left in my 6K budget.

BTW, thanks to all of you for participating. Good ideas from everybody. I had a hunch that whatever I thought of might have a hole that y'all would find, which would ruin the story for some. Plus, hopefully this is fun.

#### Umbran

Staff member
Orbital mechanics are funny.

Yes, they are. But in this situation, those are not so telling, unless you have much longer timescales to work on. The lightsail idea mentioned upthread can work on these premises - much lower thrust applied over days, months, or even years, to subtly change orbits.

Over the course of minutes as given in this scenario it is somewhat easier (and more understandable to an audience that isn't made of rocket scientists) to take this from the point of view of the Earth as the center of the coordinate system. In that, we simply have a target (the Earth) and a projectile (the asteroid).

#### Ovinomancer

##### No flips for you!
Yes, they are. But in this situation, those are not so telling, unless you have much longer timescales to work on. The lightsail idea mentioned upthread can work on these premises - much lower thrust applied over days, months, or even years, to subtly change orbits.

Over the course of minutes as given in this scenario it is somewhat easier (and more understandable to an audience that isn't made of rocket scientists) to take this from the point of view of the Earth as the center of the coordinate system. In that, we simply have a target (the Earth) and a projectile (the asteroid).
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.

There's a 6x difference in needed delta-v 1 hour out by treating the problem as an orbital one rather than a fixed Earth one (3.3 km/s vs 18 km/s). This was calculated as instantaneous delta-v at one hour out, so I have no idea where light sails come into that. Heck, at 5 mins out, ypu need to slow the object by 40km/s (80% of it's 50km/s speed!) to cause a miss, but you'd need 108km/s (216%!!) to generate the same miss by perpendicular thrust. Ignoring Earth's 30km/s speed and the fact this is a rendezvous problem not a fixed target problem is the oversight, here.

The acceleration needed will vary by object mass, but the delta-v doesn't care. For the 1 hour out case, and assuming a 1EE14 kg mass at 50km/s (roughly the same as the Chicxulbub impact), let's look at a 10 minute burn application centered at 1 hour out. The needed delta-v to slow or speed the stroid is 3.3km/s. Over 10 minutes, the a is 5.5m/(s x s). The F needed is 5.5EE14 N. The lateral shove needs an a of 30m/(ss), and an F of 3EE15 N. An order of magnitude more force!

So, no, no lightsails or slow accelerations here.

Last edited:

#### Janx

##### Hero
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.

There's a 6x difference in needed delta-v 1 hour out by treating the problem as an orbital one rather than a fixed Earth one (3.3 km/s vs 18 km/s). This was calculated as instantaneous delta-v at one hour out, so I have no idea where light sails come into that. Heck, at 5 mins out, ypu need to slow the object by 40km/s (80% of it's 50km/s speed!) to cause a miss, but you'd need 108km/s (216%!!) to generate the same miss by perpendicular thrust. Ignoring Earth's 30km/s speed and the fact this is a rendezvous problem not a fixed target problem is the oversight, here.

The acceleration needed will vary by object mass, but the delta-v doesn't care. For the 1 hour out case, and assuming a 1EE14 kg mass at 50km/s (roughly the same as the Chicxulbub impact), let's look at a 10 minute burn application centered at 1 hour out. The needed delta-v to slow or speed the stroid is 3.3km/s. Over 10 minutes, the a is 5.5m/(s x s). The F needed is 5.5EE14 N. The lateral shove needs an a of 30m/(ss), and an F of 3EE15 N. An order of magnitude more force!

So, no, no lightsails or slow accelerations here.

so to recap this (which tests my rudimentary understanding as I digest and keep it simple in the story):

the planet-killer asteroid has a mass of 1EE14 kg and is traveling at 50km/s (same size as Chicxulb)

by realizing that the rock is leading the moving target of Earth (a sniper thing to do), messing up that timing will save the day

If she can get behind it and speed the rock up, it takes less force than a pushing from the side, which is something she might be able to do

I can make it scrape the atmosphere for a lightshow and heat up, since she's personally pushing this thing, thus creating the danger through art of wordsmithing. To solve the repeat-performance problem, I can have her set the pico-printer-gel to chewing away at it, which in another hundred years, probably wouldn't be a problem. I'll come up with something to explain how she got so much force, but it's easy to justify in words that this is more feasible than pushing from the side once I throw in "order of magnitude"

I don't know how much techno-babble I'll put in, but y'all have given enough detail that if this version of the plan is plausible, I can take it from here.

Thanks!

#### Eltab

##### Is this a moon, or is it a space station?
Larry Niven's novel The Smoke Ring has a poem (6 lines or so) about what direction a thing in orbit will really move when you push on it, compared to the things around it. It might help you visualize the changing situation as Our Heroine is working on the problem.

Once in a while, borrowing everything I read from the library has a downside: I can't just copy the poem here.

#### Janx

##### Hero
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...