Redirecting asteroids on an impact course by momentum transfer

Factual Questions
1

Stranger on a Train, who has claimed many times to be a rocket scientist and made many educated seeming posts, made the following statement :

*Redirecting a much larger object, however, becomes a substantial challenge not only because of the additional mass but because the impulse will impinge upon only by a fraction of aspect of the body and would likely be absorbed as deformation within the body (inelastic transfer or liquification) rather than uniformly delivered to the entire aspect. Moving really large masses requires some fundamentally different method of propulsion than just pushing because at those scales a “solid” body doesn’t act very solid. *

As I understand it, momentum is conserved in this universe. Thus, I perform the following mental experiments :

There is a springy slushball of rocks, held together by gravitational attraction or chemical bonding.

You apply an impulse to that slushball. The mechanism is you set a nuke off near it on a specific side. The sudden flash of heat vaporizes some of the slush or rocks, and the vaporized material jets away into space. The momentum of that escaping material equals the momentum change to the slushball. Or, alternately, or both, several kilograms of material from the nuclear warhead impinge on the rocks and embed themselves in it. (a nuclear shaped charge)

mv (total) = mv (material from the nuke) + mv (asteroid) + mv (escaping material from the asteroid)

Either way, the elasticity is completely irrelevant. The total momentum of the system is affected by these impacts, and each nudge to the system pushes it. If you then consistency set the nukes off on the same side (relative to earth, the object may be spinning) each impulse changes the velocity vector and if you have enough time or enough nukes, this small change will be enough that the asteroid misses the earth.

I respect Stranger on the Train for his insights, but I would like some clarification on this subject. I don’t see how a paper on this would have survived peer review.

2

On several occasions, Stranger has made it clear he is an engineer, not a scientist. He does(did?) (still?) work in the aerospace field.

3

Point remains. Even an undergrad engineer should understand conservation of momentum, even if they aren’t in aerospace.

4

You really need to read up on the kind of mass and speed you’re dealing with before making such foolish assumption as thinking a few kilos of nuclear material will make a difference.

How the hell do you make a atomic bomb go off in one direction?

5

I once saw a bird fly into a truck. The bird bounced into the air. The truck never slowed.

6

I recall some rabbit trying to punch a tar baby.

7

Another good earth-bound example is a bullet hitting a Macy’s parade balloon (or a bean-bag chair) - the bullet has a lot of momentum but there’s not going to be an effective transmission of that momentum to the balloon (or beanbag chair).

8

Everything you said assumes a rigid body. Momentum is not conserved in elastic collisions. If you don’t believe me, drop an overripe banana on a tile floor and watch how far it bounces. Then drop a superball nearby and see how far it bounces.

For a rubble/ice pile you might well get some jetting. Which will serve to push on the outer layer where the jet escapes. Which impulse will be converted to heat as the snow & sand underneath compacts and shifts and fractures a bit. And that’s pretty much all that’ll happen.

If the bolide is a solid lump of iron nickel you’ll have effects much closer to what you’re expecting. I honestly don’t know how big those get. Is a 10km iron-nickel possible? Or is that really a gravitationally bound cluster of 10m iron-nickel chunks some of whom have cold-welded together weakly?

9

Momentum is always conserved. It’s energy that isn’t conserved in an inelastic collision. That is, when two rubber balls collide and bounce away, total kinetic energy of the balls after the collision will be the same as before. When two bean bags collide, energy is lost (the bags will be travelling much slower than before), but the momentum is conserved (i.e. center of mass of the two bags will be moving at the same speed/direction).

10

:smack: I *hate *it when I do that.

At which point a tiny mass of gas is moving one way at high speed and a large volume of asteroid is not moving much the other way.

And if the object does come apart now you’ve got a much more severe targeting problem.

11

I think you’ve got that incorrectly. Elastic collisions (such as the superball or billiard balls) conserve kinetic energy and momentum. Inelastic collisions (over-ripe banana) conserve momentum (because every interaction conserves momentum), but not kinetic energy. It’s just that the change in momentum of the Earth is so small that it’s not worth noticing, but technically, there is some momentum transfer when the banana hits it.

12

Iron asteroids come from the cores of differentiated planetestimals that have had their crusts and mantles blasted away. We have meteorite samples from a few dozen different iron asteroid parent bodies. The biggest remaining iron asteroid in our solar system is around 200 km across.

But the irons aren’t the most common type–if you play the numbers, you are more likely to run into a stony.

13

Fat kids on sleds, anyone?

FTR, I’ve never had occasion to introduce Stranger to someone, but when I do “[he] has claimed many times to be a rocket scientist and made many educated seeming posts” will not be what I say.

14

Take an extreme example. A contact binary asteroid.

You whack the smaller body on the side perpindicular to the line of contact with a super nuke. What happens?

Well, the smaller body gets a serious pounding. If it’s enough of a hit to alter its path then it may go off separately from the larger body.

The larger body keeps going. (The mutual rotation and the timing of the split will affect it’s path a bit. Let’s do a round cow and assume there’s no mutual rotation.)

So the bulk of the killer 'stroid is still coming right at us.

Remember, the goal isn’t just to make the asteroid nice and toasty with a nuke. The goal is to turn that energy into making stuff move in the right direction.

The issue people are raising about nuking a single body is how much of it is more like the contact binary setup. Does a lot of energy just result in some pieces splitting off like the contact binary with the main bulk unaffected.

It’s easier to knock stuff off a loose, soft piece of space junk than a really solid piece. We can probably assume that most worrisome asteroids are fairly solid in the center but quite loose on the outer layers. How do you knock the center hard without wasting energy on making the loose stuff warm?

Maybe if we got a team of drillers …

15

Sure it did–a little bit. If the truck was traveling through a vacuum at 60 mph, and there was going to be another truck 3000 miles away precisely 50 hours from now, that bird would have prevented a collision.

One way of redirecting an asteroid–even a “rubble pile” type–is to position a craft above it, and use gravity (between craft and asteroid) to apply a force in that direction. Gravity works on the whole mass at once, so aside from tidal forces the whole thing accelerates equally. It’s a slow process but can be sustained over a long period, and if started early enough may be sufficient.

16

Oops. I see I got ninja’d by scr4.

Yeah, that’s the method I support (if anyone ever asked me!).

17

In this analogy, there’s no air friction or rope or interaction with third parties. Just the bean bag chair/balloon in space. I’m assuming the bullet does embed itself into the target, and you obviously can design your nuclear shape charge to ensure this will happen. (for thinner asteroids you can make it more of a thin sheet)

so mv (total) = mv (original asteroid mass) + m*v (nuclear propelled material)

You can get absurd velocities with that chunk with a nuke pushing it, that’s what makes this work. Hundreds of kilometers per second. So it counts for a lot, even though it’s not very much mass.

And you keep doing this. Hundreds or thousands of times. Not just once.

18

Seems like you’re much better off with Dr. Strangelove’s idea, which doesn’t risk shattering the object into millions of harder to control pieces, and doesn’t waste energy on vaporizing any part of an object you just want to move.

19

One tricky issue with the gravity tractor approach, besides requiring a very massive spacecraft, is once you’ve positioned your spacecraft near the object, you need to accelerate the ship to drag the asteroid to the desired trajectory. A naive approach of using thrusters to accelerate the ship would have the thrusters pointed directly at the asteroid, and the exhaust hitting the asteroid would be pushing it in exactly the wrong direction. The orbiting tractor schemed described in the Wikipedia article linked to above would solve this, but would require more fuel.

20

You could also just angle a pair of engines to the sides, assuming there’s not too much exhaust divergence.

The Wikipedia example is pretty interesting in that they get a craft mass of 19 t but only need 1 t of propellant and perhaps 1 kW of power. So they could collect most of the mass from the asteroid itself–just drill into the thing and scoop the tailings into a bag.