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Comment by noman-land | Hacker News Reader

How do you prevent the asteroid from flying erratically like a balloon with the air rushing out?

A balloon does that because it has very low mass and moment of inertia, compared to the thrust provided by the escaping air.

An asteroid is not like that, even when heating one side with a nuclear bomb. The goal is to create a tiny change in its velocity vector, which still adds up to a significant trajectory change over time.

If on the other hand it IS balloon-like and hollow then let’s just let it crash into earth and pop.

Indeed, perhaps there will be some way to detect latex based asteroids in the future and avoid any unnecessary nuclear deployment.

We just need to build a spaced based array of microphones (comsaphones???) to listen for the high pitched squeal or the pfffft sounds as the air is rushing out the opening.

In a vacuum?

Really? C'mon. Walk this thread back up, and see if you can find where it went off the reality rails. Then, see if your comment fits with the seriousness of the thread.

I prefer humour to be based on plausible physics. ;0P

thank goodness you're not the one in charge of comedy for the rest of us

Ya if you go up a few I see it really goes off the rails here: https://news.ycombinator.com/item?id=38722192

so a latex asteroid is an acceptable concept in your world? or was the humor in it too witty for you to grasp?

Infinitely more probable than sound propagating through vacuum.

You don't - but it doesn't apply. The size of the "engine" compared to the mass of the asteroid is enormous. A balloon has a relatively large engine compared to it's mass, and the engine is flexible - i.e. if you stiffened where the air comes out, the balloon would fly much more predictably.

hypothetically, even if this is how it worked, that would still be fine, because literally any direction other than the one directly targeting the earth is going to be a non issue. the probability of randomly nudging and hitting anything of consequence is basically zero.

don't forget our moon, too

probability of hitting the moon with a random velocity in space is comfortably roundable to 0% provided you randomize decently far awaay

so as the occurrence of life *-*

Do you own real-estate on the moon?

Read Seven Eves, we dont want a crumbling moon raining down on the earth.

Do we actively track that now?

I don't think a balloon would fly erratically if the air was released in space or a vacuum. As far as I can understand it, the air would rush to equilibrium in all directions - the balloon would deflate rapidly - but the air 'movement' would not move the balloon, as in a vacuum there is nothing to push against. Any erratic movement would be due to the elastic of the balloon returning to its resting state. I think you would also get movement in earth based vacuum chambers due to gravity.

The air doesn’t need anything “else” to push against. It pushes against itself and the balloon. Conservation of momentum will cause propulsion.

Thank you.

I'm looking up 'Conservation of momentum': https://www.physicsclassroom.com/class/momentum/Lesson-2/Mom...

> For a collision occurring between object 1 and object 2 in an isolated system, the total momentum of the two objects before the collision is equal to the total momentum of the two objects after the collision. That is, the momentum lost by object 1 is equal to the momentum gained by object 2.

but this seems to support what I say - there is no collision occurring in a vacuum. What is the expelled air colliding with?

Same way a rocket does. That is, don't treat it as if there's a need for a second object. The release of air is applying a force in whatever the balloon still has in the direction of travel, even though it's going on the other direction.

Hope that makes sense

The air molecule collided with the inside of the balloon, and escaped out the opening.

Your explanation is much clearer than what I said, so yeah, this

You can also see this by using newton's third law (equal and opposite reactions). The air is accelerating and the only other thing for it to act on is the balloon in equal and opposite fashion.

And just as an aside, IIRC conservation of momentum only applies to your standard physics objects, real objects make noises and sometimes permanently deform. This isn't usually a significant source of error but its neat to think about how if you were playing pool on a frictionless physics surface the difference between the models predictions of where the balls go and where they actually go (very very small difference) could be directly and perhaps entirely attributed to the noise they made on collision.

You might be confusing conservation of momentum and energy with respect in the context of elastic and inelastic collisions.

In a perfectly elastic collision, both momentum and kinetic energy are conserved.

As you say, many real collisions are inelastic to a significant degree. In these cases, momentum is still conserved, but the kinetic energy is converted to another form of energy.

Conservation of linear and angular momentum are fundamental and exact laws of nature resulting from translational and rotational invariance.

No, I don't think so. If you try to apply conservation of momentum as absolutely as conservation of energy (and that linked physics page states it in simple absolute terms) you can quickly encounter gaps because real objects tend to have less than their theoretically conserved momentum due to permanent deformations and interactions with the environment, or the situation involves friction in any way.

Conservation of energy always holds, whereas conservation of momentum has several important caveats and exceptions. Seemed worth pointing that out with an anecdote that amused me.

Hmmmm, actually can you explain a little more about 'momentum is still conserved'? In the cases that I'm pointing out momentum as a measurable property seems clearly not conserved, whereas the energy is (just transformed).

Imagine your sitting on a office chair with wheels. If you're holding a large weight and suddenly throw it forward, the chair will roll backwards. This would happen in a vacuum too.

This is a nice way to express it, imo. I also think it's possible to interpret what happens differently. A balloon or something in space doesn't have anything to move against, whereas the chair's wheels are on the floor. When you move the weight in an environment without anything to respond to, ie in suspension in a vacuum, there is no opposite to respond or work against. Both seem possible to me - that a chair+person-with-weight is capable of generating it's own momentum and that nothing happens in space, in a vacuum.

My guess is the jet doesn’t last that long

well, because an average asteroid in the solar system is nothing at all like a balloon. Or maybe you're confusing comets and asteroids, though it doesn't matter anyhow because your notion doesn't apply in either case and I can't at all see why it should.

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