4 ms·

The point isn't the vaporize them, it's to nudge them ever so slightly. Consider a comet- you fire one or more lasers to a specific spot on the comet, causing

by mabbo 11y ago

The point isn't the vaporize them, it's to nudge them ever so slightly. Consider a comet- you fire one or more lasers to a specific spot on the comet, causing heat. The heat causes the water to expand, and shoot off from the comet. Does this destroy the comet? Certainly not much of it. But as that water vapor shoots away from it, Newtons Third Law comes into play and the comet, ever so slightly, is pushed in the opposite direction. You may rightfully say that this would be a very minuscule push, so what good will it do? Well, if you play Kerbal Space Program you'll quickly learn that tiny nudges can have huge effects in the long-run, especially on big orbits. Add a few mm/s to the velocity of a comet two years ahead of time, and it can make the difference between that comet hitting a planet, or not. It can mean the comet is ever so slightly closer to Jupiter, causing it to be pulled by its gravity just a bit more.

sandworm101 11y ago
But KSP does give a false impression. It is very much a "spheres in a vacuum" approach to orbital physics. It doesn't model many/most of the realworld forces. When dealing with the truly minuscule forces such as would be created by lasers aimed at asteroids, one much accommodate all other similarly-sized forces before taking action. Rocks in space are subject to a force/pressure from the sun, a pressure comparable to that from a earthbound laser. Before trying to deflect, one would have to understand how the sun will be altering the course. That calculation is not easy and requires details (albedo/rotation) that are hard to come by for tiny rocks. You don't want to be countering the force from the sun only to push the rock into an intercept it would otherwise have missed. Vaporization also need not mean destroying the rock. So long as the vaporized material achieves escape velocity (not hard on an asteroid) vaporizing the surface of an object creates a force, a thrust, which is greater than the pressure directly from the laser.
- marcosdumay 11y ago
  > Rocks in space are subject to a force/pressure from the sun, a pressure comparable to that from a earthbound laser. Hum, is it? When near the Sun, yes, of course, but when away from it, we can probably throw way more energy on it than the Sun. Yes, it does require details that we currently can't measure. The good new is that the laser doubles as an excellent instrument for measuring those details.
  - sandworm101 11y ago
    That very much depends on the laser. Most of the approaches I've seen discussed involve multiple 'attacks' on a rock spread out over a considerable period of time. You only fire when the rock is far enough above the horizon (say 25-30% of the time if you have 1 ground laser). And then you want to hit it when it is at some sort of apsis, preferably when it is far away with a relatively slow velocity. The sun has a 24/7 line of sight and is always firing.
    - chr1 11y ago
      Pressure from either sun light or laser is very small, and can be ignored for time intervals smaller than centuries. The method proposed in the article is only about thrust created by vaporization
      - sandworm101 11y ago
        The sun vaporizes. That's where comets get tails.
        Already__Taken 11y ago
        Does it? I thought that was heated gasses boiling off.
        marcosdumay 11y ago
        Boiling off == vaporized (ok pedantically, boiling off -> vaporized, but not the other way around). The Sun only does that on icy rocks, and when they are near. Lasers can (in theory) do that on icy rocks that are much further away.
        sandworm101 11y ago
        In space it often isn't so much boiling (liquid+heat=gas) as sublimation (solid+energy=gas). Sublimation can, weirdly, occur well below the freezing/boiling point. That one lone cosmic ray can heat a single molecule of water enough to cast it into space as a gas ... all in an environment where the average temp is well below freezing points. Temperatures in vacuum are strange.
jerf 11y ago
For the "larger ones" that fitzwatermellow was mentioning, the problem is that your mental model of a big rock starts breaking down. You'd be closer to think "big gravel pile sort of iced together, sort of held together by local gravity because there's nothing else pulling them apart". You shoot a little pulse at the gravel pile and break a piece off and it may simply fly out into space, taking almost all the momentum with it and not affecting the bigger body at all. You apply a big force and you might just send rocks through the pile, with, again, the rocks going through taking most of the momentum you applied with them. (Same for landing a booster on it... there may be nowhere you can boost where the booster doesn't just drill right through and come out the other side.) Hit it harder and it may simply spread out. The advantage of the gravity towing approach is that gravity affects the entire pile equally; it doesn't matter how tenuously the whole thing is held together. This approach could literally deflect a mountain-sized ball of pure sandbox-grade sand, completely and correctly. That's not something a lot of other approaches can say. It's only downside is that it's miserably slow, so you have to call the problem years in advance.
- mabbo 11y ago
  That is a really intuitive explanation. Thanks.
- Retric 11y ago
  The problem with gravity pull is you first need to mach orbits which takes a long time or a lot of delta V. From an energy and complexity perspective your best bet is to impact with a large low mass object like foam or lots of tiny objects like a cloud of sand. The advantage being just about any orbit that gets you there works and high relative velocity's are good. The problem is it's easy to miss. A solar sail is a solid candidate as you get a large low mass object and 'free' delta V at the same time. PS: Due to risk of malfunction you likely want to send several ships anyway, while the miss chance is an issue, the cheaper direct approach lets you take more chances.
- chr1 11y ago
  If we break asteroid into smaller pieces wouldn't that be enough? Small pieces won't reach surface of earth and won't cause any damage. > You shoot a little pulse at the gravel pile and break a piece off and it may simply fly out into space, taking almost all the momentum with it and not affecting the bigger body at all. this is true for shooting with projectile, but not for vaporizing with laser. With laser you don't apply any external momentum to the asteroid, vapor small debris fly off the asteroid with some momentum because of expanding under heat, and remaining part gets same momentum in the other direction.

by mabbo 11y ago

sandworm101 11y ago

But KSP does give a false impression. It is very much a "spheres in a vacuum" approach to orbital physics. It doesn't model many/most of the realworld forces. When dealing with the truly minuscule forces such as would be created by lasers aimed at asteroids, one much accommodate all other similarly-sized forces before taking action. Rocks in space are subject to a force/pressure from the sun, a pressure comparable to that from a earthbound laser. Before trying to deflect, one would have to understand how the sun will be altering the course. That calculation is not easy and requires details (albedo/rotation) that are hard to come by for tiny rocks. You don't want to be countering the force from the sun only to push the rock into an intercept it would otherwise have missed. Vaporization also need not mean destroying the rock. So long as the vaporized material achieves escape velocity (not hard on an asteroid) vaporizing the surface of an object creates a force, a thrust, which is greater than the pressure directly from the laser.

marcosdumay 11y ago
> Rocks in space are subject to a force/pressure from the sun, a pressure comparable to that from a earthbound laser. Hum, is it? When near the Sun, yes, of course, but when away from it, we can probably throw way more energy on it than the Sun. Yes, it does require details that we currently can't measure. The good new is that the laser doubles as an excellent instrument for measuring those details.
- sandworm101 11y ago
  That very much depends on the laser. Most of the approaches I've seen discussed involve multiple 'attacks' on a rock spread out over a considerable period of time. You only fire when the rock is far enough above the horizon (say 25-30% of the time if you have 1 ground laser). And then you want to hit it when it is at some sort of apsis, preferably when it is far away with a relatively slow velocity. The sun has a 24/7 line of sight and is always firing.
  - chr1 11y ago
    Pressure from either sun light or laser is very small, and can be ignored for time intervals smaller than centuries. The method proposed in the article is only about thrust created by vaporization
    - sandworm101 11y ago
      The sun vaporizes. That's where comets get tails.
      - Already__Taken 11y ago
        Does it? I thought that was heated gasses boiling off.
        marcosdumay 11y ago
        Boiling off == vaporized (ok pedantically, boiling off -> vaporized, but not the other way around). The Sun only does that on icy rocks, and when they are near. Lasers can (in theory) do that on icy rocks that are much further away.
        sandworm101 11y ago
        In space it often isn't so much boiling (liquid+heat=gas) as sublimation (solid+energy=gas). Sublimation can, weirdly, occur well below the freezing/boiling point. That one lone cosmic ray can heat a single molecule of water enough to cast it into space as a gas ... all in an environment where the average temp is well below freezing points. Temperatures in vacuum are strange.

jerf 11y ago

For the "larger ones" that fitzwatermellow was mentioning, the problem is that your mental model of a big rock starts breaking down. You'd be closer to think "big gravel pile sort of iced together, sort of held together by local gravity because there's nothing else pulling them apart". You shoot a little pulse at the gravel pile and break a piece off and it may simply fly out into space, taking almost all the momentum with it and not affecting the bigger body at all. You apply a big force and you might just send rocks through the pile, with, again, the rocks going through taking most of the momentum you applied with them. (Same for landing a booster on it... there may be nowhere you can boost where the booster doesn't just drill right through and come out the other side.) Hit it harder and it may simply spread out. The advantage of the gravity towing approach is that gravity affects the entire pile equally; it doesn't matter how tenuously the whole thing is held together. This approach could literally deflect a mountain-sized ball of pure sandbox-grade sand, completely and correctly. That's not something a lot of other approaches can say. It's only downside is that it's miserably slow, so you have to call the problem years in advance.

mabbo 11y ago
That is a really intuitive explanation. Thanks.
Retric 11y ago
The problem with gravity pull is you first need to mach orbits which takes a long time or a lot of delta V. From an energy and complexity perspective your best bet is to impact with a large low mass object like foam or lots of tiny objects like a cloud of sand. The advantage being just about any orbit that gets you there works and high relative velocity's are good. The problem is it's easy to miss. A solar sail is a solid candidate as you get a large low mass object and 'free' delta V at the same time. PS: Due to risk of malfunction you likely want to send several ships anyway, while the miss chance is an issue, the cheaper direct approach lets you take more chances.
chr1 11y ago
If we break asteroid into smaller pieces wouldn't that be enough? Small pieces won't reach surface of earth and won't cause any damage. > You shoot a little pulse at the gravel pile and break a piece off and it may simply fly out into space, taking almost all the momentum with it and not affecting the bigger body at all. this is true for shooting with projectile, but not for vaporizing with laser. With laser you don't apply any external momentum to the asteroid, vapor small debris fly off the asteroid with some momentum because of expanding under heat, and remaining part gets same momentum in the other direction.