5 ms·

The page mentions accelerating over a longer distance, but wouldn't that defeat the undetectability of the whole thing?

by t0mas88 12y ago

Arcsech 12y ago
Under certain circumstances, but since the mass can be (relatively) small due to the extremely high velocity, it can be very hard to spot. Essentially, imagine trying to pick something the size of a telephone pole, angled end-on at you, from 20 miles away out of the entire night sky. By the time it gets close enough to spot easily, it's too late to do much about it. Edit: jjaredsimpson's comment makes a very good point about velocities much closer to 1c. My comment mostly relates to the more immediately practical kinetic bombardment[0], which works on the same idea. [0] https://en.wikipedia.org/wiki/Kinetic_bombardment https://en.wikipedia.org/wiki/Kinetic_bombardment
jjaredsimpson 12y ago
If I accelerate 1000 1kg masses to .99c 1ly away from your planet you can't detect them, and I've just destroyed your entire species (sterilized the planet) with a metric ton of rocks. It doesn't matter how long I take to accelerate them, once they are going fast you are destroyed in 1 year.
- deeviant 12y ago
  You would need some pretty killer orbital prediction to do this. I would assume you'd start a bit closer.
- rwallace 12y ago
  With the caveat that at the tech level required to launch such an attack, destruction of one's home planet would constitute something closer to vandalism of a historic site than destruction of one's species. Of course, if you're thinking of a scenario where aliens at that tech level launch such an attack against us 21st-century humans, yes, we'd be toast.
- TheLoneWolfling 12y ago
  Why are you saying we couldn't detect them? There's enough interstellar medium to make any such impactor detectable. (Rough calculation: 67kw/m^2. And that's not even talking about dust grains (which ends up with much brighter instantaneous flashes))
  - letstryagain 12y ago
    By the time you see it, it will be too late to respond.
    - TheLoneWolfling 12y ago
      Why? Sure, there's time dilation effects, but they are largely cancelled out. We have less notice for a given distance, but we also can see it coming from farther away.
      - InclinedPlane 12y ago
        .99c means that you have 1/100th as much time to respond relative to the detection distance. Let's say you spot the impactors an entire light-year away. The light will only reach you less than 4 days before impact. Can you put together an effective response plan in 4 days? If instead you spotted them out in the oort cloud (say, 1000 AU away) you would have less than 2 hours to respond.
        TheLoneWolfling 12y ago
        Right, but that is compensated by the fact that that same time compression also applies to any energy released by the impactor. It'd be putting out >3.5*10^7 watts / m^2 at 0.99c, from our perspective. And a single sand grain hitting it would release 27GJ. Not to mention... How on earth are you managing to keep something together when its being bombarded by hydrogen / helium at 0.99c?
deeviant 12y ago
Not necessity. Imagine a spacecraft using a standard inertial propulsion system. It shoots mass one way and it goes in the opposite direction. If it had something like a thrust signature, it would be in exactly the wrong direction for the intended target to see, add in a shroud and it would impossible to detect unless the target had sensors in other systems perpendicular and in between the launch site and target site. Even if detected, what could be done? A planet is not going to change it's trajectory, and even if they fry it with some sort of energy weapon, it would still maintain it's velocity and the associated energy. Truly, in a universe with advanced and aggressive space faring species, a planet would be about the most unsafe place you could be, it would be ridiculous easy for said advanced space faring species to destroy/make uninhabitable a planet. And you would hardly even need a RKV to do it.
- Terr_ 12y ago
  > Even if detected, what could be done? A large-scale Whipple Shield, maybe? (http://en.wikipedia.org/wiki/Whipple_shield http://en.wikipedia.org/wiki/Whipple_shield) Throw a small moon into its path and engage it from as far away as possible. You'll end up with a lot of smaller slower pieces over a wider area.
  - deeviant 12y ago
    That might work, it would have to be detected soon enough to allow for the time to build it, though.
    - TheLoneWolfling 12y ago
      You don't need to build any sort of permanent structure. Orbits exist. Just coordinate trajectories. Throw a bunch of chaff at it from different trajectories, so that it passes through them in stages.
  - TheLoneWolfling 12y ago
    The biggest problem with Whipple shields with relativistic projectiles is that the projectiles literally don't have time to react. Partly because of the speed itself, partly because of time dilation. You need to throw enough at it that a large chunk of the nuclei in the impactor are hit by nuclei of the atoms you are throwing at it - not an insignificant feat. Small moon may work. Anything less and yes, it's rapidly expanding into a cloud of gas, but nowhere near fast enough. It'll hit the target well before it dissipates significantly if you're not careful.

The page mentions accelerating over a longer distance, but wouldn't that defeat the undetectability of the whole thing?

by t0mas88 12y ago

The page mentions accelerating over a longer distance, but wouldn't that defeat the undetectability of the whole thing?

Under certain circumstances, but since the mass can be (relatively) small due to the extremely high velocity, it can be very hard to spot. Essentially, imagine trying to pick something the size of a telephone pole, angled end-on at you, from 20 miles away out of the entire night sky. By the time it gets close enough to spot easily, it's too late to do much about it. Edit: jjaredsimpson's comment makes a very good point about velocities much closer to 1c. My comment mostly relates to the more immediately practical kinetic bombardment[0], which works on the same idea. [0] https://en.wikipedia.org/wiki/Kinetic_bombardment https://en.wikipedia.org/wiki/Kinetic_bombardment

jjaredsimpson 12y ago

If I accelerate 1000 1kg masses to .99c 1ly away from your planet you can't detect them, and I've just destroyed your entire species (sterilized the planet) with a metric ton of rocks. It doesn't matter how long I take to accelerate them, once they are going fast you are destroyed in 1 year.

deeviant 12y ago
You would need some pretty killer orbital prediction to do this. I would assume you'd start a bit closer.
rwallace 12y ago
With the caveat that at the tech level required to launch such an attack, destruction of one's home planet would constitute something closer to vandalism of a historic site than destruction of one's species. Of course, if you're thinking of a scenario where aliens at that tech level launch such an attack against us 21st-century humans, yes, we'd be toast.
TheLoneWolfling 12y ago
Why are you saying we couldn't detect them? There's enough interstellar medium to make any such impactor detectable. (Rough calculation: 67kw/m^2. And that's not even talking about dust grains (which ends up with much brighter instantaneous flashes))
- letstryagain 12y ago
  By the time you see it, it will be too late to respond.
  - TheLoneWolfling 12y ago
    Why? Sure, there's time dilation effects, but they are largely cancelled out. We have less notice for a given distance, but we also can see it coming from farther away.
    - InclinedPlane 12y ago
      .99c means that you have 1/100th as much time to respond relative to the detection distance. Let's say you spot the impactors an entire light-year away. The light will only reach you less than 4 days before impact. Can you put together an effective response plan in 4 days? If instead you spotted them out in the oort cloud (say, 1000 AU away) you would have less than 2 hours to respond.
      - TheLoneWolfling 12y ago
        Right, but that is compensated by the fact that that same time compression also applies to any energy released by the impactor. It'd be putting out >3.5*10^7 watts / m^2 at 0.99c, from our perspective. And a single sand grain hitting it would release 27GJ. Not to mention... How on earth are you managing to keep something together when its being bombarded by hydrogen / helium at 0.99c?

deeviant 12y ago

Not necessity. Imagine a spacecraft using a standard inertial propulsion system. It shoots mass one way and it goes in the opposite direction. If it had something like a thrust signature, it would be in exactly the wrong direction for the intended target to see, add in a shroud and it would impossible to detect unless the target had sensors in other systems perpendicular and in between the launch site and target site. Even if detected, what could be done? A planet is not going to change it's trajectory, and even if they fry it with some sort of energy weapon, it would still maintain it's velocity and the associated energy. Truly, in a universe with advanced and aggressive space faring species, a planet would be about the most unsafe place you could be, it would be ridiculous easy for said advanced space faring species to destroy/make uninhabitable a planet. And you would hardly even need a RKV to do it.

Terr_ 12y ago
> Even if detected, what could be done? A large-scale Whipple Shield, maybe? (http://en.wikipedia.org/wiki/Whipple_shield http://en.wikipedia.org/wiki/Whipple_shield) Throw a small moon into its path and engage it from as far away as possible. You'll end up with a lot of smaller slower pieces over a wider area.
- deeviant 12y ago
  That might work, it would have to be detected soon enough to allow for the time to build it, though.
  - TheLoneWolfling 12y ago
    You don't need to build any sort of permanent structure. Orbits exist. Just coordinate trajectories. Throw a bunch of chaff at it from different trajectories, so that it passes through them in stages.
- TheLoneWolfling 12y ago
  The biggest problem with Whipple shields with relativistic projectiles is that the projectiles literally don't have time to react. Partly because of the speed itself, partly because of time dilation. You need to throw enough at it that a large chunk of the nuclei in the impactor are hit by nuclei of the atoms you are throwing at it - not an insignificant feat. Small moon may work. Anything less and yes, it's rapidly expanding into a cloud of gas, but nowhere near fast enough. It'll hit the target well before it dissipates significantly if you're not careful.