Space Warfare II - Stealth Reconsidered

[Nyrath]

Re: Space Warfare II - Stealth Reconsidered

 

Due to a lack of effective X-ray mirrors' date=' beam spread is the sum of the aspect ratio of the lasing crystal and the diffraction limit. If we assume soft X-rays at 1 nanometer and lasing crystals 10 meters long, aspect ratio is 1e-1*width, diffraction is 1e-9/width, and we get the best value at width = 1e-4 (0.1mm) where both limits are 1e-5 so our beam divergence is 2e-5 radians, or 20cm per kilometer.[/quote']

So with about 1.8e7 kilometers per light minute, that would make the spot size grow to about 3600 kilometers, and become about as dangerous as a flashlight.

[Kristopher]

Re: Space Warfare II - Stealth Reconsidered

 

Not quite.

 

Sitting on my desk right now is a one terabyte hard drive, which I use for my iTunes music and movie files. Not gigabye. Terabyte. It cost about a hundred bucks. By the time we have space warships, a hard drive with enough memory to contain such an astronomical object database will be the size of your thumb and cost the equivalent of about fifty cents, and be available at the check-out counters of your local office-supply store.

 

And all astro-militaries will maintain such databases. Why? Do you know how much damage a re-directed asteroid can do to Earth? Think "dinosaur killer." The astro-militaries of all nations will keep close tabs of the orbits of all small bodies, just to be sure there are no unauthorized changes in their orbits.

 

I always suggest that care be taken with the word "will".

[Kristopher]

Re: Space Warfare II - Stealth Reconsidered

 

MOD: as a general note, I'd ask that people keep their arguments to the science and speculation and gaming uses, and not what they suspect other people's motives to be.

[ajackson]

Re: Space Warfare II - Stealth Reconsidered

 

Nope. CCDs suffer from blooming when the pixel well is overloaded (the equivalent of photographic overexposure). It doesn't matter how many pixels you have.

The amount of noise a pixel captures is equal to (time) * (background brightness) * (angular area covered by pixel) * (collector area). Reduce the angular area of each pixel, and you reduce noise -- but you also reduce coverage, unless you add more pixels.

It can do 10 square degree shots or go down to 25th magnitude. Not both.

No, it does both. That's what it's for -- it's a survey telescope.

[Nyrath]

Re: Space Warfare II - Stealth Reconsidered

 

I was under the impression Governments were much more active in things like Sky Guard after the Shoemaker-Levy 9 comet strike on Jupiter. Perhaps that interest was short lived.

Alas, the voting public has the attention span of a five-year-old.

The closest thing we have to Sky Guard is the NASA Near-Earth Object program. Like all things NASA, it is chronically underfunded. They track asteroid only to the point where they can determine that it has no chance of striking Earth, then it is put on the "ignore" list.

 

How long does it take to do a whole sky survey? My question here is aimed at determining how "instant" detection would be for various energy outputs.

Well, as a data point, one of my experts estimated it would take about four hours to do a full sky scan down to the 12th magnitude (a little longer than that since it takes more time to scan for objects within two degrees of the Sun's glare, using an occlusion filter).

 

The earlier post about there being a lot of junk in space was something I had been thinking about. How do we know some of the asteroids AREN'T alien space stations watching over us? How often are we looking? Apparently not very often at all. At what point does a world suddenly decide to start looking and watching? Maybe not until after it's first pearl harbor in space! At which point it is too late' date=' their planet is enslaved. So maybe you only need to be stealthy once per system, early enough.[/quote']

Currently nobody is really looking for an alien base disguising itself as an asteroid for the same reason that the military has no plans for defending themselves from an attack from Atlantis: they calculate it to be an exceedingly low probability event.

The technical term for mis-calculating the probability is "being blindsided." Yes, "Pearl Harbor" is a synonym.

 

Now, if the aliens do something stupid like using an antimatter power plant, they'll be spotted right away. Electron-positron annihilation has a characteristic signature of 511 keV. The astronomical satellites used to detect gamma ray bursters would pick it up right away. This would be quite suspicious since there are no non-artificial sources of antimatter annihilation in the asteroid belt.

 

How much energy would it take to move an asteroid into Earth orbit? Once it makes it's run how much energy would it take to prevent a collision (would that even be possible?)

Lots and lots.

 

To nudge an asteroid into a collision course with Earth takes an amount of energy determined by [1] the mass of the asteroid (higher=more), and [2] how far from the Sun the asteroid currently is (closer=more).

The energy is proportional to the asteroid's momentum, which is mass times velocity. The mass comes from [1] and the velocity change required comes from [2].

 

To prevent a collision, the energy required depends upon [1] the momentum of the asteroid, and [2] how far in advance of collision that counter-measures are taken. The farther in advance, the less the angular change in the asteroid's course will result in missing the Earth.

 

There are some techniques here:

http://www.cosmicdiary.org/blogs/nasa/franck_marchis/?p=245

 

How much to nudge the asteroid?

An example I was given:

100 megaton asteroid, assume a delta-V of 1 kps is required (the asteroid is a bit big, but the delta-V is a bit low). It is large enough to be close to a mass-extinction event.

 

So you start with a 200 megaton asteroid, and use a mass driver to use half the mass as propellant to give it 1 kps.

 

The energy requirements would be on the order of 1 x 10^17 joules. This is roughly the total output of a one-gigawatt nuclear reactor run for a bit over three years.

[ajackson]

Re: Space Warfare II - Stealth Reconsidered

 

The energy is proportional to the asteroid's momentum' date=' which is mass times velocity.[/quote']

Actually, that's not true. The lowest-energy solution involves basically splitting the asteroid in half and sending one half towards the planet while the other half becomes reaction mass, and requires energy proportional to the mass times the square of the delta-V.

[Nyrath]

Re: Space Warfare II - Stealth Reconsidered

 

Actually' date=' that's not true. The lowest-energy solution involves basically splitting the asteroid in half and sending one half towards the planet while the other half becomes reaction mass, and requires energy proportional to the mass times the square of the delta-V.[/quote']

I stand corrected.

[frgough]

Re: Space Warfare II - Stealth Reconsidered

 

The amount of noise a pixel captures is equal to (time) * (background brightness) * (angular area covered by pixel) * (collector area). Reduce the angular area of each pixel, and you reduce noise -- but you also reduce coverage, unless you add more pixels.

 

No, it does both. That's what it's for -- it's a survey telescope.

 

Yes, it will find 25th magnitude stars and 1st magnitude stars at the same time. What it won't find is 25th magnitude stars sitting next to 1st magnitude stars at the same time.

[frgough]

Re: Space Warfare II - Stealth Reconsidered

 

Well, as a data point, one of my experts estimated it would take about four hours to do a full sky scan down to the 12th magnitude (a little longer than that since it takes more time to scan for objects within two degrees of the Sun's glare, using an occlusion filter).

 

Your expert is wrong.

 

http://upload.wikimedia.org/wikipedia/commons/1/11/Galactic_Cntr_full_cropped.jpg

 

There's a ship in there somewhere. Find it for me. It could be hiding in the glare of one of the overexposed stars.

 

This also illustrates the incorrect assumption that there is no terrain in space. That picture is full of terrain.

[Kristopher]

Re: Space Warfare II - Stealth Reconsidered

 

Your expert is wrong.

 

http://upload.wikimedia.org/wikipedia/commons/1/11/Galactic_Cntr_full_cropped.jpg

 

There's a ship in there somewhere. Find it for me. It could be hiding in the glare of one of the overexposed stars.

 

This also illustrates the incorrect assumption that there is no terrain in space. That picture is full of terrain.

 

If it's on a course that keeps it, as seen from Earth, directly in front of one of the brighter stars, does that make picking it out much harder? At first thought, it seems that at the very least you'd need to be able to pick its light out from the light of the star, and be able to do fast parallax work (ie, have two stations far enough apart and synched together) on all your sky scanning.

[ajackson]

Re: Space Warfare II - Stealth Reconsidered

 

There's a ship in there somewhere. Find it for me. It could be hiding in the glare of one of the overexposed stars.

Given images separated by an hour, probably trivial. Given two sensors with enough separation to produce a parallax exceeding the sensor resolution, almost certainly trivial.

[Cancer]

Re: Space Warfare II - Stealth Reconsidered

 

The thing is, to find asteroids (or other objects), you don't simply take pictures, you use differences between pictures. Most of the "terrain" changes slowly enough that differencing takes care of it more or less completely, though as has been noted, scaling exposure times near bright objects may be needed.

 

But more to the point, things move, and in general they don't move together. A hostile object cannot hide indefinitely in the glare of a 1st-magnitude star, except by using an engine, which is almost certainly detectable.

 

After attending an unclassified talk a year or two ago about detecting objects comparable to laptops in LEO, I think "stealth" is going to be, "pick a mode of detection, and you'll be hidden to that, but you can't be invsible to everything all the time".