I think I understand your logic here but doesn't it assume that your ship itself is flying a perfect course to the planet at the point of firing your weapon and as a result the error component is only in the aiming? I would have thought that you might expect a similar error in the ships navigation as well which would then introduce the largest error in targetting?
Very true. Probably the best way to look at it is that you have launched your ship towards your target, and then the projectile you shoot is the course correction. In my above example the projectile traveled 66.2 km away from the ship after it was fired, that means that the vector of the firing ship has to pass withing 66.2 km of the target, as the largest course correction you could have would be if the ship fired at a right angle to its course. Doing the math is actually looks like that's the highest precision task of that entire maneuver, the maximum error of 66.2 km from 1e10 km away comes out to 6.62E-9 radians, 220,000 times more accurate than that South African tank cannon. I used to have some info on course corrections for a satellite mission that involved lots of planet flyby's, if I can track that down and do some math I may be able to find a ballpark number for modern space trajectories to go with the modern laser range finder and gun accuracies I found.
Now we certainly wouldn't be able to get the sort of accuracy required in my above example directly out of hyperspace, but if you came out of hyperspace at around 1E10 km, and then course corrected for the next few billion kilometers, shooting at around Uranus or Neptune, then I think you'd be back in business.
Dear All,
New to Aurora forums. Been playing for a year though. Some thoughts on the matter of extreme range planetary bombardment. I don't mean to offend, but though it is an attractive option, isn't it rather impractical?
Now imagine WE'RE all hostile aliens just entering the solar system and about to target earth with extreme range ballistic bombardment. Remember we would have VERY little pre-existing astrogravitational data on the system. What? Take time off for detailed survey in a hostile system with active defenders??? Given that our firing location is just beyond the orbit of PLUTO;-
Our problems are, assuming we can even manage to LOCATE EARTH from so far away (what a miracle that was!):-
1) All newtonian sensor data is an image of the past thanks to the speed of light limit. It takes approx 4 to 7 hours for light to travel from SOL to PLUTO; but EARTH moves at approx 108,000 km/hour around the sun. Thats like a 430K km to 750K km difference by the time we are pointed at it. Sure, lets LEAD THE TARGET, but that only takes us to the next problem:-
2) To "lead" the target is difficult given the extreme time lag, because we would need to know the object's EXACT astrogravitational movements, a study that will take years (365. 25 days???) of observation. But remember, the earth MAY NOT take the EXACT SAME path around the sun for each year due to the following problem:-
3) Predicting the exact planetary orbit given its many variations due to gravitational effects from other solar system bodies and "wobbling" due to LUNA. Then you need to study the intricate sequence of planetary alignment, but planetary alignments are kinda unique each year so you would need to study and predict ALL the significant solar system bodies to predict their effects on earth orbital variations.
4) Okay now thats done, we ALSO need to factor in the gravitational effects of all those solar system bodies ON THE PROJECTILE, including all other minor bodies that are possibly in the way (planetoids, asteroid belts, comets). Assuming we've done that, next:
5) What about the effects of solar wind and its unpredictable variations blowing on the projectile? We'd have to study SOL's internal structure, internal convections, solar flares, solar spot activity. Not to mention that solar wind interaction with the magnetosphere of each major body may have a small effect. Also, we must assume that:
6) The further away the target is, the more unreliable the sensory data (degree of uncertainty increases with range). Remeber there are such things, (no matter how minute) as gravitational lensing of light rays, interference from nearby gravity centres, diffraction, echoes, interference from solar wind (and especially actively from defenders), etc. So we roughly know where a planet WAS to the nearest +/- XXX km, but that may not be precise enough.
7) Finally we'd have to hope there are no collision whatsoever (not even a glancing blow) with micrometeorites / space dust / comets / asteroids (unlikely) that will put our projectile off course. Natural stuff are easily avoided (except the space dust and small stuff), but what about:
The last problem is a valid counter-defense in response to any such projectiles that we might launch. Moving at relativistic speeds, even a collision with a human-seeded "cloud" of floating micro-debris in space early on is enough to veer the projectile sufficiently off-course. Heck those humans could have already pre-emptively put up millions of square kms of those defensive stuff along the "predicted" optimal line of fire the moment they detected our ships warping in-system. Remember we're fighting in THEIR territory now, an unfamiliar battleground they have extensively studied and know very well more than we do.
9) And would you think they'd just let us conduct an accurate system survey in peace? Heck, bouncing all those EM signals in every spectrum off every major solar system body , blasting and lighting us up with all sorts of EM/sensor energy. Putting up false "ghost images". Partial cloaking of earth itself? I'm sure that'd have a profound effect on the accuracy of any survey.
Better we crack out those MK III 100 MSP self-guided drone missiles with the 999 radiation yield dirty warheads??? At least their courses are self-correcting with minimal sensor time-to-target (plus other interferences to be avoided) lag.
Regards,
Sam
You don't need to sit around watching the Earth's orbit for a year to know where it's going to be. From a snapshot of the current positions and velocities of the solar system bodies, you can then just propagate out their positions from that point forward. Gravity is a well understood force, and can be easily modeled. Almost immediately after arriving in a new system you will have a very accurate measurement of that Star's mass due to measuring the change in velocity of your ship, giving you the gravity acceleration, and from their (and the distance to the sun) you have the mass.
You can similarly, trivially, and almost immediately measure the mass of any planet with a moon you find, by observing the acceleration on the moon. A slightly more thorough examination would let you find the barycenter that that planet and its moons were rotating around, which would give you the mass of all of the bodies.
The only thing that would give you trouble is a planet with no moons, in that case the only way to measure its mass (except for using our sensors which somehow actively probe and detect the gravity of all objects, which I am pretending we don't have) is to examine the wobble of the sun, which does take time. While you gather that data you'll just have to use a placeholder mass based on its size and type.
So shortly (how shortly depends on how long it takes you to find all the planets, in Aurora this has always been done instantaneously upon entering a new system) you have all the information you need to propagate all of the non-trivial gravity effects for every body in the entire system. The last bit you need is the star's luminosity so you can calculate light pressure, as well as the velocity and density of the hydrogen in the solar wind. Direct observation of the star will quickly give you that.
With this information you can begin propagating the positions of all of the planets and moons in the solar system over time. Accurately, and quickly.
I have to stress again that we do this today. We have thousands of satellites up there right now, and a lot of them have to stay in fairly particular places, or we have to know where they are, or where they are going to be. We don't have to wait until they are there, we can look at where they were in the past, and then calculate their trajectory including all of the affects that you have mentioned (and more) that you seem to think are impossible or difficult to account for.
This program:
http://www.agi.com/products/by-product-type/applications/stk/stk-for-space-missions/ is one which is used to do those sorts of calculations, this is a list of the forces that it models when calculating a satellite's (Earth-orbiting) trajectory.
1. Two-body gravity
2. Gravity correction for the irregularities of the Earth's gravity field, primarily due to it being an oblate spheroid, but also due to the fact that it doesn't have a continuous density over the entire volume. This is accomplished by using corrections calculated by earlier space missions which meticulously measured the gravity over Earth. Depending on the gravity file the Earth is broken up into a couple dozen or several thousand sectors.
3. Drag, which is based on density which can be calculated in a variety of ways. The better models take into account the current strength of the magnetic field, the current solar activity, and some other variables to model how the atmosphere balloons outwards as it heats up during the day. If you really care you can also use high-altitude weather forecasts to compensate for windspeed when calculating the drag.
4. Third-body gravity from the sun and moon, and if you're really anal, Jupiter and Venus. You could do the others if you wanted to, but there is really no point.
5. The solar radiation pressure from the light emitted by the sun, as well as the solar wind. Of course this also entails modelling when you are being shaded from the sun by the Earth or the Moon.
6. The light coming from the Sun is only part of the battle, there is also a lot of light that bounces off the Earth and pushes you, and you have to measure how much light that was using the shape of the Earth and a model of its albedo.
7. It also models radiation pressure of the light being emitted by the Earth, as in the IR light being emitted as black body radiation.
8. Then you have outgassing, the mass that is from a satellites surface that throws it off course. The largest offenders of these are fuel or cryogenic tanks, which always have some amount of leakages.
9. Lastly the satellite might even have an engine, which also has to be modeled when it fires.
We've had the ability to model all of that stuff for more than 20 years, and the problem we are talking about here is way simpler, since we really just care about the sun and planet gravity. SRP is meaningless for a dense slug, and we won't be near any bodies long enough to worry about Drag or gravity field irregularities.