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The Physics of Space Battles
An anonymous reader writes PBS' It's OK to be Smart made this interesting video showing us what is and isn't physically realistic or possible in the space battles we've watched on TV and the movies. From the article: "You're probably aware that most sci-fi space battles aren't realistic. The original Star Wars' Death Star scene was based on a World War II movie, for example. But have you wondered what it would really be like to duke it out in the void? PBS is more than happy to explain in its latest It's Okay To Be Smart video. As you'll see below, Newtonian physics would dictate battles that are more like Asteroids than the latest summer blockbuster. You'd need to thrust every time you wanted to change direction, and projectiles would trump lasers (which can't focus at long distances); you wouldn't hear any sound, either."
no one can hear you explode.
...I can tell you another thing about space battles: you don't see anything aside from a few tracks on a computer screen. If you have a telescope pointed in the right direction at the exact right time you see a very unimpressive and quick flash.
The ranges, timing, and velocities involved are far too great for human perception.
I am very small, utmostly microscopic.
Actual space battles would be extremely boring to watch. It would all take place at such distances that nothing could really be observed very well or viewed as a whole. Assuming energy / laser type weapons, it's purely a matter of how sensitive and accurate the telescopes are that identify the enemy ships and direct the weapons where to fire. Stealth and cloaking would be where the real arms race would be.
Better known as 318230.
Demonstrating the physics of space fighters with Kerbals in them:
https://www.youtube.com/watch?...
Poul Anderson, The Star Fox
Larry Niven, Protector
C.J. Cherryh, Downbelow Station
My first encounter with realistic space battle physics was with Antares Dawn: a great book by Michael McCollum... Great stuff.
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"First things first -- but not necessarily in that order"
-- The Doctor, "Doctor
I'm not so sure missiles would dictate battle range. On Earth we have dedicated weapon platforms to shoot down anti ship missiles. USN has the AEGIS cruisers, the Russians the Kirov battle cruisers. The horizon means the really really fast ones give about 10-20 seconds notice before you have a massive hole in the side of your ship. Weapons firms claim they can shoot down missiles (I'm not so sure about the claims). To extent the interception time they use airborne radar. However if space battles are fought at 10,000,0000km then you have quite a lot of notice of the missile coming as there is no horizon to hide behind and no sea to skim. If the missiles manoeuvre, you can see it and it is telegraphing its attack and movement direction with hours if not days and weeks.
Well, but consider: space is big. Detecting things in it is hard, unless they've giving off light or radiation that you can detect more easily. For example, today, it is very common that we don't notice near-earth asteroids until they're less than a day away, and asteroids are a lot bigger than missiles.
Atmospheric missiles must maintain constant thrust to keep flying, in order to counteract gravity, air resistance, and to maintain course skimming the ocean, as you mentioned. That makes them easy to spot as soon as they cross the horizon, giving you that 10-20s warning.
But in space, constant thrust is not necessary. The missile can be fired initially just like a dumb projectile, and only engage its thrust once it's very close to the target and needs to adjust course to hit it. Until that time, it just looks like a very small rock hurtling through the void, giving off very little energy, making it very hard to spot.
Even if you had radar or some other kind of active sensors to detect incoming missiles before they engage their thrusters and give away their position, the attacker could simply fire their missiles inside a cloud of other flak to camouflage them. So you can see a cloud of thousands of tiny objects coming in, but you can't tell which of them are missiles with warheads until the whole cloud is close enough that those missiles activate and start homing in on your position.
I read a rebuttal to that which was fairly compelling: http://scienceblogs.com/builto...
The equation given isn’t derived. We have no idea where they’re getting that 13.4 proportionality constant. Dimensionally it’s correct, and it’s pretty easy to derive the equation up to that constant which will depend on the sensitivity of the detector. That equation modulo some uncertainty with respect to that constant is accurate as far as it goes given a spacecraft of hull temperature T and cross-sectional area A.
I would take you through the steps of the derivation, but it would be pointless because the assumption that the hull temperature has anything to do with the interior temperature is simply flat wrong. We can prove this with a potato.
Switch your oven to the “Bake” setting at a temperature of 350 F. After preheating, put in the potato. The interior of the oven, and eventually the potato, are maintained at a constant temperature of 350 degrees. How hot is the exterior surface of the oven? Depends on how well insulated your oven is, but I can guarantee it’s a lot less than 350 degrees.
The key is the understanding the relationship between heat and energy. Put hot coffee in a thermos – the hot coffee is hot because it contains thermal energy. If the energy can’t leave, the coffee will stay hot because the energy stays inside the thermos. The outside of the thermos stays at the temperature of the surroundings. Now neither the thermos nor the oven is a perfect insulator. Some energy leaks out of the oven’s interior, cooling it down. The oven thus has to pump energy into the heating elements to make up for this loss. Equilibrium is reached when the rate of energy being put into the oven equals the rate of loss through the insulation.
For a spacecraft in a vacuum, the pretty much the only way to lose energy from the interior is by radiant heat. The higher the temperature of the outside, the higher the rate of energy loss via radiation. But the temperature itself is irrelevant, since just like the oven and the thermos it’s not necessarily related to the actual temperature inside the cabin at all. It is always and everywhere a function of the total power passing through the hull. If the temperature inside the cabin is constant, the power leaving the hull by radiation is exactly equal to the power being generated inside the hull.
So how far away can we detect a given amount of emitted power? According to Wikipedia, a telescope of 24 aperture can detect stars of magnitude 22 after a half-hour exposure. I think this is a pretty good realistic limit for detection with reasonable equipment in a reasonable time frame. Now we need to compare this magnitude to something of known power output. How about the Sun? The sun has magnitude -26.73 as seen from the Earth’s surface (smaller magnitude is brighter), for a difference in magnitude of 48.73. The exponent used for magnitude is 2.512, so the difference in power per unit area of telescope is 2.512^48.73 = 3.1 x 1019. Since the Sun radiates about 1000 watts per square meter at the distance of the earth, the smallest radiant power we can reasonably detect in our telescope is about 3.123.1 x 10-17 watts per square meter.
Our hypothetical spacecraft is radiating that power into space, evenly distributed over the surface of a sphere of radius r, where r is the distance to the detector. When that power-per-area is the same as the limit of our telescopic capability, that gives us the maximum detection range. Mathematically,
Where rho is the sensitivity of our detector. Solve for r:
So what’s the power? Well, each human on board is going to produce about 100 W just from basic bodily metabolism. Computers, life support, sanitation, and all the rest will contribute more. We might assume 10,000 watts total for a futuri
Not necessarily - if you paint them black and launch them discretely (by rail gun?) a missile could coast unnoticed across the void to your apprxoimate location and only engage its engines for the final approach.
Radar is viable on Earth because the horizon is only a few dozen miles away and the area of interest is typically only a mile or two high - practicaly 2-dimensional. In 3D space you'd be talking about many orders of magnitude more power to run an broad-focus radar system with the sort of range you'd need to be useful. Especially considering projectile speeds are potentially several orders of magnitude faster than is possible to sustain within an atmosphere.
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If you're going to have reaction drive style thrusters for maneuvering, you're going to run out of fuel very quickly, dissipating mass, unless your thrusters are thrusting out little bits of mass at VERY high speed, in which case they could be used as weapons themselves. (Sci Fi writer Larry Niven came up with the idea of a reaction drive as a weapon, google the 'Kzinti Lesson' for more info.)
I think it would be interesting to have space battles where several fighters were somehow connected to each other via some sort of tractor beam, so they maneuvered by transferring momentum between each other instead of dissipating mass into the vastness of space; they might look a bit like bolas circling each other but with quick changes snapping in and out as they went in to battle, or maybe they would be tethered to a mother ship, somewhat like World War II aircraft carrier that sends out figher planes to do the fighting. The mother ship would have enough mass to let the fighters seem to be free to zap around easily.
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("Cough Cough") I wrote an unpublished Sci Fi Novel (I did send it to a bunch of publishers at the time, over 10 years ago), where interstellar travel used 'draggers'. There was no faster than light travel so it took years and years to go between even nearby stars, (The travelers themselves would be in an accelerated frame of reference so it wouldn't be so long for them.) In the novel it took a long time to set up a system between two solar systems, similar to the way it takes a long time to set up a railway between two cities, but then you could use it very efficiently. A vessel would attach itself to a dragger, and be quickly accelerated (that's the hard part, dealing with the sudden accleration that would flatten everything against the back wall like you were in a super cream separator), the dragger, much more massive than the vessel, would be slowed down some, but then, at the other end, as the dragger wheeled around a star, the vessel would transfer it's momentum back to the dragger and slow down to become part of the other solar system.
The thing about conservation of momentum is that it means the center of mass of a closed system doesn't change. If two solar systems and the draggers going between them were a closed system, then the center of mass would shift as the vessel moved between one and the other, but, if the vessel returned to the original system again, then the original center of mass would be restored, and the energy used to move between them could be recycled, plus there wouldn't be reaction mass being spewed out all over the place.
In theory, theory and practice are the same; in practice they're different. (Yogi Berra & A. Einstein)
The only two "giveaways" would be the heat signature from its power source (not just propulsion, but life-support) and whatever it accidentally occults as it moves across the background of stars. The heat can be drastically reduced by towing the power source a long way behind the main craft and having it very, very dispersed so the Watts per square metre of I.R. are very small. The occultation problem can be reduced by choosing a path that stays away from the galactic plane.
So most battles would be ones of sneak attacks and defensive fire. It might be possible to devise some sort of A.I. mines, or even simply fire a cloud of sand in the general direction (assuming the relative velocites of target + sand are high enough, that could be all that's needed).
However, I have a feeling that most "wars" in the future, whether in space on on Earth, will be economic in nature and "fought" over decades rather than wham-bam shooting battles.
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If you're going to swallow the idea of FTL drives, tractor beams and shields -- among other things -- then it's not really that much of a stretch to swallow the idea of inertial control, too. Which would make such battles not resemble a game of asteroids at all.
As for sound, presuming your vehicle maintains atmospheric integrity, you'd hear anything that causes the the craft's atmosphere to be jolted into motion. Debris hitting your vehicle, the stress caused by a sealed compartment being ruptured, people screaming when they get fried, crushed or otherwise insulted as a consequence of direct or indirect battle damage or loss of, for instance, inertial damping, equipment failures and power supplies having problems. You would also hear something if a force field of any kind was imposed upon your vehicle in such a way as to deliver any kind of uncompensated-for energy in mechanically coupled framework(s) producing direct or indirect vibrations in the audio range. And furthermore , presuming a ship has sensors to detect things like the energy outputs of other vehicles as they maneuver, seems to me that converting that to audio as a handy sound cue/warning would be hardly any trick at all. Just as one example.
Likewise, perhaps *we* can't focus a laser today, but that's not an inherent limitation of lasers even by today's known physics, that's a limitation of our technology, so that objection is kind of dead on the doorstep, so to speak. Not that a visible future beam weapon is necessarily carrying its punch in the form of light anyway. Could be just a side effect, or an aiming aid. This is the future, we're talking an imaginary scenario resulting from science and technology we don't presently have and so may speculate upon (using current knowledge... pretty boring... we can barely get off the planet's surface, much less engage in space battles... that's why most SF has at least a few pure fantasy elements in it.)
And along the lines of what we accept and what we don't, if you are blase' about the idea of a magic camera floating around your space battle and instantly changing perspective from A to B to C, perhaps it's just a little bit silly to complain about, for instance, a whoosh, or what "lasers" can do. That's entirely outside of what might be realistic in terms of what the movies subjects are up to.
So yeah, it's ok to think, but don't let someone else do your thinking for you. If there are space battles as depicted in most SF(fantasy) movies, the rules as we know them right now have long since been trashed, so there doesn't really seem to be any reason to worry about it.
All of the above is why I can really enjoy Star Wars, Firefly, Trek, etc, btw. Even though I'm fairly well grounded in how we think things work at present.
I have more trouble with obvious errors that don't take into account technologies we already have. For instance, in Red Mars, some of the characters "hide" from satellite surveillance by moving over long distances in a large hollow rock (or perhaps a thing that looks like I rock, I forget), something we would spot in an instant *today* by the simple expedient of image subtraction; Take two shots under the same or similar conditions but separated by time, align them, and subtract them. Everything that's in the same place turns to black; anything that has moved will be bright. This is *trivial* surveillance technology, and has been in use since *at least* the 1970's. And the kicker is this would work even better on Mars than it does here -- thinner atmosphere. Caused me a few snickers, that one did.
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