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Science in Space
Jul. 31st, 2006 12:19 pmI've been reading some Charles Stross novels lately (Singularity Sky and Iron Sunrise being good ones; I'm still not so sure about Accelerando, which so far is stylized-bordering-on-incoherent.) One thing I really like about him is his treatment of space physics -- specifically, that he takes into account that flying and fighting in space is fundamentally different from doing so here on Earth. Most sci-fi just doesn't get this stuff right... because it's really inconvenient.
What does most sci-fi get wrong about trying to have a battle in space? Well, there's the obvious things, of course -- there's no sound in space at all, and there's also no reason for spaceships to have wings (there's no aerodynamics, a cube maneuvers just fine.)
But then there are all the less-obvious ones, things that don't spring to mind because we're so used to the behavior of objects in an atmosphere and gravity well. Some of the more interesting ones:
1.) Aerodynamics. Without wings to push against air (and without air to push against), you have to counter your own momentum if you want to change direction. If I want to pull up in my space fighter, I can't just nose up and fire a rear engine -- I'll keep going in the direction I was going at the same speed, forever, just adding upward motion as well. I actually have to turn the ship 180 degrees and fire the rear engine for every bit as long as I did to get going in the first place, then start up. Of course, if I'm really good (or have a computer calculating vectors for me), I can do this at a diagonal, but the fact remains that for a time (probably a substantial period), I'll be flying nearly straight backwards if I was going at any reasonable speed.
2.) Distance. There is no reason to have a space battle at World War I dogfight distances. There's nothing between me and my enemy to impede accuracy. As a result, space battles would probably occur at distances of several light-seconds (a light-second is about 186,000 miles, or about the same distance as going around the world 5 times.) Even if the opposing ship was absolutely immense, you would never come within range of seeing it with the naked eye without one of you having been destroyed.
3.) Relativity. If my opponent is 10 light-seconds away, then the location he appears to be in to me is actually where he was 10 seconds ago. There is no way to pinpoint his current location. If I use radar or lidar (UV-laser-based radar), it will ping back to me where he was 20 seconds ago. Overall, space fighting would be less like an aerial dogfight and more like submarine warfare, where two vessels try to destroy each other despite being only vaguely aware of each other's location, having sensors that give away their own position far better than they reveal their opponent's, and weapons that take minutes to reach their destination.
4.) Invisibility. A ship may produce no passive emissions, except from where the engines are (which may be facing away from me.) Thus, every ship is a stealth fighter when it's not accelerating. So is every projectile -- if I fire a bullet, it's invisible until it comes into radar/lidar range (and then has the relativistic time lag.) Laser weapons might be used, but unlike lasers in the movies, you wouldn't be able to see them. A laser is tightly focused -- you can only see one because of diffusion (photons strike the air and get deflected out of the path of the beam to your eyes.) In space, there's nothing to get in the way -- any beam focused enough to cover several light-seconds would be totally invisible unless you were standing right in its path (and thus likely vaporized.) Nothing says cinematic action like a bunch of invisible ships fighting each other with invisible weapons in total silence. See what I mean about space science being inconvenient for sci-fi writers? This could even be taken advantage of for special weapons. For instance, a relativistic kill vehicle (RKV) is a missile accelerated to a significant fraction of lightspeed and then allowed to coast to its destination, weeks (or months, or years) away. With no emissions, just a black, coasting object, it will be undetectable until shortly before it hits (if it's going 50% of lightspeed, when it shows up on radar, it's already about to hit you.) And with that much momentum, the missile doesn't even have to be carrying any explosives to demolish a ship (or a planet.)
5.) Momentum. In space, once you start moving, you tend to keep moving that way. It takes as much fuel to slow down or stop as it does to get moving. In fact, it takes an amazing lot of fuel to do anything. With nothing to push against, thrusters must move a ship with nothing more than Newton's Third Law of Motion. If I throw propellant (burning rocket fuel) out one side of my ship, my ship will move the other way. Sounds good... but how much will it move? Well, the accleration provided to my ship will be equal to the mass of the propellant thrown out times the accleration imparted on it, divided by the mass of my ship. What this means is that unless I can shoot the propellant out really fast (by which I mean significant fractions of lightspeed, thousands of times faster than a rocket does), it takes a lot of propellant to get my ship moving at any decent speed, and just as much to stop later. To move at speeds that even count as moving when fighting something light-seconds away, I'd better not change direction very often or I'm going to need a fuel-tank bigger than my ship. On the other hand, when I fire projectile weapons, I risk moving my ship! Projectiles are going to have to go really fast if I want to hit something 20 light-seconds away in the near future. And projectiles will be far more massive than the (presumably) ionized particles I'm using for drive thrust.
6.) Explosives. Bombs don't work very well in space -- most of the damage done by an explosive is caused by the shock-wave. With no air, there's no shock-wave -- the only mass hitting a ship struck by an explosive will be the mass of the explosive itself. What's more, most of the explosive force will be directed along the path of least resistance -- back into space. Even a nuclear bomb's explosive power will be minimal (though the heat, radiation, and electromagnetic pulse will still be effective in that case.) You'd pretty much want to stick to lasers and projectile weapons. On the other hand, a projectile weapon fired at sufficient speed (significant fractions of lightspeed again) will cause whatever it hits to explode anyway.
7.) Physiology. Humans are gloriously unsuited to space travel. Our bodies require carefully maintained temperature and pressure. Overall, it would be a terrible idea to put any people on a ship meant to fight in space, as it would make the ship so very fragile (puncturing it will depressurize the interior.) In addition, you would limit yourself to very low accelerations, as human beings can't survive more than about 8g -- this means your ships must either be very slow or take a very long time to change direction. Far better to have the ships piloted by computers, who will need to do all the calculations anyway (there's no way a human can "lead a target" that's 10 light-seconds away.) If people must be on the ship, they should be kept in suspended animation. If you did have people on a warship, it would not be spacious like the bridge of the Enterprise -- there should be very small quarters with pressure doors anywhere they can be placed, so as to minimize the loss of air and crew whenever the ship was punctured. Of course, if there are no people on the ship, you can make it a lot smaller -- less mass makes it more able to manuever, and nanotech-based computers don't need much space.
I just find it interesting to consider how alien an environment space really is to our brains, which evolved in an environment of atmosphere and gravity. We're also not conditioned to think about the speeds and distances involved in it.
Of course, Charles Stross novels have to go and make it even more complicated by putting singularity-based stardrives on the ships that are capable of navigating a closed timelike curve. Nothing like the possibility of arriving somewhere before you left to throw a wrench in things. :-)
What does most sci-fi get wrong about trying to have a battle in space? Well, there's the obvious things, of course -- there's no sound in space at all, and there's also no reason for spaceships to have wings (there's no aerodynamics, a cube maneuvers just fine.)
But then there are all the less-obvious ones, things that don't spring to mind because we're so used to the behavior of objects in an atmosphere and gravity well. Some of the more interesting ones:
1.) Aerodynamics. Without wings to push against air (and without air to push against), you have to counter your own momentum if you want to change direction. If I want to pull up in my space fighter, I can't just nose up and fire a rear engine -- I'll keep going in the direction I was going at the same speed, forever, just adding upward motion as well. I actually have to turn the ship 180 degrees and fire the rear engine for every bit as long as I did to get going in the first place, then start up. Of course, if I'm really good (or have a computer calculating vectors for me), I can do this at a diagonal, but the fact remains that for a time (probably a substantial period), I'll be flying nearly straight backwards if I was going at any reasonable speed.
2.) Distance. There is no reason to have a space battle at World War I dogfight distances. There's nothing between me and my enemy to impede accuracy. As a result, space battles would probably occur at distances of several light-seconds (a light-second is about 186,000 miles, or about the same distance as going around the world 5 times.) Even if the opposing ship was absolutely immense, you would never come within range of seeing it with the naked eye without one of you having been destroyed.
3.) Relativity. If my opponent is 10 light-seconds away, then the location he appears to be in to me is actually where he was 10 seconds ago. There is no way to pinpoint his current location. If I use radar or lidar (UV-laser-based radar), it will ping back to me where he was 20 seconds ago. Overall, space fighting would be less like an aerial dogfight and more like submarine warfare, where two vessels try to destroy each other despite being only vaguely aware of each other's location, having sensors that give away their own position far better than they reveal their opponent's, and weapons that take minutes to reach their destination.
4.) Invisibility. A ship may produce no passive emissions, except from where the engines are (which may be facing away from me.) Thus, every ship is a stealth fighter when it's not accelerating. So is every projectile -- if I fire a bullet, it's invisible until it comes into radar/lidar range (and then has the relativistic time lag.) Laser weapons might be used, but unlike lasers in the movies, you wouldn't be able to see them. A laser is tightly focused -- you can only see one because of diffusion (photons strike the air and get deflected out of the path of the beam to your eyes.) In space, there's nothing to get in the way -- any beam focused enough to cover several light-seconds would be totally invisible unless you were standing right in its path (and thus likely vaporized.) Nothing says cinematic action like a bunch of invisible ships fighting each other with invisible weapons in total silence. See what I mean about space science being inconvenient for sci-fi writers? This could even be taken advantage of for special weapons. For instance, a relativistic kill vehicle (RKV) is a missile accelerated to a significant fraction of lightspeed and then allowed to coast to its destination, weeks (or months, or years) away. With no emissions, just a black, coasting object, it will be undetectable until shortly before it hits (if it's going 50% of lightspeed, when it shows up on radar, it's already about to hit you.) And with that much momentum, the missile doesn't even have to be carrying any explosives to demolish a ship (or a planet.)
5.) Momentum. In space, once you start moving, you tend to keep moving that way. It takes as much fuel to slow down or stop as it does to get moving. In fact, it takes an amazing lot of fuel to do anything. With nothing to push against, thrusters must move a ship with nothing more than Newton's Third Law of Motion. If I throw propellant (burning rocket fuel) out one side of my ship, my ship will move the other way. Sounds good... but how much will it move? Well, the accleration provided to my ship will be equal to the mass of the propellant thrown out times the accleration imparted on it, divided by the mass of my ship. What this means is that unless I can shoot the propellant out really fast (by which I mean significant fractions of lightspeed, thousands of times faster than a rocket does), it takes a lot of propellant to get my ship moving at any decent speed, and just as much to stop later. To move at speeds that even count as moving when fighting something light-seconds away, I'd better not change direction very often or I'm going to need a fuel-tank bigger than my ship. On the other hand, when I fire projectile weapons, I risk moving my ship! Projectiles are going to have to go really fast if I want to hit something 20 light-seconds away in the near future. And projectiles will be far more massive than the (presumably) ionized particles I'm using for drive thrust.
6.) Explosives. Bombs don't work very well in space -- most of the damage done by an explosive is caused by the shock-wave. With no air, there's no shock-wave -- the only mass hitting a ship struck by an explosive will be the mass of the explosive itself. What's more, most of the explosive force will be directed along the path of least resistance -- back into space. Even a nuclear bomb's explosive power will be minimal (though the heat, radiation, and electromagnetic pulse will still be effective in that case.) You'd pretty much want to stick to lasers and projectile weapons. On the other hand, a projectile weapon fired at sufficient speed (significant fractions of lightspeed again) will cause whatever it hits to explode anyway.
7.) Physiology. Humans are gloriously unsuited to space travel. Our bodies require carefully maintained temperature and pressure. Overall, it would be a terrible idea to put any people on a ship meant to fight in space, as it would make the ship so very fragile (puncturing it will depressurize the interior.) In addition, you would limit yourself to very low accelerations, as human beings can't survive more than about 8g -- this means your ships must either be very slow or take a very long time to change direction. Far better to have the ships piloted by computers, who will need to do all the calculations anyway (there's no way a human can "lead a target" that's 10 light-seconds away.) If people must be on the ship, they should be kept in suspended animation. If you did have people on a warship, it would not be spacious like the bridge of the Enterprise -- there should be very small quarters with pressure doors anywhere they can be placed, so as to minimize the loss of air and crew whenever the ship was punctured. Of course, if there are no people on the ship, you can make it a lot smaller -- less mass makes it more able to manuever, and nanotech-based computers don't need much space.
I just find it interesting to consider how alien an environment space really is to our brains, which evolved in an environment of atmosphere and gravity. We're also not conditioned to think about the speeds and distances involved in it.
Of course, Charles Stross novels have to go and make it even more complicated by putting singularity-based stardrives on the ships that are capable of navigating a closed timelike curve. Nothing like the possibility of arriving somewhere before you left to throw a wrench in things. :-)
