Over what distance would space combat between spacecraft probably occur given physical realities?

Question

Many science fiction books invoke a naval combat paradigm when describing space combat: sensors and battleships with batteries, carriers with fighter-bombers, etc., usually over a distance of lightseconds.

Sometimes, they might even use something akin to submarines: "stealth" craft which may try to intercept communications, or conduct submarine like attacks.

However, would it be possible for space craft to really be "stealthy" or invisible to any degree in space, given that they must release some heat, but space is in general, utterly cold?

Would there be regions (apart from commonly invoked nebulae) which may mess with sensors?

What would stop spaceships from engaging in combat over the distance of lightyears, rather than lightseconds, if they are readily detectable from distances on the scale of lightyears?

I assume no major deviation from today's known science, in particular no faster-than-light communication or travel.

Answer 1

Some useful discussion and links can be found on projectrho.com, I mentioned these in comments before the question was migrated but they were deleted in the migration, so I'll repost here. First of all, in the Space War page, at the top there are various links to posts on the "rocketpunk manifesto" blog which have good discussions of issues relating to space combat. And here are some other good pages from projectrho.com:

Detection in Space Warfare (most relevant to your questions about stealth)

Defenses in Space Warfare

Introduction to Space Weapons (mostly just devoted to classification, but has a link to this site which has a lot of interesting ideas)

Conventional Space Weapons

Exotic Space Weapons

Space Warship Designs

Combat Theater

Planetary Attack

You can find some other semi-relevant pages if you google site:www.projectrho.com and "space war" (in quotes), but the other pages I saw are almost entirely devoted to describing how space war was depicted in various science fiction works rather than discussing how it would "realistically" work.

(edited to add that I recently came across another good article about realistic space combat, The Physics of Space Battles)

Since your question is mainly about whether it would be possible to "hide", definitely look through the "Detection in Space Warfare" page, the author is of the definite opinion that none of the proposed solutions would work. For example, here's the discussion of just channeling exhaust and waste heat in a narrow beam going in the opposite direction of where the enemy is located:

Glancing at the above equation it is evident that the lower the spacecraft's temperature, the harder it is to detect. "Aha!" you say, "why not refrigerate the ship and radiate the heat from the side facing away from the enemy?"

Ken Burnside explains why not. To actively refrigerate, you need power. So you have to fire up the nuclear reactor. Suddenly you have a hot spot on your ship that is about 800 K, minimum, so you now have even more waste heat to dump.

This means a larger radiator surface to dump all the heat, which means more mass. Much more mass. It will be either a whopping two to three times the mass of your reactor or it will be so flimsy it will snap the moment you engage the thrusters. It is a bigger target, and now you have to start worrying about a hostile ship noticing that you occluded a star.

Dr. John Schilling had some more bad news for would be stealthers trying to radiate the heat from the side facing away from the enemy.

"Besides, redirecting the emissions merely relocates the problem. The energy's got to go somewhere, and for a fairly modest investment in picket ships or sensor drones, the enemy can pretty much block you from safely radiating to any significant portion of the sky.

"And if you try to focus the emissions into some very narrow cone you know to be safe, you run into the problem that the radiator area for a given power is inversely proportional to the fraction of the sky illuminated. With proportionate increase in both the heat leakage through the back surfaces, and the signature to active or semi-active (reflected sunlight) sensors.

"Plus, there's the problem of how you know what a safe direction to radiate is in the first place. You seem to be simultaneously arguing for stealthy spaceships and complete knowledge of the position of enemy sensor platforms. If stealth works, you can't expect to know where the enemy has all of his sensors, so you can't know what is a safe direction to radiate. Which means you can't expect to achieve practical stealth using that mechanism in the first place.

"Sixty degrees has been suggested here as a reasonably 'narrow' cone to hide one's emissions in. As a sixty-degree cone is roughly one-tenth of a full sphere, a couple dozen pickets or drones are enough to cover the full sky so that there is no safe direction to radiate even if you know where they all are. The possiblility of hidden sensor platforms, and especially hidden, moving sensor platforms, is just icing on the cake.

"Note, in particular, that a moving sensor platform doesn't have to be within your emission cone at any specific time to detect you, it just has to pass through that cone at some time during the course of the pre-battle maneuvering. Which rather substantially increases the probability of detection even for very narrow emission cones.

Then the page gives another quote from Ken Burnside:

"The problem with directional radiation is that you have to know both where the enemy sensor platforms are, and you have to have a way of slowing down to match orbits that isn't the equivalent of swinging end for end and lighting up the torch. Furthermore, directing your waste heat (and making some part of your ship colder, a related phenomena) requires more power for the heat pump - and every W of power generated generates 4 W of waste heat. It gets into the Red Queen's Race very quickly.

"Imagine your radiators as being sheets of paper sticking edge out from the hull of your ship. You radiate from the flat sides. If you know exactly where the enemy sensors are, you can try and put your radiators edge on to them, and will "hide". You want your radiators to be 180 degrees apart so they're not radiating into each other.

"Most configurations that radiate only to a part of the sky will be vastly inefficient because they radiate into each other. Which means they get larger and more massive, which reduces engine performance...and they still require that you know where the sensor is.

"The next logical step is to make a sunshade that blocks your radiation from the sensor. This also requires knowing where the sensor is, and generates problems if the sensor blocker is attached to your ship, since it will slowly heat up to match the equilibrium temperature of your outer hull....and may block your sensors in that direction as well.

Update: Some commenters have been asking about the possibility of having a sort of "heat battery" which absorbs waste heat generated by propulsion and other systems on the ship for the period of time where it needs to be stealthy, and is well-insulated so as not to give off detectable blackbody radiation, or to leak its energy to other parts of the ship as heat, so that from the outside the ship would not give off radiation due to heat. I found some useful equations relevant to the feasibility of this, so I thought I'd post them.

Suppose we want to have enough fuel for some set of maneuvers during the period the rocket needs to be stealthy, such that, if the same amount of fuel were spent just accelerating the rocket continuously in one direction, the rocket's change in velocity would be $\Delta v$. Then if the final mass once all this fuel is spent is $m_1$ (which will include both the mass of the weapons and other useful systems, like life support if the rocket is manned and computers and sensors if it's not, as well as the mass of the heat battery), and the initial mass including fuel is $m_0$, and the effective exhaust velocity of the propellant is $v_e$, then the Tsiolkovsky rocket equation relates these quantities:

$\Delta v = v_e \ln \frac{m_0}{m_1}$

A related equation is the amount of energy the fuel must supply to the rocket in order to achieve this $\Delta v$, given the effective exhaust velocity $v_e$ and the final mass $m_1$ that should be left over once the fuel is used up. As given in the "energy" section of the spacecraft propulsion article on wikipedia, "If the energy is produced by the mass itself, as in a chemical rocket", then the energy would be given by this formula:

$E = \frac{1}{2}m_1 (e^{\Delta v / v_e} - 1)v_e^2$

The "internal efficiency" $\eta_{int}$ of a rocket is the ratio of the actual increase in linear kinetic energy delivered per unit time to the internal chemical energy used up per unit time, as explained here, so if the the fuel delivered an amount of linear kinetic energy $E$ to the rocket while it was burned, the original chemical energy must have been a greater amount $E / \eta_{int}$, and thus the energy lost to heat must have been approximately $(E / \eta_{int}) - E = E( \frac{1}{\eta_{int}} - 1) = E\frac{1 - \eta_{int}}{\eta_{int}}$ (Note that this isn't exact, because some of the loss of efficiency is not due to energy lost to heat, but rather due to exhaust particles having some kinetic energy that isn't parallel to the direction the rocket is traveling. Also I'm assuming below that the heat battery is somehow absorbing all energy lot to heat, the calculations would be somewhat different if heat couldn't be channeled away from the exhaust trail, but only the heat that would be added to the ship itself, see the chart here for estimates of about how much fuel energy is lost to each. Maybe the best way to be stealthy would be to avoid chemical rocketry with hot exhaust trails, and instead use something like a mass driver that could fling a stream of cooled pellets backwards at high velocity.) So using the above formula for $E$, the heat generated $Q$ would be approximately:

$Q = ( \frac{1 - \eta_{int}}{\eta_{int}}) \frac{1}{2}m_1 (e^{\Delta v / v_e} - 1)v_e^2$

If the heat battery has mass $m_b$ and specific heat $c$, then rearranging the formula here, we can see that absorbing heat $Q$ will cause a temperature change $\Delta T$ of:

$\Delta T = \frac{Q}{c m_b}$

And in the equation for $Q$, we can replace the final mass after fuel is expended, $m_1$, with $m_b + m_p$, where $m_b$ is again the heat battery mass and $m_p$ is the remaining payload mass (weapons etc.). Then combining the equations gives:

$\Delta T = ( \frac{1 - \eta_{int}}{\eta_{int}}) \frac{1}{2 c m_b}(m_b + m_p) (e^{\Delta v / v_e} - 1)v_e^2$

With some algebra you can solve this for the ratio of the heat battery mass $m_b$ to the remaining payload mass $m_p$:

$m_b / m_p = \frac{( \frac{1 - \eta_{int}}{\eta_{int}}) \frac{1}{2 c } (e^{\Delta v / v_e} - 1)v_e^2 }{\Delta T \, - \, [( \frac{1 - \eta_{int}}{\eta_{int}}) \frac{1}{2 c } (e^{\Delta v / v_e} - 1)v_e^2 ]}$

The part to note is the denominator, which goes to zero if $\Delta T = [( \frac{1 - \eta_{int}}{\eta_{int}}) \frac{1}{2 c } (e^{\Delta v / v_e} - 1)v_e^2 ]$, which would make $m_b$ infinite; and since $m_b$ can't be negative either, that means for a physically realistic solution you must satisfy $\Delta T > [( \frac{1 - \eta_{int}}{\eta_{int}}) \frac{1}{2 c } (e^{\Delta v / v_e} - 1)v_e^2 ]$, which can be rearranged as:

$\Delta v < v_e \ln [(\Delta T (\frac{\eta_{int}}{1 - \eta_{int}}) \frac{2c}{v_e^2}) + 1]$

You can plug some numbers into this equation to get some sense of the limitations it puts on any such system. For example, say our heat battery starts off at 0 K, and its temperature can increase up to 1000 K before the insulation can no longer keep a system that hot hidden from the outside, so $\Delta T$ = 1000 K. And say the specific heat $c$ is 0.9 kJ/(kg K), the same as that of the tiles on the space shuttle at 400 K according to this, which converted into SI units becomes 900 J/(kg K). And suppose $\eta_{int}$ is 0.8, which would be extremely good according to the table here ($\eta_{int}$ = 1 would mean no energy lost to heat at all), which would make $(\frac{\eta_{int}}{1 - \eta_{int}})$ equal to 4. Finally, suppose the effective exhaust velocity $v_e$ is 2,500 m/s, about the same as a typical solid rocket according to the table in the "examples" section of the specific impulse wiki article. With these numbers, the formula tells us that $\Delta v$ cannot exceed 2500*ln(1000*4*(2*900)/(2500)^2 + 1), plugging that into the calculator here gives a maximum $\Delta v$ of about 1916 m/s, just slightly under the amount of fuel needed to achieve escape velocity from the moon, and equivalent in fuel use to about 196 seconds of 1G acceleration. That doesn't seem like nearly enough for hitting a target in space that may be making unpredictable changes in its own velocity to confound possible pursuers even if it can't see them yet, and with the distances involved being very large. You can change some of those numbers and plug the altered formula into the calculator to see the effects, though.

Answer 2

Space combat in the "real world" is akin to a gun fight in a bull ring.

Lethal weapons, no cover. One shot, one kill.

The ranges will be determined by "Minute of Maneuver".

In shooting, rifles tend to have their accuracy measured by "minute of angle", which is a unit of degrees (circles have 360 degrees, degrees have 60 minutes).

By "Minute of Maneuver", I mean putting the shot in to zone where the target will be.

For (a contrived) example, when hunting, say, a deer, the vital area is quite large (say, 6 inches across). A rifle with a 1 MOA accuracy, can put a bullet within 1" of the aim point at 100 yards. So, with a 6" vital area, a deer could be shot with such a rifle out to 600 yards. With a 2 MOA rifle, it would be 300 yards.

When shooting at an airplane, you "know" how fast the plane is moving, you know how fast your bullet travel, so via ballistics, you have an idea how much to "lead" the aircraft to get your bullets on target. But that doesn't consider the airplane changing direction. The farther away you are from the target aircraft, the more opportunity there is for the plane to simply move out of the way.

In WWII, anti aircraft worked for things like bombers, because they flew in straight lines at steady speeds. They didn't work so well with dog fighting fighters.

So, you can see that the ranges will depend highly on the types of weapons involved. Because "range" only applies to ballistic weapons. A guided missile has "unlimited" range in this case. It can continually adjust its impact point.

Now consider the simple case, an asteroid flying through space. Here, the minute of maneuver is very small. The asteroid simply does not maneuver. So, it's "easy" to ballistically hurl an object that will impact the asteroid. Consider the recent comet landing. Yes, there was some actual maneuvering done, but most of the "attack" was ballistic. A very complicated orbit, but ballistic nonetheless. The error wasn't so much the comet, but rather the limitations of the precision of place the probe in to it's proper orbit in the first place. Error tends to add up over such long distances, so corrections were inevitable.

For a ship that can maneuver, you have to be close enough to overcome such maneuver. If you shoot a big warhead at, say, 10km/s, and the target is 5km away, the target has .5s to react, and maneuver the craft "out of the way".

In space, maneuver is expensive. On an airplane, you budge an aileron, and the plane quickly changes direction. In space, you have to apply thrust to maneuver. You can't rely on aerodynamics to provide "free" maneuvering.

How much thrust does the ship have? How long does it take to apply such thrust? All these factor in to the combat range. If a ship takes 10s to maneuver, then you can see that with a 10km/s warhead, you have an effective engagement range of 100km.

Now, of course, all these numbers are made up. And also, 100km sure seems REALLY CLOSE for "space combat".

But see how these numbers change with light speed weapons. In space, LASERs are, effectively, "ballistic weapons". With our previous example, your LASER "flies" at 300,000km/s. With our slow target before, that puts effective range at 3,000,000km. That's, (very) roughly, 10 lunar diameters. That seems like a more "realistic" "space range". Of course, getting a LASER powerful enough to do damage at 3Mkm is a different problem. You also start running in to limitations of accuracy. A 1000 foot ship (i.e. USS Nimitz) at 3Mkm is .006 degrees large. Can you imagine controlling a device with enough power to do damage with 6/10000 of a degree of tracking accuracy? Difficult to imagine.

So, "real" space combat. Is probably going to be pretty close. It's going to be extremely lethal. There will be no cover. Like I said, rifleman in a bull ring.

Answer 3

Targeting becomes harder with distance. At lightyear distances, even directed energy weapons are far too slow to possibly hit something (remember: a lightyear is the distance light travels in a year, so you'd be pointing a flashlight at where the thing was 2 years ago).

As distance decreases, maneuverability becomes more important (ie., quick fighters can dodge better than slow dreadnoughts; they dodge directed energy by seeing where the gun is pointed and being somewhere else).

There's a point, though, at which maneuverability no longer matters: the fighter absolutely cannot outmaneuver a missile, because the pilot wouldn't survive the G-forces. Directed energy basically can't miss.

At lightyear scale, it's also pretty easy to cloak: energy power level drops off with the cube of distance, so even the "brightest" ships we could build today are likely invisible from even a few lightminutes off.

As to what might affect sensors, that depends heavily on how the sensors work. Visible-light sensors can be blocked by black paint at extremely close range, for instance.

And, a side note: it's theoretically possible for a ship to go stealthy for a time by sinking most of its heat into some internal store, or by directing it away from the thing they're trying to avoid.

Answer 4

My first thought is: The question is almost impossible to answer, because we have no idea what technology will exist. Suppose you asked someone in 1492 what the maximum range was at which naval ships in 1900 would engage. Even if he was imaginative enough to consider radical advances in technology, it would be awfully difficult to predict what would actually come about.

That said, a few random thoughts:

A guided missile should be useful at ranges comparable to the range of your star ships. If you have ships that can travel hundreds of light-years without refueling, then it seems likely you could make a guided missile that would do the same.

Laser weapons (or phasers or whatever equivalents) would be limited by the dissipation of the beam. By definition lasers are highly focused, and presumably in the future they would be more focused still. But they must have SOME spread over distance, and eventually this will result in the beam having too little energy per square meter to be effective. Are there theoretical limits to how tightly focused a laser beam can be?

At long enough distances, aiming a laser would be an issue. If it takes minutes (or hours) for the beam to reach the target, then a ship could just make random evasive maneuvers to render beam weapons worthless. I say random maneuvers because, if your laser beam travels as fast as any signals from a sensor, there would be no way to detect the laser beam before it hit. In 21st century combat, a missile travels a whole lot slower than light or radar waves, so you can bounce a signal off a missile and detect that it's coming. But how would you do that with a laser? Even if you could bounce some sort of signal off a laser beam, that signal would have to be faster than the laser or you wouldn't get the signal that the laser was coming until the same instant it hit, which wouldn't help much.

As to stealth ... the way modern stealth aircraft work is by, (a) reducing the amount of heat they give off, (b) being made of materials that absorb radar waves, and (c) having shapes that bounce radar waves off in directions other than back to the radar transmitter. I'd think similar things would work in space. Of course it all depends on how their sensors work. But if they're transmitting a beam that gets reflected back when it hits something, then presumably you could have technologies that absorb that beam or reflect it in a different direction. If they have passive sensors that detect heat or other emissions, you could have techniques that reduce those emissions to the point where they are below the sensitivity of the sensors. It's difficult to say how hard that would be to do without knowing how the sensors work, and what sort of equipment has to be aboard the space ships for them to operate.

Additional thought

Several folks on here have stressed the difficulty of hiding a spaceship's heat. But that all depends on how hot the spaceship is, how sensitive the other guy's sensors are, and how far apart you are. If someone put a nuclear reactor in orbit around the Sun at the same distance as Pluto, how difficult would that be to detect from Earth? I don't think there is any detection device that exists today that could sweep the sky and instantly find such an object. I suspect it would take a long and meticulous search. Even if there was no attempt to hide the reactor at all, the amount of energy from that reactor that would reach detectors on Earth from such a distance would be tiny.

Ah, here's a way to think of it. Energy received is going to fall off with the square of the distance. So if the energy from your spaceship divided by the square of the distance is less than energy being received from stars divided by the square of the distance, then your ship would not stand out against the background noise of the stars. How much heat would a spaceship put out compared to a star? If it was, just to make up a number, 1/10,000 as much, then if the ship is more than 1/100 of the distance away as the star, the sensor would receive the same amount of energy from each. That wouldn't make it undetectable, I suppose: You could have a map of all the stars and any other known energy source and have a computer search for anomalies. But even a very hot spaceship would not stand out brightly.

Answer 5

Space is really big, and almost any star ship would be really tiny compared to everything else out there. A few light years is a distance at which we can't consistently detect planets, let alone a space craft.

Even one light second is really far away. For reference, the moon is just over a light second from earth. At this distance, a carrier would be half the size of Neptune in the sky. An X-wing would appear smaller than Pluto. Spotting something that looks smaller than Pluto in a reasonable time at that kind of distance might be possible if it's not trying to stay hidden, but it would be quite easy to give it a stealth coating and fling it towards your target at a relativistic speed without any active propulsion. Combat at a a few light minutes away would be akin to shooting at people on earth from the surface of Venus or Mars.

While it's possible that some technology could one day exist to allow such combat, the constraints and distances that such combat would occur under would be entirely dependent on that supposed technology.

Answer 6

Sixty degrees has been suggested here as a reasonably 'narrow' cone to hide one's emissions in.

You don't need a cone of that size. Shoot molten pellets that're insulated and covered in ice. Eventually they burn through the insulation, and before that have melted the ice (insulation isn't perfect). Firing those in all directions might be a convenient deterrent vs. heat-seeking missiles (like chaff or flares). Or, another solution to the cone problem is to use a refrigerating laser. I don't see why a radiator needs to 'snap off', there's no air resistance - max force you'd apply would be your own thrust operating in reverse direction (if you didn't rotate your ship).

However stealth is hard to get in space. In normal space, you're visible against the background, even if you're black - especially if you're moving (you start occluding more stars). You could try for a chameleon effect, but you'd need to know where your observer is, in order to calibrate your brightness (falls off with cube of the distance, and you're a lot closer than the star... to get the same output at his location, you'd need to know his location, and hope he doesn't have a sensor net up. There are some bending light solutions which might work for this. I've not investigated them fully, but they're int he popular science press - I've seen them in the last couple of months. If those actually work, then it'd make life a lot easier for a moving ship.

You can, however, hide behind things. And you can use micro-drones, micro-missiles, and micro-sensors to remain undetected at closer ranges. I expect that you'd end up with remote networks doing much of the fighting, and ships with bio-crew to be so far apart that they're hard to hit with direct energy weapons - probably usually hidden behind chunks of matter in the system (which are hopefully not mined), and moving other chunks of matter to create more hide-behind spaces; Maybe with lots of remote-operated tugs.

--

An aside/comment on the deleted answer: I don't see what's wrong with answering a question with questions; it'll help OP firm up the details, or at least pick some things or offer a range of solutions so they can get a better answer from all of us (ie: it can improve the original question). But yes, answer is worded pretty snidely.