Space is big. You won't believe how big space is. Look, you might think it's a long way down to the shops, but that's peanuts compared to space... (HHGTTG)

Okay, I'm not in a very serious mood today. But it does kinda bug me how space combat is often portrayed in media. I'm going to pick on Star Wars a lot here, purely because it's the best known offender. Even the settings which try to get some stuff right (hello, B5!) get a lot wrong too.

None of this means that the show's aren't enjoyable. Suspension of disbelief is necessary for all media consumption...

Let's start with Episode 1. The Trade Federation is blockading the planet Naboo by having space ships hanging above it. But how do you blockade a planet?

To be clear, the aim of the blockade is to stop ships either reaching or leaving the surface, with "deep space" being at the other end of the journey. If you can just fly round the blockade, it's basically useless.

Our planet has a surface of a little over five hundred million square kilometres. 500,000,000 km^2. Depending how far above the surface our blockading ships are, the sphere they're occupying will be bigger than that, but perhaps less than you might think. We don't know how big Naboo is (perhaps it's smaller than the Earth) so let's just go with the 500 million figure.

So, if each ship is responsible for blockading a square kilometre of space (and the film makes them look like they're CLOSER together than that) you'd need 500 million ships. Which is roughly 500 million more than we actually see... (To put that figure in context, that's roughly one ship for every three cars on Earth at the minute...)

Even if you have that number of ships, there's another problem. Objects don't hang in space, they orbit a centre of gravity. Star Wars tends to have ships hanging statically in front of a static planet. Easier to render on film, sure, but wrong. Generally, objects in space move, orbitting the planet. Where and how fast you orbit is a function solely of height above the planet. Doing anything else - such as hanging statically - is actually very difficult, involving constant fuel expenditure to correct the natural tendancy of an object to follow it's orbit. (Okay, Star Wars has a real problem with energy consumption, which maybe I'll come back to another day. But probably not...)

There is, of course, one orbital height where you do stay static compared to a point on the ground - the geostationary or Clarke orbit. However, that (a) quickly becomes very crowded with useful things like communication satellites, and (b) is a ring around the planet's centre, and (c) only is true relative to the planet's equator 'cos that's the bit of the planet which rotates around the planet centre. Again, basically useless for a blockade. Fly towards the planet from near either pole, and you're thousands of kilometres from the nearest object in Clarke orbit.

So, the Star Wars style blockade requires hundreds of millions of ships. They're either spending huge amounts of fuel to constantly fight their tendency to orbit, or they're orbitting the planet in a series of rings at angles to each other - some going pole to pole, some over the equator, and everything in between. The former is difficult to achieve with any reasonable energy model, the latter needs even more ships as there's going to be a large number of overlaps in every orbit. You go from one ship per square kilometre to sometimes two or more per square kilometre, depending on where they are in their orbit.

How do you blockade a planet? Well, you don't. It's incredibly inefficient and almost certainly beyond the resources of any culture. What you can do is control access to the population centres by troops on the ground backed by air support. Which is exactly what we see in the original Star Wars film...

Before I move, let's pick on Babylon 5 a little... Series 4 shows that Earth has a "planetary defense grid". In a lot of ways this is quite good - the grid implies that we're looking at multiple over-lapping orbits, what we see on screen are relatively small objects (at least compared to Star Destroyers or Lurce Hulks...), and there seem to be a lot (but not enough) of them. But it still seems like it wouldn't work well enough to be worth the enormous cost. Why? I'll answer that in the next section.

Space is big... Okay, I've done that quote already. But space is also really empty. Lots of nothing with realtively little in it. What does that mean for space combat?

There's two considerations here. What can you see and can you hit it effectively? The first is relatively straight forward. Given time, we can see large objects literally billions of light years away. The James Webb Telescope has produced many beautiful images of star-size objects at that range. Equally, space is big, and we're still discovering new rocks and lumps of ice within our own solar system. Ships might be somewhat easier to see - they're consuming energy and that potentially makes them more visible. "Potentially" is carrying a lot of weight in that last sentence. An attacking ship probably doesn't want to be seen, so we'll see an arms race between better technology for spotting ships and better ways of hiding from enemy sensors. Off the top of my head, an attacker could try ensuring exhausts point away from the enemy, using the sun to swamp your energy output, or even just pointing ships in the right direction out of effective sensor range and then drift into combat range.

Eventually, though the enemies will see each other, and potentially at a great distance. Can they actually do anything about it? Probably not. At least, not yet.

Okay, let's assume some sort of beam weapon (read - powerful laser or other light-based weapon) 'cos nothing is as fast as light. If we try to hit a target a million kilometres away (and the distance from the Sun to the Earth is more than a hundred times that number) our beam weapon will take a bit over three seconds to reach the target. We're firing not at the target but where the target will be in three seconds, based on where it was three seconds ago. Can our target move out of the way in that time? Well, it can't see the beam coming, but it can move randomly, perhaps enough to not be where we think it will be. There's also ablative armour, shielding, chaff ahead of the flight path, etc... I'm going to go ahead, though, and say that a million km is probably about maximum engagement range IF you have beam weapons. Any weapon with a physical component is going to reduce that distance dramatically.

Next, do we care if we miss? If I'm attacking Earth, possibly not. This is where B5's defense grid fails. An attacker trying to punch a hole through that grid either damages part of the grid or they damage the planet behind it which they're attacking anyway. The defender may care. No one wants to be saying "Well, the good news is we wiped out the alien invasion, but the bad news is we also destroyed our own Mars colony". Keeping something your enemy might care about behind you is an obvious tactic.

We can divide space combat into three core steps. First we need to *find* the enemy. If they're defending a planet, that's fairly easy, but one side is presumably coming from "far away". If both sides are on the same planet, space combat becomes a specialist case of air support. There's a race here between techniques and technologies for spotting enemies and for hiding. At some point, though, both sides will be aware of the other. Given that space is big and empty, there's a good chance this awareness will happen well before either side is really able to do much about it.

Step two is therefore engaging the enemy. As hinted at above, you really don't want to engage someone attacking your planet at a range where they can do damage to the planet you're trying to defend. Engagement needs to happen outside the enemy's effective weapon range or you've already lost the battle.

Bringing the enemy to battle isn't easy. Yes, you know where they are, but presumably they also know where you are. Is one side able to move significantly faster than the other? Is one more manueveurable? Does one have significantly better sensor range? And everything is based on out of date information. It takes light hours to cross the solar system. Given that movement in space is based on expensive (in energy terms) changes in orbit, you may need to commit to a course of action based on what your enemy was doing yesterday...

All of which makes for interesting decisions, but it's also slow. Space travel, realistically, will take months. You might fire a missile and wait weeks to know if it hits. Imagine being on a ship targetted by such a missile - you know you're dead when it hits, but you have days before it gets there.

One side is also likely to want to avoid battle. Perhaps they're numerically or technologically inferior, so actual battle is a foregone conclusion. Perhaps their objective isn't served by direct attack. Declining a fight is often going to be preferable for one side or other.

And, finally, combat happens. So much here is going to depend on the technologies available to each side. One thing I will point out is the speed at which this happens. Travel between planets within the solar system seems slow because of the vast distances involved. But ships travelling in opposite directions will pass each other in seconds. Ships hanging in space exchanging fire just isn't going to happen. Ships will close, pass, and be gone. Turning round for a second attack isn't going to happen. It'll be slightly different with a pursuit scenario, but not significantly. Ships will reach effective combat range, start attacking, and continue until the enemy is dstroyed or out of range again.

Which, interestingly, leaves The Last Jedi as a passing decent model of space combat. The middle section with the First Order following the Resistance fleet and picking off ships one at a time while the Resistance tries to outmanueveur them is pretty good. The two fleets are shown far too close together for my taste, but that's possibly nitpicking.