You cannot land on a planet or moon, or leave it - including, notably, Earth - without passing through its surrounding orbital space. This gives orbital space great strategic importance.
I have used 'orbital space' a good deal on this blog without ever defining what it means. Any formal definition would be somewhat arbitrary (like 'the threshold of space') but generally a planet's orbital space is the region dominated by its gravity. Think of it as close enough that you orbit the planet rather than just taking up a nearby solar orbit. (Or orbit a moon instead of its parent planet.)
For orbital space to have distinctive characteristics, major orbit change maneuvers must also require a substantial effort, a delta v of at least a few hundred meters per second - enough that chemfuel burns are costly in propellant consumption, while high specific impulse burns are time consuming.
Ceres, with an escape velocity of 0.51 km/s and low orbit velocity around 0.35 km/s, is about the minimum size for strategically significant orbital space. Neatly, and not entirely by coincidence, this corresponds to the minimum size for a 'dwarf planet,' shaped (literally!) by geological forces.
Significant zones of orbital space thus surrounds the eight major planets, the Moon, Ceres itself, the four big moons of Jupiter, Titan and six other moons of Saturn, four moons of Uranus, and Triton, along with Pluto and a growing list of outer system objects. We are interested in visiting most of them, and might one day be interested in fighting over them. (This last may not really be very likely, but it is possible, and makes for good thud and blunder space stories.)
Earth and Mars have escape velocities of several km/s, on the same order as interplanetary transfer speeds. (Escape speeds from the giant planets are higher still, but in strategic terms their moon systems are like miniatures of the Solar System, and a somewhat different strategic beast.)
This means that typical encounter speeds in Earth and Marsr orbital space are fairly high, even after making the burn from interplanetary transfer orbit. In low Earth orbit, encounter speeds can range from 4 km/s for circular orbits with a 30 degree difference in inclination, up to 22 km/s for a retrograde encounter just below escape velocity. Even at lunar distance a head-on encounter at escape velocity means a relative speed of 2 km/s.
Which makes orbital space a kinetic shooting gallery. A defender can pre-position kinetic target seekers as 'mines' on retrograde orbits, while an attacker coming from deep space needs hardly more than a tap to send kinetics onto a retrograde approach. Moreover, so long as they are below escape velocity, kinetic target seekers will not hurtle off into the void, but keep coming around.
What applies to kinetics also applies to ships. Ships in orbital space do not encounter each other as ships on crossing orbits in deep space do, one flash-past and off they go into the void on their separate paths, needing dozens of km/s of delta v to reverse track and re-engage. Ships orbiting a planet, so long as they are below escape velocity, will swing back around for repeated passes.
And it gets better. Orbital space (specifically, low orbit) is the domain of the Oberth effect. Imagine a target seeker in an elongated elliptical Earth orbit, so that it whips around perigee at 11 km/s, a shade under escape velocity. Let it have a small chemfuel booster good for 3 km/s of delta v. (The booster will have about twice the mass of the target seeker itself.)
Fire the booster at perigee and the target seeker is booted to 14 km/s, well above escape velocity. And its departure speed 'at infinity' will be 8.7 km/s (14 squared - 11 squared). Any target coming from deep space will have its own approach velocity, making for encounter speeds upwards of 12 km/s. A similar boot from low Mars orbit gives a departure speed of 6.2 km/s, and encounter speeds upwards of 10 km/s.
Finally, orbital space has the planet itself at the center of the maelstrom, giving spaceships a rare opportunity to crash, and providing a big exception to the rule that 'everyone sees everything.' You don't see anything through a solid planet or moon, and remote sensor probes can be burned out.
All of this ought to make orbital space militarily ... intriguing. Maneuvering there is more complex than in 'flat' space. Kinetics can be deployed cheaply and effectively as a sort of mine warfare.
And it matters, because a large proportion of strategic objectives will surely be in some planet's or moon's orbital space - or on the planet, subject to attack or blockade by whoever controls its orbital space. In any setting where planets are important, a good case can be made that most combat will take place in their orbital space.
Serious space warfare games, like Attack Vector, respond to all of this potential by avoiding orbital combat like the plague. This is for good reason. No one has yet figured out to sim convincing orbits in a board game, and not for lack of trying. This is no bar to fiction writers, who only have the problem of getting things right, or at any rate convincing.
But there is one other important consideration for orbital combat in a setting. Most of the interesting complications belong only to the near or midfuture, and become progressively less significant at higher techlevels. The planet or moon remains a physical obstacle, but its surrounding winds and currents matter less to steamships, so to speak, than to sailing ships. The shooting gallery effect matters only if kinetics approaching at 3-15 km/s are effective weapons, while orbital maneuvers are trivial for ships with torch drives.
So if you measure speeds as a fraction of c, don't have the captain fretting over approach orbits and defensive orbital mines.
The image is from commenter Luke, via Atomic Rockets. (It actually shows a laser zap, not a kinetic strike, but still portrays realistic orbital combat.)
Related posts: See the June, July, August, and September archives for previous posts in this series, plus the battles of the spherical war cows. And a much earlier post on space fighters.