The motion of objects in orbit is counterintuitive to brains that evolved on Earth. Hollywood space battles typically resemble World War Two dogfights or 19th-century naval warfare, but real spacecraft manoeuvre in a manner utterly unlike aircraft or ships.

For a start, a satellite’s speed and altitude are inextricably linked, so it cannot ‘speed up’ without getting closer to Earth, nor ‘slow down’ without moving further away. The most distant satellites orbit at a relatively unhurried 3 kilometres per second, while their atmosphere-hugging low-orbit counterparts gallop along at almost three times that. All travel faster than a speeding bullet.

The enormous distances and speeds at which satellites move make pinpointing a satellite’s position and predicting its movement a technical feat even for an operator whose own satellite ‘wants to be found’. Tracking a satellite is a full-time job that requires constantly piecing together ‘sightings’ from cooperative partners around the globe.

The space domain also differs radically in scale. The lowest-altitude satellites orbit within the thin upper reaches of the atmosphere (where air resistance eventually slows them enough that they plummet), while the most distant orbit in widespread use is 36,000 kilometres from Earth—more than twice as far as Australia is from London by air. By comparison, the highest-flying jet aircraft record approached 38 kilometres, and the average depth of the world’s oceans is less than 4 kilometres. 

All this makes satellites awkward targets. A missile launched from the ground would take hours to reach a satellite in a high orbit, while those nearer Earth are faster and usually smaller.

In other ways, however, the physics of spaceflight make satellites vulnerable. Objects in orbit are not ‘weightless’. Rather they are perpetually falling, albeit falling ‘around’ the Earth because they are moving so fast horizontally. In this state of free-fall, changing direction isn’t much easier than it is for a person falling from a building. That makes satellites, in a sense, immobile: once they are installed on a given orbital ‘track’, they cannot generally leave it. Most do carry small amounts of propellant with which they can gradually nudge themselves into different orbits over time, but this is slow and their range of motion is heavily constrained—think of steering a speedboat with a canoe paddle—and because satellites must be light enough to launch, they can’t carry much fuel. This means that if an adversary does somehow manage to pinpoint a satellite, that satellite cannot run away.

Weapons in space

Space may in principle be ‘the ultimate high ground’ allowing domination of Earth, but realising this potential in practice is far beyond the reach of current or near-future technology. Physics dictate that armed satellites in orbits low enough to strike ballistic missiles or terrestrial targets necessarily move at high speed, so in order to ensure continuous coverage of a given location a large number of them would be needed. Global coverage would be a national undertaking on the scale of the Great Wall of China, yet without global coverage there would be little advantage over much cheaper terrestrial missile defence systems. The technical barriers and astronomical cost will be insurmountable for the foreseeable future, but the idea is so appealing that it will likely never die.

Deploying space-based weapons for use against other satellites is perhaps more feasible. All objects in orbit move at bullet-like speed and carry so much kinetic energy that collisions between them are typically explosive. That means any orbiting object that can be steered is a potential space-to-space missile.

It is impossible for an aggressive spacecraft to sneak up on a satellite—empty space by definition affords no cover—but it is also impossible for a country to determine the nature of any spacecraft that approaches one of its satellites, since most are far too distant for visual imagery. Radar can at best reveal a spacecraft’s approximate location, size and movement, but not who it belongs to, what it’s carrying, or even where it came from unless it was tracked continuously from the moment it was launched. That’s why the world’s current space defence efforts focus largely for now not on putting marines on the Moon but simply on ‘space situational awareness’—figuring out what’s up there, where it is, and what it’s doing.

The challenge for an aggressor would be to position its attack satellite onto an orbit where it could quickly close the distance, without being blatant about it, while preserving enough precious propellant to achieve the final manoeuvre. It would have to locate its target with extreme precision and then ram it, burn it with a laser, spray some kind of chemical onto it or use a robotic appendage to maul it—all technically feasible but extremely challenging. For comparison, it takes hours for a spacecraft to rendezvous with the International Space Station, and that’s with complete transparency and the space station ‘trying’ to be caught.

Ground-based anti-satellite missiles are more straightforward, and the US, Russia, China, and India all have them, but using them would be self-destructive in that the resulting debris would become a hail of de facto bullets whipping around the Earth on unpredictable trajectories, smashing into and wrecking satellites belonging to friend and foe alike.

The minefield of ‘space junk’ left by past missile tests, accidental collisions, and the hulks of dead spacecraft already poses a more serious threat to space security than hostile action does. 

Hostile action, for the foreseeable future, will focus not on the physical bodies of satellites but on their raison d’etre: the transmission of information. Cyberattacks and radio jamming are cheaper, more precise, less escalatory, and do not devastate the environment for centuries to come. Space warfare doesn’t look like Star Wars. It looks like a computer saying ‘no’.