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I'm writing a story to be set around the late 1980s/early 1990s. The story opens with a spaceship crashing to Earth, hitting and destroying a large part of central London. The idea is that, before the spaceship actually hits, it will be detected by a missile strike detection system and the operators will think that a nuclear missile has been fired at the UK. The UK will then fire a retaliatory strike at Russia, thinking that they are responsible for the London strike, thus setting off the whole mutually-assured destruction thing and triggering a nuclear war that leads to the collapse of society.

My question is this: how would a nuclear strike have been detected in that era? And could a spaceship crashing (out of control, but travelling in a straight-downward direction) theoretically be detected on such a system? My plan at the moment is that the ship could have a radiological signature, to really convince the detection system's operators that they're looking at a genuine nuclear strike, and it will only be detected once it's entered Earth's orbit (so they won't have tracked it across the solar system or anything like that, and won't realise what the object they're tracking really is) but a better idea of how the real-world systems worked (or still work, if there has been no change) would really help.

My opening scene currently has a technician in a monitoring station picking up the ship on his equipment, and telephoning a superior officer to inform them but I'm not sure how accurate that is.

What kind of ICBM/missile tracking tech did they have in the 80s?

Sonic boom Missiles really haul the mail, and the sonic boom is fairly distinctive. With listening posts throughout the world/nation, it's possible to track the movement of the missile. With fast enough computers (a bit of a problem in the 80s), this could be used to extrapolate an actual position (sonic booms happen after the fact).

Heat Missiles are notorious polluters with all that rocket thrust to keep them going. Consequently, there's a boatload of heat. To be honest, there's heat all over. A house fire can produce more therms than the exhaust of a rocket, so we're back to computers doing things like analyzing the path and speed of the heat source. Fast + ballistic = bad. Stationary and sea-level = call the fire department.

Radar Missiles are small, but not that small. Radar can pick them so long as the radar system's refresh time is fast enough to capture their passing. Some radar installations in the 80s could do it, but most couldn't. Do you remember that lovely line that sweeps around 360° on the old WWII radar screens? That's the position of the rotating antenna that's emitting/detecting the radar signal. We don't draw the line anymore, but it's still there, and if it can't get around fast enough, it can't see the missile. (Note that phased array radar do this electronically along a narrow arc, which makes them fast. They're great for seeing what's in front of a plane, they're less useful for watching a horizon, but it's better than what it was.)

Visual In the end, machines make mistakes. Nothings more useful than a pair of eyes and a set of binoculars. Pro: more trustworthy identification. Con: You don't have long before... oh, cra... BOOM!

Radiation Finally, though unlikely to have been used regularly (or at all) during the 80s, the tech did exist to track radiation sources. Usually that's not an issue until after the missile has hit, but you could believably use it in a story (hey, if The Blacklist can suggest six Soviet briefcase nukes hidden in the U.S. in the 60s, then we Americans will suck down just about anything).

Could any or all of these be used to track a spaceship? Sure! The biggest problem would be the confusion over the fact that it's the wrong kind of ballistic path. It's a "fall from orbit" ballistic path rather than a "launched from my enemy's backyard" path. That'd confuse NORAD for a bit, but it's quite believable that they'd see it.

Yeah, but what about today?

I doubt any of this could be done in the 80s. I'd be mildly surprised of some of it could actually be done today. But for the sake of being thorough.

Light reflection Objects in the sky reflect/occlude light. It's one of the ways we detect new stars and planets — by looking for light that should be there, but isn't, or that shouldn't be there, but is.

EM emissions Anything that uses electronics emits EM noise. To a degree this can be blocked, but sometimes it can't. Add to this that thermal differentials, metallic stress, communications to/from/with the missile... all of this could be detected and triangulated.

Humidity AKA contrails, the condensation of water behind the high-heat/high-pressure-sudden-cold/low-pressure exhaust of a jet/rocket engine. Whether they're visible or not, the change in humidity is there, and when light passes through it, that light changes. (And if you can see it... see "Visual," above.)

There have been several "close calls" in Soviet and US history regarding false detection of missile launches or strikes. The so-called "Great Storm" in 1967 is one example, sunlight reflecting off clouds in 1983 is another.

Detecting missile launches in late 1980s / early 1990s is extremely different than today - especially on the soviet side of the fence (the US was more forward-moving with computerized technology in the 80s/90s than the soviets, but the details of that are FAR too tedious to explain in answer to this question). The "old" (meaning pre-computerized) systems relied on radio transmissions, random bursts of light (ie explosions that resembled the bursts similar to that of rockets taking off), and so on. More advanced computerized systems came with more precise detection systems (and therefore less error rates), but these only came to the US in the early 1980s (with President Reagan's so-called "star wars" advancements), and to Russia in the mid 1990s.

So before the computerized advancements, the "crash" of a spaceship - if it caused a small or medium-sized explosion near a known US missile site - would almost certainly have triggered concern from the Russians as a nuclear launch.

Another option is an alien craft that, by nature of operation, would interfere with radio operations on either (or both) sides of the fence, much like the 1967 event. This is not far-fetched; a complex space-craft might send radio waves cross-galaxy in a way that requires such large amounts of energy it blasts through the noise floor of radio frequencies.

Yet another option is where the spaceship hits - if it hits a known target (like the White House, Congress, Supreme Court - or on the USSR side of the fence the Kremlin, Baku, etc) - might be wrongly perceived as an attack.