I'm writing a story to be set around the late 1980s or 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 U.K. The U.K. 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.