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Home Academy Debris & Re-entry Conjunctions (close approaches)
LESSON 02 OF 6

Conjunctions (close approaches)

Beginner ~7 min Slide deck Free

A conjunction is a predicted close approach between two catalogued objects. Thousands happen every day — most are harmless. The challenge is identifying the tiny fraction that might actually be dangerous, and deciding whether to maneuver.

A conjunction is a predicted close approach between two catalogued objects. Thousands happen every day — most are harmless. The challenge is identifying the tiny fraction that might actually be dangerous, and deciding whether to maneuver.

What this lesson covers

What defines a conjunction

A conjunction occurs when the predicted trajectories of two objects pass within a defined screening distance — typically a few kilometres — within a specified time window.

Conjunction scale

Most CDMs are informational — the predicted miss distance is well beyond the combined uncertainty. Only a small fraction require analysis, and fewer still trigger a maneuver decision.

Miss distance isn't enough

A 500-metre miss distance sounds alarming — but it might be perfectly safe. A 5 km miss distance might actually be more dangerous. The difference is uncertainty.

Key facts

💡There are ~50,000+ catalogued objects. The number of potential conjunction pairs scales quadratically — that's billions of checks per day.
💡Pc > 10⁻⁴ (1 in 10,000) is a common threshold for serious analysis. Pc > 10⁻² triggers red-flag maneuver decisions.
Conjunctions are common. Dangerous ones are rare. Uncertainty — not miss distance — determines risk.

Operators receive thousands of alerts weekly. The skill is filtering signal from noise using probability metrics, not reacting to raw distance numbers.

All lessons in Debris & Re-entry
01What counts as debris~7 min02Conjunctions (close approaches)~7 min03Re-entry prediction (why it shifts)~7 min04Kessler Syndrome (the cascade risk)~6 min05Notable debris events (history)~6 min06Debris mitigation (guidelines & tech)~7 min
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