Astronomy & Celestial Mechanics
the sky, computed
Concept

Space sustainability — where it appears

The aim of using Earth orbit without degrading it for later users, by limiting debris, removing dead satellites and controlling re-entries. Its rules are written per object, while its risks add up over whole fleets.

Named by 2 essays across one field — each of them below, with the objects they name alongside it.

Too slow to shatter. The kinetic energy a projectile brings per gram of a 2,000 kg satellite, against the impact speed on logarithmic axes, for projectiles of 1, 10, 100 kg. Above 40 joules per gram (dashed) the standard breakup model counts the collision as catastrophic — the whole target is shattered; below it, only a crater is made. The speeds needed are 12.6, 4.0, 1.3 km/s respectively. In the geostationary ring, where nothing crosses faster than about 0.8 km/s (shaded), only a projectile of hundreds of kilograms — another satellite — can shatter a satellite; in low orbit, at ten kilometres a second, a kilogram can.

A ring where nothing leaves and little shatters

Every argument about orbital debris in low orbit ends in the atmosphere, which pulls fragments down in years or centuries. The geostationary ring has no atmosphere, so its removal term is exactly zero and anything left there stays for millions of years. It is saved by a second fact of the same geometry. Everything in the ring moves at nearly the same speed in nearly the same direction, so collisions there are slow, and a slow collision usually leaves a crater where a fast one leaves a cloud.

spaceflight · Orbital debris A limit per object is not a limit on the sky. Expected casualties a year against the number of uncontrolled re-entries a year, for objects each at the limit of one in ten thousand, at one in a hundred thousand and at one in a million. A fleet of 40,000 satellites replaced every five years returns about 8,000 a year; at the limit it would expect 0.8 casualties a year, and at a tenth of the limit 0.08 — one every 13 years. Every object can pass the rule while the total grows without bound.

A risk limit written for one satellite at a time

A satellite left to fall uncontrolled must have less than a one-in-ten-thousand chance of hurting anyone, and the arithmetic behind the rule is simple: the area of the debris that survives, times the density of people under the orbit. The rule was written when a few dozen large objects came down each year. Applied to a fleet that returns thousands, every satellite can pass it while the expected number of people hurt grows in proportion to the fleet.

spaceflight · Orbital debris

Named alongside it

The objects these essays reach for when they reach for this one.

Orbital debrisAtmospheric dragBreakup modelCatastrophic breakupCollision velocityGeostationary orbitGraveyard orbitInclination cycleKinetic theoryOrbital lifetimePost-mission disposalSatellite constellation

All concepts