What Is Orbital Decay? The Slow Fall to Re-entry

Quick answer

Orbital decay is the gradual lowering of a satellite's orbit, chiefly by atmospheric drag, ending in re-entry. Decay rates hang on altitude and solar activity: months below 300 km, decades at 600 km, centuries near 800 km — the clock behind every re-entry forecast.

📘 Full definition✓ Reviewed 2026-09-07

Orbital decay is the progressive loss of orbital energy that spirals an object downward until it re-enters. In low Earth orbit the driver is atmospheric drag: even hundreds of kilometres up, the whisper-thin thermosphere steals momentum every second, most effectively at perigee, so eccentric orbits circularise as they shrink. The rate is brutally sensitive to altitude — roughly months of lifetime below 300 km, years around 400 km (the ISS reboosts routinely), decades at 600 km, centuries by 800 km — and to the Sun, because solar activity heats and inflates the thermosphere; a strong storm can multiply drag overnight, visibly steepening every low orbit's decay and famously claiming a batch of freshly launched Starlink satellites in 2022. Decay is tracked through the TLE drag terms and falling period, feeding re-entry predictions. It is also the environment's only self-cleaning mechanism — and the reason disposal rules push dead satellites down into the fast-decay zone.

200 km

Days to weeks

400 km

1–2 years

600 km

25 years

800 km

100–200 years

Understanding Orbital Decay

The solar heartbeat in every low orbit

The thermosphere breathes with the 11-year solar cycle and gasps during storms: extreme ultraviolet flux and geomagnetic energy can raise density at 400 km several-fold. Decay statistics inherit that rhythm — solar maximum years scrub debris from LEO measurably faster, while quiet years let it linger. Re-entry forecasting is therefore partly space-weather forecasting, and every prediction carries the Sun's error bars.

Reading decay in the data

A decaying object's telemetry is public: mean motion creeping upward, the TLE's drag term swelling, apogee and perigee converging. Trackers watch the "n-dot" acceleration to rank re-entry candidates weeks out, then hand the final days to specialised prediction centres that fold in live solar indices. The same signatures, appearing unexpectedly on an active satellite, can reveal a failure — or a deliberate deorbit under way.

Frequently Asked Questions

Why do re-entry predictions stay uncertain to the end? ▼

Because the final descent depends on an atmosphere that varies with solar weather, the object's unknown tumbling attitude, and its drag profile. Errors compound over each remaining orbit: a 10% drag misestimate a day out moves the re-entry point thousands of kilometres — which is why windows narrow only in the final hours.

Do orbits above the atmosphere decay at all? ▼

Higher orbits feel other slow influences — lunisolar gravity, radiation pressure, Earth's asymmetries — that reshape rather than shrink orbits; there is no meaningful drag sink at GEO. That permanence is exactly why GEO disposal means boosting to a graveyard orbit instead of down.

Can decay be useful? ▼

It is the disposal plan: end-of-life burns lower perigee into the fast-decay regime so the atmosphere finishes the job within the mandated window, and drag-augmentation devices — sails, tethers — accelerate the process for small satellites without propulsion.

Understanding Orbital Decay

The solar heartbeat in every low orbit

Reading decay in the data

Frequently Asked Questions

Sources & References