What Is Orbital Decay?
Orbital decay is the gradual loss of altitude a satellite suffers from atmospheric drag — even hundreds of kilometres up, there's just enough air to slow a satellite down, a little more on every single orbit.
See real decays on the live globe
Why low orbits don't last forever
There's no hard edge to space
The atmosphere thins out gradually rather than stopping — even at 400 km, where the ISS flies, there's enough residual air to create real drag over millions of orbits.
Drag costs altitude, not speed
Counterintuitively, drag makes a satellite speed up as it settles into a lower, faster orbit — right up until the descent steepens and it comes down for good.
Space weather changes the rate
Solar activity heats and expands the upper atmosphere, thickening the air low satellites fly through — the same geomagnetic storms AstroSat's Kp index tracks measurably speed up decay in low orbit.
It shows up in the numbers
AstroSat's /statistics page tracks how many satellites leave the catalogue day by day — decay and re-entry are a real part of that churn, not just new launches.
How fast an orbit decays depends on altitude, the satellite's mass and cross-sectional area (its ballistic coefficient — the same BSTAR term stored in every TLE), and how active the Sun is at the time. A few hundred kilometres up, decay can take days to weeks; the ISS needs regular reboosts to stay at ~400 km at all. Well above ~600 km, natural decay can take decades to centuries, which is exactly why modern debris-mitigation rules increasingly require satellites to deliberately deorbit themselves at end of life rather than leaving it to chance.
Frequently asked questions
What causes orbital decay?
Atmospheric drag. Even in low Earth orbit there's a trace of atmosphere, and it slows a satellite down a little on every pass, gradually lowering its orbit until it re-enters.
How long does orbital decay take?
From days at very low altitudes to centuries above about 600 km — it depends heavily on the satellite's altitude, shape and mass, and on solar activity.
Why does the ISS need reboosts?
At ~400 km it experiences enough drag to lose meaningful altitude every month. Periodic engine burns push it back up rather than letting it decay toward re-entry.
Does space weather affect decay?
Yes — solar activity heats and expands the upper atmosphere, increasing drag on low satellites. A strong geomagnetic storm can measurably speed up decay for weeks afterward.