Space Debris Decay Calculator
How long an object stays in low Earth orbit before atmospheric drag brings it down — from its altitude, mass and cross-section — and whether that meets the 25-year disposal guideline.
Even at 400 km there is enough atmosphere to slow a satellite; drag lowers the orbit, lower means denser air, and the decay accelerates until re-entry.
How the space debris decay calculator works
Even at 400 km there is enough atmosphere to slow a satellite; drag lowers the orbit, lower means denser air, and the decay accelerates until re-entry. The rate depends on the ballistic coefficient — drag area over mass — and on density, which falls roughly tenfold every 50 to 100 km.
This integrates the decay through a standard mean atmosphere. Real lifetimes vary by a factor of two or three with the solar cycle, which puffs the upper atmosphere up at solar maximum. At 800 km the answer is centuries, which is the whole debris problem.
Formula: da/dt = −ρ (C_D A / m) √(GM a), integrated from altitude down to 150 km
Worked examples
| Inputs | Orbital lifetime (years) | Note |
|---|---|---|
| A 100 kg satellite at 400 km | 0.385 | about a year |
| The same at 800 km | 190.938 | centuries |
| A 1 kg cubesat at 500 km | 3.017 | years |
FAQFrequently asked questions
How long does a satellite stay in orbit?
Months at 300 km, a few years at 400, decades at 600 and centuries at 800 for a typical density. Above 1,000 km, effectively forever on human timescales.
What is the 25-year guideline?
The international norm that a spacecraft in low Earth orbit should re-enter within 25 years of the end of its mission. Several regulators are moving to five.
What is the ballistic coefficient?
Drag coefficient times area over mass — how strongly the atmosphere grips the object. A light, wide object decays fast; a dense compact one slowly.
Why does the solar cycle matter?
Ultraviolet from an active Sun heats and expands the upper atmosphere, raising density at a given altitude several-fold. Lifetimes can differ by two or three times between solar minimum and maximum.
Does the model handle elliptical orbits?
No — this is for near-circular orbits, which is most satellites and debris. An elliptical orbit decays mainly at perigee and needs a different calculation.
Where these figures come from
- NIST — CODATA 2018 fundamental physical constants — G and the speed of light
- IAU 2015 Resolution B3 — nominal solar and planetary conversion constants — the astronomical unit, solar mass and planetary radii
- CSIRO Space and Astronomy — Australia's national science agency
Last checked: September 2026. Constants are CODATA 2018 (G, c) and IAU 2015 nominal values (solar and planetary parameters).