Drag & decay

LEO Drag & Decay Timeline

How long does a satellite stay up? Atmospheric drag at 400-800 km is small but relentless. Plot the curve.

// King-Hele simple-atmosphere drag, exponential density table fit to NRLMSISE-00. solar activity tiers, mass/area/Cd inputs. orbit-mean approximation. flight design needs MSIS-90/2000 + numerical propagation.

AI explainer Run the numbers, then let ENKI break down what they mean — diagrams and all.
How this model works & what it omits

Every satellite in LEO is slowly losing altitude. Even at 600 km, the atmosphere has enough density (~10⁻¹³ kg/m³ at moderate solar activity) that the few grams of mass swept up per second by a typical smallsat translate to several metres of altitude lost per day. Over years, the loss compounds; eventually the orbit dips into denser air below ~150 km, the satellite tumbles, and reentry happens within hours.

This tool uses the King-Hele simple-atmosphere drag formulation: da/dt = -CD · A · ρ(h) · √(μ · a) / m, integrated numerically with an adaptive timestep until either reentry (100 km threshold) or the 50-year simulation horizon. Atmospheric density comes from a 15-point reference table fit to NRLMSISE-00 (Picone et al., 2002) annual-mean output at three solar-activity tiers (F10.7 ≈ 70 / 150 / 230). Between table entries, density is interpolated linearly in log space against altitude.

The ballistic coefficient BC = m / (CD · A) is the single most important parameter: high BC (heavy, small frontal area) decays slowly; low BC (light, large frontal area like deployed solar panels) decays fast. A 3U CubeSat at 4 kg / 0.03 m² has BC ≈ 60 kg/m²; a deployed ISS-class platform at 100 t / 2500 m² has BC ≈ 18 kg/m². Solar activity multiplies density by roughly 5× from minimum to maximum at any given altitude, which translates to roughly 5× decay-rate change.

What this tool does not capture: diurnal density variation, geomagnetic-storm transients, ballistic-coefficient changes from attitude variation (tumbling vs gravity-gradient), drag-modulating sails, propulsive reboost. The 25-year reentry guideline (IADC 02-01, NASA-STD-8719.14B) is the regulatory anchor for mission lifetime planning.

// pick a class, then dial mass / area / solar activity.

Orbit

// circular orbit; King-Hele simple-atmosphere model.

Spacecraft

// ballistic coefficient = m / (Cd × A).

Atmosphere

// reference table fit to NRLMSISE-00; engineering trade study only.

Decay timeline

// BC 90.9 kg/m² · orbit period 94.6 min

2.47 years

Time to reentry

2.47 years

Same, in days

90.9 kg/m²

Ballistic coefficient

69.7 m/day

Initial decay rate

1.4e-12 kg/m³

ρ at start

100 km

Final altitude

// altitude vs time

0 km125 km250 km375 km500 km0 d7 mo1.2 y1.9 y2.5 y

// shareable URL encodes every input. no backend.

// ai-generated breakdown of what these numbers mean — with diagrams.

References

  • // King-Hele, D. (1964). Theory of Satellite Orbits in an Atmosphere. Butterworths.
  • // Picone, J. M., Hedin, A. E., Drob, D. P., Aikin, A. C. (2002). NRLMSISE-00 empirical model of the atmosphere. JGR 107(A12).
  • // Cook, G. E. (1965). Satellite drag coefficients. Planetary and Space Science 13(10), 929-946.
  • // Vallado, D. A. (2013). Fundamentals of Astrodynamics and Applications, 4th ed., ch. 8 (Perturbations).
  • // IADC 02-01, NASA-STD-8719.14B — the 25-year reentry guideline.