LEO Drag & Decay Timeline

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.

This satellite orbital decay calculator integrates the King-Hele (1964) drag formulation numerically from your starting altitude until reentry - or a 50-year horizon - with atmospheric density from a reference table fit to NRLMSISE-00 (Picone et al., 2002) at three solar-activity tiers. It reports orbital lifetime, the altitude-vs-time decay curve, the initial decay rate, and the ballistic coefficient for any satellite in low Earth orbit (LEO), from a 1U CubeSat to a station-class platform. It is an engineering trade-study estimate; flight design needs MSIS-class density models and numerical propagation (Vallado 2013, ch. 8).

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 International Space Station (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.