Altitude 0 kmPeriod 0 minDecay 0 m/dayElapsed 0RunningSolar x1FPS 0

Satellite (Blender model) in a circular low Earth orbit around the baked-texture Earth. The gap between orbit and surface is exaggerated (setting in the panel) so the slow spiral is visible; the blue haze marks the thick lower atmosphere, drawn to the same exaggeration. Dashed ring: starting altitude. Orange ring: the 120 km re-entry line. Orbit speed on screen is capped, the elapsed time is not. Drag to orbit, wheel or pinch to zoom.

Low Earth Orbit Decay Lab

Why satellites fall: thin air at 200 to 600 km still drags. Orbit-averaged decay with a layered exponential atmosphere, an 11-year solar cycle and a reboost budget. Educational model, circular orbit, no attitude or tumbling.

Satellite and orbit

Solar activity

Solar activity heats and puffs up the thermosphere: the same orbit sees several times the density at solar maximum. The effect fades below about 200 km.

Time

Live readouts

Altitude history

Atmosphere density profile

Dashed: solar min and max. Vertical line: current altitude. Each 50 to 60 km up cuts the density by about a factor e (the scale height).

Reboost and station keeping

Automatic station keeping: reboost to the target when altitude falls 5 km below it

A circular-to-circular Hohmann transfer costs two burns (dv1 + dv2); propellant follows the rocket equation.

Missions

Mission 3: predict the altitude lost per orbit at the current setting (m)

Hint: the loss per orbit is the drag impulse per revolution; see the science panel.

Experiment: lifetime against altitude

Headless sweep of the current satellite from 200 to 900 km at solar minimum, average and maximum, 51.6 degree inclination unless changed above.

Independent numerical checks

Controls

Drag or one finger: orbit camera; wheel or pinch: zoom; three camera views.
Pick a preset or move the sliders; the elapsed-time warp is logarithmic (the HUD shows how many days pass per second).
Reboost raises the orbit to the target altitude and spends propellant; automatic mode does it for you.

The science

Drag. The acceleration is a_D = ½ ρ v_rel² (C_D A / m). The group B = C_DA/m (m²/kg) is the inverse of the ballistic coefficient: a light, wide satellite decays fast, a dense compact one slowly.

Why drag makes the orbit smaller and the satellite faster. A circular orbit has v = √(μ/a) and energy −μ/(2a). Drag removes energy, so a shrinks, and the satellite speeds up as it sinks (the paradox of satellite drag).

Orbit-averaged decay. For a circular orbit da/dt = − B ρ(a) √(μ a) (1 − ω_E cos i / n)², integrated here with RK4 and an adaptive step. The last factor accounts for the atmosphere co-rotating with the Earth (ω_E = 7.292e-5 rad/s, n = mean motion). Per orbit the satellite loses Δa = −2π a² B ρ.

Atmosphere. Density is a piecewise exponential ρ = ρ₀ e^{−(h−h₀)/H} table (typical tabulated values, Vallado-style). Solar activity is a multiplier that blends in between 200 and 350 km. The multipliers 0.3 and 3.5 and the sinusoidal 11-year cycle are illustrative, not a forecast: real density depends on the F10.7 flux, geomagnetic storms, local time and latitude.

Lifetime. Integrating until 120 km (the point where re-entry becomes unavoidable in days). With one scale height the integral has the closed form t = H (1 − e^−Δ/H) / (B ρ₀ √(μ a₀)), used as one of the checks.

Reboost. A Hohmann transfer between circular orbits costs Δv₁ = √(μ/a₁)(√(2a₂/(a₁+a₂)) − 1) and Δv₂ = √(μ/a₂)(1 − √(2a₁/(a₁+a₂))). Propellant: m_p = m (1 − e^{−Δv/(I_sp g₀)}). Holding altitude costs about a_D × 1 year of delta-v.

Simplifications. Circular orbit throughout (no eccentricity growth), spherical Earth, constant C_D, no J2, no tumbling, no day-night density bulge, the 46-minute transfer time is ignored and the burn is instantaneous. The satellite shape and Earth texture are artistic. Exaggerated altitude on screen is a display choice only. Assets built with Blender; scene by Three.js.