A low-Earth-orbit station is never truly parked — faint traces of atmosphere at these altitudes still exert drag, so the orbit continuously loses energy and altitude decays. The thinner the air, the smaller the drag, so decay accelerates sharply as the station sinks lower — an exponential atmosphere model. When altitude falls below a safe threshold, the station fires its thrusters in a reboost burn, spending propellant to climb back up, and the cycle repeats — a sawtooth of decay and correction, exactly how the ISS is actually kept aloft.
dh/dt ∝ −(area/mass) · ρ₀·exp(−(h−h_ref)/H) · solarFactor
on h ≤ h_trigger: h += burnSize, Δv ≈ burnSize·k, propellant += Δv·massFactor
- Starting altitude — higher starts mean thinner air and a slower initial decay.
- Drag area / mass — a station's ballistic coefficient; more solar-panel area or less mass per unit area means faster decay, exactly like the ISS's added modules and arrays.
- Solar activity — the Sun heats and puffs up the upper atmosphere; solar maximum can multiply drag several-fold versus solar minimum.
- Reboost trigger — how low the crew lets altitude fall before correcting; a lower trigger means fewer, bigger burns, a higher one means frequent, cheap top-ups.
- Reboost burn size — how much altitude each burn restores, trading immediate propellant cost against how soon the next burn is needed.
Real-world relevance: the ISS loses roughly 2 km of altitude a month at solar minimum, far more at solar maximum, and is reboosted every few weeks by its own thrusters or a docked Progress/Cygnus vehicle — propellant for reboosts is a real, recurring line item in every space station's logistics budget.