Reentry Casualty Risk Assessment

Object Under Test

Dry mass 800 kg

Ballistic coefficient A/m 0.010 m²/kg

Population density under track 80 /km²

Simulation speed 8×

Live Telemetry

PHASE: ORBITAL DECAY

Altitude

500 km

Orbital period

94.6 min

Fragments

–

Casualty area A_c

–

E_c not yet computed — awaiting breakup

How it works

An uncontrolled reentry decays roughly exponentially once residual atmospheric drag dominates: altitude h(t) = h₀·e^−t/τ, with the decay time constant τ inversely proportional to the ballistic coefficient B = C_d·A/m (higher area-to-mass → faster decay). Breakup is modelled at the standard ≈78 km aero-thermal fragmentation altitude.

h(t) = h0 * exp(-t/tau)
tau  ~ K / (A/m)
Ac_i = pi * (rp + rd_i)^2      (casualty area per fragment)
Ec   = sum(Ac_i) * population_density   (per event)

Each surviving fragment has a "casualty area" — the cross-section within which a human would be struck — approximated as a circle of radius r_p + r_d (average person radius r_p ≈ 0.3 m plus the fragment's own radius). Multiplying the total casualty area by the population density under the ground track gives the casualty expectancy E_c, exactly the quantity NASA's Debris Assessment Software and ESA's DRAMA/SESAM compute for every reentering object.

IADC / UNOOSA space debris mitigation guideline: a reentry is only acceptable if E_c < 1×10⁻⁴ (a 1-in-10,000 chance of a human casualty). Operators failing this threshold must plan a controlled (targeted-ocean) reentry instead.
Mass and A/m jointly set how many fragments survive breakup and how large they are; population density sets the risk under the actual ground track at breakup time.
This is the same governance mechanism referenced by COSPAR/IADC debris-mitigation guidance and enforced through national licensing under UNOOSA's space traffic management framework.

Decaying orbit / ground track Surviving debris fragment Impact footprint

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