The Re-entry Process
As an object descends below ~120 km altitude, it encounters dense atmosphere at orbital speed (~7.8 km/s). Air compression ahead of the object generates extreme temperatures — exceeding 1,600°C for even small objects. Most spacecraft are not designed to survive these conditions and break apart between 80–60 km altitude. Smaller components melt or ablate completely. The process typically takes 1–2 minutes from initial heating to disintegration.
What Survives?
Dense, high-melting-point components — titanium fuel tanks, stainless steel reaction wheels, glass optics — can survive re-entry and reach the ground. NASA estimates 10–40% of a satellite's mass may survive, depending on design. Larger objects like rocket bodies have a higher survival fraction. The surviving fragments scatter across a "footprint" that can span hundreds of kilometres.
Is It Dangerous?
For any individual on the ground, the risk from re-entering debris is extraordinarily small — roughly 1 in a trillion per year. About 70% of Earth's surface is ocean, and much of the land is sparsely populated. However, as the number of objects in orbit grows, uncontrolled re-entries become more frequent, increasing the aggregate risk. To date, no confirmed human fatality has resulted from re-entering space debris.
Controlled vs Uncontrolled Re-entry
Controlled re-entries use propulsion to target a specific ocean area (typically the South Pacific Uninhabited Area, known as Point Nemo). The ISS, Mir, and large rocket stages undergo controlled re-entry. Uncontrolled re-entries occur when objects lack propulsion — the re-entry location cannot be predicted until the final hours. Chinese Long March 5B core stages have drawn particular attention for their uncontrolled re-entries.