Most satellites and rocket stages that fall back burn up in the atmosphere, but large ones can drop surviving pieces, usually into the ocean.
Nothing in low orbit stays up for ever. Thin air at those heights slowly drags satellites, debris and spent rocket stages lower, until they plunge into the thick atmosphere. This is re-entry, and it happens every day. ESA says that on average two small tracked debris objects come down daily, and an object 1 metre or larger about once a week.
Height decides. Low down, drag works fast; above about 800 km objects generally stay up for many decades. ISRO's own record shows the range:
Shortly before re-entry, a spacecraft is moving at about 28,000 km/h at around 120 km altitude. Heating from the air makes most of it melt and break apart. Not everything burns, though: for big uncontrolled re-entries such as the 74-tonne Skylab station, ESA says 20–40% of the mass may reach the ground.
The risk to any one person is tiny. ESA knows of no injuries caused by re-entering space debris, and says being struck by lightning is 60,000 times more likely. About 75% of Earth's surface is water, and much of the land is empty. ESA requires — and agencies widely accept — that an uncontrolled re-entry should carry less than a 1-in-10,000 chance of hurting anyone.
The busiest year so far was 2024. ISRO's report counts a record 2,095 catalogued objects re-entering, including 335 Starlink satellites. Nine Indian satellites came down that year, bringing India's total to 31.
Its catalogue record shows a decay date of 14 February 2024 — the day the de-orbited satellite re-entered. See Cartosat-2's record
Speed first. 28,000 km/h ÷ 3,600 s/h ≈ 7.8 km/s. Every kilogram therefore carries kinetic energy of ½ × 1 kg × (7,780 m/s)² ≈ 30 MJ. The air must remove almost all of it, mostly as heat, which is why most material melts or vaporises.
Size matters for what survives. ESA's 20–40% figure is for very large uncontrolled re-entries such as Skylab (74 tonnes) and Salyut-7/Kosmos-1686 (40 tonnes); at 20%, Skylab would have dropped about 15 tonnes, and at 40% about 30 tonnes.
The risk rule in numbers: ESA's limit is a casualty probability below 1 in 10,000 for a single uncontrolled re-entry. Suppose every 1-metre-plus object coming down — about one a week, or 52 a year — sat right at that limit. The expected number of casualties would be at most 52 × 1/10,000 ≈ 0.005 a year, or about one in two centuries. In practice many re-entries are far below the limit.
Height and time: ISRO cut Cartosat-2's remaining lifetime from more than 30 years to under 4 by lowering it. The PSLV-C58 fourth stage at about 350 km came down within about three months. ESA notes that above about 800 km, objects generally stay up for many decades. So a few hundred kilometres of height can mean the difference between months and generations.
Over the whole space age, ESA says about 75% of all the larger objects ever launched have already re-entered.
They hit the atmosphere at about 28,000 km/h, and the air turns their huge kinetic energy into heat, melting and breaking up most of the structure.
A casualty probability below 1 in 10,000 for a single re-entry, which ESA makes mandatory for its missions.
It lowered the satellite's orbit at end of life, cutting its remaining time in orbit from over 30 years to under 4; it re-entered on 14 February 2024.