A controlled de-orbit is when operators fire a satellite's engines to bring it down at a chosen time and place, usually over an empty stretch of ocean.
Left alone, a dead satellite in low orbit will eventually fall, but nobody can choose where. A controlled de-orbit fixes that. While the satellite still works, engineers use its remaining fuel to steer it into the atmosphere on purpose, aiming any surviving pieces at a remote part of the ocean. ESA's five ATV cargo ships, for example, all made controlled re-entries over an uninhabited area of the South Pacific.
Megha-Tropiques-1, a joint ISRO–CNES satellite for tropical weather and climate studies, was launched on 12 October 2011. It was not designed to be brought down in a controlled way, which made ISRO's attempt hard. From August 2022 ISRO used 20 manoeuvres to lower the low point of its orbit step by step, spending about 120 kg of fuel.
On 7 March 2023 came the last two burns, at 11:02 and 12:51 UTC. Each fired four 11-newton thrusters for about 20 minutes, dropping the low point to an estimated below 80 km. The satellite entered the dense atmosphere and broke up over the deep Pacific Ocean, and analysis showed no large fragments would survive. Every burn plan was first screened for close approaches, including with the ISS and China's space station.
Not every mission can do a fully controlled re-entry. A simpler option is to lower the orbit so the satellite falls soon, even if not at a chosen spot. ISRO lowered Cartosat-2, which re-entered on 14 February 2024, and de-orbited Scatsat-1 with 12 manoeuvres before decommissioning it on 26 September 2024. It also lowers spent PSLV fourth stages to about 350 km so they come down within months.
ISRO's Debris Free Space Missions initiative aims for every Indian mission, government or private, to leave no debris by 2030. It asks that end-of-life disposal succeed with better than 99% probability, and that objects in low Earth orbit stay up no more than 5 years after their mission ends.
Its catalogue record shows a decay date of 7 March 2023 — the day ISRO steered it into the Pacific. See Megha-Tropiques' record
A controlled de-orbit is two steps: lower the perigee (the lowest point of the orbit) far enough that the atmosphere captures the satellite on the next pass, and time the final burn so the capture point lies over empty ocean.
How big a burn? Use the vis-viva equation, v² = GM (2/r − 1/a), with GM = 398,600 km³/s² and Earth's radius 6,378 km. Take a satellite in a circular orbit at 400 km: r = 6,778 km, and its speed is v = √(398,600 / 6,778) ≈ 7.669 km/s. To drop the perigee to 80 km (r = 6,458 km), the new orbit has a = (6,778 + 6,458) / 2 = 6,618 km. Just after the burn, at r = 6,778 km:
v² = 398,600 × (2/6,778 − 1/6,618) ≈ 398,600 × 1.440 × 10⁻⁴ ≈ 57.39, so v ≈ 7.576 km/s.
The required Δv is about 7.669 − 7.576 ≈ 0.093 km/s, or about 93 m/s — a big ask for a satellite near the end of its fuel.
Megha-Tropiques-1's thrusters were small, so each burn was long. Total thrust was 4 × 11 N = 44 N; over about 20 minutes (1,200 s) that gives an impulse of 44 × 1,200 = 52,800 N·s per burn, or about 105,600 N·s for the final two. Splitting the lowering into 20 earlier manoeuvres plus two final burns let ISRO reach the target perigee with the fuel and thrusters it had.
Compare the uncontrolled path: ESA's risk limit for an uncontrolled re-entry is a casualty chance below 1 in 10,000, and large uncontrolled objects can drop 20–40% of their mass. A controlled re-entry moves that risk to an ocean area chosen in advance.
It is a vast, uninhabited ocean area, so any pieces that survive re-entry fall far from people and shipping lanes.
The satellite was not designed for a controlled end-of-life re-entry, so ISRO had to plan 20 perigee-lowering manoeuvres and two long final burns with its small 11 N thrusters.
By 2030, no debris from Indian missions: end-of-life disposal with better than 99% success, and at most 5 years in low Earth orbit after a mission ends.