Thin air at low altitude drags on a satellite, slowly lowering its orbit until it re-enters the atmosphere and burns up or falls into the ocean.
Even a few hundred kilometres up there is still a trace of air. A satellite moving at nearly 8 km/s keeps bumping into it. Each bump steals a little energy, so the orbit shrinks. This slow sinking is called orbital decay.
Height makes a huge difference. Below about 120 to 160 km, drag brings a satellite down within days, and it breaks apart at around 80 km. Above about 600 km, drag is so weak that orbits usually last more than 10 years. ESA puts the practical lower edge of low Earth orbit at about 180 km for this reason.
The upper atmosphere swells when the Sun is active. At 500 to 800 km, the air density at solar maximum can be about 100 times higher than at solar minimum. So satellites sink much faster during busy years of the Sun's cycle.
NASA's Earth-observing satellites show this. When the Sun is quiet they boost their orbits about four times a year. At solar maximum they may need a boost every two to three weeks.
Megha-Tropiques-1, an ISRO and French space agency (CNES) climate satellite, was launched on 12 October 2011 into an 867 km orbit tilted 20°. At that height it would have taken more than 100 years to come down naturally.
After its mission ended, about 125 kg of fuel was still on board. Starting in August 2022, ISRO fired its engines 18 times to lower the orbit step by step. On 7 March 2023 the final two burns sent it into the atmosphere over an empty stretch of the Pacific Ocean, between 5°S and 14°S and 119°W and 100°W.
Dead satellites in low orbit add to space debris. Guidelines ask that a satellite either be brought down or left in an orbit that will decay within 25 years. ESA now asks its new missions to clear protected orbits within five years.
The fastest-moving dots on the globe are the low-orbit satellites, and they are the ones atmospheric drag slowly pulls down. See low-orbit satellites on the globe
Drag force on a satellite is F_D = ½ · ρ · v² · C_D · A, where ρ is air density, v the speed, A the area facing the flow, and C_D the drag coefficient (about 2 to 4 for satellites). How strongly the force slows the satellite depends on the ballistic coefficient m / (C_D · A): light satellites with big surfaces decay fastest.
Drag removes energy, and a circular orbit's energy per kilogram is ε = −GM / (2r). Taking energy away makes ε more negative, so r shrinks. But the circular speed is v = √(GM/r), which grows as r shrinks. This is the drag paradox: drag makes a satellite go faster, because it lets the satellite fall to a lower, quicker orbit.
Worked numbers for Megha-Tropiques-1:
So a satellite lowered from 867 km to a circular 200 km orbit ends up about 0.36 km/s faster, even though its engines fired backwards — the energy comes from falling closer to Earth.
Density is the hard part of any decay forecast. Because it can swing a hundredfold with solar activity at 500–800 km, re-entry predictions for uncontrolled objects carry large uncertainty until the final days.
Drag from the thin upper atmosphere, which removes the satellite's energy and lowers its orbit.
The Sun's extra activity heats and swells the upper atmosphere, so the air at satellite heights becomes denser and drag increases.
From 867 km it would have stayed in orbit for over 100 years. Using its leftover fuel, ISRO brought it down into an uninhabited part of the Pacific in 2023.