Friction normally slows things down. But atmospheric drag makes a satellite lose orbital energy and fall into a lower orbit — and lower orbits are faster. So although drag initially slows the spacecraft, the resulting orbital decay can leave it moving faster around Earth. Scientists were analysing this effect as early as 1958.

Atmospheric drag never gives a satellite a forward push. It acts against the spacecraft’s motion and removes orbital energy. Yet after the orbit decays, the satellite can be circling Earth faster than before.

The apparent contradiction comes from comparing two different moments. Drag slows the spacecraft locally, so it can no longer remain on its previous path. It falls deeper into Earth’s gravitational field, where gravity accelerates it. A lower circular orbit has a higher speed than a higher circular orbit, even though its total mechanical energy is lower.

NASA’s catalog of Earth satellite orbits describes the same sequence: atmospheric resistance pulls a low-orbiting satellite closer to Earth, gravity increases its forward velocity, and the object spirals lower and faster until reentry. That wording is accurate as a long-term orbital description, but it does not mean friction itself provides the acceleration.

Drag always opposes the immediate motion

The upper atmosphere is extremely thin at satellite altitudes, but it is not empty. Molecules strike a spacecraft moving at several kilometres per second and exert a force opposite its velocity relative to the surrounding air. The US Space Weather Prediction Center’s drag guide notes that the effect is important across low Earth orbit, broadly the region below about 2,000 kilometres.

During each encounter, drag reduces the spacecraft’s kinetic energy compared with the speed it would have had without drag. It also removes angular momentum. The lost mechanical energy is transferred to the atmosphere and ultimately appears mainly as heat.

Then orbital mechanics takes over.

A small reduction in speed does not leave the satellite hovering at the same altitude. Gravity pulls it inward. As the spacecraft falls, its gravitational potential energy becomes more negative and its kinetic energy rises. The later gain in speed comes from gravity and the descent, not from the aerodynamic force.

Why a lower orbit is faster

For an ideal circular orbit, the relation is simple: orbital speed equals the square root of Earth’s gravitational parameter divided by the distance from Earth’s centre. NASA’s Kepler and orbital-velocity guide derives this balance between gravity and circular motion.

Using a two-body circular-orbit calculation, a satellite at 400 kilometres altitude travels at about 7.67 kilometres per second. At 350 kilometres, the corresponding speed is about 7.70 kilometres per second. The difference is only around 29 metres per second, but the direction is clear: the lower circle is the faster one.

The energy bookkeeping looks odd only if speed is treated as the whole energy budget. In a circular orbit, total specific mechanical energy is negative and inversely related to orbital radius. Moving lower makes that total more negative. Kinetic energy can increase at the same time because gravitational potential energy decreases by a larger amount, with drag carrying away the difference.

So the satellite can lose energy and gain speed.

Real orbital decay is not a staircase of circles

The neat circular comparison explains the result, but a decaying spacecraft does not jump from one perfect circle to another. At any instant it follows an osculating orbit, the Keplerian path it would take if the disturbing forces suddenly disappeared. Continued drag changes that path again.

In an elliptical orbit, atmospheric drag is usually strongest near perigee, where the spacecraft is lower, the air is denser and the orbital speed is greater. A short deceleration there initially lowers the altitude on the opposite side of the orbit. Repeated passes shrink the semimajor axis and tend to reduce the period. The instantaneous speed still rises and falls around each ellipse.

This is why the safest version of the claim is that orbital decay can increase a satellite’s average orbital speed or leave it moving faster in the resulting lower orbit. It is not a claim that every velocity measurement after every collision with an air molecule must be larger than the one before it.

Eventually the descent becomes a feedback loop. Lower altitude means denser air, denser air means stronger drag, and stronger drag removes orbital energy faster. Once the spacecraft reaches the denser atmosphere, the simple image of a succession of near-circular orbits breaks down and reentry begins.

Operators can use drag as a control input

The effect is not only something mission controllers resist. NASA’s eight-spacecraft CYGNSS constellation has used differential drag to adjust satellite spacing without propulsion. Operators changed a spacecraft’s orientation to expose more or less area to the atmosphere. More drag lowered its orbit and changed its mean motion relative to the other satellites.

That technique works because a lower orbit has a shorter period. A satellite allowed to drop slightly can move ahead around Earth relative to a companion kept higher, even though both continue losing some energy to the atmosphere.

The International Space Station shows the other side of the same physics. NASA says the station operates around 415 kilometres and requires regular reboosts to counter atmospheric drag. Without reboosts, NASA estimates its orbital lifetime at that altitude would be roughly one to two years, depending on solar activity. A reboost adds energy and raises the orbit. Once established higher, the station travels more slowly on average, not faster.

The Sun changes the amount of drag

Atmospheric density at orbital altitude is not fixed. Extreme ultraviolet radiation and geomagnetic storms heat the thermosphere, making it expand upward. A spacecraft can encounter more particles and much stronger drag without changing its nominal altitude.

A sharp example came after SpaceX launched 49 Starlink satellites on February 3, 2022. A geomagnetic storm heated and expanded the upper atmosphere, and a NASA visualization summary says 38 of the satellites reentered within days. Their low deployment altitude left little room for the orbit to decay.

Longer-term changes matter too. TerraDaily has reported on how changing thermospheric density may alter storm-time satellite drag. The exact lifetime of a low-orbiting object therefore depends on its mass, cross-sectional area, attitude and altitude, as well as solar and geomagnetic conditions.

Researchers were calculating the effect in 1958

The historical part of the title is well supported. On September 20, 1958, D. G. King-Hele and D. C. M. Leslie published a Nature letter titled “Effect of Air Drag on the Orbit of an Earth Satellite”. They discussed how drag altered a satellite’s period of revolution and how assumptions about atmospheric density affected the calculation.

This was less than a year after Sputnik 1 opened the artificial-satellite era. Early spacecraft were not only engineering demonstrations. Their changing orbits offered a way to infer properties of the upper atmosphere, where direct measurements were sparse.

The 1958 paper is evidence of early analysis, not a claim that its authors were the first people to understand every part of the effect. Modern orbit prediction includes atmospheric rotation, changing density, solar activity, spacecraft attitude and detailed gravity models that the first satellite analysts could not treat with today’s precision.

The central result has survived that added detail. Drag takes energy from the orbit at every pass. Gravity turns the resulting descent into greater speed around a smaller path, until increasing atmospheric resistance ends the orbit altogether.

Edited by Lachlan Brown