When you put your hand out the window of a moving car, you feel a force pushing against you called drag. This force opposes a moving vehicle, and it’s part of the reason why your car naturally slows to a stop if you take your foot off the gas pedal. But drag doesn’t just slow down cars.

Aerospace engineers are working on using the drag force in space to develop more fuel-efficient spacecraft and missions, deorbit spacecraft without creating as much space junk, and even place probes in orbit around other planets.

Space is not a complete vacuum − at least not all of it. Earth’s atmosphere gets thinner with altitude, but it has enough air to impart a force of drag on orbiting spacecraft, even up to about 620 miles (1,000 kilometres).

As an aerospace engineering professor, I study how drag affects the movement of spacecraft in orbit. Aerobraking, as the name suggests, is a type of manoeuvre that uses the thin air in space to apply a drag force in the direction opposite to a spacecraft’s motion, much like braking in a car.

Changing an orbit

In space, aerobraking can change the orbit of a spacecraft while minimising the use of its propulsion system and fuel.

Spacecraft that orbit around Earth do so in two types of orbits: circular and elliptical. In a circular orbit, the spacecraft is always at the same distance from the centre of the Earth. As a result, it’s always moving at the same speed. An elliptical orbit is stretched, so the distance from Earth − and the speed the craft moves at − change as the spacecraft travels along the orbit.

The closest point in an elliptical orbit around Earth, where the satellite or spacecraft is moving fastest, is called the perigee. The farthest point, where it’s moving slowest, is called the apogee.

A diagram showing an oval path around Earth, represented by a blue dot. The point on the oval farthest from Earth is labeled 'apogee' while the point closest is labeled 'perigee.' — The apogee is the point farthest from Earth in an elliptical orbit, while the perigee is the point closest to Earth. Iketsi/Wikimedia Commons, CC BY-SA

The general idea behind aerobraking is to start in a large circular orbit and manoeuvre the spacecraft into a highly elliptical orbit, so that the lowest point in the orbit − the perigee − lies in the denser part of the upper atmosphere. For Earth, that’s between about 62 and 310 miles (100 and 500 kilometres), with the choice depending on the time required to complete the orbit change.

As the spacecraft passes through this lowest point, the air exerts a drag force on it, which reduces the stretch of the orbit over time. This force pulls the craft toward a circular orbit smaller than the original orbit.

A diagram showing two orbits around circles representing planets, with the orbit labeled elliptical shaped more like an oval or stretched circle, while the orbit labeled circular is the shape of a circle. — Aerobraking brings a spacecraft from a large, circular orbit into a highly elliptical orbit, into a smaller, more circular one. Moneya/Wikimedia Commons, CC BY-SA

The first manoeuvre to put the spacecraft in an elliptical orbit so that drag can take effect does require using a propulsion system and some fuel. But once it’s in the elliptical orbit, drag from the atmosphere slows the craft, and it doesn’t need to use much, if any, fuel.

Aerobraking brings a craft from a large orbit to a small orbit and is not reversible − it can’t increase the size of an orbit. Increasing the size of an orbit or raising the spacecraft to a higher orbit requires propulsion and fuel.

Aerobraking uses

A common case where spacecraft controllers use aerobraking is when changing the craft’s orbit from a geostationary orbit − GEO − to a low Earth orbit, LEO. A GEO orbit is a circular orbit with an altitude of roughly 22,236 miles (35,786 km). In GEO, the spacecraft makes one orbit around Earth in 24 hours, so the spacecraft always stays above the same point on Earth’s surface.

A drawing of Earth, with an arrow pointing clockwise, with a satellite and its circular path above it, with an arrow pointing clockwise as well. — In GEO orbit, a spacecraft orbits with Earth and stays above the same point on the surface the whole time. MikeRun/Wikimedia Commons, CC BY-SA

Before aerobraking, the spacecraft’s onboard propulsion system thrusts in the opposite direction of the GEO orbit’s motion. This thrust puts it into an elliptical orbit. The craft passes through the atmosphere multiple times, which eventually circularizes the orbit.

Once it makes it to LEO, the spacecraft may need to use a little bit of fuel to propel itself up into its target orbit. Usually, the lowest point of the original elliptical orbit is lower than the final target circular orbit.

This process is conceptually similar to how the U.S. Space Force’s X-37B used aerobraking in early 2025.

The U.S. Space Force reported that its unmanned spaceplane, X-37B, used aerobraking. This test demonstrated the craft’s agility and manoeuvrability.

Another application for aerobraking is to make a spacecraft deorbit − or reenter the atmosphere − after it has stopped working. This way, the company or agency can dispose of the spacecraft and avoid creating space junk, since it will burn up in the lower atmosphere.

An illustration of a spacecraft with two large solar panels and a metal sheet in a circular shape orbiting around a dusty planet. — NASA’s Mars Reconnaissance Orbiter used aerobraking to orbit around Mars. NASA/JPL

Aerobraking for interplanetary missions

A few Mars missions, including the Mars reconnaissance orbiter and the Mars Odyssey orbiter, have used aerobraking to reach their target orbits around the red planet.

For interplanetary missions like these, scientists use aerobraking in conjunction with the craft’s onboard propulsion system. When a spacecraft arrives at Mars, it does so in a hyperbolic orbit.

A diagram showing an oval around a dot, which represents an elliptical orbit, and two curved lines getting close to but not going all the way around the dot to represent parabolic and hyperbolic orbits. — While an elliptical orbit is closed, a hyperbolic orbit doesn’t go all the way around a planet. Maxmath12/Wikimedia Commons

Unlike a circular or an elliptical orbit, the spacecraft’s path in a hyperbolic orbit won’t keep it orbiting around Mars. Instead, it would fly through and depart Mars − unless it uses thrust from its propulsion system to get “captured” into a closed elliptical orbit.

As the spacecraft arrives at Mars, the onboard propulsion system fires to provide the force necessary to capture the spacecraft into a highly elliptical orbit around Mars. Once captured, scientists use aerobraking over several orbital passes through the atmosphere to achieve the final orbit, generally a circular one.