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 kilometers).
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 maneuver 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 minimizing 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 center 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 − changes 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.
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 maneuver the spacecraft into a highly elliptical orbit, so that the lowest point in the orbit − the perigree − lies in the denser part of the upper atmosphere. For Earth, that’s between about 62 and 310 miles (100 and 500 kilometers), with the choice depending on 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.
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 maneuver 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…
