While you put your hand out the window of a transferring automobile, you’re feeling a drive pushing towards you known as drag. This drive opposes a transferring automobile, and it’s a part of the explanation why your automobile naturally slows to a cease in case you take your foot off the fuel pedal. However drag doesn’t simply decelerate vehicles.
Aerospace engineers are engaged on utilizing the drag drive in area to develop extra fuel-efficient spacecraft and missions, deorbit spacecraft with out creating as a lot area junk, and even place probes in orbit round different planets.
Area just isn’t an entire vacuum − at the very least not all of it. Earth’s environment will get thinner with altitude, but it surely has sufficient air to impart a drive of drag on orbiting spacecraft, even as much as about 620 miles (1,000 kilometers).
As an aerospace engineering professor, I examine how drag impacts the motion of spacecraft in orbit. Aerobraking, because the title suggests, is a sort of maneuver that makes use of the skinny air in area to use a drag drive within the path reverse to a spacecraft’s movement, very similar to braking in a automobile.
Altering an orbit
In area, aerobraking can change the orbit of a spacecraft whereas minimizing the usage of its propulsion system and gas.
Spacecraft that orbit round Earth achieve this in two forms of orbits: round and elliptical. In a round orbit, the spacecraft is all the time on the similar distance from the middle of the Earth. Because of this, it’s all the time transferring on the similar pace. An elliptical orbit is stretched, so the space from Earth − and the pace the craft strikes at − adjustments because the spacecraft travels alongside the orbit.
The closest level in an elliptical orbit round Earth, the place the satellite tv for pc or spacecraft is transferring quickest, is known as the perigee. The farthest level, the place it’s transferring slowest, is known as the apogee.
The apogee is the purpose farthest from Earth in an elliptical orbit, whereas the perigee is the purpose closest to Earth.
Iketsi/Wikimedia Commons, CC BY-SA
The final thought behind aerobraking is to begin in a big round orbit and maneuver the spacecraft right into a extremely elliptical orbit, in order that the bottom level within the orbit − the perigree − lies within the denser a part of the higher environment. For Earth, that’s between about 62 and 310 miles (100 and 500 kilometers), with the selection relying on time required to finish the orbit change.
Because the spacecraft passes by way of this lowest level, the air exerts a drag drive on it, which reduces the stretch of the orbit over time. This drive pulls the craft towards a round orbit smaller than the unique orbit.
Aerobraking brings a spacecraft from a big, round orbit right into a extremely elliptical orbit, right into a smaller, extra round one.
Moneya/Wikimedia Commons, CC BY-SA
The primary maneuver to place the spacecraft in an elliptical orbit in order that drag can take impact does require utilizing a propulsion system and a few gas. However as soon as it’s within the elliptical orbit, drag from the environment slows the craft, and it doesn’t want to make use of a lot, if any, gas.
Aerobraking brings a craft from a big orbit to a small orbit and isn’t reversible − it could possibly’t improve the dimensions of an orbit. Growing the dimensions of an orbit or elevating the spacecraft to a better orbit requires propulsion and gas.
Aerobraking makes use of
A typical case the place spacecraft controllers use aerobraking is when altering the craft’s orbit from a geostationary orbit − GEO − to a low Earth orbit, LEO. A GEO orbit is a round orbit with an altitude of roughly 22,236 miles (35,786 km). In GEO, the spacecraft makes one orbit round Earth in 24 hours, so the spacecraft all the time stays above the identical level on Earth’s floor.
In GEO orbit, a spacecraft orbits with Earth and stays above the identical level on the floor the entire time.
MikeRun/Wikimedia Commons, CC BY-SA
Earlier than aerobraking, the spacecraft’s onboard propulsion system thrusts in the other way of the GEO orbit’s movement. This thrust places it into an elliptical orbit. The craft passes by way of the environment a number of instances, which ultimately circularizes the orbit.
As soon as it makes it to LEO, the spacecraft might have to make use of somewhat little bit of gas to propel itself up into its goal orbit. Often, the bottom level of the unique elliptical orbit is decrease than the ultimate goal round orbit.
This course of is conceptually much like how the U.S. Area Pressure’s X-37B used aerobraking in early 2025.
The U.S. Area Pressure reported that its unmanned spaceplane, X-37B, used aerobraking. This check demonstrated the craft’s agility and maneuverability.
One other utility for aerobraking is to make a spacecraft deorbit − or reenter the environment − after it has stopped working. This manner, the corporate or company can get rid of the spacecraft and keep away from creating area junk, since it should dissipate within the decrease environment.
NASA’s Mars reconnaissance orbiter used aerobraking to orbit round Mars.
NASA/JPL
Aerobraking for interplanetary missions
Just a few Mars missions, together with the Mars reconnaissance orbiter and the Mars Odyssey orbiter, have used aerobraking to succeed in their goal orbits across the purple planet.
For interplanetary missions like these, scientists use aerobraking at the side of the craft’s onboard propulsion system. When a spacecraft arrives at Mars, it does so in a hyperbolic orbit.
Whereas an elliptical orbit is closed, a hyperbolic orbit doesn’t go all the way in which round a planet.
Maxmath12/Wikimedia Commons
Not like a round or an elliptical orbit, the spacecraft’s path in hyperbolic orbit gained’t hold it orbiting round Mars. As an alternative, it will fly by way of and depart Mars − except it makes use of thrust from its propulsion system to get “captured” right into a closed elliptical orbit.
Because the spacecraft arrives at Mars, the onboard propulsion system fires to supply the drive essential to seize the spacecraft right into a extremely elliptical orbit round Mars. As soon as captured, scientists use aerobraking over a number of orbital passes by way of the environment to attain the ultimate orbit, typically a round one.
Aerobraking maneuvers may end up in important gas financial savings. As people get nearer to touchdown on the floor of the purple planet, the gas financial savings enabled by aerobraking may save mass and permit every spacecraft headed to Mars to take extra provides.
Within the grand arc of area exploration, aerobraking isn’t just a maneuver. It has an important position to play in the way forward for area operations and planetary missions and colonization.
