Spacecraft can use atmospheric friction to brake into planetary orbit
Instead of firing heavy retrorockets and burning tons of propellant to slow down, spacecraft can use 'aerobraking.' By dipping carefully into a planet's wispy upper atmosphere over dozens or hundreds of orbits, a probe lets atmospheric drag bleed off its orbital energy for free. Missions like NASA's Mars Reconnaissance Orbiter cut hundreds of kilograms of fuel mass this way, paving the path for larger exploration payloads.
The Physics of Orbital Drag
When an interplanetary spacecraft arrives at another planet, it typically travels along a hyperbolic trajectory with far too much velocity to remain bound by the target planet's gravity. To stay in orbit, the vehicle must shed an immense amount of kinetic energy. Traditionally, this is accomplished by firing powerful rocket engines opposite the direction of motion. However, carrying the propellant needed for these massive braking burns increases the launch mass exponentially, requiring larger rockets and severely limiting the weight available for scientific instruments.
Aerobraking offers a propellant-sparing alternative by exploiting a planet's atmospheric envelope. Once the spacecraft achieves an initial, highly elongated elliptical orbit, it dips its lowest point—the periapsis—into the thin outer reaches of the planetary atmosphere. As the probe sweeps through the upper gas layers at speeds of thousands of meters per second, friction and dynamic pressure create aerodynamic drag. This drag gently bleeds off kinetic energy, which is dissipated as low-level thermal energy across the spacecraft's surfaces.
Crucially, orbital mechanics dictates that applying a braking force at the lowest point of an orbit primarily reduces the altitude of the opposite, highest point, known as the apoapsis. Because the spacecraft only grazes the atmosphere near periapsis, the low point remains relatively stable while the high point drops significantly with each pass. Over hundreds of repeated passes, an unwieldy, elongated orbit gradually shrinks and circularizes into a low-altitude operational path suitable for high-resolution planetary science.