Orbital Decay

Lore & Background

In the early days of the KSC, orbital decay was less a named phenomenon and more a recurring mystery. Junior Kerbals would launch a payload into what they assumed was a stable low orbit, turn off their engines, and drift back to the launch pad to find their craft had simply… vanished. The flight recorder showed nothing dramatic—no explosion, no structural failure—just a slow, almost imperceptible lowering of the periapsis over several orbits until the vessel kissed the upper atmosphere and burned up in a streak of green-tinged plasma over the ocean.

Senior flight engineers began calling it 'the slow kiss' in the break room. The Kerbinian atmosphere, while far thinner than Earth's in many layers, still exerts a cumulative toll on anything whose orbit dips below the effective drag threshold. The effect is nonlinear: a craft at 70 km altitude loses altitude in hours, while the same craft at 120 km might coast for weeks. This gradient made orbital planning a genuine engineering discipline rather than a 'point the nose up and hope' exercise.

Over time, orbital decay became woven into KSC culture. Mission planners built 'drag margins' into every trajectory. The famous Kerbin Orbital Relay Station was deliberately parked at a high-inclination, high-periapsis orbit partly to minimize long-term decay costs. And every new pilot's first solo mission traditionally includes a deorbit exercise, where they must use the very same drag force that threatens them as a tool, shaping a controlled reentry corridor and touching down on the KSC runway with their craft intact. It is, in the words of one veteran flight director, 'the atmosphere teaching you to respect it—gently, or all at once.'

In Their Own Story

The telemetry was wrong. That was the first thought that hit Jem, staring at the navball on the cramped cockpit of *KSC-7 'Tin Can'*, the little two-seat trainer she'd been ferrying to the Kerbin Orbital Relay Station. Periapsis: 82 km. Two minutes ago it had been 84. She blinked, checked the fuel gauge, confirmed the engines were cold, and watched the number tick down again. 83. 82. 81.

'Control, this is Tin Can. I'm… I'm losing altitude. Slowly. No thrust. No damage. Just… sinking.'

A pause. Then the calm, almost amused voice of Flight Director Wernher: 'Jem. You're in the soup. You know the soup.'

She knew. Everyone knew. The upper atmosphere, that invisible ceiling they'd all learned to fear in sim. She'd cut her parking orbit two kilometres too low to save a handful of fuel, and now the Kerbinian air was eating her orbit one metre at a time. She had fuel for one burn. Maybe two, if she was careful.

'Copy, Control. Tin Can is climbing out of the soup. Hold my beer.'

She lined up prograde at apoapsis, held her breath, and squeezed the throttle. The little ship groaned, the stars steadied, and slowly—so slowly—the periapsis number crept upward. 84. 86. 90. She cut the engines, exhaled, and watched the Kerbinian horizon glow amber below her. The soup was patient. But so, apparently, was she.

Reader's Guide

Rule: Any spacecraft whose periapsis falls within a body's atmosphere will experience a continuous drag force opposing its velocity. The force scales with the square of velocity, the local atmospheric density, and the vessel's cross-sectional area. In practice, this means a fast, wide, low-flying craft bleeds altitude far faster than a slow, slim, high-flying one.

Why it matters: You will not get a warning chime. The navball will simply show your periapsis creeping down by a few metres per orbit. By the time the drop is obvious, you may have already lost the fuel margin needed to correct it. On Kerbin, anything below roughly 80 km is in the danger zone; on Duna the atmosphere is thinner but still present, so the threshold is lower but the same principle applies.

Common failure modes: (1) Launching into a 'parking orbit' that is a few km too low and forgetting about it while assembling a stack. (2) Performing a deorbit burn that is slightly too strong, sending the periapsis into the atmosphere and triggering an uncontrolled, high-speed reentry. (3) Ignoring decay during long-duration station-keeping and discovering the orbit has dropped 5 km over a session.

Pro tips: Always check your periapsis after every burn. When raising orbit, burn prograde at apoapsis for maximum efficiency. If you need to deorbit intentionally, a small retrograde burn at periapsis is far more fuel-efficient than a large one at apoapsis. Keep a fuel reserve of at least 100–200 m/s for emergency orbit raises. And if you see your periapsis trending down by more than a few metres per orbit, do not wait—correct it now.

Did You Know?

• On airless bodies like the Mun and Minmus, orbital decay from atmospheric drag simply does not occur, which is why low-altitude orbits around them are essentially stable indefinitely unless perturbed by other forces.
• The drag force in KSP follows the standard aerodynamic drag equation, meaning a craft with a large flat cross-section (like a wide solar array) will decay noticeably faster than a slender rocket of the same mass at the s
• Orbital decay is the same mechanic that makes uncontrolled reentries possible: if you let your periapsis fall low enough, the increasing atmospheric density creates a positive feedback loop that accelerates the descent u
• Experienced KSP pilots sometimes exploit a small amount of intentional drag in the upper atmosphere to fine-tune their orbit's shape without spending fuel, a technique that works only in very specific altitude windows.