Orbital Decay (Atmosphere)
Satellites in low Earth orbit still encounter trace atmosphere; drag removes energy and angular momentum, lowering the semi-major axis and often reducing eccentricity when drag acts predominantly near perigee in simplified models. The International Space Station near ~400 km would re-enter on a timescale of months to a few years without periodic reboosts; solar extreme ultraviolet activity expands the thermosphere and strongly affects drag, so operational lifetime forecasts are empirical. This page integrates a deliberately schematic per-orbit decay tied to perigee height via an exponential density proxy—not a numerical propagator like SGP4—so students see qualitative rounding and spiraling rather than exact decay predictions.
Who it's for: Follows circular orbit and Hohmann pages; pairs with orbital debris and real-world mission operations discussions.
Key terms
- Orbital decay
- Atmospheric drag
- Perigee
- Semi-major axis
- Eccentricity
- LEO
- ISS
- Reboost
How it works
**Low orbits** feel **residual atmosphere**: drag is strongest near **perigee**, removing energy so the ellipse **shrinks** and often becomes **rounder** in this toy model. The ISS near **~400 km** would re-enter in **months to a few years** without **reboosts**; real lifetime depends on **solar activity**, **ballistic coefficient**, and **area-to-mass**. This page uses a **schematic** per-orbit kick tied to perigee height—not a **numerical propagator**—to show **decay + circularization** qualitatively. Press **Reset** after changing initial altitudes.
Key equations
Frequently asked questions
- Why does eccentricity sometimes decrease in the animation?
- When drag is modeled as strongest at perigee, each orbit preferentially bleeds energy near closest approach, which tends to circularize many toy parameterizations. Real satellites can show more complex eccentricity evolution depending on atmosphere models and orientation.
- Can I read off an exact re-entry date?
- No. The coefficients are tuned for visualization. Mission analysts use detailed density models (e.g., NRLMSISE), ballistic coefficients, and tracking data.
- How do engineers fight decay?
- Propulsive reboosts, often from visiting vehicles or onboard thrusters, raise the orbit. Drag sails or end-of-life deorbit plans are alternative strategies.
More from Gravity & Orbits
Other simulators in this category — or see all 22.
Mercury Perihelion Precession
GR Δω per orbit vs Newton; ~43″/century readout; amplified animation.
Earth–Moon Barycenter Wobble
Heliocentric path of Earth’s center: barycentric ellipse plus lunar epicycle (exaggerated).
Oberth Effect
Same prograde Δv at peri vs apo on one ellipse; higher ε when burning deep.
Three-Body Figure-Eight
Equal masses: Chenciner–Montgomery choreography in 2D (RK4, periodic orbit).
Restricted 3-Body (map)
CRTBP: escape vs collision vs chaos proxy; μ slider.
Multistage Rocket (Tsiolkovsky)
Δv per stage; sum vs single-stage with same total propellant.