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.

Orbital Decay (Atmosphere)

Toy perigee drag: shrinking, circularizing ellipse; ISS lifetime intuition.

About this model

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

Mean motion n = √(GM/a³); drag proxy ∝ ρ(h_p) — exponential scale-height sketch

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.

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About this model

How it works

Key equations

Frequently asked questions

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About this model

How it works

Key equations

Frequently asked questions