Orbit Lab: the physical model
A satellite with negligible mass moves in the gravitational field of a fixed, spherical Earth. Acceleration points towards its centre and scales with μ/r². The model is integrated with the velocity-Verlet method, using a fixed step for each predicted path.
Distances are calculated in units of the initial orbital radius. The interface converts results using Earth's gravitational parameter, 398,600.435507 km³/s², from JPL DE440. We use an approximate mean Earth radius of 6,371 km.
Circular speed is √(μ/r); escape speed is √(2μ/r). An impact is detected when the path reaches the model surface. Time is compressed and the view zooms to fit. The displayed trace is calculated in advance. Traces stop at impact, eight initial orbital radii, or a finite calculation interval (at most 60 natural time units for bound paths and 14 for escape paths). A clipped trace does not imply that gravity stops there. This is an educational experiment, not a live tracker or navigation tool.
Air resistance, other bodies, Earth's rotation, oblateness and manoeuvres are omitted. Moving the altitude control sets a new initial condition; it does not simulate a launch.
Light and gravity field notes
Images and diagrams
Earth texture: NASA Visible Earth / Blue Marble, “The Blue Marble: Land Surface, Ocean Color and Sea Ice.” The globe is rendered from this texture with illustrative lighting. It is not a live view. Diagrams are original explanatory graphics produced by EXOVAS.
Editorial responsibility
Source links accompany the claims they support. Simplifications and computed examples are identified. Dates describe the published explanation, not a claim that every linked source has just been updated.
Published: 18 October 2026.