Physics
Gravity and orbits
Run Earth’s orbit, binary stars or a gravity assist and discover how the starting speed decides the shape of an orbit.
Launch simulationHow it works
Every body pulls on every other body. In each step the simulation adds up these forces and moves the bodies according to Newton’s laws of motion. The shape of an orbit depends on the speed relative to the central body: at 1 AU, Earth follows a circle at about 29.8 km/s. A higher speed gives a longer ellipse, and above √2 times the circular speed (about 42.1 km/s) the body escapes for good.
F = G·m₁·m₂ / r²v_circ = √(G·M / r)v_esc = √2 · v_circ
Try it yourself
- Choose the “Circular orbit” scenario and press “▶ Play”. The “Simulation time” readout shows Earth completing an orbit in about 365 days.
- Pause, select Earth and change Vy from 29.78 to 35 km/s in the body editor, then press “Apply changes”. Predict first: the orbit becomes a long ellipse and takes longer.
- Set Vy to 43 km/s. The speed is now above the escape speed of 42.1 km/s, so Earth leaves on an open path.
- In the “Gravity assist” scenario, watch “Speed history”: the probe speeds up as it passes the moving planet. A planet at rest would only change the probe’s direction, not its speed.
Model limitations
The model is two-dimensional and includes only Newtonian gravity between the bodies (no relativity or other forces). Distances are to scale; bodies are drawn larger so you can see them. It uses a Verlet integrator with an adaptive step: fine for learning, not for navigating real spacecraft.