Moon systems use separate enlarged scales. Diamond markers identify spacecraft.
Build a trajectory
Experiment with a route between planets, then examine how a close encounter changes its course. These simplified models explain orbital mechanics; Replay a mission uses recorded historical trajectories.
Chain gravity assists
Build consecutive encounters. Each flyby changes the vessel’s outgoing orbit, which determines which planet orbits it can reach next.
Sun-centered linear distance scale; markers are enlarged. Departure is a tangential impulse from Earth’s circular solar orbit. Sun-only propagation connects instantaneous, unpowered flybys. Planet phases are freely placed at encounter points, so this tests an idealized sequence rather than real launch dates. Rings, atmospheres, moons and multi-body perturbations are excluded. Positive solar orbital energy means Solar System escape, not escape from the Milky Way. Limit: 10 encounters, 80 years per leg.
Sources: NASA · Gravity assists · JPL
Mission designer
Plan a circular, coplanar transfer. The explorer date sets the starting planetary longitudes; ideal alignment places the destination where the spacecraft can meet it.
Gold: spacecraft. Blue: departure planet. Red: destination planet. Distance scale is linear; markers are enlarged. The thin line at arrival shows the miss distance when alignment is wrong.
Advanced: destination flyby
Assume ideal alignment and skip the arrival burn. Change flyby altitude to see the outgoing solar orbit. This explores one encounter, not an optimized multi-planet mission.
Hohmann transfer model: circular orbits, impulsive burns and Sun-only gravity between encounters. Velocity changes are relative to circular solar orbits, excluding surface launch, capture, parking orbits and propulsion losses. Flyby output uses a patched two-body approximation. These are educational estimates, not real launch windows or flight plans.
Sources: NASA · Trajectories · JPL · Astronomy Engine
Gravity-assist playground
Bend a spacecraft’s path around a moving planet. Compare its incoming and outgoing speeds in each reference frame.
Gold dot: spacecraft. Blue dot: planet. Planet motion points right. Paths share a linear distance scale; markers are enlarged.
Idealized unpowered flyby: spherical gravity, circular planet orbit and constant planet velocity during the encounter. Atmospheres, rings, moons and solar tides are excluded. Jupiter uses its system mass. The speed comparison uses asymptotic approach and departure, far from the planet; local speed rises near closest approach. This is a learning model, not mission navigation.
Sources: NASA · Gravity assists · JPL · Astrodynamic parameters · JPL · Planet sizes
Beyond the Solar System
Does leaving the Sun also mean leaving the Galaxy? Start with an outward solar speed, coast through a model galaxy, then compare the route after a hypothetical stellar encounter.
Gold dot: vessel. Center dot: model center. Gray ring: starting orbital radius. Dashed path: coast without the latest encounter. Distances share a linear scale; markers are enlarged. 1 kpc is about 3,262 light-years; 1 Myr is one million years.
Try a hypothetical stellar encounter
Place a Sun-mass star at the vessel’s current position. Its motion is tangent to the model center; a negative speed reverses that direction. The encounter rotates the vessel’s relative velocity without changing its magnitude in the star’s frame.
Model and limits
This is a spherical Hernquist thought experiment, not a fitted Milky Way model or a flight plan. The starting radius is 8.2 kpc, circular speed 220 km/s, and scale radius 20 kpc. Imported solar speed is aligned with galactic rotation; the planetary route’s direction and travel time are not transferred. Stars are placed by hand, with instantaneous unpowered flybys. No real star rendezvous, disk, central black hole, radiation, propulsion or relativity is modeled. Limit: 20 steps. Escape means nonnegative orbital energy in this model, not crossing a drawn boundary.
Sources: Hernquist potential · galpy · NASA · Gravity assists