Physics · Astronomy
Orbits & Gravity
Launch a moon, find escape velocity, fly a Hohmann transfer and watch three-body chaos.
Also on your phone · Orbit Lab
The course
7 worlds · 42 lessons
World 1 · Orbit Basics
FREEHow gravity makes a path- 01What is an orbit?Falling forever, sideways · 6 min
- 02Circular orbit speedThe one speed that stays round · 7 min
- 03Escape velocityThe speed that never comes back · 8 min
- 04Kepler's First LawOrbits are ellipses · 7 min
- 05Kepler's Second LawEqual areas in equal times · 6 min
- 06Kepler's Third LawBigger orbits take longer · 8 min
- 07Fall, orbit, or escapeOne slider, three destinies · 7 min
- 08Eight planets, one distance scale
World 2 · Solar System Missions
PROFly to every world- 01Mission: MercuryOrbiting deep in the Sun's well · 7 minPRO
- 02Mission: VenusEarth's scorched twin · 7 minPRO
- 03Mission: EarthHome orbit, 29.8 km/s · 7 minPRO
- 04Mission: MarsThe outbound step · 8 minPRO
- 05Mission: JupiterKing of the planets · 8 minPRO
- 06Mission: SaturnLord of the rings · 8 minPRO
- 07Mission: UranusThe sideways ice giant · 7 minPRO
- 08Mission: NeptuneThe far frontier · 7 minPRO
- 09Bonus: Moon transferEarth to lunar orbit · 8 minPRO
- 10Bonus: Mars transferThe interplanetary Hohmann · 8 minPRO
World 3 · Satellite & Rocket Engineering
PRODesign real orbits- 01Altitude vs. speedThe engineer's trade-off · 7 minPRO
- 02Low Earth OrbitWhere most satellites live · 7 minPRO
- 03Geostationary orbitHanging over one spot · 8 minPRO
- 04Hohmann transfersThe cheapest way up · 8 minPRO
- 05The delta-v budgetCurrency of spaceflight · 8 minPRO
- 06Inclination & plane changesThe expensive turn · 7 minPRO
- 07Gravity assistsStealing speed from planets · 8 minPRO
- 08Reentry & deorbitComing home · 7 minPRO
World 4 · Deep Space: Multi-Body
PROWhere Kepler breaks- 01When Kepler breaksTwo masses, no formula · 8 minPRO
- 02The three-body problemOrder dissolves into chaos · 8 minPRO
- 03The figure-eightA rare perfect choreography · 8 minPRO
- 04Lagrange pointsWhere forces balance · 8 minPRO
- 05Binary star systemsTwo suns, tangled orbits · 7 minPRO
- 06Chaos & sensitivityThe butterfly in the sky · 8 minPRO
World 5 · The scale of our Solar System
PROSeparate size from distance, then combine them
World 6 · Stars and galaxies
PROHow a star’s mass sets its life, its death and the shapes of galaxies- 01Heavy stars burn out fastMass, luminosity and main-sequence lifetime · 9 minPRO
- 02The Sun as a red giantWalking across the Hertzsprung–Russell diagram · 9 minPRO
- 03White dwarf, neutron star or black hole?How the initial mass decides the ending · 9 minPRO
- 04Sorting galaxies on Hubble’s tuning forkEllipticals, lenticulars, spirals, barred spirals and irregulars · 9 minPRO
World 7 · Around a black hole
PROGeneral relativity: precessing orbits, the last stable orbit, bent light and slow clocks- 01Orbits that do not closePeriapsis precession in curved spacetime · 10 minPRO
- 02The last stable orbitWhy nothing circles closer than 6M for long · 10 minPRO
- 03Light that escapes, light that falls inBending of light and the photon sphere · 10 minPRO
- 04Slow clocks near a black holeGravitational time dilation · 10 minPRO
Read first
All guides →Guides for Orbits & Gravity
Exam prepForces & MotionAP Physics 1 Forces and Motion: A Free Study GuideMap the forces and motion topics of an introductory physics course to free interactive lessons, with a four-week study plan and tips for the practice exams.
ExplainerForces & MotionHow Free Fall and Projectile Motion Work: KinematicsWhy falling speed grows steadily, why distance grows with time squared, and how splitting a throw into two parts predicts range and height. With a live physics lab.
ExplainerOrbits & GravityHow Orbits Work: Falling Sideways Around a PlanetWhy satellites never fall down, what sets orbital speed and escape velocity, and how Kepler’s three laws describe every bound orbit. With a live 3D orbit lab.