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Gravity, Kepler's laws, orbital elements, speeds, escape, the rocket equation and how spacecraft travel between worlds.

NASA/JPL/Space Science Institute

  1. Newton's gravity

    • Newton's gravity · 12 min

      One law explains falling apples, the Moon's orbit and the path of every planet. Fire Newton's cannon and find out how.

  2. Kepler's laws

    • Kepler's laws · 14 min

      Three rules found from careful observations in the early 1600s still describe every orbit, from moons to exoplanets.

  3. Orbital elements

    • Orbital elements · 14 min

      Six numbers pin down any orbit in space: its size, shape, tilt, orientation and where the body is on it.

  4. Orbital speed: the vis-viva equation

    • Orbital speed: the vis-viva equation · 12 min

      One short equation gives the speed of anything in orbit at any point, if you know how far it is and the size of its orbit.

  5. Escape velocity

    • Escape velocity · 12 min

      How fast do you need to go to leave a world for good? And why rockets do not actually need to reach it at the surface.

  6. Why an orbit is a fall

    • Why an orbit is a fall · 10 min

      Astronauts float not because there is no gravity up there, but because they are falling all the time, and missing.

  7. Reaching orbit: the gravity turn

    • Reaching orbit · 12 min

      Why rockets go up for only a few seconds, then spend ten minutes going sideways: the gravity turn, staging and orbital insertion.

  8. The rocket equation and delta-v

    • The rocket equation and delta-v · 12 min

      Why rockets are mostly fuel, why they come in stages, and the one equation behind every mission's budget.

  9. Hohmann transfers

    • Hohmann transfers: changing orbits · 12 min

      To go higher you speed up, and then you arrive slower. The two-burn manoeuvre that moves satellites from low orbit to geostationary orbit.

  10. Launch windows

    • Launch windows: the road to Mars · 12 min

      You cannot aim at Mars, only at where Mars will be in eight months. Why Mars missions leave every 26 months, all at once.

  11. Orbital insertion: getting captured

    • Orbital insertion: getting captured · 12 min

      Arriving at a planet is the most dangerous minute of a mission: one burn decides whether you stay or fly past forever.

  12. Gravity assists

  13. Lagrange points

    • Lagrange points: balance in the sky · 11 min

      Five places around every pair of worlds where a spacecraft can ride along for free, and why the James Webb Space Telescope lives at one of them.

  14. Rendezvous and docking

    • Rendezvous: catching the space station · 11 min

      To catch something ahead of you in orbit, you drop lower and go slower-looking but faster. The counter-intuitive art of meeting in space.

  15. Coming home: reentry

    • Coming home: reentry · 11 min

      A small push slows a spacecraft by a hundred metres per second, and the air takes care of the other 7.8 kilometres per second, as a fireball.

  16. Porkchop plots

    Coming soon

  17. Asteroids and comets

    Coming soon

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