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1. 1

   牛顿引力

   • Newton's gravity · 12 分钟

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

2. 2

   开普勒定律

   • Kepler's laws · 14 分钟

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

3. 3

   轨道元素

   • Orbital elements · 14 分钟

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

4. 4

   轨道速度：活体方程

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

     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. 5

   逃逸速度

   • Escape velocity · 12 分钟

     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. 6

   为什么一个轨道是一个下降

   • Why an orbit is a fall · 10 分钟

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

7. 7

   达到轨道:重力转向

   • Reaching orbit · 12 分钟

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

8. 8

   火箭方程和delta-v

   • The rocket equation and delta-v · 12 分钟

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

9. 9

   霍曼转移

   • Hohmann transfers: changing orbits · 12 分钟

     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. 10

    發射窗口

    • Launch windows: the road to Mars · 12 分钟

      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. 11

    轨道插入：被捕获

    • Orbital insertion: getting captured · 12 分钟

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

12. 12

    重力辅助

    • Gravity assists: the free slingshot · 12 分钟

      How Voyager, Cassini and New Horizons stole speed from planets, and why it never breaks the conservation of energy.

13. 13

    拉格朗日点

    • Lagrange points: balance in the sky · 11 分钟

      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. 14

    会合和对接

    • Rendezvous: catching the space station · 11 分钟

      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. 15

    回家：重返大气层

    • Coming home: reentry · 11 分钟

      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. 16

    豬肉塊

    即将上线

17. 17

    小行星和彗星

    即将上线

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