L12. Why Orbits Don’t Fall Down

Unit 3 · Gravity and Motion

R-report

L12. Why Orbits Don’t Fall Down

The Moon has been falling toward Earth for four and a half billion years and has never landed. That is not a riddle — it is exactly what an orbit is.

Newton’s cannonball

Newton imagined a cannon on top of an impossibly tall mountain, firing horizontally. - Fire it gently and the ball arcs downward and hits the ground fairly close by. - Fire it harder and it travels farther before landing, because it is still falling at the same rate but covering more ground horizontally. - Fire it hard enough and something remarkable happens. The ball falls toward Earth, but Earth’s surface curves away beneath it at exactly the same rate. The ball keeps falling and never reaches the ground. It has entered orbit. That is the whole idea. An orbit is not a balance between gravity and some outward force. There is no outward force. There is only gravity pulling inward, plus enough sideways speed that the falling path curves right around the planet. Fire it harder still and the ball escapes Earth altogether. For our planet that escape speed is about 40,000 kilometres per hour.

Two motions, one curve

It helps to separate an orbiting object’s motion into two parts happening at once. - Sideways motion: the satellite is travelling across your line of sight at high speed. Left alone, it would continue in a straight line forever. - Inward pull: gravity constantly bends that straight line toward the planet. Add them together and you get a closed curve. Change either one and the orbit changes. - Too slow and gravity wins; the object spirals in and eventually hits the atmosphere. - Too fast and it climbs away into a wider orbit or escapes entirely. - Just right and it circles indefinitely. The required speed depends on altitude. The space station, 400 km up, must travel about 28,000 km/h and completes an orbit every 90 minutes. A satellite 35,786 km up needs only about 11,000 km/h and takes a full 24 hours — which means it stays above the same point on the rotating Earth, ideal for television and weather satellites.

Orbits everywhere, and why they decay

The same physics repeats at every scale in the universe. - The Moon orbits Earth once every 27.3 days. - Earth orbits the Sun once a year at about 107,000 km/h. - The Sun orbits the centre of the Milky Way once every 230 million years or so. Orbits in space are extremely stable because there is almost nothing to slow an object down. Almost — but not quite. At a few hundred kilometres up, Earth’s atmosphere is still faintly present, and over months and years that thin drag steals energy. The space station would gradually spiral down and burn up if it were not periodically boosted back to a higher orbit. This is also how spacecraft change orbit. Firing an engine forward or backward changes the speed, and changing the speed changes the shape and size of the orbit. Every rendezvous, docking, and Moon landing is an exercise in adjusting these curves precisely.

Putting the investigation together

An orbit is falling that never ends. Gravity pulls an object inward while its sideways motion carries it past the planet, and the two combine into a curve that closes on itself. The speed required depends only on how high you are.

Newton’s cannonball is a model worth remembering because it turns something mysterious into something ordinary. The Moon is doing exactly what a thrown ball does — it is simply moving fast enough that the ground it is falling toward keeps curving away.