Gravity is always pulling each planet straight toward the Sun, so why don't they just crash in? Because they are also moving sideways, fast. By the time a planet falls a little closer, it has already shot past the Sun, so it falls toward it from a new angle instead. It keeps falling and keeps missing, looping round and round forever. Slide the launch speed in the simulator: too slow and it spirals in, too fast and it flies away, just right and it circles.
Most people think an orbit is a balance between gravity and some outward force. In fact there is no outward push: the planet is simply in free-fall, moving sideways fast enough to keep missing the Sun forever.
What's actually happening
The puzzle hides inside the word 'orbit'. We picture planets as gently circling, held at a polite distance, when in truth gravity is yanking each one straight at the Sun with enormous force, the same force that makes an apple fall. Left to gravity alone, every planet would drop into the Sun within months. Something has to be saving them, and it is not an outward push. There is no outward push.
Isaac Newton cracked it with a thought experiment about a cannon on an impossibly tall mountain. Fire the cannonball gently and it arcs down and lands nearby. Fire it harder and it lands farther, because while it falls, the curved Earth is also dropping away beneath it. Fire it hard enough and the ground curves away exactly as fast as the ball falls — so it never reaches the ground at all. It keeps falling and keeps missing, all the way around the planet, back to where it started. That is an orbit: not a balance of forces, but free-fall that goes sideways fast enough to perpetually overshoot. A planet is a cannonball that was fired so fast it has been missing the Sun for billions of years.
The speed has to be tuned. Too slow and the sideways motion can't keep up with the fall, so the path spirals inward toward a crash. Too fast and the planet outruns the Sun's grip and escapes on a one-way path into deep space. In between lies a whole family of stable ellipses. This same arithmetic explains the lineup of the solar system: a planet close to the Sun feels a fiercer pull and must travel faster to keep missing, so Mercury races around in 88 days while distant Neptune crawls through a 165-year lap. Astronauts on the space station are doing the very same thing in miniature: they aren't weightless because there's no gravity up there (there's almost as much as on the ground) but because they, and their station, are in constant free-fall, forever missing the Earth.
An orbit is just falling and endlessly missing, which is also why astronauts float despite feeling nearly full Earth gravity.
- 1Half-fill a small bucket with water and swing it in a fast vertical circle on a sturdy handle, over grass in case it slips.
- 2At the top, upside down, the water stays in: it is trying to fly off in a straight line, and the bucket keeps pulling it inward, exactly as the Sun pulls a planet.
- 3Now swing too slowly and the water falls out: too little sideways speed and the inward pull wins. That is a planet spiralling into its star.
Common questions
They do. The station feels nearly full Earth gravity, but the whole thing is in continuous free-fall, curving around the planet at about 28,000 km/h. The crew float only because they are falling alongside it.
A planet closer to the Sun feels a fiercer pull and must travel faster to keep missing it. Mercury races round in 88 days at 47 km/s, while distant Neptune ambles at 5 km/s and takes 165 years.
Too slow and the sideways motion can't keep up with the fall, so the path spirals inward to a crash. Above escape speed, about √2 times circular speed, the body outruns the Sun's grip and leaves on a one-way path.