Orbita

Meet Orbita, a satellite who is always falling toward the planet, and always moving sideways fast enough to miss.

Isaac Newton imagined a cannon on a mountain firing ever faster. At a high enough speed, the ground curves away beneath the falling ball as fast as it falls, and it never lands. For a circular orbit, gravity supplies exactly the centripetal force needed: GMm/r² = mv²/r, which gives v = √(GM/r). The satellite's own mass cancels.

For an orbit 400 kilometres above Earth, about the height of the International Space Station, this gives about 7.67 kilometres per second, a lap roughly every 92 minutes. Farther out, orbits are slower.

The same formula sets the speeds of GPS satellites, the Moon and the planets. Astronomers apply it to stars in galaxies; the fact that outer stars orbit faster than visible matter can account for is key evidence for dark matter.

At 400 km there is still a trace of atmosphere, so low orbits slowly decay and need occasional boosts. Real orbits are also ellipses, not perfect circles.