Orbits

What is an orbit?

An orbit is what happens when an object is falling toward another body but also moving sideways fast enough to keep missing it. The Moon, for example, doesn’t fall to Earth: it falls around Earth, its path continually bent by gravity.^[2]

Newton’s cannon

Isaac Newton explained orbits with a thought experiment. Imagine a cannon on a very high mountain, firing cannonballs horizontally. A slow cannonball falls to the ground nearby. A faster one lands farther away. Fire it fast enough, and it falls all the way around Earth in a complete circle, never reaching the ground.^[1] That is an orbit.

How fast?

• A satellite in a circular orbit near Earth travels at about 8 km/s (about 17,500 miles per hour).^[1]
• At that speed it circles the planet in about 90 minutes.^[1]
• To escape Earth’s gravity entirely takes about 11 km/s, Earth’s escape velocity.^[1]

What shape are orbits?

Orbits are ellipses, stretched circles. Each planet moves around the Sun in an ellipse, with the Sun at one of its two foci.^[3] Two numbers describe an ellipse:^[3]

• the semimajor axis, half the longest distance across, which sets the orbit’s size;
• the eccentricity, which says how stretched it is.

Bigger orbits take longer

The farther a planet is from the Sun, the longer it takes to complete an orbit. Kepler’s third law puts numbers on this: P² = a³, where P is the period in years and a is the semimajor axis in astronomical units (Earth–Sun distances).^[3] Newton later showed that the full version also depends on the masses of both bodies.^[2]

Travelling between planets

Getting from one planet to another is a matter of changing orbits. Spacecraft can use gravity-assisted encounters with planets along the way. Voyager 2 used a chain of them to visit Jupiter, Saturn, Uranus and Neptune between 1979 and 1989.^[1] To go into orbit around a destination, a spacecraft must fire a rocket to slow down when it arrives.^[1]