What does it mean for something to be in orbit?
Before we dive in, let’s do a quick review to appreciate what it actually means for an object to be in orbit.
As Isaac Newton’s famous first law of motion states, an object in motion will stay in motion at a constant speed and travel in the same direction unless acted upon by an unbalanced force. For satellites, that “unbalanced force” is the pull of gravity. An object launched to space would otherwise keep moving in a straight line, but Earth’s gravity pulls it back. The result is a kind of tug-of-war in which an object’s momentum is counteracted by the pull of gravity.
An object’s forward momentum needs to be balanced with the force of gravity; otherwise, it won’t stay in orbit. An object moving too slowly will re-enter the atmosphere and fall to Earth, while an object moving too quickly will zoom past our planet. This happy place for satellites, whether natural or artificial, is known as the orbital velocity. For example, a satellite flying 150 miles (242 kilometers) above Earth requires an orbital velocity of roughly 17,000 miles per hour (27,400 km/hr), according to NASA. The higher the orbit, the slower that orbital velocity needs to be.
It’s a misnomer to refer to satellites as “floating” in space. More accurately, objects in orbit are free-falling, but because they’re moving so quickly and in a direction that’s horizontally aligned with the curvature of the Earth, they just keep spinning around and around the planet.
All orbits are elliptical, though some can be nearly circular. The planets orbit the Sun in near-circular orbits, while comets coming in from the Kuiper Belt are caught in highly elliptical orbits. These same rules apply to artificial satellites, though on a much smaller scale.
