How does an orbit work?

The universe is filled with stars, planets and satellites that interact with one another in a variety of ways. Gravity, the same force that attracts objects to the ground on Earth, also works on objects in space, changing the speed and direction of asteroids, satellites, planets and even stars. An orbit generally occurs when one body in space is dragged into movement around another.

Cause

Newton's First Law states that an object in motion will keep moving with the same speed and in the same direction unless an unbalanced force acts upon it. The Earth and the remaining planets of the solar system orbit the sun because the sun's gravity is pulling all of the planets away from their straight paths.

Period

The period of an object in orbit is the time that it takes that object to make one trip around the primary body. This value varies depending on its distance from the body. Planets in the solar system orbit the sun with a period that is proportional to the cube of its average distance from the sun, a principle determined by Johannes Kepler.

Decay

An orbit rarely remains stable. Instead, the orbiting body loses the velocity necessary to fight the pull of gravity and eventually falls into its larger companion. This can be caused by gravitational waves or the drag of a planet's atmosphere.

Eccentricity

Most planets do not trace out a perfect circle when orbiting a star. On the contrary, planets generally follow an elliptical path. This means that their mean distance from the star changes throughout the orbit. An orbit of high eccentricity is one that follows a highly elliptical path.

Types

There are many types of orbits, each with their own peculiarities. Communications satellites orbit the Earth in a geostationary or geosynchronous orbit; since they take 24 hours to orbit the Earth, it appears that they never move. Surveillance satellites often follow a polar orbit, a 90-degree orbit that offers a wide view of Earth's surface.

Misconceptions

It's easy to imagine that only the moon or a satellite is capable of orbiting the Earth, but in fact any object that is given enough speed is capable of falling into orbit around the planet. A June 2000 count revealed almost 9,000 objects orbiting the Earth, only some 2,700 of them being satellites or space probes.

Source:

Orbital Mechanics

Types of Orbits

Space Junk

More Information:

An Overview of the Solar System