Table of Contents

Phase 2 - In Orbit

Teacher Backgrounder

In Orbit!

An orbit is the path of an object, typically called a satellite, as it moves around a second object or point. For example, the Moon is a satellite of the Earth and the Earth is a satellite of the Sun.

Orbits can be characterized by their eccentricity, period and inclination.

Eccentricity

A satellite's orbit is typically in the shape of an ellipse. An ellipse can be thought of as a circle that is slightly stretched in either the vertical or horizontal axes. Unlike the perfect circle that has a single point in its center called the focus, which is equidistant from any point on the circle, the ellipse has two focal points.

As the focal points on one of the axes move farther and farther apart, the shape of the orbit becomes more and more elliptical, and thus more eccentric.

Satellite orbits range from nearly circular orbits (with a low eccentricity) to very highly elliptical orbits (with a high eccentricity).

As a satellite becomes more elliptical in shape its distance from the Earth's surface changes. The point at which the satellite is closest to the Earth is called perigee. The point when the satellite is farthest from Earth is called apogee. A satellite’s orbital velocity will increase as it moves toward perigee. Conversely, its orbital velocity will decrease as it approaches apogee.

Period

The amount of time it takes for a body to complete one full orbit around a second body is called a period. The closer an object is to the body that it orbits, the higher its velocity and the shorter its period. For example, Earth’s period around the Sun is 365 ¼ days or one Earth year. Mars, the next planet in our solar system that is farther from the Sun than Earth, has a period of 687 days or one Martian year.

Inclination

The tilt of the orbit plane with respect to the Equator is referred to as the inclination. An orbit with an angle of 0º would orbit the Earth in the same plane as the Equator and is called an equatorial orbit. Conversely, an orbit with an inclination angle of 90º, called a polar orbit, would move perpendicularly to the equatorial plane and cross the North and South Poles. Most satellites have orbits with inclinations between 0 and 90º.

For further information on orbits and manoeuvring in orbit please consult the CSA Orbital Mechanics web resource.