Some of these are natural satellites, called moons, orbiting the planets. Note, Mercury and Venus do not have any moons. In addition two classes of smaller objects orbit the Sun. These objects are the asteroids and the comets which are believed to be small chunks of material left over form the formation of the solar system, and they will be discussed later in lecture 6. Asteroids and comets differ from each other in composition and in the nature of their orbits.
Asteroids have smaller semimajor axes than comets.
One of the most popular comet is Haley's comet with orbital revolution time of 76 years. Although there is no difficulty in maintaining an artificial satellite in orbit a great deal of energy and planning is required involving complex calculations to lift it off the Earth and accelerate it to its required orbital speed. Once the satellite is in orbit, if it is high enough to be free of retarding atmospheric frictional effects, it will remain in orbit forever. Let's illustrate how a satellite is launched. Having a mind fig.