• Satellites are classed as either natural or artificial.satellite orbiting earth and moon image by patrimonio designs from Fotolia.com
  
  Millions of objects orbit Earth. One is large and obvious, while many more lurk largely unseen. Earth has one natural satellite, thousands of artificial ones and millions of particles of spacecraft debris circling it. The first artificial satellite, Sputnik I, was launched by the Soviet Union in 1957. Since then, the United States, Russia, France, the United Kingdom, Japan, China, India, Israel and Iran have launched thousands more.
  
  

The Moon

• The moon shines only by reflected light. (See References 1)moon image by Stefan H?¡èuselmann from Fotolia.com
  
  The moon is Earth's only known natural satellite. Its diameter is about a quarter that of Earth's and its mass approximately one-eightieth. The moon's surface is heavily cratered--indicating that it is very old and inert--unlike Earth's surface which is constantly changing due to vulcanism, plate tectonics, the oceans and weather. There is no weather on the moon. In fact, there is effectively no atmosphere at all, meaning that from the surface the lunar sky appears black, even in daytime. The moon orbits at an average distance of thirty times Earth's diameter, and does so in the same period of time that it rotates once. Consequently, one hemisphere constantly faces Earth, while the far side can never be seen from our planet.
  
  

Unmanned Artificial Satellites

• A communications satellite can relay thousands of telephone calls simultaneously.Illustration of a satellite image by Nicemonkey from Fotolia.com
  
  According to NASA, out of 6,000 artificial satellites launched, 3,000 still orbit Earth in a useful condition. Communications satellites relay TV programs and telephone calls from one part of the world to another; scientific satellites study our planet's geology, vegetation and weather, and observe the bodies of the solar system and beyond; navigation satellites provide reference points for GPS, while military satellites spy on real or potential enemies. Artificial satellites must be launched accurately into their orbits, which take a variety of forms. Communications and weather satellites often use a geosynchronous orbit--in the Earth's equatorial plane, at an altitude of 22,300 miles and taking 24 hours to complete--meaning that they remain in the same place in the sky, enabling Earth-based antennae to remain fixed on them. Satellites that require a close look at Earth use lower orbits, some of which are polar--the satellite flies over both poles--so that the satellite can closely observe every part of the planet over time.
  
  

Manned Spacecraft

• The International Space Station is easily visible from Earth with the naked eye.satellite image by photlook from Fotolia.com
  
  Since Yuri Gagarin became the first human being to orbit Earth in 1961, Russia, the United States and China have launched 500 more men and women of many nations into space. The International Space Station, a collaborative project of the space agencies of the United States, Russia, the European Union, Japan and Canada, is the only permanently inhabited spacecraft in orbit. Construction of the ISS in space began in 1998 and will end in 2010, by which time the station will be 361 feet long--by far the largest artificial object ever to orbit Earth. (See References 5, page 1)
  
  

Debris

• A considerable amount of junk produced by human activity orbits our planet. Spent rocket stages, derelict satellites, discarded spacecraft components and trash from accidental explosions and collisions litter Earth's vicinity. NASA reports that 19,000 pieces of debris larger than four inches orbit, along with tens of millions of smaller objects, mostly within an altitude of 1,300 miles. Some of this debris will remain in orbit for decades or longer and can threaten operational spacecraft. In 2009, the first accidental collision between orbiting satellites occurred when an American communications satellite, Iridium 33, and a dead Russian military satellite, Kosmos-2251, smashed into each other at a combined speed of 7.3 miles per second, scattering yet more debris.