The earliest space probes were sent to the planet Venus. The reason for this was the fact that Venus is continually shrouded by dense clouds and any visual observations using telescopes from Earth are impossible. With crude equipment they gave scientists an insight into the planet's makeup. Later probes actually landed on the Venusian surface and were able to send back pictures of the surface before the high temperatures and great atmospheric pressures destroyed them.
At the beginning of the 20th century many scientists were convinced that there was a great civilization on the planet Mars but the space probe Mariner 4 discounted this theory forever showing that Mars was a dry barren world. The Viking Landers touched down on the martian surface in 1976 and immediately began their programmed experiments to determine whether there was any life in the martian soil. Details of the experiments were transmitted to the orbiter in Mars orbit. The orbiter carried the Viking Lander on its 500 million mile journey and remained in orbit to act as a relay station sending information back to Earth.
Space probes have to travel millions of miles to rendezvous with their given targets but by utilising very powerful computers back on Earth the probe's flight path can be predetermined enabling the probe to arrive at its destination. As the probe hurtles towards its destination scientists back on Earth are able to track it using very large radio antenna. Any deviations from the probe's course can be altered via this antenna then transmitted to the probe's on-board computers. The antenna is also used to pick up the probe's weak transmissions of data from their on-board cameras. Some of these transmissions may take hours to reach Earth such are the vast distances involved in exploring our solar system.
