Are we alone in the universe? That is a question that we have faced since 1958. Planetary and lunar missions have been trying to prove that we are not alone in this universe since 1958 and are scheduled to do so up until 2004. Scientists are looking for water, organic molecules and an energy source that can support life on planets other than Earth. Life is defined as being a quality that separates a vital and functional being from an inanimate object. There are three properties that are considered attributes of living organisms: metabolism, growth, and reproduction. Living things use energy from their environment to live. Living things are organized into units called cells and living things also respond to stimuli. Living things develop and adapt to their surroundings, to survive in the world. They have to reproduce for future generations to keep their species from going extinct. Scientists use these three attributes of life while looking for life existence on other planets and moons in our solar system. Many voyages and expeditions have been made into outer space in the search for extraterrestrial life. .
One of the largest expeditions into space was in 1975, when NASA sent its Viking Mission to Mars. It became the first project in history to land a spacecraft safely on the surface of another planet. This mission was composed of two spacecrafts, Viking 1 and Viking 2. Each of these spacecrafts consisted of an orbiter and a lander. Each orbiter-lander pair flew together and entered Mars orbit; after some time the landers then separated and descended to Mars" surface. Viking 1 was launched on August 20, 1975 and arrived at Mars on June 19, 1976. Viking 2 was launched on September 9, 1975, and entered Mars orbit on August 7, 1976. The main mission objectives were to obtain high-resolution images of the Martian surface, characterize the structure and composition of the atmosphere and surface, and search for evidence of life on the planet.