America's unmanned 'Viking' space mission to Mars is entering its final stages for landing two craft on the planet later this year.

The Viking Control Centre is in Pasadena, California. It is there that the mission's progress is constantly monitored. Recently, all of the Centre's staff spent eleven days rehearsing the first Viking landing.

Viking One was launched last August and is now about 111 million miles (about 180 million kilometres) from Mars. Viking Two launched last September is trailing close behind. Both craft are travelling at a speed of 59,000 miles per hour (95,000 kilometres per hour).

The Vikings are due to go into orbit around Mars on 19 June and 7 August respectively. Viking One's landing craft is scheduled to land on America's 200th. anniversary of Independence - 4 July, 1976.

Both craft have equipment used for probing the planet's surface in search of any evidence of life. Soil samples taken at the landing will be examined and the results transmitted to Earth.

Part of this film is serviced with a commentary by Dr. Carl sagan, Director of Planetary Studies at Cornell University and one of the Viking scientists. A transcript appears below.

SYNOPSIS: The planet Mars.....soon it's hoped to reveal some of its secrets. Two unmanned American spacecraft - Vikings One and Two - are preparing to go into orbit around Mars later this year. Viking One is due to land its probe craft on the planet on America's bicentennial day - 4 July. The second should land two months later. One of the mission's scientists - Dr Carl Sagan - explained recently what it is hoped the probes will reveal about Mars.

The Viking Control Centre in Pasadena monitors the mission's progress continuously. A full dress-rehearsal for the first landing took place and lasted eleven days. If all goes well, tests carried out by the Vikings could reveal if life does exist on Mars.