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An engine burn at 2337 GMT put the craft into an elliptical orbit around the Red Planet set to take it within 800 miles (1,290 km) of the surface.
Three previous missions, Mariners 4, 6 and 7, have flown past Mars, but none has gone nearer than 2000 miles (3,200 km).
Mariner 9 is due to circle the planet twice a day for three months, sending back more than 5000 pictures covering 70% of the surface.
It's hoped it will map the planet's white polar caps, believed to consist of carbon dioxide, as well as provide vital clues about the possible existence of life.
However, the first photographs, taken on the approach to the planet, have been disappointing.
A vast dust storm which began on 22 September has been sending huge red clouds at high speeds across much of the planet's surface.
The setback led to a groundbreaking exercise by Nasa scientists, who had to re-program the space probe over millions of miles of space to wait until the dust storm had abated before continuing with its mission.
Geologists hoping for a closer look at the features on Mars are dismayed, but atmospheric scientists are delighted.
Dr Bradford Smith, one of the Mariner investigators, said the storm was "unprecedented", and said it was "an unusual opportunity to study a dynamic atmospheric phenomenon on Mars".
Experts say the storm is beginning to clear, however, and already some features are visible through the gloom.
Some of the first to be released by the Jet Propulsion Laboratory in California, which runs the Mariner missions, show four striking black dots rising above the mist.
Scientists say they are mountain peaks near the south polar cap.
The Soviet authorities have informed Nasa that two Russian probes, Mars 2 and Mars 3, will, as had been suspected, attempt a landing on Mars once they arrive.
The two probes have been trailing closely behind Mariner 9 on its five-month journey to the planet.
For the first time, a "hot line" has been set up between the Jet Propulsion Laboratory and the Soviet Academy of Sciences to exchange important findings.
The Americans are due to attempt their first landing on Mars with the Viking mission in 1976.
It was another month before the storm died away and scientists saw the first truly clear pictures of the surface of Mars.
They showed a planet of huge seismic activity, with gigantic volcanoes over twice the height of the largest found on Earth, and a grand canyon stretching 3000 miles (4800km) across its surface - the largest in the solar system.
There were also channels which suggested that water once flowed on Mars.
Mariner 9 orbited the planet for far longer than expected. Its two television cameras returned 7329 photographs before contact ended on 27 October 1972.
The Soviet probe, Mars 2, arrived shortly before Mariner 9 and crashed due to the dust storm. Mars 3 touched down on the surface, but malfunctioned after 20 seconds.
The first successful landing on the Martian surface was carried out by Viking 1 five years later in 1976.
Another period of intense exploration of Mars began when the Mars Observer arrived in 1993.
The most successful was the Mars Odyssey in 2001, which drew up the most detailed geological map of the planet ever, transforming our knowledge of what Mars is made of.
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