1971: American probe orbits Mars

The American space probe, Mariner 9, has become the first spacecraft to orbit another planet, swinging into its planned trajectory around Mars without a hitch.

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.

Dust storm

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".

Mountains

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.

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