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Mariner 5 passed within 2,480 miles (3,990 km) of Venus and sent back the most detailed data yet gathered about what it is like there.
The probe was travelling at about 19,122 miles an hour (30,774 km/h) as it began the fly-by at 1734 GMT. After three minutes, communications were broken as it went behind the planet and out of the line of sight of the Earth.
It resumed transmissions 23 minutes later.
During the fly-by, the spacecraft's instruments measured the amount of hydrogen and oxygen in the upper fringes of the planet's atmosphere.
From this information, US scientists hope to work out how much carbon dioxide is there, and also double-check the data sent back from the Soviet probe, Venera 4, yesterday.
That probe was the first to penetrate the dense layer of cloud which permanently shrouds Venus.
It sent back information for 94 minutes after entering the atmosphere, but then suddenly disappeared.
Before it went, Venera 4 had already registered a temperature of 536 degrees Fahrenheit (280 degrees Celsius) - hot enough to melt lead.
It also found high-level winds whipping round the planet at six times the force of a hurricane.
Venus is often called the Earth's sister planet as it is similar in size and density, but scientists have now ruled out any possibility of life there.
The logistics involved in getting Mariner 5 to curve around the front of Venus with only about 2,500 miles (4,000 km) to spare was likened by scientists to an insect being able to cut across the front of a thrown ball with just half an inch (1.25cm) to spare.
The tiny spacecraft was powered by four solar panels and weighed just 244.9kg (540 lbs), as opposed to the Venera 4 which tipped the scales at about a ton.
It was originally a back-up for the Mariner 4 fly-by past Mars, but was refurbished for its trip past Venus instead.
More and more future planetary missions are expected to use a "standard basic design", with only key components being different for each mission.
The vision of Venus revealed after Mariner 5's data was analysed was described by Nasa scientists as "a hell-hole".
The planet was revealed as extremely hot and unpleasant, with an atmosphere high in carbon dioxide.
The heat is partly caused by an extreme "greenhouse effect" - now informing scientists researching the same effect on Earth.
The Mariner probes continued to explore deep space in the years that followed, taking detailed photographs of Venus, Mercury and Mars.
Mariners 11 and 12 were renamed the Voyager probes.
In 1989, Nasa launched the Magellan spacecraft from the space shuttle.
Between 1990 and 1994 it orbited Venus, mapping the majority of the surface in unprecedented detail before plunging into the atmosphere and disintegrating under the massive pressures.
The European Space Agency is planning to launch a further mission, known as the Venus Express, in October 2005.
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