Mariner 4 (1965): The First Close-Up Photographs of Mars

Mariner 4 was the NASA space probe that made the first successful flyby of Mars, passing the planet on 14-15 July 1965. Its 21 grainy images were the first close-up pictures ever taken of another planet. They showed a cratered, Moon-like surface and, with other measurements, ended long-held hopes that Mars might be a living world crossed by canals.

Background: the Mars of imagination

For centuries Mars was seen through telescopes only as a small reddish disc. In the late 19th century the Italian astronomer Giovanni Schiaparelli described dark lines he called canali (“channels”), and the American Percival Lowell popularised the idea that they were canals built by an intelligent civilisation. By the 1960s most scientists rejected the canals, but many still thought Mars might have a fairly thick atmosphere and perhaps plant life that caused seasonal colour changes.

Reaching Mars was difficult. The Soviet Union attempted several Mars probes from 1960 onwards, and its Mars 1 spacecraft lost contact in 1963 on the way. NASA’s Jet Propulsion Laboratory (JPL) in Pasadena built two identical probes, Mariner 3 and Mariner 4, for the launch window of late 1964.

Launch and voyage

Mariner 3 was launched on 5 November 1964, but the protective shroud around it failed to separate and the mission was lost. Engineers quickly redesigned the shroud, and Mariner 4 was launched on an Atlas-Agena D rocket from Cape Kennedy on 28 November 1964.

The spacecraft weighed about 260 kilograms. It was powered by four solar panels and carried a television camera, a magnetometer, instruments to detect cosmic dust, charged particles and radiation, and a tape recorder to store images. It kept its orientation by locking on to the Sun and the bright star Canopus. The journey to Mars took about seven and a half months.

The flyby

Mariner 4 made its closest approach to Mars at about 9,800 kilometres on 15 July 1965 (Universal Time), which was the evening of 14 July in California. In the preceding minutes its camera took 21 complete pictures and part of a 22nd, recording them on tape. Each image was only 200 by 200 pixels, and the pictures together covered roughly 1 per cent of the planet.

As the spacecraft passed behind Mars, its radio signal travelled through the Martian atmosphere on the way to Earth. By measuring how the signal changed, scientists calculated that the surface pressure was only about 4 to 7 millibars, less than 1 per cent of Earth’s. No significant magnetic field was detected.

Mariner 4 then played back its pictures at a data rate of about 8.3 bits per second. Each one took several hours to transmit, and the complete set took more than a week. While the first image was arriving, impatient JPL staff printed out the brightness numbers on strips of paper and coloured them in by hand with pastel crayons, producing a picture before the official processing was finished.

What the pictures showed

The clearest images, especially frame 11, showed many impact craters, some tens of kilometres across. Mars looked far more like the Moon than like Earth. Combined with the thin atmosphere, this suggested a cold, dry and ancient surface where liquid water and life seemed unlikely.

The results were widely reported, and many newspapers described Mars as a dead world. Later missions showed that this view was too simple. Mariner 4 had happened to photograph some of the oldest, most heavily cratered terrain. When Mariner 9 became the first spacecraft to orbit Mars in 1971, it revealed giant volcanoes, the vast canyon system now called Valles Marineris and dried-up river channels.

After Mars

Mariner 4 also carried a symbolic importance beyond science. It was the first successful mission to Mars, following Mariner 2’s flyby of Venus in 1962, and it showed that a spacecraft could work reliably for months in deep space and send useful data across more than 200 million kilometres.

Mariner 4 continued in orbit around the Sun. It was designed to last about eight months, but engineers stayed in contact with it, apart from a long gap while it was too far away, until it ran low on attitude-control gas. The mission officially ended on 21 December 1967.

Timeline

• 1964, 5 November: Mariner 3 is launched but fails because its shroud does not separate.
• 1964, 28 November: Mariner 4 is launched from Cape Kennedy.
• 1965, 14-15 July: Mariner 4 flies past Mars and takes 21 complete images.
• 1965, late July: The pictures are played back to Earth over more than a week.
• 1967, 21 December: The Mariner 4 mission is ended.
• 1971, 14 November: Mariner 9 becomes the first spacecraft to orbit Mars.

Why it matters

Mariner 4 was the first spacecraft to show humans the surface of another planet up close. It replaced centuries of speculation with data, and its results shaped every later mission, from the Viking landers of 1976 to the Curiosity rover, as scientists searched for the wetter Mars that its few grainy pictures had missed.