By the time of humanity’s first close encounter with the Solar System’s first planet, when the Mariner 10 space probe passed 700 km from Mercury on 29 March 1974, missions to our planetary neighbours had been underway for more than a decade. Just over four dozen missions to Mars and Venus had been attempted by the Soviet Union and United States, with 17 successful.
The repeat visits to our two closest neighbours before the attempt on Mercury were due to the degree of difficulty involved: for a planet closer to the Sun, the greater the gravitational forces a spaceship must overcome to change its trajectory and speed. To send a spaceship to Mars (an average 78.3 million km away) is therefore easier than to Venus (an average 41.4 million km, but closer to the Sun). Sending a probe to Mercury (an average 91.7 million km from Earth, and just 57.9 million km from the Sun) requires more fuel than sending one to the outer limits of the Solar System.
So Mariner 10 was notable – and not just for being the first mission to Mercury. It was also the first mission to visit two planets, reaching its final destination by sling-shotting around Venus in the first use of the gravitational assist manoeuvre that is now a standard part of space navigation; and despite being the seventh probe to reach Venus, it provided the first detailed images of the planet. Another first was the use of an experimental ‘X-band’ high-frequency transmitter.
The Mariner was built around an octagonal body 1.4 m across and 4.5 m deep, with eight separate electronics compartment protected by thermal blankets. It was equipped with just two solar panels, unlike the rest of the Mariner series that had four. There were two sets of reaction jets for stabilising the craft on three axes, and 29 kg of liquid fuel to power them. All up the craft weighed about 500 kg.
Onboard instruments included a radiometer to detect infrared radiation; two spectrometers to detect ultraviolet radiation and emissions; plasma detectors to study solar wind; magnetometers to detect magnetic fields; telescopes to detect whether any magnetic fields had captured charged particles; and two 15 cm telescopes connected to a video imaging system. These were positioned on the side of the spacecraft intended to face away from the Sun, to protect the electronics from overheating. On the other side of of the craft an umbrella-like sunshade provided extra shielding from solar radiation.
These photos of the the Earth and Moon were taken from 2.6 million km away.
Shortly after its launch on 3 November 1973, Mariner 10 turned to take pictures of the Earth and Moon. The purpose was to calibrate its television cameras in preparation for its main mission. It was the first spacecraft capable of returning high-resolution digital color image data. Two images, taken at a distance of about 2.6 million km, are combined here to show their relative sizes.
Travelling at a speed of about 38,600 km/h, Mariner 10 reach its first mission objective three months later. On February 5, 1974 the spacecraft came within 5,631.5 km of the surface of Venus.
During its time in close proximity to Venus, Mariner 10 took about 4000 images and carried out several important scientific observations despite severe technical problems that meant only a few of the thousands of commands for subsystems were executed by the probe’s onboard computer.
While Venus appears featureless in normal light, due to its blanketing cloud, ultraviolet camera filters showed a much more dynamic atmosphere. The marbled look of the planet’s atmosphere indicates where more or less ultraviolet radiation is being absorbed. Based on observations from Earth, scientists had expected to see a blotchy surface, but the degree captured by Mariner 10 was a surprise to most, leading to much theorising about the causes. That speculation has continued to this day.
Venus’ gravity field first slowed the velocity of Mariner 10 to about 16,000 km/h and then slung it onto a new path toward Mercury.
About five weeks later, on March 23, Mariner 10 began taking intermittent pictures of Mercury. At first, from at a distance of 5.3 million km, the images were little better than what could be observed through a telescope on Earth. But steadily they improved. By the time Mariner 10 was within 3.5 million km of Mercury the images being received back on Earth indicated how mottled the planet’s surface was, giving scientists an inkling of it being a crater-scarred landscape like the Moon.
On March 28, Mariner 10’s surveillance kicked into high gear, taking a picture every 42 seconds. On March 29 it reached its closest point to the planet, passing within 705 km of the surface. This flyby was over the planet’s dark side, so provided no usable photos, but the probe did return high-quality images before and after. It continued to take and send photos until April 3, when it was more than 3.5 million km from Mercury. All up it took and transmitted more than 2,000 photographs, which were received by tracking stations of the Deep Space Network in Goldstone, California; Tidbinbilla, Australia; and Madrid, Spain.
From the observations Mariner sent home, scientists learned Mercury, while dense, has a magnetic field that is just 0.016% as strong as Earth’s.
The images of Mercury revealed a planet very much like our own Moon – indeed the planet is only 1.4 times the diameter of Earth’s satellite, though with a mass 4.6 times greater. Its surface is a barren scape of basins, craters, scarps, ridges and plains.
“Where the plains are absent, overlapping craters and basins form rugged terrain,” noted the technical report prepared by the Jet Propulsion Laboratory for NASA. “The plains materials have many of the same characteristics of the lunar maria and have been cratered to approximately the same degree. This twofold division of the morphology of Mercury is similar to that of the Moon.”
Mariner 10’s trajectory would bring it back into contact with Mercury twice more, looping around the Sun to intercept and complete close flybys in September 1974 and March 1975. On its third flyby it came within 330 km of the planet. Soon after it depleted all its fuel, and NASA turned off its transmitter on 24 March 1975.
All told, Mariner 10 took about 7,000 photos of the Earth, Moon, Venus and Mercury.
It would be the last mission in NASA’s Mariner program (with two slated missions to visit Saturn and Jupiter becoming the first missions the Voyager program to explore the outer Solar System). It was almost 33 years before the next mission, MESSENGER, visited Mercury, arriving in January 2008.
Despite being the last of the Mariners, Mariner 10 remains a mission replete with firsts.
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