Mercury, the smallest planet in our solar system and the closest to the sun, took on a number of personas to ancient civilizations.
To the Egyptians it was Thoth, the god of knowledge, speech, writing and arithmetic. To the Greeks it was alternatively known as Apollo (the god of truth, arts, archery, plagues, and divination) and Hermes (the god of writing and messenger to the other gods), depending on whether it rose in the morning or evening. But details of the planet beyond its orbit were unknown until we visited it with the Mariner 10 spacecraft 40 years ago.
The Mariner program was born during the Apollo era to explore the inner planets of the solar system. It was a compliment to the Pioneer missions that aimed at the outer giant planets, and the final two spacecraft, Mariners 11 and 12, were eventually repurposed into the Voyager 1 and 2 spacecraft.
There were ten Mariner spacecraft built and launched. Three were lost to launch vehicle failures, four went to Mars, two were aimed squarely at Venus, and one was designed to explore Mercury. It was Mariner 10, the last of the series.
Mariner 10, like all the Mariner spacecraft, was octagonal in shape and made of magnesium and aluminum. It was squat and long, measuring about 18 inches high and 4.5 feet in diameter. With its solar panels deployed -- flying toward the sun, it used solar power for electricity -- it measured a substantial 26 feet across. Its science payload was also fairly common for the Mariner family. Its suite of instruments included cameras, a magnetometer, a plasma science experiment, a charged particle telescope, an ultraviolet spectrometer and an infrared radiometer.
Mariner 10 launched on an Atlas Centaur rocket on Nov. 3, 1973. Not long after leaving the Earth, it turned around and looked back at our planet, snapping a stunning picture of our home planet and the moon together in one frame.
Its first planetary encounter was with Venus. Mariner 10 took some 4,000 images of the second planet from the sun, revealing little beyond the thick clouds enveloping the planet. Other instruments revealed Hadley-type circulation in Venus' atmosphere, as well as confirmed the existence of a weak magnetic field surrounding the planet and an ionosphere that interacted with the solar wind to form a bow shock.
Mariner 10 used its encounter with Venus to slingshot itself on a path to Mercury, the first time a gravity-assist maneuver was done on a multi-planet mission. The spacecraft reached Mercury and made its first flyby on March 29, 1974.
Photos taken as the spacecraft approached its target showed scientists a moon-like surface: the planet was heavily cratered and barren. The spacecraft's magnetometers found only a weak magnetic field. Radiometer readings found temperatures vary significantly on the small world, from -297°F in the dark to 368°F in the sunlight.
This first came on March 29, 1974 at a range of 436 miles from Mercury’s surface. A second flyby happened on Sept. 21, 1974; the spacecraft passed 29,868 miles from the surface. The third and final flyby came on March 16, 1975 and it was the closest pass. Mariner 10 flew within 203 miles of the planet’s surface. NASA’s last contact with the spacecraft came less than two weeks later on March 24.
Photographs and scientific measurements taken during the Mariner 10’s three passes revealed a wealth of information about the planet. The mission confirmed that Mercury has no appreciable atmosphere, a small magnetic field, and a relatively large iron-rich core. Just under half the planet was imaged by the spacecraft, and though it was far from uniform in appearance, the planet was found to be uniformly cratered and ragged.
Like so many early survey missions, Mariner 10 raised more questions about Mercury than it answered, like, what is the planet’s internal structure? Is there volcanic activity? What is the story behind Mercury’s atmosphere and magnetosphere? Ground-based observations after the mission ended shed some light on the remaining mysteries, as has NASA’s MESSENGER mission, the only spacecraft dedicated to exploring Mercury since Mariner 10 launched.