The First Probe to Visit Two Planets Launched on this Day in 1973
Concept art of mission. Image: NASA

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The First Probe to Visit Two Planets Launched on this Day in 1973

Mariner 10 was the first probe to visit Mercury, to use a gravitational slingshot maneuver, to hit up two planets in one mission, and to photograph Venus and Mercury up close. Not bad for a drama queen.

As the innermost and smallest planet in our solar system, Mercury is a uniquely fascinating place, so it's no wonder it was one of the earliest targets for spaceflight. NASA's Mariner 10 probe, launched 43 years ago on November 3, 1973, was the explorer that finally pulled this ambitious dream into reality.

Not only was it the first spacecraft with multiple planetary stops—Venus and Mercury—it was also the first mission to use the gravitational pull of a planet to propel it onward to a new destination. These gravity slingshot maneuvers have become an essential tool for scooting around the solar system, a convenience we owe to Mariner 10's trailblazing acrobatics some four decades ago.

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For context, the nine previous Mariner probes had been split between Earth's two closest neighbors: Mariners 1, 2, and 5 were Venus probes, and the rest were Martian flyby spacecraft or orbiters.

READ MORE: Why the First Close-Up Image of Mars Was Hastily Painted in Pastels

By the time 1973 rolled around, Mars had been posing for splashy Mariner photo shoots for nearly a decade. The Venusian flybys, meanwhile, resulted in no pictures because those missions had not been equipped with cameras due to budget constraints. (Plus, the Venus-bound Mariner 1 failed shortly after liftoff due to a typo that Arthur C. Clarke quipped was "the most expensive hyphen in history").

Point being: Mariner 10 had a lot on the line. NASA had shown that it could snap stunning pictures of Mars, but portraits of the two inner solar system worlds had never been captured, let alone by the same probe.

The mission presented unique challenges, including packing enough fuel to ensure the spacecraft could nail the gravitational pivot at Venus with precision. To survive the intense solar radiation and searing heat near Mercury, mission leads armed Mariner 10 with thermal blankets, sunshade cloth, and special heat-resistant paint, plus they allowed it to shift its solar panels into partial shade to prevent them from frying.

In spite of all this preparation and its eventual achievements, the probe threw its creators some nerve-wracking curve balls and was "very demanding to operate," according to one mission lead Donna Shirley.

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"It was ultimately a very successful mission; the spacecraft just broke all the time."

"It seemed as if we were always just patching Mariner 10 together long enough to get it on to the next phase and next crisis," Shirley said in her book Managing Martians. "It was ultimately a very successful mission; the spacecraft just broke all the time."

For instance, there were all the times that the probe's navigational star-tracker locked onto dislodged flakes of paint instead of its guiding stars, or the weird occasions when its computer randomly rebooted itself, requiring leads to reset the probe's clock to the right time. The list goes on and on; you can scroll through it in this scanned document of the original 1976 report on Mariner 10's many quirks and glitches.

But despite its drama queen leanings, Mariner 10 did end up delivering NASA a gravity-assisted victory lap around its targets. It flew by Venus on February 5, 1974, at a distance of 5,768 kilometers (3,584 miles), and snapped over 4,000 shots of the fascinating hell world.

While it was far from the only spacecraft to have approached Venus at that point, its images provided the first-ever glimpses of the planet from the perspective of a visiting spacecraft. The data collected from its suite of instruments shed new light on the atmospheric complexities and composition of Venus, and detected its weak magnetic field for the first time.

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Hurtling past Venus, Mariner 10 reached Mercury a few weeks later on March 29, passing by at a distance of only 703 kilometers (437 miles). It snapped a few images and recorded measurements on its first flyby, which revealed that Mercury was a cratered world with no atmosphere, a small magnetic field, and an iron-rich core.

It then swooped out and returned for another, more distant flyby of Mercury on September, 21, 1974, followed by the last and closest encounter on March 16, 1975. With its mission goals accomplished and its gas almost exhausted, Mariner 10 was allowed to retire into orbit around the Sun, where it is presumably still hanging out to this day.

Only a few years after the probe's historic achievements, NASA built on the experience with the Voyager probes, which used the gravitational slingshot maneuver to slalom all the way through the outer solar system. In this way, Mariner 10's enduring legacy is not just about reaching unexplored vistas, but also leaving a roadmap for adventurers to follow.

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