How a Theory Helped Mariner 10 Reach the Planet Mercury

In ‘This Week in History,’ the last of NASA’s Mariner missions pursued the impossible task of reaching the elusive planet Mercury.

An artist's impression of the Mariner 10 mission. It used a flyby of the planet Venus to increase its chance of reaching Mercury when it was furthest from the sun. This would allow the spacecraft to meet Mercury on three occasions in 1974 and 1975. Public Domain

Nicolaus Copernicus once said of the planet Mercury that “the planet has tortured us with its many riddles and with the painstaking labor involved as we explored its wanderings,” according to Dava Sobel’s “The Planets.”

Copernicus, arguably history’s greatest astronomer, published his “De Revolutionibus Orbium Coelestium“ (”On the Revolutions of the Heavenly Spheres") in 1543, the same year of his death. His work established the heliocentric model of the planetary system, which upended the longstanding geocentric model.

His theory suggested that instead of the planets and sun orbiting around Earth, the Earth and the other planets in fact orbited around the sun. Along with establishing a new centricity, Copernicus also flipped Venus and Mercury, placing the latter closest to the sun. Despite his switch, a long-enduring legend says that Copernicus never actually saw Mercury.

Order of the heavenly spheres annotated with periods of revolution from Chapter 10 of Copernicus's manuscript. Public Domain

Mercury is the smallest planet in our system—about a third the size of Earth. It zooms around the sun at 29 miles per second, completing its orbit in about 88 days, hence, the reason for it being named after the fleet-footed Roman god. It maneuvers in an egg-shaped orbit, moving it as close as 29 million miles and as far as 43 million miles from the sun. Additionally, it’s about 50 million miles from Earth. If, indeed, the great astronomer had never laid eyes on the first planet in the solar system, he can certainly be excused for missing it.

The Space Race

More than 400 years later, the world still knew very little about Mercury. Its size, distance, location, and speed kept it a rather elusive planet. On Sept. 30, 1958, the employees of the National Advisory Committee for Aeronautics (NACA) were informed their company would witness a name change with one very important addition: space. On Oct. 1, the National Aeronautics and Space Administration (NASA) began operations with plans to eventually reach the celestial bodies.

NASA was embedded in the Space Race with the Soviet Union. The world witnessed a flurry of rockets blasting into space during the 1950s and 1960s. The two space programs challenged each other to be the first to put a man into orbit and then a man on the moon. The Americans lost the first race in 1961, but won the second in 1969. When the Soviet, Yuri Gagarin, became the first man in orbit, his spacecraft reached 203 miles into space. When the Americans of Apollo 11 reached the moon, they were approximately a quarter of a million miles into space.  There were some distances, however, that man could not go.

The planets of Venus, Mars, and Mercury are the closest to the Earth, but those distances vary. Although Mercury is the closest planet to the sun, it actually spends more time closer to Earth than does Venus. Venus ranges as close as 24 million miles and as far away as 162 million miles from Earth. On average, Mercury is 97 million miles from Earth. Mars, the first of the outer planets, is an average of 140 million miles from Earth.

JPL and a Theory

Just as with putting a man into orbit or on the moon, the most brilliant mathematicians and scientists in the world were required to formulate ways to visit these other solar relatives. These visitations, however, would be unmanned. The Jet Propulsion Laboratory (JPL) near Pasadena, California, which was building NASA’s unmanned lunar and planetary spacecraft, began working on a way to reach these planets.

Reaching Venus and Mars by way of an unmanned spacecraft was rather straightforward. It would require immense thrust from its rocket boosters and a precise trajectory so as not to be so far from the planet that taking images would be difficult. Thus began NASA’s Mariner missions.

The first mission, Mariner 1, launched on July 22, 1962, toward Venus, but was an immediate failure when the rocket shifted off course. Thirty-six days later, Mariner 2 launched toward Venus and became the “first successful planetary science mission in history.” Next was Mars. On Nov. 5, 1964, Mariner 3 launched, though unsuccessfully. It was followed later that month with the Mariner 4, which returned “the very first photos of another planet from deep space.” Over the next five Mariner missions, Venus and Mars remained NASA’s targets.

Reaching Venus

It was not until Mariner 10 that NASA set its sights on the elusive Mercury. To accomplish this mission, NASA had to utilize a mathematical theory that had been consistently tested by the scientists at JPL. This theory was called “gravity assist,” which found its origins in Newton’s third law of motion. The belief was that a spacecraft, upon entering and then exiting a planet’s gravitational pull, could greatly increase its velocity as well as alter its trajectory. NASA approved the Venus-Mercury mission in 1969.

JPL built a 1,100-pound and 32-foot end-to-end satellite, armed with an X-band radio transmitter, two TV cameras, an electron spectrometer, magnetometer, radiometer, high-gain and low-gain antennas, two solar panels for energy, and a protective sun shade. The mission was four years in the making, and it had finally arrived. With 430,000 pounds of thrust, the rockets propelled the satellite from the ground into space.

It was during this week in history, on Nov. 3, 1973, that Mariner 10—the last of the Mariner missions—blasted off at a speed of 25,458 miles per hour toward Venus and, hopefully, toward Mercury.

An illustration showing the instruments of Mariner 10. NASA/Jet Propulsion Laboratory. Public Domain

The journey to Venus lasted three months. It approached Venus on Feb. 5, 1974, getting within 3,584 miles of the cloudy planet. Mariner 10 relayed more than 4,000 photos of Venus, providing exceptional data about the planet. But the most important step of the mission was about to begin.

The NASA scientists had configured the timing of the launch and approach of Venus and then that of the gravity boost from Venus to coincide with a particular time of Mercury’s orbit around the sun. The goal was to reach Mercury when it was at its furthest distance from the sun in order to avoid overheating and destroying the satellite.

One Italian scientist, Professor Giuseppe “Bepi” Colombo, of the University of Padua, suggested that if Mariner 10 was placed in a specific orbit after its first Mercury encounter, then it could reconnect with the planet every 176 days, or every second trip of the planet’s orbit of the sun.

Aiming for Mercury

Having entered Venus’s gravitational pull, it was time to exit it. Like a slingshot, Mariner 10 was thrust toward Mercury at a speed of 72,215 miles per hour—nearly three times its original speed, though approximately approximately 34,000 miles per hour slower than Mercury’s orbit.

After a course correction by NASA scientists on March 16, the satellite reached Mercury on March 29, coming within an incredible 438 miles of its surface. The seemingly impossible—for so very many reasons—had been achieved. From March 29 to April 3, Mariner 10 took approximately 2,000 images of the planet’s lit surface, destined to return to the planet in 176 days.

The images show a planet that upon first glance appeared very similar to that of Earth’s moon with its many craters. But further inspection showed large cliffs, something not seen on the moon. Mercury proved incapable of possessing an atmosphere. Its proximity to the sun and slow turn allows the solar heat to pummel the lit surface, reaching approximately 800 degrees Fahrenheit, while the dark side, protected from the sun’s rays, indicates temperatures as low as -290 degrees Fahrenheit, 320 degrees below freezing. The magnetometer detected the planet possessed a magnetic field, though a much weaker one than Earth. Also, compared to Earth, Mercury was a much more perfect sphere.

Mariner 10's first image of Mercury acquired on March 24, 1974. NASA/JPL/USGS. Public Domain

As Mariner 10 conducted its orbits around the sun, it returned to Mercury on Sept. 21, 1974 at a distance of 29,869 miles, and a third time on March 16, 1975 from only 200 miles away (a maneuver conducted by NASA scientists ensured the satellite would not crash into the planet). Due to some technical difficulties, Mariner 10 relayed less than 1,000 images from its second and third encounters combined.

If there was a downside to the three successful encounters it was that, due to the timing of Mariner 10’s arrival combined with the planet’s slow rotation, the images were of the same lit section. Therefore, Mariner 10 was only able to scan and photograph approximately 40 to 45 percent of Mercury. Despite this, the mission had been a grand success, solving several of those “riddles” Copernicus and many others had been tortured with.

Additionally, Mariner 10’s successful use of gravity assist proved the theory, and was used shortly thereafter by Pioneer 11 on Dec. 3, 1974, when it used Jupiter’s gravity to reach Saturn.

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