To the ancient Romans, Mercury was the messenger of the gods, his winged sandals adroitly whisking the maestro of luck and trickery to do his divine masters’ bidding. Small wonder that the nearest planet to the Sun was named in his honor: Crater-pocked Mercury whips around our parent star in 88 days at a mean distance of 36 million miles (58 million kilometers), two-thirds closer than Earth.
Brutalized by aeons of remorseless meteoroid bombardment, Mercury is a barren, broiling world of extremes. In its coal-dark sky, the Sun appears 11 times brighter, three times bigger, and many times hotter than here on Earth. Its 3:2 spin-orbit resonance and orbital eccentricity produced a Moon-like surface of mountains and plains, scarps and valleys, frozen to minus 170 degrees Celsius (minus 270 degrees Fahrenheit) at night then baked as high as 420 degrees Celsius (790 degrees Fahrenheit) in the mercurian daytime.
Fifty years ago, humanity gained its first up-close glimpse of this battered world when NASA’s Mariner 10 spacecraft made the first of three Mercury encounters. Its seven science instruments revealed heavily cratered highlands and rolling lowlands, the Caloris impact basin, and surprisingly (in view of Mercury’s geological deadness), an intrinsic magnetic field and hints of an iron-rich core.
But if Mariner 10’s mission was historic, so was the Boeing-built spacecraft itself, which for the first time used the gravity of one world (Venus) to reach another (Mercury) and in doing so visited two planets for the first time. It used extant hardware and technology to keep costs down, but engineers recognized that voyaging within 0.4 astronomical units (AU; 1 astronomical unit is equal to the average Earth-Sun distance of 93 million miles or 150 million km) of the Sun would impose 4.5 times greater solar radiation at Mercury than at Earth.
Get on the Mariner 10 bus
As such, Mariner 10’s octagonal spacecraft “bus” was guarded with thermal blankets of beta cloth (a combination of aluminized Kapton and glass-fiber sheets treated with Teflon) plus a sunshade and specialized louvers to regulate internal temperatures. Its two solar arrays, with a wingspan of 26 feet (7.6 meters), were rotatable through their long axes to minimize solar heating and keep them below 115 C (239 F).
Mariner 10 bristled with cameras, an infrared radiometer to assess the thermal properties of Mercury’s surface, an ultraviolet spectrometer to sniff out a putative atmosphere, sensors to analyze interactions between the solar wind and cosmic radiation, two magnetometers on a 19-foot-long (5.8 m) hinged boom to seek evidence of an intrinsic magnetic field and a radio science toolkit to take mass and gravitational measurements. A 4.6-foot (1.4 m) high-gain antenna transmitted data at rates of 117.6 kilobits per second.
Previous Mariners flew in pairs to hedge against failure, but budget constraints had cost-capped Mariner 10 at $98 million. That allowed a spare spacecraft to be built in case of failure, but NASA decreed it could only fly if the fatal flaw was identified and resolved within two weeks. With Mariner 10 set to fly on Nov. 3, 1973, in the event of disaster the spare had to be ready to go no later than Nov. 21.
Meanwhile, Italian physicist Giuseppe Colombo suggested that Mariner 10’s trajectory following its March 1974 encounter of Mercury would permit a second flyby of the planet six months later. That layered greater complexity onto an already complex mission but carried the potential to greatly expand the scientific yield from an unknown world.
Mariner 10 launches in 1973
Mariner 10 launched atop an Atlas-Centaur rocket from Cape Canaveral’s Pad 36B at 12:45 a.m. EST on Nov. 3, 1973. Roaring aloft under 430,000 pounds (195,000 kilograms) of thrust, the Atlas booster shut down as planned four minutes into the flight, leaving the Centaur upper stage to perform two burns to firstly insert Mariner 10 into an Earth-parking orbit, then boost it into deep space.
It was a risky maneuver. Never before had two upper-stage burns been done on a U.S. planetary mission; an engine fault could spell the end for Mariner 10. With the exception of a drifting gyro, the Centaur performed admirably, pushing the 1,100-pound (500 kg) spacecraft out of Earth’s gravitational clutches and into interplanetary space at 25,458 mph (40,969 km/h). Ahead in 94 days lay its first planetary target: Venus.
But the cruise proved far from untroubled. Hours after launch, a cover failed to open on an electrostatic analyzer, nixing its use in an electron spectrometer experiment. Mariner 10’s star tracker repeatedly lost its lock on Canopus, locking its gaze instead onto stray flecks of paint. Multiple computer resets and high-gain antenna glitches turned a low-level headache for NASA engineers into a full-blown, three-month migraine.
Worse, short-circuiting heaters on the spacecraft’s cameras failed to activate, threatening permanent damage from the cold of deep space. Fortunately, the cameras’ normal mode of operation kept them warm enough to hover above critical temperature limits; they were left switched on during the cruise to Venus. Then, in an unexpected piece of good fortune, in January 1974 the heaters started working again.
On Feb. 5, Mariner 10 swept 3,584 miles (5,768 km) over Venus’ cloud-cloaked façade, acquiring 4,165 photographs and gathering data on atmospheric structures and near-surface temperatures. Near-ultraviolet measurements were taken of high-altitude, cirruslike chevron clouds.
But it proved a whistlestop tour. Venus’ gravity shaved the spacecraft’s velocity from 82,785 mph (133,230 km/h) to 72,215 mph (116,220 km/h), bending its flight path and reshaping its elliptical solar orbit to intersect Mercury. Seven weeks after passing Venus, and 146 days since departing Earth, the tiny planet emerged from the gloom.
Mariner 10 began photographing Mercury several days before closest approach, at first intermittently, then shooting a frame every 42 seconds by March 28. Next day, the spacecraft hurtled within 437 miles (703 km) of a world known for centuries as little more than a faint blob in telescope eyepieces, almost lost in the Sun’s glare. Heading outbound, it continued shuttering images until April 3.
Two thousand pictures streamed to expectant scientists back home. The Mercury I data uncovered a global magnetic field two orders of magnitude feebler than Earth’s own, arguably Mariner 10’s most unexpected find. And a vast impact basin crept teasingly into view, its yawning maw spanning over 830 miles (1,340 km), wider than Texas. Ringed by mile-high mountains, its location on the planet’s broiling, Sun-facing hemisphere earned it the apt moniker of Caloris Basin – Latin for “hot.”
Looping back around the Sun, Mariner 10 returned for its second visit, Mercury II, over the planet’s southern hemisphere on Sept. 21. Although the flyby distance of 29,870 miles (48,070 km) was wider, the viewing geometries were better. More data was also harvested from the spacecraft via improvements to NASA’s Deep Space Network (DSN), including the microwave antennas in Goldstone, California.
Six months later, on March 16, 1975, Mariner 10 encountered the planet a third time for Mercury III, passing over the north polar region at 203 miles (327 km), its closest yet. A failed tape recorder and restrictions in data-reception rates meant only the central quarter of 300 high-resolution photographs was received, but Mariner 10 managed to the image surface features as small as 328 feet (100 m) across.
Much of this remarkable extended mission was only possible thanks to engineers tripling the amount of hydrazine propellant for course correction maneuvers and extra nitrogen gas for its thrusters. But after Mercury III, fuel was low and the end was nigh. After traveling for 506 days and covering a billion miles (1.6 billion km), Mariner 10’s transmitter was shut down on March 24, 1975, and the mission concluded.
It had been a remarkable triumph, although the geometry of Mercury’s orbit during all three encounters meant the same side of the planet was sunlit each time, enabling Mariner 10 to map barely 40 to 45 percent of the surface. Nevertheless, the spacecraft’s 2,800 photographs revealed rugged highlands and smooth lowlands. Chains and clusters of craters embayed the highlands, and Mariner 10 data pointed to the existence of a tenuous exosphere of hydrogen and helium atoms captured from the solar wind, plus a bulk density consistent with a massive core, rich in iron and nickel.
Ground observations and NASA’s Messenger spacecraft have since mapped the planet in its entirety, hinting at water-ice in permanently shadowed polar craters. Europe’s BepiColombo mission will reach Mercury in December 2025 for its own extended orbital survey. And Mariner 10, which first shed light on this odd little world, now drifts silently in heliocentric space, unseen and untracked since contact went dead a half-century ago.
