How Did One Spacecraft Visit Mercury Three Times and Still Photograph Only Half of It?

Mariner 10 Went to Venus First So That Venus's Gravity Could Steer It to Mercury

Mariner 10 left Cape Canaveral, Florida, on an Atlas-Centaur rocket on November 3, 1973. It weighed about 500 kilograms (1,100 pounds), roughly as much as a small motorcycle with a rider, and carried cameras and instruments to study Mercury’s surface and surroundings [1]. NASA is the United States space agency, and the mission was run by its Jet Propulsion Laboratory (JPL) in California, which builds and flies robotic spacecraft.

Its first stop was not Mercury but Venus. Mercury is hard to reach because a spacecraft launched from Earth is already moving fast around the Sun, and to drop toward Mercury it has to lose a lot of that speed. On February 5, 1974, Mariner 10 passed within 5,768 kilometers (3,584 miles) of Venus. Venus’s gravity bent the path and left the spacecraft heading toward Mercury [1]. According to Wikipedia, its speed relative to the Sun fell from about 37 to about 32 kilometers per second during the pass [4].

This is called a gravity assist, and Mariner 10 was the first spacecraft to use one to change its flight path. It was also the first mission to explore two planets, Venus and Mercury, on a single trip [1]. On the way it sent back 4,165 pictures of Venus [1].

An Italian Mathematician Saw That Mercury Could Be Visited Again Every Six Months

The idea of a return trip came from Giuseppe Colombo, a mathematician who taught theoretical mechanics at the University of Padua in Italy. ESA, the European Space Agency, records that he noticed the spacecraft’s orbit after the first Mercury flyby would take about twice as long as one of Mercury’s own orbits (the ESA page words this as twice Mercury’s “rotational period,” but the orbit comparison is the one that fits the numbers). JPL then ran an analysis confirming that a carefully chosen flyby point would let Mercury’s gravity send the spacecraft back to the planet about six months later [2].

NASA’s history office describes the same rhythm: the spacecraft goes around the Sun once while Mercury goes around twice, so they meet again about every six months [3]. Mercury’s year is about 88 Earth days, so twice that is about 176 days.

A simple way to picture it is two runners on a track, one who needs 176 seconds per lap and one who needs 88. Every time the slow runner finishes one lap, the fast runner has finished exactly two, and they are side by side again. Real orbits are stretched ovals, not circles, so this is only a picture, but it captures the timing.

The flybys happened on March 29, 1974 (703 kilometers, or 437 miles, above Mercury), September 21, 1974 (about 48,000 kilometers away), and March 16, 1975 (just 327 kilometers away, passing over the north pole) [1]. You can check the rhythm yourself at the end of this note.

The Spacecraft Kept Breaking, and Ground Controllers Kept Fixing It

The journey was not smooth. NASA’s history office says the three-month trip to Venus included several malfunctions that the ground team overcame [3], and NASA’s mission page mentions trouble with the main antenna and the system that keeps the spacecraft pointed correctly [1].

One failure sounds almost silly. Wikipedia reports that the star tracker, a small sensor that steers by locking on to a bright star, Canopus, once locked on to a bright flake of paint instead, and that flaking paint kept happening during the mission [4]. I found this detail on Wikipedia only, so treat it as reported, not confirmed by a second source.

The biggest limit was gas. Mariner 10 used small jets of nitrogen gas to keep itself pointed. By the third Mercury flyby it was nearly out. Eight days after that flyby the supply ran out, and on March 24, 1975, controllers turned off its transmitter [3][1]. The third flyby was also hurt by a failed tape recorder: only the central quarter of each of about 300 high-detail images could be sent home [1].

Mercury Looked Like the Moon, but It Had a Magnetic Field the Moon Does Not

The first close pictures showed a gray, crater-pocked world that looked a lot like the Moon, with ridges and jumbled terrain. Instruments suggested nights as cold as minus 183 °C (minus 297 °F) and days as hot as 187 °C (369 °F), and showed almost no atmosphere [1]. Mariner 10 also photographed part of the huge Caloris basin, a ringed crater-like scar about 2,500 kilometers (1,550 miles) wide [1].

The big surprise was invisible. The spacecraft’s magnetometers found a weak magnetic field around Mercury [1]. A summary of the magnetic-field analysis by Norman Ness and colleagues says that before the flyby, Mercury’s slow spin and Moon-like surface had pointed to a planet with essentially no field of its own, and that the data showed the field really belongs to the planet. Where it comes from, whether from a core that is still stirring or from magnetism left over from the past, was left uncertain [5].

The Same Six-Month Rhythm Meant Every Visit Saw the Same Lit Half of Mercury

Mariner 10 returned more than 2,700 pictures from its three Mercury flybys, but they covered only nearly half of the planet [1]. Wikipedia gives the figure as 40–45% and says the reason was that the same side of Mercury was sunlit each time the spacecraft returned [4].

The link to the rhythm is this. Mercury turns about three times for every two trips around the Sun, so from the surface one sunrise-to-sunrise “day” lasts about 176 Earth days, the same length as Mariner 10’s repeat cycle. (Our note on Mercury’s long day explains where that 3:2 pattern comes from.) A camera can only photograph the part that the Sun lights. After every 176 days Mercury had returned to the same position in its rotation relative to the Sun, so the same hemisphere was in daylight each time Mariner 10 arrived. That step is my own reasoning from the dates and the 3:2 pattern, which Wikipedia and the ESA summary support, not a quotation from NASA.

In other words, the clever orbit that made a second and third visit possible was also the reason the visits kept showing the same face.

It Took More Than 30 Years for Another Spacecraft to Show the Rest of Mercury

NASA calls Mariner 10 the last robotic visit to Mercury for more than 30 years [1]. The next one was MESSENGER, launched in 2004. It flew past Mercury three times in 2008 and 2009 and in March 2011 became the first spacecraft to orbit the planet [6]. By early May 2012 its camera had mapped the entire surface, in black and white and in color [6]. The mission ended on April 30, 2015, when MESSENGER ran out of fuel and struck Mercury’s surface [6].

Mariner 10 had told scientists where to look, and MESSENGER filled in the rest. That is why later missions to Mercury, such as the joint European–Japanese BepiColombo, were planned with a much larger view of the planet in mind.

Check the 176-Day Rhythm Yourself With a Calendar

You only need a calendar and a pencil. Count the days from the first Mercury flyby (March 29, 1974) to the second (September 21, 1974). Then count from the second to the third (March 16, 1975). Both gaps come out at 176 days.

Next, do the sum Colombo’s idea depends on: 88 days is one Mercury year, so 2 × 88 = 176. Finally, open NASA’s Mariner 10 page (listed below) and compare its dates with the ones above. If you like, write down a number that surprised you. The 176-day gap is one to look for.

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