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Why did Mariner 10 photograph only part of Mercury’s surface?
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Mercury
Mercury as seen by the Messenger probe, January 14, 2008. This image shows half of the hemisphere missed by Mariner 10 in 1974–75 and was snapped by Messenger's Wide Angle Camera when it was about 27,000 km (17,000 miles) from the planet.
Mariner 10 was able to photograph only about 45 percent of Mercury’s surface because of the specific orbit the spacecraft followed around the Sun.1
The spacecraft was placed in a solar orbit that matched one Mercurian solar day. As a result, each of the three flybys occurred when the exact same hemisphere of the planet was illuminated by sunlight. Consequently, the mission could only capture images of the sunlit side, leaving the remainder of the surface unmapped by the probe.1