1975: Mariner 10, the first spacecraft to visit two planets, completes its third and final fly-by of Mercury.

Mariner 10, the last of the Mariner family, launched on Nov. 3, 1973 from Cape Canaveral, Florida. With Mariner 10's narrow-angle cameras, ultraviolet spectrometers and infrared radiometers, scientists hoped to ascertain whether Mercury had an atmosphere, what its topography was like, how much it weighed and how big it was.

En route to Mercury, the 952-pound robotic space probe first swung by Venus on Feb. 5, 1974. It captured the first close-up images of the planet, including its chevron clouds. Mariner 10 used Venus' gravity to pull it along toward its final destination, Mercury. This maneuver is pretty standard now, but Mariner 10 was the first spacecraft to use the "gravity assist" tactic.

After leaving Venus behind, Mariner 10 encountered Mercury on March 29, 1974 for the first time, at a altitude of just under 440 miles, roughly the distance between Boston and Washington, D.C. Scientists positioned the first flyby on Mercury's dark side so they could study the interactions between the planet and solar winds more easily. This way, they could look at infrared thermal emissions that might help them determine if the planet had an atmosphere.

Mariner 10 flew by Mercury again on Sept. 21, 1974, this time on the sunlit side, at a whopping distance of almost 30,000 miles, or the equivalent of about six round-trip flights between New York and Los Angeles. The second flyby was another NASA milestone: It was "the first time any spacecraft had returned to its target planet for a second look," according to NASA's "Voyage of Mariner 10." The purpose of the second flyby was to gather more photographs of the closest planet to the sun.

Mariner 10 passed over Mercury one final time on this day in 1975 at a distance of about 200 miles. The third flyby's main objective was to study the planet's weak magnetic field, which scientists had detected during the first flyby. Scientists found that Mercury, like Earth, has a magnetically neutral tail that spurts electrons and protons. These bursts may be the result of magnetic fields canceling each other out. Because of its close proximity during its final approach, Mariner 10 was also able to gather some very detailed images of Mercury's bumpy surface.

The spacecraft ran out of its nitrogen fuel on March 24, 1975. Scientists turned off its transmitters, and communication with Earth ceased. Mariner 10 continues to orbit the sun, but solar radiation has probably scorched its equipment.

The mission helped scientists learn that Mercury, the smallest planet in the solar system since Pluto was demoted in 2006, is an airless, cratered chunk of rock cooler than Venus.

Source: Various

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