On November 3, 1973, Mariner Venus Mercury, or Mariner 10, set out see what it could learn about Venus and Mercury. Packed aboard an Atlas-Centaur rocket, Mariner 10 launched successfully about of Earth's atmosphere and began its long trip to Venus. It would perform 1 flyby of Venus then continue its journey to conduct 3 flybys of Mercury. It was the first spacecraft to visit 2 planets in a mission and the first to get a close look at Mercury.
Mysterious Planets
Mercury, the innermost planet to the sun, and Venus, the second planet, both remained mysteries to mankind. Mercury's small size and close proximity to sun prevented earth-bound astronomers from studying the planet in any detail. Venus, shrouded in its thick atmosphere, was seen from earth as a hazy sphere, with faint indications of swirling atmospheric patterns.
Mission Objectives
The goal of the Mariner 10 mission was to study the atmospheres, and if possible, the surfaces of Venus and Mercury. The spacecraft was equipped with instruments to measure magnetic fields, plasma, infrared radiometry and ultraviolet, as well as a camera to photograph the planets. It also boasted an experimental high-frequency X-band transmitter that was being used in space for the first time.
Mariner 10 Reaches Venus
On February 5, 1974, Mariner 10 reached its closest point to Venus at 5,679 kilometers. It transmitted back to Earth the first close-up photographs of the shrouded planet ever to be seen. These photographs showed nothing more than a hazy atmosphere, a slight disappointment to scientists and a great disappointment to the media. According to "The Voyage of Mariner 10: Mission to Venus and Mercury," published by NASA in 1978, much of the press left the von Karman auditorium at JPL after seeing this first batch of photos. Those that stayed however, were treated to a very exciting view of Venus when scientists switched to ultraviolet imagery. These pictures revealed sharp detail of the stark patterns in the atmosphere. Excitement grew quickly as scientists realized they were looking at close-ups of ultraviolet markings that had only been seen as faint indications in the best Earth-bound photos of Venus.
Mercury Flybys
Using the gravity of Venus to slingshot around the planet, Mariner 10 gained enough speed to reach Mercury. According to NASA's NSSDC Master Catalog, on March 29, 1974, the spacecraft passed by Mercury at a distance of 704 kilometers from the surface and took photographs. 6 months later, on September 21, Mariner 10 passed by Mercury for a second time, but at a much greater distance of 48,069 kilometers. Finally, on March 16, 1975 the spacecraft made its final pass, this time at a distance of 327 kilometers. Tremendous photos were sent back to Earth from this flyby. In addition, the spacecraft conducted measurements of Mercury's magnetic field, which was found to faintly exist.
Scientific Discoveries of the Mariner 10 Mission
Thanks to Mariner 10, scientists learned that Hadley-type circulation existed in Venus' atmosphere, and that the planet had a very weak magnetic field. They also were able to study the interactions between the ionosphere and solar wind. Mercury, as predicted, had virtually no atmosphere. Even so, it was shown to have more helium than the Moon, possibly a result of radioactive decay or capture from solar wind. Over 2,800 photographs of the planet revealed that its surface was cratered and quite similar to Earth's moon. Scientists discovered that Mercury has a small magnetic field and relatively large iron-rich core. Temperatures on Mercury, as measured by the spacecraft's infrared radiometer reached 90K (-297 F) at night and 460K (369 F) during the day.
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