Mariner 10 was the first and only mission to Mercury until NASA's MESSENGER mission more than 30 years later. It was also the first spacecraft to reach one planet by using the gravity of another planet (in this case, Venus) to alter its speed and trajectory. This has become an extremely important technique.
During three flybys of Mercury, Mariner 10 photographed half the planet's moonlike surface and transmitted data indicating a surprising magnetic field, a metallic core comprising about 80 percent of the planet's mass, and temperatures ranging from 369 degrees Fahrenheit (187 degrees Celsius) on the dayside to -297 degrees Fahrenheit (-183 degrees Celsius) on the nightside.
NASA
1973-11-03
<a rel="nofollow" class="external free" href="http://solarsystem.nasa.gov/multimedia/display.cfm?IM_ID=13065">http://solarsystem.nasa.gov/multimedia/display.cfm?IM_ID=13065</a> (<a rel="nofollow" class="external text" href="http://solarsystem.nasa.gov/multimedia/gallery/Mariner_10_Launchpad.jpg">image link</a>)