History: NASA Archive Photography

The first mission was Explorer 1, which started as an ABMA/JPL project during the early space race. It was launched in January 1958, two month after Sputnik. At the creation of NASA it was transferred to this agency and still continues to this day (2011). Its missions have been focusing on the Earth and the Sun measuring among others magnetic fields and solar wind. A more recent Earth mission, not related to the Explorer program, was the Hubble telescope, was brought into orbit in 1990.
The closest planets Mars, Venus and Mercury have been the goal of at least 4 programs. The first was Mariner in the 1960s and ‘70s, which visited all three of them. Mariner was also the first to make a planetary flyby, to take the first pictures from another planet, the first planetary orbiter, and the first to make a gravity assist maneuver. This is a technique where the satellite takes advantage of the gravity and velocity of planets to reach its destination.
The first successful landing on Mars was made by Viking I in 1976. 20 years later a rover was landed on Mars by Mars Pathfinder.
Outside Mars, Jupiter was first visited by Pioneer 10 in 1973. More than 20 years later Galileo sent a probe into the planets’ atmosphere. The first spacecraft to leave the solar system was Pioneer 10 in 1983. At a time it was the most distant spacecraft, but it is now passed by Voyager 2.