NASA Photography

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.
Both the Pioneer and the Voyager program carries messages from the Earth to extraterrestrial life. A problem with far space travel is communication. For instance, at present it takes about 3 hours for a radio signal to reach the New Horizon spacecraft at a point more than half way to Pluto. Contact with Pioneer 10 was lost in 2003.