Although it was only the fourth to be launched, Vanguard 1 is the oldest artificial earth satellite to still be in orbit! The orb was part of the Vanguard project in the 1950s - it was named "vanguard" because scientists hoped that it would be the first element of human space exploration. This was not the case, however, since the Russian Sputnik satellites and America's Explorer beat Vanguard 1 to the punch.
Vanguard 1 was launched on March 17th, 1958, achieving the highest altitude of any man-made vehicle up to that point, and was also the first satellite to be powered by solar energy. Since that time, it has completed more than 200,000 Earth orbits and racked up well over 6 billion miles of travel! It has almost outlived all its creators and is predicted to remain in orbit well into the 22nd century.
(Sources: 1, 2)