1 Explorer 1 was the first US satellite to go into orbit around Earth in 1958. Although it was first a military project, NASA took over the Explorer program with 1959’s Explorer 6. The Explorer program is ongoing.
Modern Explorer missions are split into two main classes, Small and Medium. Medium missions were originally designed to be launched on a new, smaller, cheaper rocket, but this was never developed, and a modified Delta II is used instead. Examples include 1997’s ACE, which is still operational and studying the solar wind’s particles, and WISE, launched in 2009.
Small missions are even smaller, with a mission cost cap of $165m (around £130m) in 2017. Examples include the Sun-observing IRIS telescope, which has discovered plasma jets and tornadoes in the Sun’s chromosphere.
Explorers will continue into at least 2019, and there’s no reason to believe NASA is going to stop there.