any of a series of six unmanned scientific satellites launched by the United States from 1964 to 1969. Equipped with a complex of magnetometers, these orbiting satellites were designed to study the Earth's magnetosphere (i.e., zone of strong magnetic forces around the planet) and its effect on high-energy particles emitted by the Sun. These studies included investigations of auroral displays, magnetic storms, and other related phenomena.

OGO-1, the first in the series, was launched on Sept. 4, 1964. It weighed about 250 pounds (113 kilograms) and carried instrumentation for 20 to 25 experiments. The other OGO satellites were identical in size and equipment.