Astronomy Picture of the Day Discover the cosmos! Each day a different image or photograph of our fascinating universe is featured, along with a brief explanation written by a professional astronomer. 2018 January 31



The First Explorer

Image Credit: NASA

Explanation: Sixty years ago, on January 31, 1958, the First Explorer was successfully launched by the Army Ballistic Missile Agency on a Jupiter-C rocket. Inaugurating the era of space exploration for the United States, Explorer I was a thirty pound satellite that carried instruments to measure temperatures, and micrometeorite impacts, along with an experiment designed by James A. Van Allen to measure the density of electrons and ions in space. The measurements made by Van Allen's experiment led to an unexpected and then startling discovery of two earth-encircling belts of high energy electrons and ions trapped in the magnetosphere. Now known as the Van Allen Radiation belts, the regions are located in the inner magnetosphere, beyond low Earth orbit. Explorer I ceased transmitting on February 28, 1958, but remained in orbit until March of 1970.