Explorer 1, the First U.S. Satellite to Orbit Earth

Explorer 1 post-launch press conference
William Hayward Pickering, James Van Allen, and Wernher von Braun display a full-scale model of Explorer 1 at a crowded news conference in Washington, DC after confirmation the satellite was in orbit. NASA

America's First Satellite in Space

Explorer 1 was the first satellite launched by the United States, sent into space on January 31, 1958. It was a very exciting time in space exploration, with the race to space heating up. The U.S. was particularly interested in gaining the upper hand in space exploration. This was because the then-Soviet Union had made humanity's first-ever satellite launch on October 4, 1957. That was when the U.S.S.R sent Sputnik 1 on a short orbital journey. The U.S. Army Ballistic Missile Agency in Huntsville, Alabama (charged with launches before NASA was formed later in 1958) was directed to send up a satellite using its Jupiter-C rocket, developed under the direction of Dr. Wernher von Braun. This rocket had been flight tested, making it a good choice to loft the satellite into orbit.

Before scientists could send a satellite to space, they had to design and build it. The Jet Propulsion Laboratory (JPL) received the assignment to design, build and operate the artificial satellite that would serve as the rocket's payload. Dr. William H. "Bill" Pickering, was the rocket scientist who took charge of developing the Explorer 1 mission and also worked at JPL as its director until his retirement in 1976. There is a full-scale model of the spacecraft hanging at the entry to JPL's Von Kármán Auditorium, commemorating the team's achievement. The teams went to work building the satellite while teams in Huntsville got a rocket ready for the launch.  

The mission was very successful, returning never-before-seen science data for several months. It lasted until May 23, 1958, when controllers lost communication with it after the spacecraft's batteries ran out of charge. It stayed aloft until 1970, completing more than 58,000 orbits of our planet. Finally, atmospheric drag slowed the spacecraft down to the point where it couldn't stay up any longer, and it crashed into the Pacific Ocean on March 31, 1970. 

Explorer 1 Science Instruments

The primary science instrument on Explorer 1 was a cosmic ray detector designed to measure high-speed particles and the radiation environment near Earth. Cosmic rays come from the Sun and also from distant stellar explosions called supernovae. The radiation belts surrounding Earth are caused by the interaction of the solar wind (a stream of charged particles) with the magnetic field of our planet.

Once in space, this experiment — provided by Dr. James Van Allen of the State University of Iowa — revealed a much lower cosmic ray count than expected. Van Allen theorized that the instrument may have been saturated by very strong radiation from a region of highly charged particles trapped in space by Earth's magnetic field.

The existence of these radiation belts was confirmed by another U.S. satellite launched two months later, and they became known as the Van Allen Belts in honor of their discoverer. They capture incoming charged particles, preventing them from reaching Earth. 

The spacecraft's micrometeorite detector picked up 145 hits of cosmic dust in the first days it was on orbit, and the spacecraft's motion itself taught mission planners some new tricks about how satellites behave in space. In particular, there was a lot to learn about how Earth's gravity affected the motion of a satellite. 

Explorer 1's Orbit and Design

Explorer 1 circled around Earth in a looping orbit that took it as close as 354 km (220 mi.) to Earth and as far as 2,515 km (1,563 mi.). It made one orbit every 114.8 minutes, or a total of 12.54 orbits per day. The satellite itself was 203 cm (80 in.) long and 15.9 cm (6.25 in.) in diameter. It was spectacularly successful and opened up new possibilities for scientific observations in space via satellites. 

The Explorer Program

A launch attempt of a second satellite, Explorer 2, was made on March 5, 1958, but the fourth stage of the Jupiter-C rocket failed to ignite. the launch was a failure. Explorer 3 was successfully launched on March 26, 1958, and operated until June 16th. Explorer 4 was launched July 26, 1958, and sent back data from orbit until October 6 , 1958. Launch of Explorer 5 on August 24, 1958, failed when the rocket's booster collided with its second stage after separation, changing the firing angle of the upper stage. The Explorer program ended, but not before teaching NASA and its rocket scientists some new lessons about lofting satellites to orbit and gathering useful data.