The proposal, using a military Redstone missile, was rejected in 1955 by the Eisenhower administration in favor of the Navy's Project Vanguard, using a booster advertised as more civilian in nature.

The Jupiter-C design used for the launch had already been flight-tested in nose cone reentry tests for the Jupiter intermediate-range ballistic missile (IRBM) and was modified into Juno I.

The instrument section at the front end of the satellite and the empty scaled-down fourth-stage rocket casing orbited as a single unit, spinning around its long axis at 750 revolutions per minute.

[8][9] Because of the limited space available and the requirements for low weight, the payload instrumentation was designed and built with simplicity and high reliability in mind, using germanium and silicon transistors in its electronics.

The final color scheme was determined by studies of shadow–sunlight intervals based on firing time, trajectory, orbit and inclination.

The Explorer 1 payload consisted of the Iowa Cosmic Ray Instrument without a tape data recorder which was not modified in time to make it onto the spacecraft.

The scientific instrumentation of Explorer 1 was designed and built under the direction of Dr. James Van Allen of the University of Iowa containing:[8] After a jet stream-related delay on 28 January 1958, at 03:47:56 GMT on 1 February 1958 [14] the Juno I rocket was launched, putting Explorer 1 into orbit with a perigee of 358 km (222 mi) and an apogee of 2,550 km (1,580 mi) having a period of 114.80 minutes, and an inclination of 33.24°.

[14] At about 06:30 GMT, after confirming that Explorer 1 was indeed in orbit, a news conference was held in the Great Hall at the National Academy of Sciences in Washington, D.C. to announce it to the world.

The elongated body of the spacecraft had been designed to spin about its long (least-inertia) axis but refused to do so, and instead started precessing due to energy dissipation from flexible structural elements.

Later it was understood that on general grounds, the body ends up in the spin state that minimizes the kinetic rotational energy for a fixed angular momentum (this being the maximal-inertia axis).

This motivated the first further development of the Eulerian theory of rigid body dynamics after nearly 200 years – to address this kind of momentum-preserving energy dissipation.

Later, after Explorer 3, it was concluded that the original Geiger counter had been overwhelmed ("saturated") by strong radiation coming from a belt of charged particles trapped in space by the Earth's magnetic field.