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Explorer 1, also designated 1958 Alpha 1, was the first American Earth satellite. It was launched on January 31, 1958 atop a Juno rocket. It was the first spacecraft to detect the Van Allen radiation belt. The satellite returned data for four months before depleting its batteries. Explorer 1 was the first in a series of over 90 satellites.
The satellite weighed a total of 13.97 kg, the majority(8.3 kg) of which was instrumentation. The instrument section and the fourth-stage rocket casing orbited as a single unit. Data was transmitted by two antennas. The first was a dipole antenna made up of two fiberglass slot antennas that were fed by a 60 milliwatt transmitter operating on 108.03 MHz. The other was a group of four flexible whips that formed a turnstile antenna that were fed by a 10 milliwatt transmitter operating on 108.00 MHz. All of the instrumentation aboard the satellite were built based on simplicity in order to meet weight requirements. Transistors made of germanium and silicon were used for the first time. Forty percent of the payload weight of the satellite consisted of the mercury chemical batteries used by the spacecraft. Temperature control was provide through alternate strips of white and green paint. This passive system proved adequate throughout the mission.
The science payload consisted of the Iowa Cosmic Ray Instrument which included: an Anton 314 omnidirectional Geiger-Muller tube, five temperature sensors, an acoustic detector to detect micrometeorite impacts, and a wire grid detector for further micrometeorite detection. The mission and instruments provided some unexpected, but exciting results. First, Explorer 1 changed rotation axis after launch. The spacecraft had been designed to spin about its long axis, but started precessing from energy dissipation due to structural elements. That led to scientists knowing that a body ends up in the spin state that minimizes the kinetic rotational energy(the maximal-inertia axis), which was the first new development in the Eulerian theory of rigid body dynamics in over 200 years. Secondly, the Geiger counter onboard would report the expected cosmic ray count in general, but would show a peculiar zero count at regular intervals. All of these zero count incidents occurred at an altitude of 2,000 km over South America. It wasn’t until Explorer 3 that scientists understood that the Geiger counter was being overwhelmed by the amount of radiation in what is now known as the Van Allen radiation belt.
The success of Explorer 1 signaled the real entry of the United States into the Space Race with Russia. Given the cooperation between NASA and the Russian Space Agency, it is hard to imagine that a short 50 years ago each was in a race with implications that ran deep during the Cold War.
We’ve also recorded an episode of Astronomy Cast about the space shuttle. Listen here, Episode 127: The US Space Shuttle.