Explorer 1 (1958): First US Satellite and the Van Allen Belts

Explorer 1 was the first satellite launched by the United States, sent into orbit from Cape Canaveral, Florida, on 31 January 1958. Built in under three months by the Army and the Jet Propulsion Laboratory after the shock of the Soviet Sputnik launches, it restored American confidence in the early Space Race. Its small scientific payload also produced the first major discovery of the space age: the belts of radiation that surround the Earth, named after the physicist James Van Allen.

Background: the Sputnik shock

During the International Geophysical Year of 1957–58, both superpowers had promised to launch scientific satellites. The United States chose the Navy’s Project Vanguard in 1955, partly because it was seen as a civilian scientific effort, separate from military missiles. A rival Army proposal, Project Orbiter, built around Wernher von Braun’s Redstone rocket, was turned down.

On 4 October 1957 the Soviet Union launched Sputnik 1, and on 3 November it followed with the much heavier Sputnik 2, which carried the dog Laika. Americans were alarmed: if Soviet rockets could orbit satellites, they could also carry nuclear warheads across continents. The pressure grew worse on 6 December 1957, when the first Vanguard satellite attempt, broadcast live, lost thrust and exploded on the launch pad a couple of seconds after ignition. Newspapers mocked it as “Flopnik” and “Kaputnik”.

The Army had already been told, on 8 November 1957, to prepare its own launch. Major General John B. Medaris, commander of the Army Ballistic Missile Agency (ABMA) in Huntsville, Alabama, and von Braun had argued for months that they could launch quickly. They had a head start, because their Jupiter-C rocket had already flown test missions in 1956 and 1957.

Building the satellite and the rocket

The launch vehicle, renamed Juno I for the satellite mission, was a four-stage rocket. Its first stage was a lengthened Redstone burning alcohol and liquid oxygen. On top sat a spinning “tub” holding clusters of small solid-fuel rockets supplied by the Jet Propulsion Laboratory (JPL) in Pasadena: eleven in the second stage, three in the third and a single one in the fourth. Spinning the tub kept the upper stages stable without a complex guidance system.

The satellite itself was the fourth stage plus an instrument section, a slim cylinder about 2 metres long and 15 centimetres across. JPL, directed by the New Zealand-born engineer William H. Pickering, built it and its radio systems. The main scientific instrument was a Geiger-Müller counter to measure cosmic rays, designed by James Van Allen and his students at the State University of Iowa. Van Allen’s team, including George Ludwig, had originally built the instrument for Vanguard and adapted it in a matter of weeks.

Explorer 1 also carried temperature sensors and simple detectors for micrometeorite impacts. Its total mass in orbit was about 14 kilograms (30.8 pounds), a fraction of Sputnik 1’s 83.6 kilograms.

The launch

Bad weather and high-altitude winds delayed the launch for several days. On the night of 31 January 1958, at 10:48 p.m. Eastern time, Juno I lifted off from Launch Complex 26A at Cape Canaveral. Because the upper stages had no guidance, the timing of their ignition was set by hand from the ground, based on tracking of the rocket’s climb.

Confirmation took an anxious wait. Tracking stations had to hear the satellite’s signal after it completed its first orbit, and the signal arrived several minutes later than expected, because the orbit was higher than planned. Early on 1 February, at a crowded press conference at the National Academy of Sciences in Washington, Pickering, Van Allen and von Braun held a full-scale model of Explorer 1 above their heads, an image that became one of the most famous photographs of the early Space Race.

The orbit was strongly elliptical, ranging from roughly 360 kilometres to about 2,550 kilometres above the Earth, with each circuit taking around 115 minutes.

Discovering the Van Allen belts

Van Allen’s Geiger counter worked as expected at lower altitudes, but at higher points in the orbit it sometimes reported no counts at all. Explorer 1 had no tape recorder, so data came only when the satellite passed over a ground station, which made the readings hard to interpret.

When Explorer 3, launched on 26 March 1958, carried a small tape recorder, the pattern became clearer. Van Allen’s group, including Carl McIlwain and Ernest Ray, concluded that the counter was not failing but being overwhelmed by radiation far stronger than expected. On 1 May 1958 Van Allen announced at a joint meeting of the National Academy of Sciences and the American Physical Society that the Earth was surrounded by a zone of energetic charged particles trapped by its magnetic field.

Later missions showed that there are two main belts, an inner one dominated by high-energy protons and an outer one mainly of electrons. Soviet instruments on Sputnik 3 had recorded similar signals in 1958, but the Iowa team was first to identify and publish the explanation.

Key people

• James Van Allen (1914–2006), physicist at the State University of Iowa, designed the cosmic-ray experiment and gave his name to the radiation belts.
• Wernher von Braun (1912–1977), German-born rocket engineer who led ABMA’s development team.
• William H. Pickering (1910–2004), director of JPL from 1954 to 1976, responsible for the satellite and its upper stages.
• John B. Medaris (1902–1990), Army general commanding ABMA, who pushed for the Army’s satellite effort.

Consequences

Explorer 1 eased, though it did not end, the American sense of having fallen behind. Sputnik 2 weighed over 500 kilograms, while Explorer 1 weighed 14, a gap that reflected the far more powerful Soviet R-7 rocket. The Navy’s Vanguard finally orbited a satellite on 17 March 1958.

The rivalry between the Army, Navy and Air Force over space projects convinced President Dwight D. Eisenhower and Congress that a single civilian agency was needed. In 1958 they created the National Aeronautics and Space Administration, which absorbed JPL later that year and von Braun’s team in 1960. The Explorer name continued as a long series of NASA science satellites.

Explorer 1 also taught engineers an unexpected lesson. Designed to spin around its long axis, it soon began to tumble end over end because its flexible wire antennas dissipated energy. The problem helped establish an important rule of spacecraft design: a spinning body only spins stably around the axis with the greatest moment of inertia.

By the numbers

Timeline

• 1957, 4 October: The Soviet Union launches Sputnik 1.
• 1957, 8 November: The Army is authorised to prepare a satellite launch.
• 1957, 6 December: The first Vanguard satellite attempt explodes on the launch pad.
• 1958, 31 January: Explorer 1 is launched on a Juno I rocket from Cape Canaveral.
• 1958, 1 February: Pickering, Van Allen and von Braun announce the success in Washington.
• 1958, 26 March: Explorer 3 is launched with a tape recorder for its radiation data.
• 1958, 1 May: Van Allen announces the discovery of the radiation belts.
• 1958, 23 May: Explorer 1 stops transmitting.
• 1970, 31 March: Explorer 1 re-enters the atmosphere.

Why it matters

Explorer 1 put the United States into the Space Race and showed that small, well-designed scientific satellites could make real discoveries. The Van Allen belts became a founding finding of space physics and a practical concern for every later spacecraft and astronaut. The project’s urgency and rivalries also fed directly into the creation of NASA.