Explorer 1

Explorer 1 was the first artificial satellite launched by the United States and marked the nation’s entry into space exploration during the Cold War. Sent into orbit on 1 February 1958 as part of the American contribution to the International Geophysical Year (IGY), it followed the Soviet Union’s successful launches of Sputnik 1 and Sputnik 2 in 1957. The achievement represented a pivotal moment in the early Space Race, demonstrating new scientific and technological capabilities. Explorer 1 remained active for nearly four months before its batteries were depleted and stayed in orbit until March 1970.

Background and development

The roots of Explorer 1 lay in Project Orbiter, a joint proposal by the US Army and Navy in 1954 to launch a scientific satellite during the IGY. The plan, relying on a Redstone missile, was rejected in favour of the Navy’s Project Vanguard, which aimed to provide a civilian front for American space activity. The Soviet launch of Sputnik 1 on 4 October 1957 transformed the geopolitical landscape and prompted the revival of Project Orbiter as the Explorer programme.
Development was carried out jointly by the Army Ballistic Missile Agency (ABMA) and the Jet Propulsion Laboratory (JPL). A Jupiter-C missile, initially designed for re-entry testing of the PGM-19 Jupiter missile, was modified into the Juno I launch vehicle. ABMA and JPL completed the conversion of the rocket and the design and construction of the satellite in an accelerated period of 84 days. Before Explorer 1 was completed, the Soviet Union launched Sputnik 2 in November 1957, intensifying the urgency of the American effort. The Navy’s attempted Vanguard TV3 launch failed on 6 December 1957, further elevating the importance of the Army–JPL mission.

Spacecraft design

Explorer 1 was designed and built at JPL under the direction of Dr William Hayward Pickering. It was a slender, cylindrical satellite with a total mass of roughly 14 kilograms, of which about eight kilograms comprised scientific instrumentation. Its dimensions and configuration enabled it to spin along its long axis at approximately 750 revolutions per minute.
Communications were maintained via two radio transmitters: a 60-watt system operating at 108.03 MHz using a dipole antenna, and a 10-milliwatt transmitter at 108.00 MHz using a turnstile antenna formed from four flexible whips. To conserve mass and ensure reliability, the satellite’s electronics used a minimal number of components, including 20 transistors—an advanced feature for its time. Power was provided by mercury batteries, accounting for nearly 40 per cent of the payload weight.
Its external surface consisted of stainless steel with painted stripes to optimise thermal regulation during orbit. Several test colour schemes were considered before the final pattern of white stripes on steel was adopted.

Scientific payload

The primary scientific aim of Explorer 1 was to investigate cosmic rays, meteoroids, and environmental conditions in low Earth orbit. The payload, designed under the supervision of Dr James Van Allen of the University of Iowa, consisted of:

• Geiger–Müller tube (Anton 314) for detecting high-energy cosmic rays. The instrument frequently saturated in areas of intense radiation, an observation key to later discoveries.
• Temperature sensors, including one internal and three external sensors, plus another on the nose cone.
• Acoustic micrometeorite detector, using a crystal transducer and amplifier to measure impacts from tiny particles.
• Wire-grid micrometeorite detector, comprising copper–nickel wires arranged to break upon impact from particles roughly 10 micrometres in size.

Due to time constraints, Explorer 1 did not include a tape recorder, so only real-time data could be transmitted when the spacecraft passed over receiving stations.

Launch and orbital operations

Explorer 1 was successfully launched on 1 February 1958 (GMT), corresponding to the evening of 31 January in the United States, from Cape Canaveral Launch Complex 26. The Juno I booster placed the satellite into an elliptical orbit with an orbital period of about 114.8 minutes and an inclination of 33.24°. Because the orbit was higher than expected, ground stations required additional time to confirm its insertion before the mission was publicly announced.
The high-power transmitter operated for 31 days and the low-power transmitter for roughly 105 days. Explorer 1 ceased sending data on 23 May 1958 once its batteries were exhausted. It remained a silent object in orbit for more than 12 years before re-entering Earth’s atmosphere over the Pacific Ocean on 31 March 1970, having completed more than 58,000 revolutions.

Scientific results and discoveries

Explorer 1 made one of the most significant scientific contributions of the IGY. Its Geiger–Müller counter occasionally returned zero counts, a puzzling finding that was later understood to indicate instrument saturation in regions of very high radiation. This phenomenon, confirmed by Explorer 3 and subsequent satellites, revealed the existence of what are now known as the Van Allen radiation belts—regions of trapped charged particles within Earth’s magnetosphere.
The micrometeorite detectors recorded 145 impacts during the first 78,750 seconds of operation, translating to an average of 29 impacts per hour per square metre. The satellite’s rotational behaviour also contributed to advances in the understanding of rigid body dynamics, as its spin evolved toward the axis of maximum inertia, prompting renewed attention to Euler’s rotational theory.

Legacy and subsequent missions

Explorer 1 inaugurated the long-running Explorer programme, which remains active today as one of NASA’s principal lines of scientific satellites. Four further satellites were launched on Juno I vehicles in 1958: Explorer 3 and Explorer 4 were successful, whereas Explorer 2 and Explorer 5 failed to reach orbit. The programme’s final Juno I flight, carrying Project Beacon, also failed.