Juno I

Juno I was a four-stage American space launch vehicle used to place lightweight payloads into low Earth orbit during the early phase of the United States space programme. Operated between January 1958 and December 1959, it formed part of the Redstone family of rockets and was directly derived from the Jupiter-C sounding rocket. Owing to the similarity of names, it is often confused with the later Juno II, which was instead based on the PGM-19 Jupiter medium-range ballistic missile. Juno I achieved historical significance by launching Explorer 1, the first satellite of the United States, in 1958.

Historical background

Juno I emerged within the wider framework of the Explorers Program, which aimed to place artificial satellites into orbit. The political and scientific landscape changed dramatically after the Soviet Union launched Sputnik 1 on 4 October 1957, triggering the Sputnik crisis and galvanising the United States to accelerate its own satellite efforts. A failed Vanguard launch attempt added to the urgency.
In November 1957, Jet Propulsion Laboratory Director Bill Pickering proposed the name “Juno” after the Roman goddess, aligning the vehicle with the Jupiter-C lineage while designating it as the satellite-launching variant. The design incorporated a potential fourth stage, a concept developed after a test flight of a Jupiter-C in September 1956 for the Army Ballistic Missile Agency. That test had demonstrated that, had an active fourth stage been carried, the projectile could have achieved Earth orbit.
The first operational Juno I launch occurred on 1 February 1958 (GMT), successfully orbiting Explorer 1. The mission had previously been delayed by two scrubs in late January. With this success, the United States entered the space age. Explorer 1 provided the first evidence of the Van Allen radiation belts, a hallmark scientific discovery of the early space era.
Juno I made five additional launch attempts in 1958. Of these, two—Explorer 3 and Explorer 4—were successful, and three ended in failure. Its final launch, on 23 October 1958 from Cape Canaveral, failed to achieve orbit. By the end of 1959 the system had been replaced by the more capable Juno II.

Launch vehicle design

The Juno I comprised four stages arranged to provide sufficient impulse to achieve orbital velocity. The first stage was a Jupiter-C booster, itself based on the PGM-11 Redstone missile. The second, third, and fourth stages were clusters of solid-fuelled motors derived from the MGM-29 Sergeant.
The fourth stage sat atop a rotating “tub”, mounted above the third stage and spun up while the rocket remained on the launch pad. This spin stabilisation generated gyroscopic forces that eliminated the need for a full upper-stage guidance system, making the design relatively simple. The first-stage guidance package, still attached to the spinning tub, separated after burnout and controlled orientation until second-stage ignition.
After third-stage burnout, the fourth stage fired to accelerate the payload to orbital velocity, attaining accelerations of roughly 25–51 g. While this multistage arrangement offered a straightforward method of reaching orbit, the absence of active upper-stage guidance meant that payloads could not be placed into precise orbits. Both the four-stage Juno I and the three-stage Jupiter-C shared the same overall height, with the Juno I fourth stage enclosed within the third-stage nose cone.

Launch record

The Army Ballistic Missile Agency conducted six Juno I launches during 1958, all with the aim of deploying satellites into low Earth orbit. The flight record was as follows:

• Explorer 1 (1 February 1958) — Success
• Explorer 2 — Failure
• Explorer 3 — Success
• Explorer 4 — Success
• Explorer 5 — Failure
• Final Juno I launch (October 1958) — Failure

Overall, the Juno I achieved a success rate of 50 per cent. Despite the mixed performance, the early success of Explorer 1 had an outsized impact on public perception and national morale. Following high-profile failures of the Vanguard and Viking programmes, the Juno I launch boosted confidence in the U.S. space effort and highlighted the capabilities of Wernher von Braun and his team.
The limited capability and reliability of Juno I eventually led to its replacement by the more powerful Juno II, which adopted a PGM-19 Jupiter first stage.